The cover image was created by the transcriber and is placed in the public domain.
THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE ANSWER
A PROGRESSIVE COURSE OF STUDY FOR ENGINEERS, ELECTRICIANS, STUDENTS AND THOSE DESIRING TO ACQUIRE A WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
COPYRIGHTED, 1914,
BY
THEO. AUDEL & CO.,
New York.
Printed in the United States.
ALTERNATING CURRENT MOTORS | 1,267 to 1,376 |
---|---|
Classification—synchronous motors—essential parts—synchronous motor principles: condition for starting; effective pressure; dead centers; speed; limit of lag; effect of load changes—effect of altering the field strength—disadvantages of synchronous motors; advantages—the "V" curve—adaptation—efficiency—hunting of synchronous motors; mechanical analogy—use as condenser—surging—characteristics of synchronous motors: starting; running; stopping; effect upon circuit; power factor; auxiliary apparatus; adaptation—induction (asynchronous) motors—essential parts—types—oscillating magnetic field—rotating magnetic field—operation of single phase motor; why not self starting; provision for starting—operation of polyphase induction motor; why called asynchronous—speed; classification according to speed—the terms primary and secondary—why polyphase induction motors are explained before single phase—polyphase induction motors—features—essential parts—principles—production of rotating field—Tesla's rotating field—method of obtaining resultant flux of Tesla's field—Arago's rotations; explanation—Faraday's experiment—production of two phase rotating field; resultant poles—six and eight pole two phase rotating fields—physical conception of two phase rotating field—production of three phase rotating field; with ring winding—physical conception of three phase rotating field—three phase six pole winding—slip—copper cylinder illustrating principle of operation of induction motor—calculation of slip—table of synchronous speeds—variation of slip; why so small; variation with load; table of variation—sector method of measuring slip—evolution of [Pg iv]the squirrel cage armature; construction—the field magnets; parts; construction—field windings for induction motors—calculation for revolutions of rotating field; objection to high speed of field—difficulty with low frequency currents—general character of field winding—formation of poles—grouping of coils—starting of induction motors: external resistance, auto-transformer, internal resistance methods—internal resistance induction motors; adaptation—how resistance is cut out—why not desirable for large sizes—external resistance or slip ring motors—operation—armature connections—single phase induction motors—service suitable for—disadvantage—parts—why not self-starting—how started—phase splitting; production of rotating field from oscillating field—methods—starting coils—shading coils—character of the starting torque—modification of armature for starting with heavy load—clutch type of single phase induction motor; its action in starting—commutator motors—classification—action of closed coil rotating in alternating field—the transformer pressure—generated pressure—self-induction pressure—local armature currents; reason for sparking; how reduced—high resistance connectors—effect of low power factor—effect of frequency—series motor—features—adaptation—neutralized series motor—conductive method—inductive method—shunt motors—repulsion motors—difficulty with early motors—means employed to stop sparking—essentials of single repulsion motors—the term repulsion induction motor—compensated repulsion motor—power factor of induction motors—its importance—false ideas in regard to power factor—speed and torque of motors. | |
TRANSFORMERS | 1,377 to 1,456 |
Their use—essential parts—basic principles—the primary winding—the secondary winding—magnetic leakage—the induced voltage—no load current—magnetizing current—action of transformer with load—classification—step up transformers—use—construction—copper economy—step down transformers—use—construction—core transformers—construction—advantages—shell transformers—comparison of core and shell types—choice—combined core and shell transformers—economy of construction—single and polyphase transformers—features of each type—choice of types for polyphase currents—operation of three phase transformer with one phase damaged[Pg v]—transformer losses—hysteresis—what governs the loss—how reduced—eddy currents—lamination—thickness of laminæ—importance of iron losses—how to reduce iron losses—copper losses—how caused—effect on power factor—effect of resistance—cooling of transformers—cooling mediums employed—heating of transformers—objection to heating—dry transformers—air cooled transformers—natural draught type—forced draught or air blast type—construction of coils for air cooling—requirements with respect to air supply—quantity of air used—oil cooled transformers—circulation of the oil—action of the oil—objection to oil—kind of oil used—oil requirements—moisture in oil—water cooled transformers—internal coil type—external coil type—thermo-circulation—quantity of circulating water required—transformer insulation—the "major" and "minor" insulation—mica—outdoor transformers for irrigation service—oil insulated transformers—efficiency of transformers—efficiency curve—all day efficiency of transformers—transformer fuse blocks—auto-transformers—constant current transformers for series arc lighting; elementary diagram illustrating principles—regulation—transformer connections—single phase connections—combining transformers—precautions—operating secondaries in parallel—connections for different voltages—precautions—two phase connections—three phase connections: delta, star, delta star, star-delta—comparison of star and delta connections—three phase transformers—comparison of air blast, water cooled, and oil cooled transformers—standard transformer connections—how to test transformers—transformer operation with grounded secondary—transformer capacity for motors—transformer connections for motors—arc lamp transformer—transformer installed on pole—static booster or regulating transformer. | |
CONVERTERS | 1,457 to 1,494 |
Where used—kinds of converter—A.I.E.E. classification—rotary converters—operation—speed—principles—relation between input and output pressures—single and polyphase types—advantage of polyphase converters—armature connections of polyphase converter—pressure relation—voltage variation—advantage of unity power factor—effect of field too strong—compounding of rotary converters—[Pg vi]ratio of conversion—voltage regulation—split pole method—regulating pole method—best location of regulating poles—reactance method—multi-tap transformer method—synchronous booster method—winding connections—field connections—adaptation—motor generator sets—classification—standard practice—behavior of rotary when hunting; comparison with motor generator sets—racing—frequency changing sets—parallel operation of frequency changers—cascade converter—speed—action in motor armature winding—advantages—how started—comparison of cascade converter with synchronous converter. | |
RECTIFIERS | 1,495 to 1,530 |
Classification—mechanical rectifiers—essential features—construction—application—electrolytic rectifiers—principles of operation—Mohawk rectifier—the term "valve"—metals for electrodes—electrolyte—Nodon valve—Audion valve—Buttner valve—Churcher valve—De Faria valve—Fleming oscillation valve—Grisson valve—Pawlowski valve—Giles electric valve—Buttner valve—mercury vapor rectifiers—principles—the terms "arc" and "vapor"—three phase mercury vapor rectifier—construction—auxiliary apparatus—series mercury arc rectifier—dissipation of heat from bulb—replacement of bulb—advantages of rectifier—precautions in installing—electromagnetic rectifiers—construction and operation. |
The almost universal adoption of the alternating current system of distribution of electrical energy for light and power, and the many inherent advantages of the alternating current motor, have created the wide field of application now covered by this type of apparatus.
As many central stations furnish only alternating current, it has become necessary for motor manufacturers to perfect types of alternating current motor suitable for all classes of industrial drive and which are adapted for use on the kinds of alternating circuit employed. This has naturally resulted in a multiplicity of types and a classification, to be comprehensive, must, as in the case of alternators, divide the motors into groups as regarded from several points of view. Accordingly, alternating current motors may be classified:
1. With respect to their principle of operation, as
a. SYNCHRONOUS MOTORS;
b. ASYNCHRONOUS MOTORS:
1. Induction motors;
{series;
2. Commutator motors {compensated;
{shunt;
{repulsion.
2. With respect to the current as
3. With respect to speed, as
a. Constant speed;
b. Variable speed.
4. With respect to structural features, as
a. Enclosed;
b. Semi-enclosed;
c. Open;
d. Pipe ventilated;
e. Back geared;
f. Skeleton frame;
g. Riveted frame;
h. Ventilated; etc.
Of the above divisions and sub-divisions some are self-defining and need little or no explanation; the others, however, will be considered in detail, with explanations of the principles of operation and construction.
Synchronous Motors.—The term "synchronous" means in unison, that is, in step. A so called synchronous motor, then, as generally defined, is one which rotates in unison or in step with the phase of the alternating current which operates it.
Strictly speaking, however, it should be noted that this condition of operation is only approximately realized as will be later shown.
Any single or polyphase alternator will operate as a synchronous motor when supplied with current at the same pressure and frequency as it produces as a generator, the essential condition, in the case of a single phase machine, being that it be speeded up to so called synchronism before being put in the circuit.
In construction, synchronous motors are almost identical with the corresponding alternator, and consist essentially of two elements:
[Pg 1271] either of which may revolve. The field is separately excited with direct current.
The principles upon which such motors operate may be explained by considering the action of two elementary alternators connected in circuit, as illustrated in the accompanying illustrations, one alternator being used as a generator and the other as a synchronous motor.
Suppose the motor, as in figs. 1,585 and 1,586, be at rest when[Pg 1272] it is connected in circuit with the alternator. The alternating current will flow through the motor armature and produce a reaction upon the field tending to rotate the motor armature first in one direction, then in another.
Because of the very rapid reversals in direction of the torque thus set up, there is not sufficient time to overcome the inertia of the armature before the current reverses and produces a torque in the opposite direction, hence, the armature remains stationary or, strictly speaking, it vibrates.
Now if the motor armature be first brought up to a speed corresponding in frequency to that of the alternator before connecting the motor[Pg 1274] in the circuit, the armature will continue revolving at the same frequency as the alternator.
The armature continues revolving, because, at synchronous speed, the field flux and armature current are always in the same relative position, producing a torque which always pulls the armature around in the same direction.
A polyphase synchronous motor is self starting, because, before the current has died out in the coils of one phase, it is increasing in those of the other phase or phases, so that there is always some turning effort exerted on the armature.
The speed of a synchronous motor is that at which it would have to run, if driven as an alternator, to deliver the number of cycles which is given by the supply alternator.
For instance a 12 pole alternator running at 600 revolutions per minute will deliver current at a frequency of 60 cycles a second; an 8 pole synchronous motor supplied from that circuit will run at 900 revolutions per minute, which is the speed at which it would have to be driven as an alternator to give 60 cycles a second—the frequency of the 12 pole alternator.
The following simple formula gives the speed relations between generators and motors connected to the same circuit and having different numbers of poles.
P × S | ||
s | = | |
p |
in which
Question. If the field strength of a synchronous motor be altered, what effect does this have on the speed, and why?
Ans. The speed does not change (save for a momentary variation to establish the phase relation corresponding to equilibrium), because the motor has to run at the same frequency as the alternator.
Ques. How does a synchronous motor adjust itself to changes of load and field strength?
Ans. By changing the phase difference between the current and pressure.
If, on connecting a synchronous motor to the mains, the excitation be too weak, so that the voltage is lower than that of the supply, this phase difference will appear resulting in wattless current, since the missing magnetization has, as it were, to be supplied from an external source. A phase difference also appears when the magnetization is too strong.
Ques. State the disadvantages of synchronous motors.
Ans. A synchronous motor requires an auxiliary power for starting, and will stop if, for any reason, the synchronism be destroyed; collector rings and brushes are required. For some purposes synchronous motors are not desirable, as for driving shafts in small workshops having no other power available for starting, and in cases where frequent starting, or a strong torque[Pg 1278] at starting is necessary. A synchronous motor has a tendency to hunt[1] and requires intelligent attention; also an exciting current which must be supplied from an external source.
Ques. State the advantage of synchronous motors.
Ans. The synchronous motor is desirable for large powers where starting under load is not necessary. Its power factor may be controlled by varying the field strength. The power factor can be made unity and, further, the current can be made to lead the pressure.
A synchronous motor is frequently connected in a circuit solely to improve the power factor. In such cases it is often called a "condenser motor" for the reason that its action is similar to that of a condenser.
[Pg 1279]The design of synchronous motors proceeds on the same lines as that of alternators, and the question of voltage regulation in the latter becomes a question of power factor regulation in the former.
Ques. For what service are they especially suited?
Ans. For high pressure service.
High voltage current supplied to the armature does not pass through a commutator or slip rings; the field current which passes through slip rings being of low pressure does not give any trouble.
Ques. How do synchronous and induction motors compare as to efficiency?
Ans. Synchronous motors are usually the more efficient.
Hunting of Synchronous Motors.—Since a synchronous motor runs practically in step with the alternator supplying it with current when they both have the same number of poles, or some multiple of the ratio of the number of poles on each machine, it will take an increasing current from the line as its speed drops behind the alternator, but will supply current to the line as a generator if for any reason the speed of the alternator[Pg 1281] should drop behind that of the motor, or the current wave lag behind, which produces the same effect, and due to additional self-induction or inductance produced by starting up or overloading some other motor or rotary converter in the circuit.
When the motor is first taking current, then giving current back to the line, and this action is continued periodically, the motor is said to be hunting.
Ques. What term is applied to describe the behavior of the current when hunting occurs?
Ans. The term surging is given to describe the current fluctuations produced by hunting.
The mechanical analogy of hunting illustrated in fig. 1,611 will help to an understanding of this phenomenon. In alternating current circuits a precisely similar action takes place between the alternators and synchronous motors, or even between the alternators themselves.
CHARACTERISTICS OF SYNCHRONOUS MOTORS
Starting.—The motor must be brought up to synchronous speed without load, a starting compensator being used. If provided with a self-starting device, the latter must be cut out of circuit at the proper time. The starting torque of motor with self-starting device is very small.
Running.—The motor runs at synchronous speed. The maximum torque is several times full load torque and occurs at synchronous speed.
Stopping.—If the motor receive a sudden overload sufficient to momentarily reduce its speed, it will stop; this may be brought about by momentary interruption of the current, sufficient to cause a loss of synchronism.
Effect upon Circuit.—In case of short circuit in the line the motor acts as a generator and thus increases the intensity of the short circuit. The motor impresses its own wave form upon the circuit. Over excitation will give to the circuit the effect of capacity, and under excitation, that of inductance.
Power Factor.—This depends upon the field current, wave form and hunting. The power factor may be controlled by varying the field excitation.
Necessary Auxiliary Apparatus.—Power for starting, or if self-starting, means of reducing the voltage while starting; also, field exciter, rheostat, friction clutch, main switch and exciter switch, instruments for indicating when the field current is properly adjusted.
Adaptation.—If induction motors be connected to the same line with a synchronous motor that has a steady load, then the field of the synchronous motor can be over excited to produce a leading current, which will counteract the effect of the lagging currents induced by the induction motors. Owing to the weak starting torque, skilled attendance required, and the liability of the motor to stop under abnormal working conditions, the synchronous motor is not adapted to general power distribution, but rather to large units which operate under a steady load and do not require frequent starting and stopping.
Induction (Asynchronous) Motors.—
An induction motor consists essentially of an armature and a field magnet, there being, in the simplest and most usual types, no electrical connection between these two parts.[2]
[2] NOTE.—The author prefers the terms armature and field magnet, instead of "primary," "secondary," "stator," "rotor," etc., as used by other writers, the armature being the part in which currents are induced and the field magnet (or magnets) that part furnishing the field in which the induction takes place.
According to the kind of current that an induction motor is designed to operate on, it may be classified as:
The operation of an induction motor depends on the production of a magnetic field by passing an alternating current through field magnets.
The character of this field is either
according as single phase or polyphase current is used.
[3] NOTE.—"The word oscillating is becoming specialized in its application to those currents and fields whose oscillations are being damped out, as in electric 'oscillations.' But for this, we should have spoken of an oscillating field."—S. P. Thompson. The author believes the word oscillating, notwithstanding its other usage, best describes the single phase field, and should be here used.
Ques. Describe briefly the operation of a single phase motor.
Ans. A single phase current being supplied to the field magnets, an oscillating field is set up. A single phase motor is[Pg 1285] not self-starting; but when the armature has been set in motion by external means, the reaction between the magnetic field and the induced currents in the armature being no longer zero, a torque is produced tending to turn the armature.
The current flowing through the armature produces an alternating polarity such that the attraction between the unlike armature and field poles is always in one direction, thus producing the torque.
Ques. Why is a single phase induction motor not self-starting?
Ans. When the armature is at the rest, the currents induced therein are at a maximum in a plane at right angles to the magnetic field, hence there is no initial torque to start the motor.
Ques. What provision is made for starting single phase induction motors?
Ans. Apparatus is supplied for "splitting the phase" (later described in detail) of the single phase current furnished, converting it temporarily into a two phase current, so as to obtain a rotating field which is maintained till the motor is brought up to speed. The phase splitting device is then cut out and the motor operated with the oscillating field produced by the single phase current.
Ques. Describe briefly the operation of a polyphase induction motor.
Ans. Its operation is due to the production of a rotating magnetic field by the polyphase current furnished. This field "rotating" in space about the axis of the armature induces currents in the latter. The reaction between these currents and the rotating field creates a torque which tends to turn the armature, whether the latter be at rest or in motion.
Ques. Why are induction motors called "asynchronous"?
Ans. Because the armature does not turn in synchronism with the rotating field, or, in the case of a single phase induction motor, with the oscillating field (considering the latter in the light of a rotating field).
Ques. How does the speed vary?
Ans. It is slower (more or less according to load) than the "field speed," that is, than "synchronism" or the "synchronous speed."
Ques. What is the difference of speed called?
Ans. The slip.
This is a vital factor in the operation of an induction motor, since there must be slip in order that the armature inductors shall cut magnetic lines to induce (hence the name "induction" motor) currents therein so as to create a driving torque.
Ques. What is the extent of the slip?
Ans. It varies from about 2 to 5 per cent. of synchronous speed depending upon the size.
Ques. Why are induction motors sometimes called constant speed motors?
Ans. They are erroneously and ill advisedly, yet conveniently so called by builders to distinguish them from induction motors[Pg 1290] fitted with special devices to obtain widely varying speeds, and which are known as variable speed induction motors.
The term adjustable would be better.
Motor, Constant Speed.—A motor in which the speed is either constant or does not materially vary; such as synchronous motors, induction motors with small slip, and ordinary direct current shunt motors.—Paragraph 46 of 1907 Standardization Rules of the A.I.E.E.
Motor, Variable Speed.—A motor in which provision is made for varying the speed as desired. The A.I.E.E. has unfortunately introduced the term varying speed motor, to designate "motors in which the speed varies with the load, decreasing when the load increases, such as series motors." The term is objectionable, since by the expression variable speed motor a much more general meaning is intended.
Ques. Why do some writers call the field magnets and armature the primary and secondary, respectively?
Ans. Because, in one sense, the induction motor is a species of transformer, that is, it acts in many respects like a transformer, the primary winding of which is on the field and the secondary winding on the armature.
In the motor the function of the secondary circuit is to furnish energy to produce a torque, instead of producing light and heat as in the case of the transformer. Such comparisons are ill advised when made for the purpose of supplying names for motor parts. There can be no confusion by employing the simple terms armature and field magnets, remembering that the latter is that part that produces the oscillating or rotating field (according as the motor is single or polyphase), and the former, that part in which currents are induced.
Ques. Why are polyphase induction motors usually presented in text books before single phase motors?
Ans. Because the latter must start with a rotating field and come up to speed before the oscillating field can be employed.
A knowledge then of the production of a rotating field is necessary to understand the action of the single phase motor at starting.
Polyphase Induction Motors.—As many central stations put out only alternating current circuits, it has become necessary for motor builders to perfect types of alternating current motor suitable for all classes of industrial drive and which are adapted for use on these commercial circuits. Three phase induction motors are slightly more efficient at all loads than two phase motors of corresponding size, due to the superior distribution of the field windings. The power factor is higher, especially at light loads, and the starting torque with full load current is also greater. Furthermore, for given requirements of load and voltage, the amount of copper required in the distributing system is less; consequently, wherever service conditions will permit, three phase motors are preferable to two phase.
The construction of an induction motor is very simple, and since there are no sliding contacts as with commutator motors, there can be no sparks during operation—a feature which adapts the motor for use in places where fire hazards are prominent.
The motor consists, as already mentioned, simply of two parts: an armature and field magnets, without any electrical connection between these parts. Its operation depends upon:
Production of a Rotating Field.—It should at once be understood that the term "rotating field" does not signify that part of the apparatus revolves, the expression merely refers to[Pg 1295] the magnetic lines of force set up by the field magnets without regard to whether the latter be the stationary or rotating member.
A rotating field then may be defined as the resultant magnetic field produced by a system of coils symmetrically placed and supplied with polyphase currents.
A rotating magnetic field can, of course, be produced by spinning a horse shoe magnet around its longitudinal axis, but with polyphase currents, as will be later shown, the rotation of the field can be produced Without any movement of the mechanical parts of the electro magnets.
The original rotating magnetic field dates back to 1823, when Francois Jean Arago, an assistant in Davy's laboratory, discovered that if a magnet be rotated before a metal disc, the latter had a tendency to follow the motion of the magnet, as shown in fig. 290, page 270 and also in fig. 1,656. This experiment led up to the discovery which was made by Arago in 1824, when he observed that the number of oscillations which a magnetized[Pg 1296] needle makes in a given time, under the influence of the earth's magnetism, is very much lessened by the proximity of certain metallic masses, and especially of copper, which, may reduce the number in a given time from 300 to 4.
The explanation of Arago's rotations is that the magnetic field cutting the disc produces eddy currents therein and the reaction between the latter and the field causes the disc to follow the rotations of the field.
The induction motor is a logical development of the experiment of Arago, which so interested Faraday while an assistant in Davy's laboratory and which led him to the discovery of the laws of electromagnetic induction, which are given in Chapter X.
[4]In 1885, Professor Ferraris, of Turin discovered that a rotating field could be produced from stationary coils by means of polyphase currents.
[4] Note.—Walmsley attributes the first production of rotating fields to Walter Bailey in 1879, who exhibited a model at a meeting of the Physical Society of London, but very little was done, it is stated, until Ferraris took up the subject.
[5]This discovery was commercially applied a few years later by Tesla, Brown, and Dobrowolsky.
[5] Note.—The Tesla patents were acquired in the U.S. by the Westinghouse Co. in 1888, and polyphase induction motors, as they were called, were soon on the market. Brown of the Oerlikon Machine Works developed the single phase system and operated a transmission plant over five miles in length at Kassel, Germany, which operated at 2,000 volts.
The principles of polyphase motors can be best understood by means of elementary diagrams illustrating the action of polyphase currents in producing a rotating magnetic field, as explained in the paragraphs following.
Production of a Rotating Magnetic Field by Two Phase Currents.—Fig. 1,659 represents an iron ring wound with coils of insulated wire, which are supplied with a two phase current at the four points A, B, C, D, the points A and B, and C and D, being electrically connected.
According to the principles of electromagnetic induction, if only one current a entered the ring at A, and the direction of the winding be suitable, a negative pole (-) will be produced at A and a positive pole (+) at B, so that a magnetic needle pivoted in the center of the ring would tend to point vertically[Pg 1299] upward towards A. Now suppose that at this instant, corresponding to the beginning of an alternating current cycle, a second current b, differing in phase from the first by 90 degrees, is allowed to enter the ring at C. As shown in fig. 1,659, when the pressure of the current a is at its maximum, that of the current b is at its minimum; therefore, even a two phase current, at the beginning of the cycle, the needle will point toward A.
As the cycle continues, however, the strength of a will diminish and that of b increase, thus shifting the induced pole toward C, until b attains its maximum and a falls to its minimum at 90° or the end of the first quarter of the cycle, when the needle will point toward C. At 90°, the phase a current reverses in direction and produces a negative pole at B, and as its strength increases from 90° to the 180° point of the cycle, and that of phase b diminishes, the resultant negative pole is shifted past C toward B, until a attains its maximum and b falls to its minimum at 180°, and the needle points in the direction of B.
At the 180° point of the cycle, b reverses in direction and produces a negative pole at D, and as the fluctuation of the pressure of the two currents during the second half of the cycle, from 180° to 360°, bear the same relation to each other as during the first half, the resultant poles of the rotating magnetic field thus produced carry the needle around in continuous rotation so long as the two phase current traverses the windings of the ring.
Production of Rotating Magnetic Field by Three Phase Current.—A rotating magnetic field is produced by the action of a three phase current in a manner quite similar to the action of a two phase current. Fig. 1,685 shows a ring suitably wound and supplied with a three phase current at three points A, B, C, 120° of a cycle apart.
At the instant when the current a, flowing in at A, is at its maximum, two currents b and c, each one-half the value of a, will flow out B and C, thus producing a negative pole at A and a positive pole at B and at C. The resultant of the latter will be a positive pole at E, and consequently, the magnetic needle will point towards A.
As the cycle advances, however, the mutual relations of the fluctuations of the pressures of the three currents, and the time of their reversals of direction will be such, that when a maximum current is flowing at any one of the points A, B, and C, two currents each of one-half the value of the entering current will flow out of the other two points, and when two currents are entering at any two points, a current of maximum value will flow out of the other point. This action will produce one complete rotation of the magnetic field during each cycle of the current.
Slip.—Instead of the magnetic needle as was used in the preceding figures, a copper cylinder may be placed in a rotating magnetic field and it will be urged also to turn in the same direction as the rotation of the field.
The torque tending to turn the cylinder is due to the induction of currents of opposite polarity in the cylinder.
For simplicity, the rotating magnetic field may be supposed to be produced by a pair of magnetic poles placed at opposite sides of the cylinder and revolved around it as in fig. 1,710.
Now, for instance in starting, the cylinder being at rest any element or section of the surface as the shaded area AB, will, as it comes into the magnetic field of the rotating magnet, cut
[Pg 1312] magnetic lines of force inducing a current therein, whose direction is easily determined by applying Fleming's rule.[6]
[6] Note.—In order to avoid confusion in applying Fleming's rule, it may be well to regard the pole as being stationary and the cylinder as in motion; for, since motion is "purely a relative matter" (see fig. 1,393), the inductive action will be the same as if the pole stood still while the cylinder revolved from left to right, that is, counter clockwise, looking down on it. Regarding it thus (pole stationary and cylinder revolving counter clockwise) Fleming's rule (see fig. 132, page 133) is easily applied to ascertain the direction of the induced current, which is found to flow upward in the shaded area as shown.
Since the field is not uniform, but gradually weakens, as shown, on either side of the shaded area (which is just passing the center), the pressure induced on either side will be less than that induced in the shaded area, giving rise to eddy currents (as illustrated in fig. 291, page 271). These eddy currents induce poles as indicated at the centers of the whorls, the polarity being determined by applying the right hand rule (fig. 119, page 117).
By inspection of fig. 1710, it is seen that the induced pole toward which the magnet is moving is of the same polarity as the magnet; therefore it is repelled, while the induced pole from which the magnet is receding, being of opposite polarity, is attracted. A torque is thus produced tending to rotate the cylinder.
It must be evident that this torque is greatest when the cylinder is at rest, because the magnetic lines are cut by any element on the cylindrical surface at the maximum rate.
Moreover, as cylinder is set in motion and brought up to speed, the torque is gradually reduced, because the rate with which the magnetic lines are cut is gradually reduced.
Ques. What is the essential condition for the operation of an induction motor?
Ans. The armature, or part in which currents are induced, must rotate at a speed slower than that of the rotating magnetic field.
In the elementary induction motor, fig. 1,710, the cylinder is the armature, and the rotating magnets are the equivalent of a rotating magnetic field.
Ques. What is the difference of speed called?
Ans. The slip.
Ques. Why is slip necessary in the operation of an induction motor?
Ans. If the armature had no weight and there was no friction offered by the bearings and air, it would revolve in synchronism with the rotating magnetic field, that is, the slip would be zero; but since weight and friction are always present and constitute[Pg 1314] a small load, its speed of rotation will be a little less than that of the rotating magnetic field, so that induction will take place, in amount sufficient to produce a torque that will balance the load.
Ques. How is slip expressed?
Ans. In terms of synchronism, that is, as a percentage of the speed of the rotating magnetic field.
The slip is obtained from the following formula:
Slip (rev. per sec.) = Sf - Sa
or, expressed as a percentage of synchronism, that is, of the synchronous speed,
(Sf - Sa) × 100 | ||
Slip (%) | = | |
Sf |
where
The synchronous speed is determined the same as for synchronous motor by use of the following formula:
2f | ||
Sf | = | |
P |
where
The following table gives the synchronous speed for various frequencies and different numbers of poles:
Frequency | R.P.M. of the rotating magnetic field, when number of poles is | |||||
---|---|---|---|---|---|---|
2 | 6 | 10 | 16 | 20 | 24 | |
25 | 1,500 | 500 | 300 | 188 | 150 | 125 |
60 | 3,600 | 1,200 | 720 | 450 | 360 | 300 |
80 | 4,800 | 1,600 | 960 | 600 | 480 | 400 |
100 | 6,000 | 2,000 | 1,200 | 750 | 600 | 500 |
120 | 7,200 | 2,400 | 1,440 | 900 | 720 | 600 |
125 | 7,500 | 2,500 | 1,500 | 938 | 750 | 625 |
Ques. How does the slip vary?
Ans. It varies from about 1 per cent. in a motor designed for very close regulation to 40 per cent. in one badly designed, or designed for some special purpose.
Ques. Why is the slip ordinarily so small?
Ans. Because of the very low resistance of the armature, very little pressure is required to produce currents therein, of sufficient strength to give the required torque. Hence, the necessary rate of cutting the magnetic lines to induce this pressure in the armature is reached with very little difference between the field speed and armature speed, that is, with very little slip.
Ques. How does the slip vary with the load?
Ans. The greater the load the greater the slip.
In other words, if the load increase, the motor will run slower, and the slip will increase. With the increased slip, the induced currents and the driving force will further increase. If the motor be well designed so that the field strength is constant and the lag of the armature currents is small, the driving force developed or torque will be proportional to the slip, that is the slip will increase automatically as the load is increased, so that the torque will be proportional to the load.
According to Weiner, the slip varies according to the following table:
Capacity of motor H. P. | Slip at full load per cent. | Capacity of motor H. P. | Slip at full load per cent. | ||
---|---|---|---|---|---|
Usual limits | Average | Usual limits | Average | ||
⅛ | 20 to 40 | 30 | 15 | 5 to 11 | 8 |
¼ | 10 " 30 | 20 | 20 | 4 " 10 | 7 |
½ | 10 " 20 | 15 | 30 | 3 " 9 | 6 |
1 | 8 " 20 | 14 | 50 | 2 " 8 | 5 |
2 | 8 " 18 | 13 | 75 | 1 " 7 | 4 |
3 | 8 " 16 | 12 | 100 | 1 " 6 | 3.5 |
5 | 7 " 15 | 11 | 150 | 1 " 5 | 3 |
7½ | 6 " 14 | 10 | 200 | 1 " 4 | 2.5 |
10 | 7 " 12 | 9 | 300 | 1 " 3 | 2 |
Ques. Describe one way of measuring the slip.
Ans. A simple though rough way is to observe simultaneously the speed of the armature and the frequency, calculating the slip from the data thus obtained, as on page 1,315.
This method is not accurate, as, even with the most careful readings, large errors cannot be avoided. A better way is shown in fig. 1,736.
Evolution of the Squirrel Cage Armature.—In the early experiments with rotating magnetic fields, copper discs were used; in fact, it was then discovered that a mass of copper or any conducting metal, if placed in a rotating magnetic field, will be urged in the direction of rotation of the field.
Ferraris used a copper cylinder as in figs. 1,710 and 1,738, which was the first step in the evolution of the squirrel cage armature. The trouble with an armature of this kind is that there is no definite path provided for the induced currents.
Obviously, a better result is obtained if, in fig. 1,738, the downward returning currents of the eddies are led into some path where they will return across a field of opposite polarity from[Pg 1323] that across which they ascended, as in such case, the turning effect will be doubled. Accordingly the design of fig. 1,738 was modified by cutting a number of parallel slits which extended nearly to the ends, leaving at each end an uninterrupted "ring" of metal. This may be called the first squirrel cage armature, and in the later development Dobrowolsky was the first to employ a built-up construction, using a number of bars joined together by a ring at each end, as in fig. 1,740, and embedded in a solid mass of iron, as in fig. 1,741; he regarding the bars merely as veins of copper lying buried in the iron.
A solid cylinder of iron will of course serve as an armature, as it is magnetically excellent; but the high specific resistance of iron prevents the flow of induced currents taking place sufficiently copiously; hence a solid cylinder of iron is improved by surrounding it with a mantle of copper, or by a squirrel cage of copper bars (like fig. 1,740), or by embedding rods of copper (short circuited together at their ends with rings) in holes just beneath its surface. However, since all eddy currents that circle round, as those sketched in fig. 1,738, are not so efficient in their mechanical effect as currents confined to proper paths, and as they consume power and spend it in heating effects, the core was then constructed with laminations lightly insulated from each other, and further the squirrel cage copper bar inductors were fully insulated from contact with the core. Tunnel slots were later replaced by designs with open tops.
Fig. 1,744 shows a modern squirrel cage armature conforming to the latest practice, other designs being illustrated in the numerous accompanying cuts.
In the smaller sizes, the core laminæ are of the solid type as shown in fig. 1,745, but for larger motors the core consists of a spider and segmental discs as shown in figs. 1,750 and 1,751.
Fig. 1,748 shows a soldered form of end ring construction, and figs. 1,752 and 1,753 the method of welding the end ring to the inductors.
The Field Magnets.—The construction of the field magnets, which, when energized with alternating current produce the rotating magnetic field, is in many respects identical with the armature construction of revolving field alternators.
[Pg 1327] Broadly, the field magnets of induction motors consists of:
Ques. What is the construction of the yoke and laminæ?
Ans. They are in every way similar to the armature frame and core construction of revolving field alternators.
Field Windings for Induction Motors.—The field windings of induction motors are almost always made to produce more than two poles in order that the speed may not be unreasonably high. This will be seen from the following:
If P be the number of pairs of poles per phase, f, the frequency, and N, the number of revolutions of the rotating field per minute, then
60 × f | ||
N | = | |
P |
Thus for a frequency of 100 and one pair of poles, N = 60 × 100 ÷ 1 = 6,000. By increasing the number of pairs of poles to 10, the frequency remaining the same, N = 60 × 100 ÷ 10 = 600. Hence, in design, by increasing the number of pairs of poles the speed of the motor is reduced.
Ques. State an objection to very high speed of the rotating field.
Ans. The more rapid the rotation of the field, the greater is the starting difficulty.
Ques. Besides employing a multiplicity of poles, what other means is used to reduce the speed?
Ans. Reducing the frequency.
Ques. What difficulty is encountered with low frequency currents?
Ans. If the frequency be very low, the current would not be suitable for incandescent lamp lighting, because at low frequency the rise and fall of the current in the lamps is perceptible.
Ques. What is the general character of the field winding?
Ans. The field core slots contain a distributed winding of substantially the same character as the armature winding of a revolving field polyphase alternator.
Ques. Are the poles formed in the usual way?
Ans. They are produced by properly connecting the groups of coils and not by windings concentrated at certain points on salient or separately projecting masses of iron, as in direct current machines.
Ques. How are the coils grouped?
Ans. Three phase windings are usually Y connected.
Ques. What other arrangement is sometimes used?
Ans. In some cases Y grouping is used for starting and Δ grouping for running.
Starting of Induction Motors.—It must be evident that if the field winding of an induction motor whose armature is at rest, be connected directly in the circuit without using any starting device, the machine is placed in the same condition as a transformer with the secondary short circuited and the primary connected to the supply circuit. Owing to the very low resistance[Pg 1332] of the armature, the machine, unless it be of very small size, would probably be destroyed by the heat generated before it could come up to speed. Accordingly some form of starting device is necessary. There are several methods of starting, as with:
Ques. Explain the method of inserting resistances in the field.
Ans. Variable resistances are inserted in the circuits leading to the field magnets and mechanically arranged so that the[Pg 1333] resistances are varied simultaneously for each phase in equal amounts. These starting resistances are enclosed in a box similar to a direct current motor rheostat.
Ques. Is this a good method?
Ans. It is more economical to insert a variable inductance in the circuit, by using an auto-transformer.
Ques. What is the auto-transformer or compensator method of starting?
Ans. It consists of reducing the pressure at the field terminals by interposing an impedance coil across the supply circuit and feeding the motor from variable points on its windings.
Internal Resistance Induction Motors.—The armature of this type of induction motor differs from the squirrel cage variety in that the winding is not short circuited through copper rings, but, in starting, is short circuited through a resistance mounted directly on the shaft in the interior of the armature.
When the motor is thrown in circuit, a very low starting current is drawn from the line due to the added resistance in the armature. As the motor comes up to speed, this resistance is gradually cut out, and at full speed the motor operates as a squirrel cage motor, with short circuited winding.
Ques. How is the resistance gradually cut out in internal resistance motors?
Ans. By operating a lever which engages a collar free to slide horizontally on the shaft. The collar moves over the internal resistance grids (located within the armature spider), thus gradually reducing their value until they are cut out.
Ques. For what size motors is the internal resistance method suited?
Ans. Small motors.
Ques. Why is it not desirable for large motors?
Ans. The excessive I2R loss in the resistances, if confined within the armature spider, would produce considerable heating, and on this account it is best placed external to the motor.
Ques. On what class of circuit are internal resistance motors desirable?
Ans. On circuits devoted to lighting service as well as power service, where a high degree of voltage regulation is essential.
The initial rush of current when a squirrel cage motor is thrown on the line is more or less objectionable and there are central stations which allow only resistance type of induction motor to be used on their lines.
External Resistance or Slip Ring Motors.—In large machines, and those which must run at variable speed, such as is required in the operations of cranes, hoists, dredges, etc., it is[Pg 1337] advisable that the regulating resistances be placed externally to the motor. Motors having this feature are commercially known as slip ring motors, because connections are made between the external resistances and the armature inductors by means of slip rings.
As with the internal resistance motor the armature winding of a slip ring motor is not short circuited through copper rings in starting, but through a resistance, which in this case is located externally.
Ques. How is the armature winding connected?
Ans. It is connected in Y grouping and the free ends connected to the slip rings, leads going from the brushes to the[Pg 1338] variable external resistances, these being illustrated in fig. 1,779.
Single Phase Induction Motors.—The general utility of single phase motors, particularly the smaller sizes, is constantly[Pg 1339] being enlarged by the growing practice of central stations generating polyphase current, of supplying their lighting service through single phase distribution, and permitting the use of single phase motors of moderate capacity on the lighting circuit.
The simplicity of single phase systems in comparison with polyphase systems, makes them more desirable for small alternating current plants.
The disadvantage of single phase motors is that they are not self-starting.
A single phase motor consists essentially of an armature and field magnet having a single phase winding and also some phase splitting arrangement for starting.
Ques. Why is a single phase motor not self-starting?
Ans. Because the nature of the field produced by a single phase current is oscillating and not rotating.
Ques. How is a single phase motor started?
Ans. By splitting the phase, a field is set up normal to the[Pg 1343] axis of the armature, and nearly 90° displaced in phase from the field in that axis. This cross field produces the useful torque.
Phase Splitting; Production of Rotating Field from Oscillating Field.—As previously stated, an oscillating field, that is, one due to a single phase current, does not furnish any starting torque. It is therefore necessary to provide a rotating[Pg 1344] field for a single phase induction motor to start on, which, after the motor has come up to speed, may be cut out and the motor will then operate with the oscillating field.
A rotating field may be obtained from single phase current by what is known as splitting the phase.
Ques. Describe one method of splitting the phase.
Ans. The field of the motor is provided, in addition to the main single phase winding, with an auxiliary single phase winding, and the two windings are connected in parallel to the single phase supply mains with a resistance or a condenser placed in series with the single phase winding, as shown in diagram fig. 1,830, the two windings being displaced from each other[Pg 1345] on the armature about 90 magnetic degrees, just as in the ordinary two phase motor.
Ques. What is the construction of the two windings?
Ans. The main coils are of more turns than the auxiliary, being spread over more surface, and are heavier because they are for constant use; whereas the auxiliary coils are used only while starting.
Ques. What are the auxiliary coils sometimes called?
Ans. Starting coils.
Ques. What are "shading" coils?
Ans. Auxiliary coils as placed on fan motors in the manner shown in fig. 1,863.
Ques. How can single phase motors be started without the use of external phase splitting devices?
Ans. Such apparatus may be avoided by having the auxiliary winding of larger self-inductance than the main winding.
Ques. What is the character of the starting torque produced by splitting the phase?
Ans. It does not give strong starting torque.
Ques. How is the plain squirrel cage armature modified to enable the motor to start with a heavier load?
Ans. An automatic clutch is provided which allows the armature to turn free on the shaft until it accelerates almost to running speed.
This type motor is known as the clutch type of single phase induction motor. In operation when the circuit is closed, the armature starts to revolve upon the shaft; when it reaches a premeditated speed, a centrifugal clutch expands and engages the clutch disc, which is fastened to the shaft.
Ques. Explain in detail the action of the clutch type of motor in starting.
Ans. It can start a load which requires much more than full load torque at starting, because the motor being nearly up to full speed, has available not only its maximum overload capacity, but also the momentum of the armature to overcome the inertia of the driven apparatus. In this it is assisted by a certain amount of slippage in the clutch, which is the case when the armature speed is pulled down to such a point as to reduce the grip of the centrifugal clutch.
Commutator Motors.—Machines of this class are similar in general construction to direct current motors. They have a closed coil winding, which is connected to a commutator.
There are several types of commutator motor, namely:
Since, as stated, commutator motors are similar to direct current motors, the question may be asked: Is it possible to run a direct current motor with alternating current? If the mains leading to a direct current motor be reversed, the direction or rotation remains the same, because the currents through both the field magnets and armature are reversed. It must follow then that an alternating current applied to a direct current motor would cause rotation of the armature.
Action of Closed Coil Rotating in Alternating Field.—When a closed coil rotates in an alternating field, there are several different pressures set up and in order to carefully distinguish between them, they may be called:
These pressures may be defined as follows:
The transformer pressure is that pressure induced in the armature by the alternating flux from the field magnets.
For instance, assuming in fig. 1,872 the armature to be at rest, as the alternating current which energizes the magnets rises and falls in value, the variations of flux which threads through the coils of the ring winding, induce pressure in them in just the same way that pressure is induced in the secondary of a transformer.
A ring winding is used for simplicity; the same conditions obtain in a drum winding.
The generated pressure is that pressure induced in the armature by the cutting of the flux when the armature rotates.
The self-induction pressure is that pressure induced in both the field and armature by self-induction.
Nature of the Generated Pressure.—In fig. 1,872, the generated pressure induced by the rotation of the armature is minimum at the neutral plane C D and maximum at A B. It tends to cause current to flow up each half of the armature from D to C, producing poles at these points.
Nature of the Transformer Pressure.—This is caused by variations of the flux passing through each coil of the armature winding. Evidently this variation is least at the plane A B because at this point[Pg 1353] the coils are inclined very acutely to the flux, and greatest at the plane C D where the coils are perpendicular to the flux. Accordingly, the transformer pressure induced in the armature winding is least at A B and greatest at C D.
The transformer pressure acts in the same direction as the generated pressure as indicated by the long arrows and gives rise to what may be called local armature currents.
Nature of the Self-induction Pressure.—The self-induction pressure, being opposite in direction to the impressed pressure, it must be evident that in the operation of an alternating current commutator motor, the impressed pressure must overcome not only the generated
pressure but also the self-induction pressure. Hence, as compared to an equivalent direct current motor, the applied voltage must be greater than in the direct current machine, to produce an equal current.
The Local Armature Currents.—These currents produced by the transformer pressure occur in those coils undergoing commutation. They are large, because the maximum transformer action occurs in them, that is, in the coils short circuited by the brushes.
Ques. Why do the local armature currents cause sparking?
Ans. Because of the sudden interruption of the large volume of current, and also because the flux set up by the local currents being in opposition to the field flux, tends to weaken the field just when and where its greatest strength is required for commutation.
Ques. What is the strength of the local current?
Ans. They may be from 5 to 15 times the strength of the normal armature current.
Ques. Upon what does the local armature current depend?
Ans. Upon the number of turns of the short circuited coils, their resistance, and the frequency.
Ques. How can the local currents be reduced to avoid heavy sparking?
Ans. 1. By reducing the number of turns of the short circuited coils, that is, providing a greater number of commutator bars; 2, reducing the frequency; and 3, increasing the resistance of the short circuited coil circuit: a, by means of high resistance connectors; or b, by using brushes of higher resistance.
Ques. What are high resistance connectors?
Ans. The connectors between the armature winding and the commutator bars, as shown in fig. 1,885.
Ques. Does the added resistance of preventive leads, or high resistance brushes, materially reduce the efficiency of the machine?
Ans. Not to any great extent, because it is very small in comparison with the resistance of the whole armature winding.
Ques. What is the objection to reducing the number of turns of the short circuited coils to diminish the tendency to sparking?
Ans. The cost of the additional number of commutator bars and connectors as well as the added mechanism.
Ques. What effect has the inductance of the field and armature on the power factor?
Ans. It produces phase difference between the current and impressed pressure resulting in a low power factor.
Ques. What is the effect of this low power factor?
Ans. The regulation and efficiency of the system is impaired.
The frequency, the field flux and the number of turns in the winding have influence on the power factor.
Ques. How does the frequency affect the power factor?
Ans. Lowering the frequency tends to improve the power factor.
The use of very low frequencies has the disadvantage of departing from standard frequencies, and the probability that the greater cost of transformers and alternators would offset the gain.
Series Motors.—This class of commutator motor is about the simplest of the several types belonging to this division. In general design the series motor is identical with the series direct current motor, but all the iron of the magnetic circuit must be laminated and a neutralizing winding is often employed.
It will be readily understood that the torque is produced in the same way as in the direct current machine, when it is[Pg 1360] remembered that the direction of rotation of the direct current series motor is independent of the direction of the voltage applied.
At any moment the torque will be proportional to the product of the current and the flux which it is at that moment producing in the magnetic system, and the average torque will be the product of the average current and the average flux it produces, so that if the iron parts be unsaturated, as they must be if the iron losses are not to be too high, the torque will be proportional simply to the square of the current, there being no question of power factor entering into the consideration.
Ques. What are the characteristics of the series motor?
Ans. They are similar to the direct current series motor, the torque being a maximum at starting and decreasing as the speed increases.
Ques. For what service is the series motor especially suited?
Ans. On account of its powerful starting torque it is particularly desirable for traction service.
Neutralized Series Motor.—A chief defect of the series motor is the excessive self-induction of the armature, hence in almost every modern single phase series motor a neutralizing coil is employed to diminish the armature self-induction.
The neutralizing coil is wound upon the frame 90 magnetic degrees or half a pole pitch from the field winding and arranged to carry a current equal in magnetic pressure and opposite in phase to the current in the armature.
The current through the neutralizing winding may be obtained, either
In the conductive method, fig. 1,888, the winding is connected in series as shown.
In the inductive method, fig. 1,889, the winding is short circuited upon itself and the current obtained inductively, the neutralizing winding being virtually the secondary of a transformer, of which the armature is the primary.
Ques. When is the conductive method to be preferred?
Ans. When the motor is to be used on mixed circuits.
Shunt Motors.—The simple shunt motor has inherently many properties which render it unsuitable for practical use, and accordingly is of little importance. Owing to the many turns of the field winding there is large inductance in the shunt field circuit.
The inductance of the armature is small as compared with that of the field; accordingly, the two currents differ considerably in phase.
The phase difference between the field and armature currents and the corresponding relation between the respective fluxes results in a weak torque.
It is necessary to use laminated construction in the field circuit to avoid eddy currents, which otherwise would be excessive. Fig. 1,890 is a diagram of a simple shunt commutator motor.
Repulsion Motors.—In the course of his observations on the effects of alternating currents, in 1886-7, Elihu Thomson observed that a copper ring placed in an alternating magnetic field tends either to move out of the field, that is, it is repelled by the[Pg 1365] field (hence the name repulsion motor), or to return so as to set itself edgeways to the magnetic lines.
The explanation of the repulsion phenomenon is as follows:
When a closed coil is suspended in an alternating field so that lines of force pass through it, as in fig. 1,893, an alternating pressure will be induced in the coil which will be 90° later in phase than the inducing flux, and since every coil contains some inductance the resulting current will lag more or less with respect to the pressure induced in the coil.
The cosine of this phase relation becomes a negative quantity which means that the coil is repelled by the field.
It is only when the ring is in an oblique position that it tends to turn. If it be placed with its plane directly at right angles to the[Pg 1366] direction of the magnetic lines, it will not turn; if ever so little displaced to the right or left, it will turn until its plane is parallel to the lines.
The production of torque may be explained by saying that the current induced in the ring produces a cross field which being out of phase with, and inclined to the field impressed by the primary alternating current, causes a rotary field, and this in turn, reacting on the conductor, a turning moment results.
Elihu Thompson took an ordinary direct current armature, placed it in an alternating field, and having short circuited the brushes, placed them in an oblique position with respect to the direction of the field. The effect was to cause the armature to rotate with a considerable torque.
The inductors of the armature acted just as an obliquely placed ring, but with this difference, that the obliquity was continuously preserved[Pg 1368] by the brushes and commutator, notwithstanding that the armature turned, and thus the rotation was continuous. This tendency of a conductor to turn from an oblique position was thus utilized by him to get over the difficulty of starting a single phase motor. With this object in view he then constructed motors in which the use of commutator and brushes was restricted to the work of merely starting the armature, which when so started was then entirely short circuited on itself, though disconnected from the rest of the circuit, the operation then being solely on the induction principle.
Ques. What difficulty was experienced with Thomson's motor?
Ans. Since an open coil armature was used, the torque developed was due to only one coil at a time, which involved a necessarily high current in the short circuited coil resulting in heavy sparking.
Ques. How was this remedied?
Ans. By the use of closed coil armatures in later construction.
Ques. Did this effectually stop sparking?
Ans. No.
Ques. What other means is employed in modern designs to reduce sparking?
Ans. Compensation and the use of a distributed field winding, high resistance connectors, high resistance brushes, etc.
Ques. What are the names of the two classes of repulsion motor?
Ans. The simple and the compensated types.
Ques. Describe a simple repulsion motor.
Ans. It consists essentially of an armature, commutator and field magnets. The armature is wound exactly like a direct current armature, and the windings are connected to a commutator. The carbon brushes which rest on this commutator are not connected to the outside line, however, but are all connected together through heavy short circuiting connectors. The brushes are placed about 60° or 70° from the neutral axis. The field is wound exactly like that of the usual induction motor.
Ques. What is the action of this type of motor?
Ans. If nothing be done to prevent, the motor will increase in speed at no load until the armature bursts, just as it will in a series direct current motor.
Ques. What provision is made to avoid this danger?
Ans. A governor is usually mounted on the armature which short circuits the windings, after the motor has been started. The motor then runs as a squirrel cage induction motor. As a[Pg 1370] rule the brushes are lifted off the commutator when the armature is short circuited, so as to prolong their life.
This is a very successful motor, but it is of course more costly than the simple squirrel cage motor used on two and three-phase circuits.
Ques. What name may appropriately be applied to the motor?
Ans. It may be called the repulsion induction motor, because it is constructed for repulsion start and induction running.
Ques. Describe a compensated repulsion motor.
Ans. In its simplest form it consists of a simple repulsion motor in which there are two independent sets of brushes, one[Pg 1371] set being short circuited, while the other set is in series with the field magnet winding, as in the series alternating current motor.
Ques. What names are given to the two sets of brushes on a compensated repulsion motor?
Ans. The energy or main short circuiting brushes, and the compensating brushes.
Ques. What is the behavior of the armature of a compensated repulsion motor at starting?
Ans. It possesses at starting most of the apparent reactance of the motor, and the effect of speed is to decrease such apparent reactance, the latter becoming zero at either positive or negative synchronism, and negative at higher speeds in either direction.
Ques. What is the nature of the field circuit of the compensated repulsion motor at starting?
Ans. At starting it is practically non-inductive, the effect of speed being to introduce a spurious resistance which increases directly with the speed, and becomes negative when the speed is reversed.
Ques. For what use is the compensated repulsion motor especially adapted?
Ans. For light railroad service.
Ques. When employed thus what is the method of control?
Ans. A series transformer is used in the field circuit.
Ques. What frequencies are employed with this motor?
Ans. 25 to 60, the preferred frequency being 40.
Ques. To what important use is the repulsion principle put?
Ans. It is sometimes employed for starting on single phase induction motors.
In this method, after bringing the motor up to speed, the winding is then short circuited upon itself, and the motor then operates on the induction principle.
Ques. What name is given to this type of motor?
Ans. It is called the repulsion induction motor.
Power Factor of Induction Motors.—In the case of a direct current motor, the energy supplied is found by multiplying the current strength by the voltage, but in all induction motors the effect of self-induction causes the current to lag behind the pressure, thereby increasing the amount of current taken by the motor. Accordingly, as the increased current is not utilized by the motor in developing power, the value obtained by multiplying the current by the voltage represents an apparent energy which is greater than the real energy supplied to the motor.
It is evident, that if it were possible to eliminate the lag entirely, the real and apparent watts would be equal, and the power factor would be unity.
The importance of power factor and its effect upon both alternator capacity and voltage regulation is deserving of the most careful consideration with all electrical apparatus, in which an inherent phase difference exists between the pressure and the current, as for instance in static transformers and induction motors.
While the belief is current that any decrease in power factor from unity value does not demand any increase of mechanical output, this is not true, since all internal alternator and line losses manifest themselves as heat, the wasted energy to produce this heat being supplied by the prime mover.
Apart from the poor voltage regulation of alternating current generators requiring abnormal field excitation to compensate for low power factor, some of the station's rated output is rendered unavailable and consequently produces no revenue. The poor steam economy of underloaded engines is also a serious source of fuel wastage.
Careful investigations have shown that the power factor of industrial plants using induction motor drive with units of various sizes will average between 60 and 80 per cent. With plants supplying current to underloaded motors having inherently high lagging current values, a[Pg 1375] combined factor as low as 50 per cent. may be expected. Since standard alternators are seldom designed to carry their rated kilowatt load at less than 80 per cent. power factor, the net available output is, therefore, considerably increased.
Speed and Torque of Motors.—The speed of an induction motor depends chiefly on the frequency of the circuit and runs within 5 per cent. of its rated speed; it will produce full torque if the line voltage do not vary more than 5 to 10 per cent.
At low voltage the speed will not be greatly reduced as in a direct current motor, but as the torque is low the motor is easily stopped when a light load is thrown on.
The current taken by an induction motor from a constant pressure line varies with the speed as in a direct current motor. When a load is thrown on, the speed is reduced correspondingly and as the self-induction or reactance is diminished, more current circulates in the squirrel cage winding, which in turn reacts on the field coils in a similar manner and more current flows in them from the line. In this manner the motor automatically takes current from the line proportional to the load and maintains a nearly constant speed.
The so-called constant speed motors require slight variations in speed to automatically take current from the line when the load varies.
Induction motors vary in speed from 5 to 10 per cent., while synchronous motors vary but a fraction of one per cent.
Single phase motors to render efficient service must be able, where requisite, to develop sufficient turning moment or torque to accelerate, from standstill, loads possessing large inertia or excessive static friction; for example, meat choppers and grinders, sugar or laundry centrifugals; heavy punch presses; group driven machines running from countershafts with possibly over taut belting, poor alignment, lubrication, etc.
The developments in the field of electrical engineering which have rendered feasible the transmission of high pressure currents over long distances, together with the reliability and efficiency of modern generating units, have resulted in notable economies in the generation and distribution of electric current.
This has been accomplished largely by the use of distant water power or the centralization of the generating plants of a large territory in a single power station.
The transformer is one of the essential factors in effecting the economical distribution of electric energy, and may be defined as an apparatus used for changing the voltage and current of an alternating circuit. A transformer consists essentially of:
Basic Principles.—If a current be passed through a coil of wire encircling a bar of soft iron the iron will become a magnet; when the current is discontinued the bar loses its magnetization.
Conversely: If a bar of iron carrying a coil of wire be magnetized in a direction at right angles to the plane of the coil a momentary electric pressure will be induced in the wire; if the current be reversed, another momentary pressure will be induced in the opposite direction in the coil.
These actions are fully explained in chaps. X and XI, and as they are perfectly familiar phenomena, a detailed explanation of the principles upon which they depend is not necessary here.
From the first two statements given above it is evident that if a bar of iron be provided with two coils of wire, one of which is supplied from a source of alternating current, as shown diagrammatically by fig. 1,916, at each impulse of the exciting current a pressure will be induced in the secondary coil, the direction of these impulses alternating like that of the exciting current.
Ques. What name is given to the coil through which current from the source flows?
Ans. The primary winding.
Ques. What name is given to the coil in which a current is induced?
Ans. The secondary winding.
Similarly, the current from the source (alternator) is called the primary current and the induced current, the secondary current.
Ques. What is the objection to the elementary transformer shown in fig. 1,916?
Ans. The non-continuous core. With this type core, the flux emanating from the north pole of the bar has to return to[Pg 1379] the south pole through the surrounding air; and as the reluctance of air is much greater than that of iron, the magnetism will be weak.
Ques. How is this overcome?
Ans. By the use of a continuous core as shown in fig. 1,917.
Ques. Is this the best arrangement, and why?
Ans. No. If the windings were put on as in fig. 1,917, the leakage of magnetic lines of force would be excessive, as indicated by the dotted lines. In such a case the lines which leak through air have no effect upon the secondary winding, and are therefore wasted.
Ques. How is the magnetic leakage reduced to a minimum in commercial transformers?
Ans. In these, and even in ordinary induction coils (the operating principle of which is the same as that of transformers) the magnetic leakage is reduced to the lowest possible amount by arranging the coils one within the other, as shown in cross section in fig. 1,918.
The Induced Voltage.—The pressure induced in the secondary winding will depend on the ratio between the number of turns in the two windings. For example, a transformer with 500 turns of wire in its primary winding and 50 turns in its secondary winding would have a transformation ratio of 10 to 1, and if it were supplied with primary current at 1,000 volts, the secondary pressure at no load would be 100 volts.
EXAMPLE.—If ten amperes flow in the primary winding and the transformation ratio be 10, then 10 × 10 = 100 amperes will flow through the secondary winding.
Thus, a direct proportion exists between the pressures and turns in the two windings and an inverse proportion between the amperes and turns, that is:
From the above equations it is seen that the watts of the primary circuit equal the watts of the secondary circuit.
Ques. Are the above relations strictly true, and why?
Ans. No, they are only approximate, because of transformer losses.
In the above example, the total wattage in the primary circuit is 1,000 × 10 = 10 kw., and that in the secondary circuit is 100 × 100 = 10 kw. Hence, while both volts and amperes are widely different in the two circuits, the watts for each are the same in the ideal case, that is, assuming perfect transformer action or 100% efficiency. Now, the usual loss in commercial transformers is about 10%, so that the actual watts delivered in the secondary circuit is (100 × 100) × 90% = 9 kw.
The No Load Current.—When the secondary winding of a transformer is open or disconnected from the secondary circuit no current will flow in the winding, but a very small current called the no load current will flow in the primary circuit.
The reason for this is as follows: The current flowing in the primary winding causes repeated reversals of magnetic flux through the iron core. These variations of flux induce pressures in both coils; that induced in the primary called the reverse pressure is opposite in direction and very nearly equal to the impressed pressure, that is, to the pressure applied to the primary winding. Accordingly the only force available to cause current to flow through the primary winding is the difference between the impressed pressure and reverse pressure, the effective pressure.
The Magnetizing Current.—The magnetizing current of a transformer is sometimes spoken of as that current which the primary winding takes from the mains when working at normal pressure. The true magnetizing current is only that component of this total no load current which is in quadrature with the supply pressure. The remaining component has to overcome the various iron losses, and is therefore an "in phase" component. The relation between these two components determines the power factor of the so called "magnetizing current."
The true magnetizing component is small if the transformer be well designed, and be worked at low flux density.
Action of Transformer with Load.—If the secondary winding of a transformer be connected to the secondary circuit by closing a switch so that current flows through the secondary winding, the transformer is said to be loaded.
The action of this secondary current is to oppose the magnetizing action of the slight current already flowing in the primary winding, thus decreasing the maximum value reached by the alternating magnetic flux in the core, thereby decreasing the induced pressure in each winding.
[Pg 1384]The amount of this decrease, however, is very small, inasmuch as a very small decrease of the induced pressure in the primary coil greatly increases the difference between the pressure applied to the primary coil and the opposing pressure induced in the primary coil, so that the primary current is greatly increased. In fact, the increase of primary current due to the loading of the transformer is just great enough (or very nearly) to exactly balance the magnetizing action of the current in the secondary coil; that is, the flux in the core must be maintained approximately constant by the primary current whatever value the secondary current may have.
When the load on a transformer is increased, the primary of the transformer automatically takes additional current and power from the supply mains in direct proportion to the load on the secondary.
When the load on the secondary is reduced, for example by turning off lamps, the power taken from the supply mains by the primary coil is automatically reduced in proportion to the decrease in the load. This automatic action of the transformer is due to the balanced magnetizing action of the primary and secondary currents.
Classification of Transformers.—As in the case of motors, the great variety of transformer makes it necessary that a classification, to be comprehensive, must be made from several points of view, as:
1. With respect to the transformation, as
a. Step up transformers;
b. Step down transformers.
2. With respect to the arrangement of the coils and magnetic circuit, as
a. Core transformers;
b. Shell transformers;
c. Combined core and shell transformers.
3. With respect to the kind of circuit they are to be used on, as
a. Single phase transformers;
b. Polyphase transformers.
4. With respect to the method employed in cooling, as
a. Dry transformers;
b. Air cooled transformers {natural draught;
{forced draught, or air blast;
c. Oil cooled transformers;
d. Water cooled transformers.
5. With respect to the nature of their output, as
a. Constant pressure transformers;
b. Constant current transformers;
c. Current transformers;
d. Auto-transformers.
6. With respect to the kind of service, as
a. Distributing;
b. Power.
[Pg 1386]
7. With respect to the circuit connection that the transformer is constructed for, as
a. Series transformers;
b. Shunt transformers.
Step Up Transformers.—This form of transformer is used to transform a low voltage current into a high voltage current. Such transformers are employed at the generating end of a transmission line to raise the voltage of the alternators to such value as will enable the electric power to be economically transmitted to a distant point.
Copper Economy with Step Up Transformers.—To comprehend fully the bearing of the matter, it must be remembered that the energy supplied per second is the product of two factors, the current and the pressure at which that current is supplied; the magnitudes of the two factors may vary, but the value of the power supplied depends only on the product of the two; for example, the energy furnished per second by a current of 10 amperes supplied at a pressure of 2,000 volts is[Pg 1387] exactly the same in amount as that furnished per second by a current of 400 amperes supplied at a pressure of 50 volts; in each case, the product is 20,000 watts.
Now the loss of energy that occurs in transmission through a well insulated wire depends also on two factors, the current and the resistance of the wire, and in a given wire is proportional to the square of the current. In the above example the current of 400 amperes, if transmitted through the same wire as the 10 amperes current, would, because it is forty times as great, waste sixteen hundred times as much energy in heating the wire. It follows that, for the same loss of energy, the 10 ampere current at 2,000 volts may be carried by a wire having only 1/1,600th of the sectional area of the wire used for the 400 ampere current at 50 volts.
The cost of copper conductors for the distributing lines is therefore very greatly economized by employing high pressures for distribution of small currents.
Step Down Transformers.—When current is supplied to consumers for lighting purposes, and for the operation of motors, etc., considerations of safety as well as those of suitability,[Pg 1388] require the delivery of the current at comparatively low pressures ranging from 100 to 250 volts for lamps, and from 100 to 600 volts for motors.
This involves that the high pressure current in the transmission lines must be transformed to low pressure current at the receiving or distributing points by step down transformers, an elementary transformer being shown in fig. 1,925.
Transformers of this type have a large number of turns in the primary winding and a small number in the secondary, in ratio depending on the amount of pressure reduction required.
Core Transformers.—This type of transformer may be defined as one having an iron core, upon which the wire is wound in such a manner that the iron is enveloped within the coils, the outer surface of the coils being exposed to the air as shown in figs. 1,926 and 1,927.
Shell Transformers.—In the shell type of transformer, as shown in fig. 1,928, the core is in the form of a shell, being built around and through the coils. A shell transformer has, as a rule, fewer turns and a higher voltage per turn than the core type.
Ques. What is the comparison between core and shell transformers?
Ans. The relative advantages of the two types has been the subject of considerable discussion among manufacturers; the companies who formerly built only shell type transformers, now build core types, while with other builders the opposite practice obtains.
Ques. Upon what does the choice between the two types chiefly depend?
Ans. Upon manufacturing convenience rather than operating characteristics.
The major insulation in a core type transformer consists of several large pieces of great mechanical strength, while in the shell type, there are required an extremely large number of relatively small pieces of insulating material, which necessitates careful workmanship to prevent defects in the finished transformer, when thin or fragile material is used.
Both core and shell transformers are built for all ratings; for small ratings the core type possesses certain advantages with reference to insulation, while for large ratings, the shell type possesses better cooling properties, and has less magnetic leakage than the core type.
Combined Core and Shell Transformers.—An improved type of transformer has been introduced which can be considered either as two superposed shell transformers with coils in common, or as a single core type transformer with divided magnetic circuit and having coils on only one leg. It is best considered however, as a combined core and shell transformer,[Pg 1391] and for small sizes it possesses most of the advantages of both types. It can be constructed at less cost than can either a core or a shell transformer having the same operating characteristics and temperature limits.
Fig. 1,932 shows a cross section of the first transformer of this type to be developed commercially, and known as an "iron clad" transformer; this construction has been used in England for some time. Fig. 1,933 shows the American practice.
Ques. How is economy of construction obtained in designing combined core and shell transformers?
Ans. The cross section of iron in the central leg of the core is made somewhat less than that external to the coils, in order to reduce the amount of copper used in the coils.
Single and Polyphase Transformers.—A single phase transformer may be defined as one having only one set of primary and secondary terminals, and in which the fluxes in the one or more magnetic circuits are all in phase, as distinguished from a polyphase transformer, or combination in one unit of several one phase transformers with separate electric circuits but having certain magnetic circuits in common. In polyphase transformers[Pg 1393] there are two or more magnetic circuits through the core, and the fluxes in the various circuits are displaced in phase.
Ques. Is it necessary to use a polyphase transformer to transform a polyphase current?
Ans. No, a separate single phase transformer may be used for each phase.
Ques. Is there any choice between a polyphase transformer and separate single phase transformers for transforming a polyphase current?
Ans. Yes, the polyphase transformer is preferable, because less iron is required than would be with the several single phase transformers. The polyphase transformer therefore is somewhat lighter and also more efficient.
Ques. Name two varieties of polyphase transformer?
Ans. The core, and the shell types as shown in figs. 1,936 and 1,937.
Ques. How should a three phase transformer be operated with one phase damaged?
Ans. The damaged windings should be separated electrically from the other coils.
The pressure winding of the damaged phase should be short circuited upon itself and the corresponding low pressure winding should also be short circuited upon itself. The winding thus short circuited will choke down the flux passing through the portion of the core surrounded by them without producing in any portion of the winding a current greater than a small fraction of the current which would normally exist in such portion at full load.
Transformer Losses.—As previously mentioned, the ratio between the applied primary voltage and the secondary terminal voltage of a transformer is not always equal to the ratio of primary to secondary turns of wire around the core.
The commercial transformer is not a perfect converter of energy, that is, the input, or watts applied to the primary circuit is always more than the output or watts delivered from the secondary winding.
This is due to the various losses which take place, and the[Pg 1396] difference between the input and the output is equal to the sum of these losses. They are divided into two classes:
The iron or core losses are due to
Those which are classed as copper losses are due to
Hysteresis.—In the operation of a transformer the alternating current causes the core to undergo rapid reversals of magnetism. This requires an expenditure of energy which is converted into heat.
This loss of energy as before explained is due to the work required to change the position of the molecules of the iron, in reversing the magnetization. Extra power then must be taken from the line to make up for this loss, thus reducing the efficiency of the transformer.
Ques. Upon what does the hysteresis loss depend?
Ans. Upon the quality of the iron in the core, the magnetic density at which it is worked and the frequency.
Ques. With a given quality of iron how does the hysteresis loss vary?
Ans. It varies as the 1.6 power of the voltage with constant frequency.
Ques. In construction, what is done to obtain minimum hysteresis loss?
Ans. The softest iron obtainable is used for the core, and a low degree of magnetization is employed.
Eddy Currents.—The iron core of a transformer acts as a closed conductor in which small pressures of different values are induced in different parts by the alternating field, giving rise to eddy currents. Energy is thus consumed by these currents which is wasted in heating the iron, thus reducing the efficiency of the transformer.
Ques. How is the loss reduced to a minimum?
Ans. By the usual method of laminating the core.
The iron core is built up of very thin sheet iron or steel stampings, and these are insulated from each other by varnish and are laid face to face at right angles to the path that the eddy currents tend to follow, so that the currents would have to pass from sheet to sheet, through the insulation.
Ques. In practice, upon what does the thickness of the laminæ or stampings depend?
Ans. Upon the frequency.
The laminæ vary in thickness from about .014 to .025 inch, according as the frequency is respectively high or low.
Ques. Does a transformer take any current when the secondary circuit is open?
Ans. Yes, a "no load" current passes through the primary.
Ques. Why?
Ans. The energy thus supplied balances the core losses.
Ques. Are the iron or copper losses the more important, and why?
Ans. The iron losses, because these are going on as long as the primary pressure is maintained, and the copper losses take place only while energy is being delivered from the secondary.
Strictly speaking, on no load (that is when the secondary circuit is open) a slight copper loss takes place in the primary coil but because of its smallness is not mentioned. It is, to be exact, included in the expression "iron losses," as the precise meaning of this term signifies not only the hysteresis and eddy current losses but the copper loss in the primary coil when the secondary is open.
The importance of the iron losses is apparent in noting that in electric lighting the lights are in use only a small fraction of the 24 hours, but the iron losses continue all the time, thus the greater part of each day energy must be supplied to each transformer by the power company to meet the losses, during which time no money is received from the customers.
Some companies make a minimum charge per month whether any current is used or not to offset the no load transformer losses and rent of meter.
Ques. How may the iron losses be reduced to a minimum?
Ans. By having short magnetic paths of large area and using iron or steel of high permeability. The design and construction must keep the eddy currents as low as possible.
As before stated the iron losses take place continually, and since most transformers are loaded only a small fraction of a day it is very important that the iron losses should be reduced to a minimum.
With a large number of transformers on a line, the magnetizing current that is wasted, is considerable.
During May, 1910, the U. S. Bureau of Standards issued a circular showing that each watt saving in core losses was a saving of 88 cents, which is evident economy in the use of high grade transformers.
Copper Losses.—Since the primary and secondary windings of a transformer have resistance, some of the energy supplied will be lost by heating the copper. The amount of this loss is proportional to square of the current, and is usually spoken of as the I2R loss.
Ques. Define the copper losses.
Ans. The copper losses are the sum of the I2R losses of both the primary and secondary windings, and the eddy current loss in the conductors.
Ques. Is the eddy current loss in the conductors large?
Ans. No, it is very small and may be disregarded, so that the sum of the I2R losses of primary and secondary can be taken as the total copper loss for practical purposes.
Ques. What effect has the power factor on the copper losses?
Ans. Since the copper loss depends upon the current in the primary and secondary windings, it requires a larger current when the power factor is low than when high, hence the copper losses increase with a lowering of the power factor.
Ques. What effect other than heating has resistance in the windings?
Ans. It causes poor regulation.
This is objectionable, especially when incandescent lights are in use, because the voltage fluctuates inversely with load changes, that is, it drops as lamps are turned on and rises as they are turned off, producing disagreeable changes in the brilliancy of the lamps.
Cooling of Transformers.—Owing to the fact that a transformer is a stationary piece of apparatus, not receiving ventilation from moving parts, its efficient cooling becomes a very strong feature of the design, especially in the case of large high pressure transformers. The effective cooling is rendered more difficult because transformers are invariably enclosed in more or less air tight cases, except in very dry situations, where a perforated metal covering may be permitted.
The final degree to which the temperature rises after continuous working for some hours, depends on the total losses in iron and copper, on the total radiating surface, and on the facilities afforded for cooling.
There are various methods of cooling transformers, the cooling mediums employed being
The means adopted for getting rid of the heat which is inevitably developed in a transformer by the waste energy is one of the important considerations with respect to its design.
Ques. What is the behaviour of a transformer with respect to heating when operated continuously at full load?
Ans. The temperature gradually rises until at the end of some hours it becomes constant.
The difference between the constant temperature and that of the secondary atmosphere is called the temperature rise at full load. Its amount constitutes a most important feature in the commercial value of the transformer.
Ques. Why is a high rise of temperature objectionable?
Ans. It causes rapid deterioration of the insulation, increased hysteresis losses, and greater fire risk.
Dry Transformers.—This classification is used to distinguish transformers using air as a cooling medium from those which employ a liquid such as water or oil to effect the cooling.
Air Cooled Transformers.—This name is given to all transformers which are cooled by currents of air without regard to the manner in which the air is circulated. There are two methods of circulating the air, as by
Ques. Describe a natural draught air cooled transformer.
Ans. In this type, the case containing the windings is open at the top and bottom. The column of air in the case expands as its temperature rises, becoming lighter than the cold air on the outside and is consequently displaced by the latter, resulting in a circulation of air through the case. The process is identical with furnace draught.
Ques. Describe a forced draught or air blast transformer.
Ans. The case is closed at the bottom and open at the top. A current of air is forced through from bottom to top as shown in fig. 1,964 by a fan.
Ques. How are the coils best adapted to air cooling?
Ans. They are built up high and thin, and assembled with spaces between them, for the circulation of the air.
Ques. What are the requirements with respect to the air supply in forced draught transformers?
Ans. Air blast transformers require a large volume of air at a comparatively low pressure. This varies from one-half to one ounce per square inch. The larger transformers require greater pressure to overcome the resistance of longer air ducts.
Ques. How much air is used ordinarily for cooling per kw. of load?
Ans. About 150 cu. ft. of air per minute.
In forced draught transformers, the air pressure maintained by the blower varies from ½ to 1½ oz. per square inch. Forced draught or air blast transformers are seldom built in small sizes or for voltages higher than about 35,000 volts.
Oil Cooled Transformers.—In this type of transformer the coils and core are immersed in oil and provided with ducts to allow the oil to circulate by convection and thus serve as a medium to transmit the heat to the case, from which it passes by radiation.
Ques. Explain in detail the circulation of the oil.
Ans. The oil, heated by contact with the exposed surfaces of the core and coils, rises to the surface, flows outward and[Pg 1409] descends along the sides of the transformer case, from the outer surface of which the heat is radiated into the air.
Ques. How may the efficiency of this method of cooling be increased?
Ans. By providing the case with external ribs or fins, or by "fluting" so as to increase the external cooling surface.
Ques. In what types of transformer is this mode of oil cooling used?
Ans. Lighting transformers.
In such transformers, the large volume of oil absorbs considerable heat, so that the rise of temperature is retarded. Hence, for moderate periods[Pg 1410] of operation, say 3 or 4 hours, the average lighting period, the maximum temperature would not be reached.
Ques. In what other capacities except that of cooling agent, does the oil act?
Ans. It is a good insulator, preserves the insulation from oxidation, increasing the breakdown resistance of the insulation, and generally restores the insulation in case of puncture.
Ques. What is the special objection to oil?
Ans. Danger of fire.
Ques. What kind of oil is used in transformers?
Ans. Mineral oil.
Ques. What are the requirements of a good grade of transformer oil?
Ans. It should show very little evaporation at 212° Fahr., and should not give off gases at such a rate as to produce an explosive mixture with the air at a temperate below 356°. It should not contain moisture, acid, alkali or sulphur compounds.
The presence of moisture can be detected by thrusting a red hot nail in the oil; if the oil "crackle," water is present. Moisture may be removed by raising the temperature slightly above the boiling point, 212° Fahr., but the time consumed (several days) is excessive.
Water Cooled Transformers.—A water cooled transformer is one in which water is the cooling agent, and, in most cases, oil is the medium by which heat is transferred from the coils to the water. In construction, pipes or a jacketed casing is provided through which the cooling water is passed by forced circulation, as shown in figs. 1,970 and 1,971.
In some cases tubular conductors are provided for the circulation of the water.
Water cooled transformers may be divided into two classes, as those having:
Ques. Describe the first named type.
Ans. Inside the transformer case near the top is placed a coil of wrought iron pipe, through which the cooling water is pumped. The case is filled with oil, which by thermo-circulation flows upward through the coils, transferring the heat absorbed from the coils to the water; on cooling it becomes more dense (heavier) and descends along the inside surface of the casing.
Ques. How much circulating water is required?
Ans. It depends upon the difference between the initial and discharge temperatures of the circulating water.
Ques. In water cooled transformers how much cooling surface is required for an internal cooling coil?
Ans. The surface of the cooling coil should be from .5 to 1.3 sq. in. per watt of total transformer loss, depending upon the amount of heat which the external surface of the transformer case will dissipate.
For a water temperature rise of 43° Fahr., 1.32 lbs. of water per minute is required per kw. of load.
Transformer Insulation.—This subject has not, until the last few years, been given the same special attention that many other electrical problems have received, although the development[Pg 1415] of the transformer from its original form, consisting of an iron core enclosed by coils of wire, to its present degree of refinement and economy of material, has been comparatively rapid.
In transformer construction it is obviously very important that the insulation be of the best quality to prevent burn outs and interruptions of service.
Ques. What is the "major" insulation?
Ans. The insulation placed between the core and secondary (low pressure) coils, and between the primary and secondary coils.
It consists usually of mica tubes, sometimes applied as sheets held in place by the windings, when no ventilating ducts are provided, or moulded to correct form and held between sheets of tough insulating material where ducts are provided for air or oil circulation.
Ques. Describe the "minor" insulation.
Ans. It is the insulation placed between adjacent turns of the coils.
Since the difference of pressure is small between the adjacent turns the insulation need not be very thick. It usually consists of a double[Pg 1416] thickness of cotton wrapped around each conductor. For round conductors, the ordinary double covered magnet wire is satisfactory.
Ques. What is the most efficient insulating material for transformers?
Ans. Mica.
It has a high dielective strength, is fireproof, and is the most desirable insulator where there are no sharp corners.
Oil Insulated Transformers.—High voltage transformers are insulated with oil, as it is very important to maintain careful insulation not only between the coils, but also between the coils and the core. In the case of high voltage transformers, any accidental static discharge, such as that due to lightning, which might destroy one of the air insulated type, might be successfully withstood by one insulated with oil, for if the oil insulation be damaged it will mend itself at once.
By providing good circulation for the oil, the transformer can get rid of the heat produced in it readily and operate at a low temperature, which not only increases its life but cuts down the electric resistance of the copper conductors and therefore the I2R loss.
Efficiency of Transformers.—The efficiency of transformers is the ratio of the electric power delivered at the secondary terminals to the electric power absorbed at the primary terminals.
Accordingly, the output must equal the input minus the losses. If the iron and copper losses at a given load be known, their values and consequently the efficiency at other loads may be readily calculated.
EXAMPLE.—If a 10 kilowatt constant pressure transformer at full load and temperature have a copper loss of .16 kilowatt, or 1.6 per cent., and the iron loss be the same, then its
output | 10 | |||||
efficiency | = | = | = | 96.9 per cent. | ||
input | 10 + .16 + .16 |
At three-quarters load the output will be 7.5 kilowatts; and as the iron loss is practically constant at all loads and the copper loss is proportional to the square of the load, the
output | 7.5 | |||||
efficiency | = | = | = | 96.8 per cent. | ||
input | 7.5 + .16 + .09 |
The matter of efficiency is important, especially in the case of large transformers, as a low efficiency not only means a large waste of power in the form of heat, but also a great increase in the difficulties encountered in keeping the apparatus cool. The efficiency curve shown in fig. 1,975, serves to indicate, however, how slight a margin actually remains for improvement in this particular in the design and construction of large transformers.
The efficiency of transformers is, in general, higher than that of other electrical machines; even in quite small sizes it reaches over 90 per cent., and in the largest, is frequently as high as 98.5 per cent.
To measure the efficiency of a transformer directly, by measuring input and output, does not constitute a satisfactory method when the efficiency is so high. A very accurate result can be obtained, however, by measuring separately, by wattmeter, the core and copper losses.
The core loss is measured by placing a wattmeter in circuit when the transformer is on circuit at no load and normal frequency.
[Pg 1419]The copper loss is measured by placing a wattmeter in circuit with the primary when the secondary is short circuited, and when enough pressure is applied to cause full load current to flow.
If it be desired to separate the load losses from the true I2R loss, the resistances can be measured, and the I2R loss calculated and subtracted from the wattmeter reading. The losses being known, the efficiency at any load is readily found by taking the core loss as constant and the copper loss as varying proportionally to the square of the load. Thus,
output | |||
efficiency | = | × 100 | |
output + losses |
All Day Efficiency of Transformers.—This denotes the ratio of the total watt hour output of a transformer to the total watt hour input taken over a working day. To compute this efficiency it is necessary to know the load curve of the transformer over a day. Suppose that this is equivalent to 5 hours at full load, and 19 hours at no load. Then, if W1 be the core loss in watts, W2 the copper loss at rated load, and W the rated output,
and the all day efficiency is equal to
5W × 100 | |
per cent. | |
5(W + W1 + W2) + 19W1 |
Commercial or all day efficiency is a most important point in a good transformer. The principal factor in securing a high all day efficiency is to keep the core loss as low as possible. The core loss is constant—it continues while current is supplied to the primary, while copper loss takes place only when the secondary is delivering energy.
In general, if a transformer is to be operated at light loads the greater part of the day, it is much more economical to use one designed for a small iron loss than for a small full load copper loss.
Transformer Fuse Blocks.—These may be of either the single pole or double pole type. Fig. 1,976 shows a double pole fuse box opened, and fig. 1,977, the fuse box opened and the tubes removed. Of the four wires, W, W, W, W, entering the box from beneath, two are from the primary mains, and two lead to the primary coil of the transformer. These wires terminate in metallic receptacles R, R, R, R, in the porcelain plate P, fig. 1,977, which are bridged over in pairs by fuse wires placed inside porcelain tubes T, T, as shown in fig. 1,976. These tubes are air tight except for a small outlet O in each, which fit into the receptacles B, B, in the porcelain plate and open out at the back of the block, as shown in fig. 1,977.
The fuse wires are connected between metallic spring tubes S, S, S, S, which fit into the receptacles R, R, R, R.
If a sudden load or a short circuit occur in the transformer, the intense heat,[Pg 1421] accompanying the melting or blowing of the fuse, causes a rapid expansion of the air inside the tube, so that a strong blast of air rushes through the outlet O of the tube and immediately extinguishes the arc.
By this arrangement, sustained arcing is prevented, as the action of the tube causes the arc to extinguish itself automatically when the current is interrupted.
The porcelain tubes are held in position by the spring K, and the primary of the transformer becomes entirely disconnected from the circuit when the tubes are lifted out.
This form of construction enables the lineman to detach the tubes from the fuse box, and insert the fuse at his convenience. Furthermore, when inserting a fuse in a short circuited line, he does not run the risk of being hurt, as the heated vapor of the exploding fuse can escape through the outlet provided for that purpose, and in a predetermined direction.
The method of attaching the lid not only permits of quick access to the interior of the box, but enables the lineman to tighten the joints by means of the thumb screws L, L, so as to keep the box waterproof.
Auto-transformers.—In this class of transformer, there is only one winding which serves for both primary and secondary. On account of its simplicity it is made cheaply.
Auto-transformers are used where the ratio of transformation is small, as a considerable saving in copper and iron can be effected, and the whole transformer reduced in size as compared with one having separate windings.
Fig. 1,978 illustrates the electrical connections and the relations between the volts and number of turns.
By using the end wire and tapping in on turn No. 20 a current at 20 volts pressure is readily obtained which may be used for starting up motors requiring a large starting current and yet not draw heavily on the line.
Since the primary is connected directly to the secondary it would be dangerous to use an auto-transformer on high pressure circuits. This type of transformer has only a limited use, usually as compensator for motor starting boxes.
Constant Current Transformers for Series Arc Lighting.—The principle of the constant current transformer as used for series arc lighting is readily understood by reference[Pg 1423] to the elementary diagram shown in fig. 1,981. A constant alternating current is supplied to the stationary primary coil which induces a current in the movable secondary coil. The pressure induced in the coil will depend on the number of lines of flux which pass through it and by changing its position in the magnetic field over the primary a variable e.m.f. can be produced and a constant current maintained in the lighting circuit when the lamps are turned on or off, or if the resistance of the circuit be lowered by the consumption of the carbons.
Since the induced currents in the secondary are repelled by the primary there is a tendency for the secondary coil to jump out of the primary field, and in case of a very large current due to a short circuit in the lamp circuit, the secondary current is quickly reduced to normal by the rapid movement of the coil upward.
By adjusting the counterweight for a given number of amperes required by the arc, the current will be maintained constant by the movement of the secondary coil.
The magnetic field produced by the primary must be kept the same by a constant current from the alternator, therefore, when the lamp load is increased the primary voltage increases similar to that of an ordinary series wound direct current dynamo. In other words the alternator and regulating transformer supply a constant current and variable voltage.
Constant current incandescent lighting systems for use in small towns also use this method for automatically regulating the current.
Regulation.—This term applies to the means adopted either to obtain constancy of pressure or current. In the transformer, regulation is inherent, that is, the apparatus automatically effects its own regulation. The regulation of a transformer means, the change of voltage due to change of load on the secondary; it may be defined more precisely as: the percentage increase in the secondary voltage as the load is decreased from its normal value to zero. Thus, observation should be made of the secondary voltage, at full load and at no load, the primary pressure being held constant at the normal value.
The regulation is said to be "good" or "close," when this change is small. In the design of a transformer, good regulation and low iron losses are in opposition to one another when the best results are desired in[Pg 1426] both. A well designed transformer, however, should give good results, both as to regulation and iron losses, the relative value depending upon the class of work it has to do, and size.
Transformer Connections.—The alternating current has the advantage over direct current, in the ease with which the pressure and current can be changed by different connections of transformers.
On single phase circuits the transformer connections can be varied to change current and pressure, and in addition on polyphase circuits the phases can also be changed to almost any form.
Single Phase Connections.—The method of connecting ordinary distributing transformers to constant pressure mains is shown by the elementary diagram, fig. 1,984, where a transformer of 10 to 1 ratio is indicated with its primary winding connected to a 1,000 volt main, and a secondary winding to deliver 100 volts.
Fig. 1,986 shows a transformer with each winding divided into two sections. Each primary section is wound for 1,000 volts, and each secondary section for 50 volts. By connecting the entire primary winding in series, the transformer may be supplied from a 2,000 volt main, as indicated, and if the secondary winding be also connected all in series, as shown, the no load voltage will be 100 between the secondary terminals.
The sections of the primary winding may be connected in parallel to a 1,000 volt main, and 100 volts obtained from the secondary, or the primary and secondary windings may be connected each with its two sections in parallel, and transformations made from 1,000 to 50 volts as represented in fig. 1,987.
This is a very common method of construction for small transformers, which are provided with convenient terminal blocks for combining the sections of each winding to suit the requirements of the case. When the two sections of either winding are connected in parallel as shown in fig. 1,987, care must be taken to connect corresponding ends of the two sections together.
Combining Transformers.—Two or more transformers built to operate at the same pressure and frequency may be connected together in a variety of ways; in fact, the primary and secondary terminals may each be considered exactly as the terminals of direct current dynamos, with certain restrictions.
Ques. What are the two principal precautions which must be observed in combining transformer terminals?
Ans. The terminals must have the same polarity at a given instant, and the transformers should have practically identical characteristics.
The latter condition is not absolutely essential, but it is emphatically preferable. For example, if a transformer, which has 2 per cent. regulation, be connected in parallel, as indicated in fig. 1,988, with one which has 3 per cent. regulation, at no load the transformers will give exactly the same voltage at the secondary terminals, but at full load one will have a secondary pressure of, say, 98 volts, while the other has 97 volts. The result is that the transformer giving only 97 volts will be subject[Pg 1429] to a reverse pressure of one volt from its mate. This will not cause excessive current to flow backward through the secondary winding of the low voltage transformer, but it will disturb the phase relations and lower the power factor and efficiency of the combination. In such a case it is much better to work the secondary circuits of the two transformers separately.
In case the transformers have practically the same characteristics it is necessary, as stated above, to make sure that the secondary terminals connected together have the same polarity at a given instant; it is not necessary to find out definitely what the polarity is, merely that it is the same for both terminals. This can be easily done as shown in fig. 1,989.
Ques. What may be said with respect to operating transformer secondaries in parallel?
Ans. It is seldom advantageous. Occasionally it may be necessary as a temporary expedient, but where the load is such as to require a greater capacity than that of a transformer already installed, it is much better to replace it by a large transformer than to supplement it by an additional transformer of its own size.
Ques. How are the secondaries arranged in modern transformers and why?
Ans. The secondary windings are divided into at least two sections so that they may be connected either in series or parallel.
Ques. Explain how secondary connections are made for different voltages.
Ans. If, for instance, the secondary pressure of a transformer having two sections be 100 volts with the terminals in parallel, as in fig. 1,990, then connecting them in series will give 200 volts at the free secondary terminals, as indicated in fig. 1,991.
Ques. What precaution should be taken in connecting secondary sections in parallel in core type if the two sections be wound on different limbs of the cores?
Ans. It will be advisable to make the connections ample and permanent, so that there will not be any liability to a difference between the current flowing in one secondary winding and that flowing through the other.
Two Phase Connections.—In the case of two phase distribution each circuit may be treated as entirely independent of the other so far as the transformers are concerned. Two[Pg 1431] transformers are used, one being connected to one primary phase and supplying one secondary phase, the other being connected to the other primary phase and supplying the other secondary phase as indicated in fig. 1,996, exactly as though each primary and secondary phase were an ordinary single phase system, independent of the other phase.
Ques. Is the above method usually employed?
Ans. No, the method shown in fig. 1,997 is generally used.
Three Phase Connections.—There is not so much freedom in making three phase transformer connections, as with single or two phase, because the three phases are inseparably interlinked. However, the system gives rise to several methods of transformer connection, which are known as:
Delta Connection.—In the delta connection both primaries and secondaries are connected in delta grouping, as in fig. 1,992.
Star Connection.—This method consists in connecting both the primaries and secondaries in star grouping, as in fig. 1,993.
Delta-star Connection.—In this method the primaries are connected in delta grouping and the secondaries in star grouping, as in fig. 1,994.
Star-delta Connection.—This consists in connecting the primaries in star grouping, and the secondaries in delta grouping, as in fig. 1,995.
Ques. What advantage has the star connection over the delta connection?
Ans. Each star transformer is wound for only 58% of the line voltage. In high voltage transmission, this admits of much smaller transformers being built for high pressure than possible with the delta connection.
Ques. What advantages are obtained with the delta connection?
Ans. When three transformers are delta connected, one may be removed without interrupting the performance of the circuit, the two remaining transformers in a manner acting in series to carry the load of the missing transformer.
The desire to guard against a shut down due to the disabling of one transformer has led to the extensive use of the delta connection, especially for the secondaries or low pressure side.
It should be noted that if one transformer be disabled, the efficiency of the other two will be greatly reduced. To operate a damaged three phase transformer, the damaged windings must be separated electrically from the other coils, the damaged primary and secondary being respectively short circuited upon themselves.
Ques. What kinds of transformers are used for three phase current?
Ans. Either a three phase transformer, or a separate single phase transformer for each phase.
Ques. What points are to be considered in choosing between three phase and single phase transformers for the three phase current transformation?
Ans. No specific rule can be given regarding the selection of single phase or three phase transformers since both designs are equally reliable; local conditions will generally determine which type is preferable.
The following general remarks may, however, be helpful:
Single phase transformers are preferable where only one transformer group is installed and where the expense of a spare transformer would not be warranted. In such installations the burn out of one phase of a three phase unit would cause considerable inconvenience for the reason that the whole transformer would have to be disconnected from the circuit before repairs could be made.
If single phase transformers be used and connected in delta on both primary and secondary,[Pg 1438] the damaged transformer can be cut out with a minimum amount of trouble and the other two transformers can be operated at normal temperature open delta at 58 per cent. of the normal capacity of the group of three transformers, until the third unit can be replaced.
With a three phase shell type transformer, if both the primary and secondary be delta connected, trouble in one phase will not prevent the use of the other two phases in open delta. By short circuiting both primary and secondary of the defective phase, and cutting it out of circuit the magnetic flux in that section is entirely neutralized. This cannot be done, however, with any but delta connected shell type transformers.
Where a large number of three phase transformers can be used, it is generally advisable to install three phase units, the following advantages being in their favor as compared with single phase units:
[Pg 1439]Ques. What is the character of the construction of three phase transformers?
Ans. The three phase transformer is practically similar to that of the single phase, except that somewhat heavier and larger parts are required for the core structure.
COMPARISON OF AIR BLAST, WATER COOLED, AND OIL COOLED TRANSFORMERS | ||
---|---|---|
Air blast type | Water cooled type | Oil cooled type |
[8] NOTE.—No special foundations are necessary for any type of transformer other than a good, even floor, having sufficient strength to support the weight. | ||
1. COST | ||
A. First cost | ||
Necessarily more expensive than the water cooled type of similar rating. | Least expensive of all types. | Necessarily more expensive than the air blast and water cooled type of similar rating. |
B. | ||
The installation is extremely simple. Moisture that may have collected on the surfaces during transportation or storage should be thoroughly dried out. |
Being heavier than the air blast type, these transformers,
as a rule, require heavier apparatus for installing. Both
transformer and tank should be thoroughly dried out before
being filled with oil. The oil is usually supplied in 50 gal. hermetically sealed steel barrels to minimize possibility of moisture during transportation. |
Being heavier than the air blast and water cooled type, these transformers require heavier apparatus for installing. Both transformer and tank should be thoroughly dried out before being filled with oil.[Pg 1440][Pg 1441] |
C. Auxiliary apparatus | ||
A duct, or chamber, of considerable size is required
under the transformers in order to conduct the cooling air to
them. A blower outfit for supplying air is required. |
In most cases, cooling water may be obtained with
sufficient natural head. However, there are frequent cases
in which it can be obtained only by the use of pumps. A system of piping for the cooling water and oil drainage is required, the cost of which depends, of course, on the station layout. |
Do not require cooling water or blower. |
D. Maintenance | ||
An occasional cleaning, for which a supply of compressed
air at about 20 lb. pressure is recommended. The blower outfit requires no more care than any other similar apparatus. |
A water pumping outfit would possibly require a trifle more attention than a blower outfit in which there are no valves or piping. | No air or water circulation to demand attention. |
2. FLOOR SPACE | ||
Always requires space for cooling apparatus. | Extra space only required when auxiliary pumping apparatus is necessary. | Only require space for the transformer as no extra apparatus is necessary |
3. LOCATION | ||
As the transformers are open at the top they should not be
located where there is much dust or dirt nor where water from
any source is liable to fall on them. The blower should be so situated as to obtain clean dry air of a temperature not greater than 77° Fahr. |
Transformers are completely enclosed but location should
be such that no water will fall on leads or bushings. Location of auxiliary apparatus will depend on the station layout. |
Transformers are completely enclosed but location should be
such that no water will fall on leads or bushings. The building should be well ventilated. There is no auxiliary apparatus. |
4. GENERAL APPEARANCE | ||
Terminal leads may be located in the base and the air
chamber may be used for conducting and distributing the
connecting wiring. The absence of overhead wiring aids in simplifying the appearance of the station. |
Leads are brought out of the top of the transformers. Water cooling pipes are connected at the top in most cases. |
Leads are brought out of the top of the transformers. |
5. OPERATION | ||
Equal reliability in all three types. While full load efficiencies are practically equal in the three designs, it is necessary to change the proportion of iron and copper losses somewhat as the copper loss of the air blast transformer is a smaller part of its total loss than of the water cooled and oil cooled types. As a result, the regulation of the air blast transformer is a trifle better. |
||
6. GENERAL | ||
The above information regarding selection of type is not applicable to
air blast transformers for circuits materially in excess of 33,000 volts. On account of the great thickness of the solid insulation needed and the consequent difficulty in radiating heat from the copper, it is impracticable to design the air blast type for more than this voltage. The oil immersed designs are therefore recommended for transformers above 33,000 volts. Both oil cooled and water cooled types are available for all voltages, being restricted in this respect only by the limitations of transmission facilities.[8] |
Ques. How are transformers connected for four wire three phase distribution?
Ans. When the secondaries of three transformers are star [Pg 1443]connected, a fourth wire may be run from the neutral point, thus obtaining the four wire system.
The voltage between any main wire and the neutral will be 57 per cent. of the voltage between any two main wires. For general distribution this system is desirable, requiring less copper and greater flexibility than other systems.
Three phase 200 volt motors may be supplied from the main wires and 115 volt lamps connected between each of the three main wires and the neutral; if the lamp load be very nearly balanced the current flowing in the neutral wire will be very small, as in the case of the ordinary three wire direct current system.
How to Test Transformers.—The troubles incident to gas or water service have their parallels in electric power distribution.
Companies engaged in the former, credit a large percentage of their losses to leaky valves and defective mains. The remedy may involve heavy expense and the loss is often tolerated as the lesser of two evils.
In electric power distribution the transformer takes in part the place of the valve and pipe system. An inferior or defective transformer usually treats both the central station and its customers badly, being in this respect more impartial than the gas or water pipe which may annoy but one of the interested parties at a time.
Like a neglected or defective gas fixture a transformer can menace life, failing, however, to give the warning the former gives, and with a more hidden threat on account of its location.
Apart from this, corresponding to an exasperated customer who complains at home and to his friends of dim lamps, blackened lamps, you will find in the power station the manager, who, also worried and in no better humor, contemplates the difference in meter readings at the end of the line.
His business does not increase and would not increase even if he could lower the rates, which he cannot do because of these meter readings.
He may be confident of his engines and generators, and that his line is up and all right, but he very seldom knows what the transformers are doing on top of[Pg 1446] the poles. Perhaps he feels that this waste is so slight that it makes no material difference. This can be readily ascertained by means of a set of testing instruments.
Perhaps the transformers were purchased because of their attractive prices and never tested.
Water, plumbing, gas and steam fittings are subjected to test. Why not transformers? Even more so because transformers take constant toll from the company installing them, while gas and water fittings, once passed, are off the contractor's hands.
The busy manager has little time for complicated treatises and monographs on electrical measurements and even handbooks confront him with forbidding formulæ. Accordingly the methods of transformer testing, which are very simple, are illustrated in the accompanying cuts. Managers of electric power and lighting companies should study them carefully.
An ammeter, voltmeter and wattmeter are required to make the tests. Losses are small in good transformers and hence the instruments should be accurate. For the same reason instruments should be chosen of the proper capacity to give their best readings. If there be any doubt about the testing instruments being correct, they should be calibrated before being used. The testing circuits should be properly fused for the protection of the instruments. It is hardly logical, but a very common practice is to mistrust meters and to watch them closely, while the transformers are guilty of theft unchallenged, and keep busily at it on a large scale.
Transformer Operation with Grounded Secondary.—The operation of a transformer with a grounded secondary has been approved by the American Institute of Electrical Engineers, and by the National Board of Fire Underwriters.
This method of operation effectually prevents a high voltage occurring upon the low tension wires in case of a breakdown or other electrical connections occurring between the primary and secondary windings.
In case of a breakdown without the secondary grounded, any one touching a part of the low tension system, such as a lamp socket, might receive the full high pressure voltage. With the low tension grounded, the fuse in the high tension circuit will blow and the fault be discovered upon replacing it.
Transformer Capacity for Motors.—The voltage regulation of a well designed transformer is within 3 per cent. of its rated voltage on a non-inductive load such as incandescent lamps, but when motors are connected to the circuit their self-induction causes a loss of 5 per cent. or more, and if the load be fluctuating, it is better to use independent transformers for the motor, which will prevent considerable fluctuations in the incandescent lamps. Arc lamps do not show slight voltage changes as much as incandescent lamps. The proper rating of transformers for two phase and three phase induction motors is given in table on the next page.
A three phase induction motor may be operated from three single phase transformers or one three phase transformer. While the one three phase transformer greatly reduces the space and simplifies the wiring, the use of three single phase transformers[Pg 1450] is more flexible and, in case one transformer burns out, the connection can be readily changed so that two transformers will operate the motor at reduced load until the burned out transformer is replaced or repaired.
It is well to allow one kilowatt per horse power of the motor in selecting the size for the transformers, excepting in the small sizes when a little larger kilowatt rating is found to be the most desirable.
Delivered voltage of circuit | Single phase transformer voltages | |||
---|---|---|---|---|
110 volt motor | 220 volt motor | |||
Primary | Secondary | Primary | Secondary | |
1,100 | 1,100 | 122 | 1,100 | 244 |
2,200 | 2,200 | 122 | 2,200 | 244 |
Very small transformers should not be used, even when the motor is large compared to the work it has to do, as the heavy starting current may burn them out.
The following tables give the proper sizes of transformer for three types of induction motor and the approximate current taken by three phase induction motors at 220 volts.
Size of motor horse power | Kilowatts per transformer | ||
---|---|---|---|
Two single phase transformers | Three single phase transformers | One three phase transformers | |
1 | 0.6 | 0.6 | |
2 | 1.5 | 1.0 | 2.0 |
3 | 2.0 | 1.5 | 3.0 |
5 | 3.0 | 2.0 | 5.0 |
7 | 4.0 | 3.0 | 7.5 |
10 | 5.0 | 4.0 | 10.0 |
15 | 7.5 | 5.0 | 15.0 |
20 | 10.0 | 7.5 | 20.0 |
30 | 15.0 | 10.0 | 30.0 |
50 | 25.0 | 15.0 | 50.0 |
75 | 40.0 | 25.0 | 75.0 |
100 | 50.0 | 30.0 | 100.0 |
Horse power of motor | Approximate full load current | Horse power of motor | Approximate full load current |
---|---|---|---|
1 | 3.2 | 20 | 50. |
2 | 6.0 | 30 | 75. |
3 | 9.0 | 50 | 125. |
5 | 14.0 | 75 | 185. |
10 | 27.0 | 100 | 250. |
15 | 40.0 | 150 | 370. |
Transformer Connections for Motors.—Fig. 2,020 shows the connection of a three phase so called delta connected transformer with the three primaries connected to the lines leading from the alternator and the three secondaries leading to the motor.
The connections for a three phase motor using two transformers is shown in fig. 2,021 and is identical with the previous[Pg 1452] arrangement, except that one transformer is left out and the other two made correspondingly larger.
The copper required in any three wire three phase circuit for a given power and loss is 75 per cent. that necessary with the two wire single phase or four wire two phase system having the same voltage between lines.
The connections of three transformers for a low tension system of distribution by the four wire three phase system are shown in fig. 2,022. The three transformers have their primaries joined in delta connection and the secondaries in "Y" connection. The three upper lines of the secondary are the three main three phase lines, and the lowest line is the common neutral.
The voltage across the main conductors is 200 volts, while that between either of them and the neutral is 115 volts; 200 volt motors should be joined to the mains while 115 volt lamps are connected between the mains and neutral. The arrangement is similar to the[Pg 1453] Edison three wire system and the neutral carries current only when the lamp load is unbalanced.
The voltage between the mains should be used in calculating the size of wires, and the size of the neutral wire should be made in proportion to each of the main conductors that the lighting load is to the total load.
When lights only are used the neutral should be the same as the main conductors. The copper required in such a system for a given power and loss is about 33.3 per cent. as compared with a two wire single phase system or a four wire two phase system using the same voltage.
Monocyclic Motor System.—Motors on the monocyclic system are operated from two transformers connected as shown in fig. 2,023. In the monocyclic system the single phase current is used to supply the lighting load and two wires only are necessary, but if a self-starting induction motor be required, a third or teaser wire is brought to the motor and two transformers used.
The teaser wire supplies the quarter phase current required to start the motor, which afterwards runs as a single phase synchronous motor and little or no current flows through the teaser circuit as long as the motor keeps in synchronism; in case it fall behind, the teaser current tends to bring it up to speed instead of the motor stopping, as would be the case of a single phase motor.
The voltage of the transformers should be tested by means of a voltmeter or two incandescent lamps joined in series, before starting up the motor, to see if the proper transformer connections have been made and prevent an excessive flow of current.
If one of the transformers be reversed the voltage will be almost doubled; in fact, it is a good plan to check up all the transformer connections with the voltmeter or lamps which will often save a burn out.
Arrangement of links on the connecting board | Primary coils will be connected in | For circuit voltage normal at | Ratio of transformation at no load | |
---|---|---|---|---|
with secondary coils in multiple | with secondary coils in series. | |||
Figs. 2,025 to 2,029.—Diagrams of Wagner transformer connection board, and table showing various arrangements of the terminal links, corresponding transformation ratios, and suitable primary voltages.[Pg 1456] | ||||
Multiple | 1,100 | 10:1 | 5:1 | |
Multiple | 1,100 | 9.05:1 | 4.52:1 | |
Series | 2,200 | 20:1 | 10:1 | |
Series | 2,200 | 19.05:1 | 9.5:1 | |
Series | 2,200 | 18.1:1 | 9.05:1 |
The alternating current must change to a direct current in many cases as in railroad work because the induction motor is not so satisfactory as the direct current series motor and the alternating current series motor is slow in coming into general use.
In all kinds of electrolytic work, transformation must be made, and in many cities where the direct current system was started, it is still continued for local distribution, but the large main stations generating alternating currents and frequently located some distance away from the center of distribution have replaced a number of small central stations.
Transformation may be made by any of the following methods:
[9] NOTE.—Rectifiers are explained in detail in Chapter LIV.
Strictly speaking, a converter is a revolving apparatus for converting alternating current into direct current or vice versa; it is usually called a rotary converter and is to be distinguished from the other methods mentioned above.
Broadly, however, a converter may be considered as any species of apparatus for changing electrical energy from one form into another.
According to the standardization rules of the A. I. E. E. converters may be classified as:
A direct current converter converts from a direct current to a direct current.
A synchronous converter (commonly called a rotary converter) converts from an alternating current to a direct current.
A motor converter is a combination of an induction motor with a synchronous converter, the secondary of the former feeding the armature of the latter with current at some frequency other than the impressed frequency; that is, it is a synchronous converter in combination with an induction motor.
A Frequency Converter (preferably called a frequency changer) converts alternating current at one frequency into alternating current of another frequency with or without a change in the number of phases or voltages.
A Rotary Phase Converter changes alternating current of one or more phases into alternating current of a different number of phases, but of the same frequency.
Rotary Converters.—The synchronous or rotary converter consists of a synchronous motor and a direct current generator combined in one machine. It resembles a direct current generator with an unusually large commutator and an auxiliary set of collector rings.
Ques. In general, how does a rotary converter operate?
Ans. On the collector ring side it operates as a synchronous motor, while on the commutator side, as a dynamo.
Its design in certain respects is a compromise between alternating current and direct current practice most noticeably with respect to the number of poles and speed.
Ques. Upon what does the speed depend?
Ans. Since the input side consists of a synchronous motor, the speed is governed by the frequency of the alternating current supplied, and the number of poles.
Fig. 2,034 is a diagram of a ring wound rotary converter. This style winding is shown to simplify the explanation. In practice drum wound armatures are used, the operation, however, is the same.
With this simple machine the following principles can be demonstrated:
1. If the coil be rotated, alternating currents can be taken from the collector rings and it is called an alternator.
2. By connecting up the wires from the commutator segments, a direct current will flow in the external circuit making a dynamo.
3. Two separate currents can be taken from the armature, one supplying alternating current and the other direct current; such a machine is called a double current generator.
4. If a direct current be sent in the armature coil through the commutator, the coil will begin to rotate as in a motor and an alternating current can be taken out of the collector rings. Such an arrangement is called an inverted rotary converter.
5. If the machine be brought up to synchronous speed by external means and then supplied with alternating current at the collector rings, then if the direction of the current through the armature coil and the pole piece have the proper magnetic relation, the coil will continue to rotate in synchronism with the current. A direct current can be taken from the commutator, and when used thus, the machine is called simply a rotary converter.
Ques. What is the relation between the impressed alternating pressure and the direct pressure at the commutator?
Ans. The ratio between the impressed alternating pressure and the direct current pressure given out is theoretically constant, therefore, the direct pressure will always be as 1 to .707 for single phase converters or if the pressure of the machine used above indicate 100 volts at the direct current end, it will indicate 70.7 volts at the alternating current side of the circuit.
Ques. Name two different classes of converter.
Ans. Single phase and polyphase.
Ques. What is the advantage of polyphase converters?
Ans. In the majority of cases two or three phase converters are used on account of economy of copper in the transmission line.
Ques. How is the armature of a polyphase converter connected?
Ans. Similar to that of an alternator with either delta or Y connections.
Figs. 2,037 to 2,041 show various converter connections between the collector rings and commutator.
Fig. 2,037 indicates how the armature is tapped for two phase connections.
Fig. 2,038 shows three phase delta connections, and fig. 2,039 the three phase Y or star connections.
Six phase delta and Y connections are frequently used as shown in fig. 2,040 and fig. 2,041, both of which require two secondary coils in the transformer, one set of which is reversed, so as to supply the current in the proper direction.
Ques. With respect to the wave, what is the relation between the direct and alternating pressures?
Ans. The direct current voltage will be equal to the crest of the pressure wave while the alternating voltage will depend[Pg 1464] on the virtual value of the maximum voltage of the wave according to the connections employed.
DIRECT CURRENT | SINGLE PHASE | TWO PHASE | THREE PHASE | SIX PHASE | TWELVE PHASE | n PHASE | |
---|---|---|---|---|---|---|---|
VOLTS BETWEEN COLLECTOR RING AND NEUTRAL POINT | 1 | 1 / (2√2) = .354 | 1 / (2√2) = .354 | 1 / (2√2) = .354 | 1 / (2√2) = .354 | 1 / (2√2) = .354 | 1 / (2√2) = .354 |
VOLTS BETWEEN ADJACENT COLLECTOR RINGS | 1 | 1 / √2 = .707 | ½ = .5 | √3 / (2√2) = .612 | 1 / (2√2) = .354 | .183 | (SIN(π / n)) / √2 |
AMPERES PER LINE | 1 | √2 = 1.414 | 1 / √2 = .707 | (2√2) / 3 = .943 | √2 / 3 = .472 | .236 | (2√2) / n |
AMPERES BETWEEN ADJACENT LINES | 1 | √2 = 1.414 | ½ = .5 | (2√2) / (3√3) = .545 | √2 / 3 = .472 | .455 | (√2SIN(π / n)) / n |
In a single phase rotary, the value of the direct pressure is 1 to .707, therefore a rotary which must supply 600 volts direct current must be supplied by 600 × .707 = 424 volts alternating current. For three phase rotaries the ratio is 1 to .612, or in order to produce 600 volts direct current, 600 × .612 = 367 volts on the alternating current side of the rotary is required.
Fig. 2,034 shows a complete diagram of the electrical connections. A single phase rotary is illustrated so as to simplify the wiring.
The table of Steinmetz on page 1,464 gives the values of the alternating volts and amperes in units of direct current.
Ques. How is the voltage of a rotary varied on the direct current side?
Ans. Pressure or potential regulators are put in the high tension alternating current circuit and may be regulated by small motors operated from the main switchboard or operated by hand.
Ques. What is the advantage of unity power factor for rotary converters?
Ans. It prevents overheating when the rotary is delivering its full load in watts.
Ques. What greatly influences the power factor of the high tension line?
Ans. The strength of the magnetic field.
Ques. Does variation of the field strength materially affect the voltage?
Ans. No.
Since variation of the field strength does not materially affect the voltage, by adjusting the resistance in series with the magnetic circuit, the strength of the field can be changed and the power factor kept 1 or nearly 1 as different loads are thrown on and off the rotary.
Ques. What is the effect of a field too strong or too weak?
Ans. If too strong, a leading current is produced, and if too[Pg 1467] weak, the current lags, both of which reduce the power factor and are objectionable.
Usually there is a power factor meter connected up in the main generating station and one also in the rotary substation, and it is the duty of the attendant at the substation to maintain the proper power factor.
Ques. What is the ordinary range of sizes of rotaries?
Ans. From 3 kw. to 3,000 kw.
Ques. What is the general construction of a rotary converter?
Ans. It is built similar to a dynamo with the addition of suitable collector rings connected to the armature windings at points having the proper phase relations.
Standard rotary converters have been developed for 25 and 60 cycles. The standard railway machines are compound wound, the series field being designed for a compounding of 600 volts at no load and full load when supplied from a source of constant pressure with not more than 10 per cent. resistance drop and with 20 to 30 per cent. reactance in the circuit. The large size machines are usually wound for six phase operation.
Compounding of Rotary Converters.—Compounding is desirable where the load is variable, such as is the case with interurban railway systems. The purpose of the compounding is to compensate automatically for the drop due to line, transformer, and converter impedance.
On account of the low power factor caused by over compounding, and the fact that substations are customarily connected to the trolley at its nearest point without feeder resistance, over compounding is not recommended. An adjustable shunt to the series field is provided with each machine.
Shunt wound converters are satisfactory for substations in[Pg 1469] large cities and similar installations where due to the larger number of car units demanding power, the load is more nearly constant.
Ratio of Conversion.—The relation between the alternating and direct current voltages varies slightly in different machines, due to differences in design. The best operating conditions exist when the desired direct current voltage is obtained with unity power factor at the converter terminals when loaded.
Ques. Upon what does the ratio of conversion depend?
Ans. Upon the number of phases and method of connecting the windings.
For single phase or two phase machines it is 1 to .7; for three phase, 1 to .612, or six phase, 1 to .7 or 1 to .613 depending upon the kind of connection used for the transformer.
For example, a two phase rotary receiving alternating current at 426 volts will deliver direct current at 600 volts, while a three phase rotary receiving alternating current at 367 volts will deliver direct current at 600 volts.
Ques. What difficulty would be encountered if other ratios of conversion than those given above were required?
Ans. An armature with a single winding could not be used.
It would be necessary to use a machine with two distinct armature windings or else a motor generator set.
Ques. What change in voltage is necessary between a converter and the alternator which furnishes the current?
Ans. The voltage must be reduced to the proper value by a step down transformer.
Voltage Regulation.—As the ratio of the alternating to the direct current voltage of a converter is practically constant, means must be provided to compensate for voltage variation due to changes of load in order to maintain the direct current pressure constant.
There are several methods of doing this, as by:
Shifting the Brushes.—Were it not for the difficulties encountered, this would be a most convenient method of voltage regulation, since by this procedure the direct current voltage may be varied from maximum to zero. It is, however, not practical because of the excessive sparking produced when the brushes are shifted out of the neutral plane.
Split Pole Method.—In order to overcome the difficulty encountered in shifting the brushes the split pole method was devised by Woodbridge in which each field pole is split into two or three parts.
The effect of this is the same as shifting the brushes except that no sparking results.
The other part is arranged so that its excitation may be varied, thus shifting the resultant plane of the field with respect to the direct current brushes.
One of these parts is permanently excited and it produces near its edge the fringe of field necessary for sparkless commutation.
Regulating Pole Method.—As applied to the rotary converter regulating poles fulfill the same functions as commutating or interpoles (see page 385) on motors and dynamos, that is, they insure sparkless commutation from no load to heavy overloads with a fixed brush position.
The regulating poles are used in order to vary the ratio between the alternating current collector rings and the direct current side without the use of auxiliary apparatus such as induction regulators or dial switches which involve complicated connections and many additional wires. The regulating poles are arranged with suitable connection so that the current through them can be raised, lowered or reversed.
The characteristics of the regulating pole converter being novel, a detailed explanation of the principles involved is given to facilitate a clear understanding of its operation.
Consider a machine with a field structure as shown in fig. 2,056 resembling in appearance a machine with commutating poles, but with the brushes so set that one of the regulating poles adds its flux to that of one main pole, cutting the inductors between two direct current brushes. The regulating pole is shown with a width equal to 20 per cent. of that of the main pole.
To obtain definite figures, it will be assumed that the machine at normal speed, with the main poles excited to normal density, but with no excitation on the regulating poles, gives 250 volts direct current pressure. Then with each regulating pole excited to the same density[Pg 1476] as the main poles, and with a polarity corresponding to that of the main pole in the same section between brushes, the direct current pressure will rise to 300 volts at the same speed, since the total flux cutting the inductors in one direction between brushes has been increased 20 per cent.
If, on the other hand, the excitation of the regulating poles be reversed and increased to the same density as that of the main poles, the direct current pressure will fall to 200 volts, since in this case the regulating poles give a reverse pressure, that is, a pressure opposing that generated by the main poles.
Now, if the machine be equipped with collector rings, that is, if it be a converter, this method of varying the direct current voltage from 200 to 300 volts does not give nearly as great a variation of the alternating current voltage; in fact, the latter voltage will be the same when delivering 200 volts as when delivering 300 volts direct current pressure, if the field excitation be the same.
This may be seen by reference to fig. 2,057, which is a diagram of the alternating current voltage developed in the armature windings by the two sets of poles.[10]
[10] NOTE.—In the Burnham split pole rotary converter, each pole is divided into only two sections, one larger than the other. A main shunt winding is arranged on the large sections, and a winding for providing the voltage regulation is placed on the other section. When the current is sent through this latter winding in one direction the voltage is raised, when in the other direction the voltage is lowered.
The horizontal line OA represents the alternating current voltage [Pg 1477]generated by the main poles, alone, with the regulating poles unexcited, that is, when delivering 250 volts direct current pressure.
For a six phase converter OA measures about 180 volts diametrically, that is, between electrically opposite collector rings.
If now the regulating poles be excited to full strength, to bring the direct current pressure up to 300 volts, the alternating current voltage generated by the regulating poles will be 90 degrees out of phase with that generated by the main poles (since they are placed midway between the main poles), and will be about 40 volts as shown by the line AB.
The resultant alternating current volts across the collector rings will be represented by the line OB with a value equal to 184.
Again, if the regulating poles be reversed at full strength, to cut the direct current pressure down to 200 volts, the alternating current voltage of the main and regulating poles will be OA and AC respectively, giving the resultant OC equal to OB with a value of 184 volts. Accordingly, the direct current pressure may be either 200 or 300 volts with the same alternating current pressure, and if the main field be kept constant, the direct current pressure may range between 200 or 300 volts, while the alternating current pressure varies only between 180 and 184 volts.
The alternating current pressure can be kept constant through the full range of direct current voltage by changing the main field so as always to give an equal and opposite flux change to that of the regulating field. A constant total flux may thus be obtained equal to the radius of the arc BC, fig. 2,057. In this case the line OA, representing the main field strength, will equal OB when the regulating field is not excited, and 250 volts can only be obtained at this adjustment.
This method of operation gives unity power factor with a constant impressed pressure of 184 volts alternating current with a range of direct current voltage from 200 to 300 volts.
Ques. Where should the regulating poles be located for best results?
Ans. A better construction is obtained by placing them closer to the corresponding main pole, as in fig. 2,060, than when spaced midway between the main poles as in fig. 2,056.
Ques. When the regulating poles are spaced as in fig. 2,060, what is the effect on the direct current voltage?
Ans. The effect is the same as for the midway position (fig. 2,056) except for magnetic leakage from the main poles to the regulating poles when the latter is opposed to the former, that is, when the direct current voltage is being depressed.
Ques. What is the effect on the alternating current voltage?
Ans. It is somewhat altered as explained in figs. 2,058 and 2,059.
Reactance Method.—This consists in inserting inductance in the supply circuit and running the load current through a few turns around the field cores. This method is sometimes called compounding, and as it is automatic it is generally used where there is a rapidly fluctuating load.
If a lagging current be passed through an inductance, the collector ring voltage will be lowered, but will be raised in case of a leading current. The degree of excitation governs the change in the phase of the current to the converter, the excitation, in turn, being regulated by the load current. Accordingly[Pg 1481] with series inductance, the effect of the series coils on the field of the converter is quite similar to that of the compounding of the ordinary railway dynamo.
Multi-tap Transformer Method.—The employment of a variable ratio step down transformer for voltage regulation is a non-automatic method of control and, accordingly, is not desirable except in cases where the load is fairly constant over considerable periods of time. It requires no special explanation.
Synchronous Booster Method.—This consists of combining with the converter a revolving armature alternator having the same number of poles.
Ques. How is the winding of the booster alternator armature connected?
Ans. It is connected in series with the input circuits on the converter.
Ques. How are the field windings connected?
Ans. They are either fed with current regulated by means of a motor operated field circuit rheostat, or joined in series with the commutator leads of the converter.
Ques. For what service is the synchronous booster method desirable?
Ans. For any application where a relatively wide variation in direct current voltage is necessary.
It is particularly desirable for serving incandescent lighting systems where considerable voltage variation is required for the compensation of drop in long feeders, for operation in parallel with storage batteries and for electrolytic work where extreme variations in voltage are required by changes in the resistance of the electrolytic cells.
Motor Generator Sets.—The ordinary rotary converter is the most economical machine for converting alternating currents into direct currents, and where slight variations in the direct current voltage is necessary, they are mostly used on account of their high efficiency, and because they are compact.
In many central stations where they supply a great variety of apparatus, the motor generator sets are employed as the generator is independent of the alternating current line voltage and any degree of voltage regulation can be performed.
Motor Generator Combinations.—The following combinations of motor generators are made and used to suit local conditions:
Synchronous motor | dynamo |
Induction motor | dynamo |
Direct current motor | dynamo |
Direct current motor | alternator |
Synchronous motor | alternator |
Induction motor | alternator |
Standard practice has adopted high tension alternating current for transmission systems, but direct current distribution[Pg 1487] is very frequently used. This is particularly true where alternating current apparatus has been introduced in old direct current lighting systems.
The synchronous motor or the induction motor connected to a generator stands next in importance to the rotary converter because it is easy to operate and the pressure may be changed by a rheostat placed in the field circuit of the generator.
The line wires carrying full voltage can usually be connected direct to the motor and thus do away with the necessary step-down transformer required by the rotary.
Ques. What is the behavior of a rotary converter when hunting?
Ans. It is liable to flash over at the direct current brushes, which is common in high frequency converters where there are a great number of poles and the brushes are necessarily spaced close together around the commutator.
Ques. Is this fault so pronounced with motor generator sets?
Ans. The motor generator operating on a high frequency circuit, the generator can be designed with a few poles and the brushes set far apart which will greatly reduce the chance of flashing over.
A synchronous motor will drive a generator at a constant speed during changes in load on it, and by having a field regulating resistance it can be used to improve the power factor of the system.
When an induction motor is used its speed drops off slowly as the load comes on the generator, and it is necessary to regulate the voltage of the generator by means of a field rheostat, or compound wound machines may be used.
While an induction motor requires no separate excitation of the field magnets like the synchronous motor, its effect on the power factor of the system is undesirable.
Although it is seldom necessary to convert direct current to alternating, such an arrangement of a direct current motor driving an alternator is often justified in place of an inverted rotary converter, as in this case the alternating current voltage can be changed independent of the direct current voltage.
The racing of an inverted rotary under a heavy inductive load or short circuit does not take place in motor generator set mentioned above.
Frequency Changing Sets.—A frequency of 25 cycles is generally used on railway work and in large cities using the Edison three wire system, and as a 25 cycle current is not desirable for electric lighting it is necessary to change it to 60 cycles by means of a frequency changer shown in fig. 2,069 for distribution in the outlying districts.
The two machines in this combination are of the same construction, only the synchronous motor would have eight poles and have the 25 cycle current passing through it, while the generator would have 20 poles and produce 62½ cycles per second at 300 revolutions per minute. By supplying the motor with 24 cycles, the generator would produce 60 cycles.
It will be seen from the figure that the separate exciter is fastened on the base plate and has its armature directly connected to the shaft.
Parallel Operation of Frequency Changers.—It is very difficult to construct two or more frequency changers and join them to synchronous motors so that the current wave of one[Pg 1490] machine will be in phase with the other, since the speed of the motor will depend on the frequency of the line and be independent of the load thrown on it.
When alternators are run in parallel, if one machine lag behind, the other carries the load with the result that the lightly loaded machine will speed up and get in step with the other, or in other words a synchronizing current will flow between the two alternators and tend to keep them in proper relation with respect to phase and load.
Cascade Converter.—This piece of apparatus was introduced by Arnold and La Cour. Briefly, it consists of a combination of an induction motor having a wound armature and[Pg 1491] a dynamo, the armatures being placed on the same shaft. The windings are joined in cascade, that is, in series with those of the armature of the induction motor. The line supplies three phase currents at high voltage direct to the field of the induction motor and drives it, generating in it currents at a lower voltage depending on the ratio of the windings.
Part of the current thus generated in the armature passes into the armature of the dynamo and is converted by the[Pg 1492] commutator into direct current as in a rotary converter, but is also increased by the current induced in the winding of the dynamo armature.
Ques. At what speed does the machine run?
Ans. Assuming equal numbers of pole, the armatures rotate at a speed corresponding to one half the circuit frequency.
Thus if the motor have six poles and the frequency be 50, the rotary field revolves at 50 × 60 ÷ 3 = 1,000 R.P.M. and the motor will revolve at one-half that speed or 1,000 ÷ 2 = 500 R.P.M.
Since the connections are so arranged that these currents tend to set up in the armature a revolving field, rotating at half speed in a sense opposite to that in which the shaft is rotating at half speed, it follows that by the super-position of this revolving field upon the revolutions of the machine, the magnetic effect is equivalent to a rotation of the armature at whole speed, so that it operates in synchronism, as does the armature of a rotary converter.
Half the electric input into the motor part is, therefore, turned into mechanical energy to drive the shaft, the other half acts inductively on the armature winding, generating currents therein.
As to the dynamo part it is half generator, receiving mechanical power by transmission along the shaft to furnish half its output, and it is half converter, turning the currents received from the armature into direct current delivered at the brushes.
Ques. What action takes place in the motor armature winding?
Ans. Since it runs at one-half synchronous speed, it generates alternating current of half the supply current frequency, delivering these to the armature of the dynamo.
Ques. What claim is made for this type of apparatus?
Ans. The cost is said to be less than a motor generator set, and it is claimed to be self-synchronizing and to require no special starting gear, also to be 2.5 per cent. more efficient than a motor generator.
Ques. How is the machine started from the high pressure side?
Ans. The field winding is connected directly to the high pressure leads. The three slip ring brushes are connected with external resistances which are used while starting, the external resistances being gradually cut out of the circuit as the machine comes up to speed (the same as with an ordinary slip ring motor).
Ques. How does a cascade converter compare with a synchronous converter?
Ans. It is about equally expensive as the synchronous converter with its necessary bank of transformers, but is about one per cent. less efficient. It is claimed to be more desirable for frequencies above 40 on account of the improved commutation at the low frequency used in the dynamo member. For lower frequencies the synchronous converter is preferable.
The purpose of a rectifier is to change alternating current into a uni-directional or pulsating current. There are several classes of apparatus to which the term rectifier may be applied, as
Mechanical Rectifiers.—By definition, a mechanical rectifier is a form of commutator operating in synchronism with the generator and commutating or rectifying the negative waves of the alternating current as shown graphically in figs. 2,076 and 2,078. The essential features of construction are shown in fig. 2,079.
Ques. Mention some application of a mechanical rectifier.
Ans. It is used on a compositely excited alternator as illustrated on page 1,192.
Electrolytic Rectifiers.—If two metals be placed in an electrolyte and then subjected to a definite difference of pressure, they will (under certain conditions) offer greater resistance to the[Pg 1496] passage of a current in one direction, than in the other direction. On account of this so called valve effect, electrolytic rectifiers are sometimes called "valves."
Ques. What metal is generally used for the cathode?
Ans. Aluminum.
Ques. What is generally used for the other electrode?
Ans. Lead or polished steel.
Metals of low atomic weight exhibit the valve effect at high differences of pressure, and heavier metals at low differences of pressure.
Ques. Describe the "Nodon valve."
Ans. The cathode is of aluminum or aluminum alloy, and the[Pg 1497] other electrode, which has considerably more surface, is the containing vessel. The electrolyte is a neutral solution of ammonia phosphate.
Ques. Describe its action.
Ans. It is due to the formation of a film of normal hydroxide of aluminum, over the surface of the aluminum electrode. This film presents a very high resistance to the current when flowing in one direction but very little resistance, when flowing in the reverse direction.
Ques. What is the effect when a Nodon cell is supplied with alternating current?
Ans. Half of the wave will be suppressed and an intermittently pulsating current will result as shown in fig. 2,077.
Ques. How may both halves of the alternating waves be utilized?
Ans. By coupling a series of cells in opposed pairs as in fig. 2,080.
Ques. Upon what does the efficiency of the film depend?
Ans. Upon the temperature.
It should not for maximum efficiency exceed 86 degrees Fahr. There is also a certain critical voltage above which the film breaks down locally, giving rise to a luminous and somewhat disruptive discharge accompanied by a rapid rise of temperature and fall in efficiency.
Ques. When an electrolytic rectifier is not in use for some time what happens?
Ans. The electrodes will loose the film.
Ques. What must be done in such case?
Ans. The electrodes must be reformed.
Ques. How is the loss of film prevented?
Ans. By Removing the electrodes from the electrolyte and drying them.
Ques. What attention must be given to the electrolyte?
Ans. Water must be added from time to time to make up for evaporation.
This is necessary to keep the solution at the proper density.
Ques. What is the indication that the rectifier needs recharging?
Ans. Excessive heating of the solution with normal load.
Ques. What is the indication that a rectifier is passing alternating current?
Ans. It will heat, and if the solution be very weak, it will cause a buzzing sound.
Ques. What harm is caused by operating a rectifier with a weak electrolyte?
Ans. The electrodes will eat away.
A few of the so called electrolytic valves are here briefly described:
The Audion Valve.—This valve was invented by De Forest in 1900 and is practically identical with the Fleming oscillation valve, the latter being illustrated in fig. 2,086.
Grisson Valve.—In this valve the cathode is a sheet of aluminum, and the anode, a sheet of lead, supported, in the original form, horizontally in a vessel containing the electrolyte, consisting of a solution of sodium carbonate. Cooling is effected by circulating water through metal tubes in the electrolyte itself.
Pawlowski Valve.—This is an electrolytic valve employing a solid electrolyte. It consists of a copper plate which has been coated with a crystalline layer of carefully prepared copper hemisulphide, prepared by melting sulphur and copper together out of contact with air. The prepared plate is placed in contact with an aluminum sheet and the combination is then formed by submitting it to an alternating pressure until sparking, which at first occurs, ceases.
Giles Electric Valve.—This consists of a combination of spark gaps and capacity used to protect electrical apparatus against damage due to atmospheric discharges and resonance surges. The spark gaps are formed between the edges of sharp rimmed discs of non-arcing metal. These discs are insulated from each other, and from the central tube, which provides a support for the apparatus and also an earth. The condenser effect is obtained by means of the annular discs and the tube; an adjustable spark gap, a high resistance, and a fuse all connected in series, complete the valve.
Buttner Valve.—It is of the Nodon type employing a cathode of magnesium-aluminum alloy, and probably iron or lead as anode, with an electrolyte of ammonium borate. Buttner claims that the borate is superior to the phosphate in that it does not attack iron, and will keep in good working condition for longer periods.
Mercury Vapor Rectifiers.—The Cooper Hewitt mercury vapor rectifier, as shown in fig. 2,093 consists essentially of a hermetically sealed glass bulb filled with mercury vapor and provided with four electrodes. The two upper electrodes are of solid material and the two lower of mercury.
The solid electrodes are the positive electrodes; the mercury electrodes are the negative electrodes.
The mercury pools of the two lower electrodes are not in contact when the bulb is vertical, but the bulb is so mounted that it can be tilted to bring these two pools temporarily in contact for starting.
The bulb contains highly attenuated vapor of mercury, which, like other metal vapors, is an electrical conductor under some conditions. The positive electrodes are surrounded by this[Pg 1508] vapor. Current can readily pass from either of the solid electrodes to the mercury vapor and from it to the mercury electrode, but when the direction of flow tends to reverse, so that current would pass from the vapor to the solid electrode, there is a resistance at the surface of the electrode, which entirely prevents the flow of current.
The alternating current supply circuit is connected to the two positive electrodes as shown in the diagram, and as the electrodes will allow current to flow in only one direction and oppose any current flow in the opposite direction, the pulsations of the current pass alternately from one or the other of the positive electrodes into the mercury.
As these currents cannot pass from the vapor into either positive electrode, they are constrained to pass out all in one direction through the mercury electrode, from which they emerge as a uni-directional current. The positive electrodes of the rectifier thus act as check valves, permitting current to pass into the mercury vapor but not allowing it to pass from the vapor to the solid electrodes.
Ques. What condition prevails before the bulb starts to rectify?
Ans. There appears to be a high resistance at the surface
[Pg 1512] of the mercury, which must be broken down so that the current can pass.
Ques. What is this apparent surface resistance called?
Ans. The negative electrode resistance.
Ques. What must be done before any current can pass?
Ans. The negative electrode resistance must be overcome.
When once started the current will continue to flow, meeting with practically no resistance as long as the current is uninterrupted.
Ques. What will happen if the current be interrupted even for the smallest instant of time?
Ans. The negative electrode resistance will re-establish itself, and stop the operation of the bulb.
Ques. How is the negative electrode resistance overcome?
Ans. The bulb is tilted or shaken so that the space between the mercury electrodes is bridged by the mercury.
Ques. What happens when the bulb is tilted?
Ans. Current then passes between the two mercury electrodes from the starting transformer and the little stream of mercury which bridges the space between the electrodes breaks with a spark as the bulb is returned to its vertical position.
Ques. What duty is performed by the spark?
Ans. It breaks down the negative electrode resistance.
Ques. What conditions are now necessary for continuous operation of the rectifier?
Ans. The rectifier will now operate indefinitely as long as the current supply is uninterrupted and the direct current load does not fall below the minimum required for the arc.
Ques. Is the rectifier self-starting?
Ans. After the bulb has been started a few times, as described above, it becomes self-starting, so that under all ordinary operating conditions it will commence to operate when the switches connecting it with the load and the alternating current supply are closed.
Ques. What provision is made in the Westinghouse-Cooper Hewitt rectifier to render it self-starting?
Ans. It is rendered self-starting by means of a condenser.
Ques. Describe the arrangement and operation of the condenser.
Ans. The condenser is connected between one of the positive electrodes and a coating of tinfoil outside[Pg 1514]
[Pg 1515] the part of the bulb containing the mercury, and induces static sparks on the surface of the mercury which break down the negative electrode resistance.
The action of the rectifier will be better understood by reference to the diagram of current waves and impressed pressure as shown in figs. 2,103 to 2,106.
Ques. Describe a mercury vapor rectifier outfit for series arc lighting.
Ans. It consists of a constant current regulating transformer,[Pg 1516] a rectifier bulb, and a control panel containing the necessary switches, meters, etc. The transformer and rectifier bulb are mounted in the same tank.
Ques. Describe the construction and operation of the mercury arc[11] rectifier shown in fig. 2,108.
[11] NOTE.—The terms vapor and arc as applied to rectifiers, do not indicate a different principle; the Westinghouse Co. employ the former, and the General Electric Co., the latter.
Ans. Fig. 2,108 is an elementary diagram of connections. The rectifier tube is an exhausted glass vessel in which are two graphite anodes A, A', and one mercury cathode B. The small starting electrode C is connected to one side of the alternating circuit, through resistance; and by rocking the tube a slight arc is formed, which starts the operation of the rectifier tube. At the instant the terminal H of the supply transformer is positive, the anode A is then positive, and the arc is free to flow between A and B. Following the direction of the arrow still further, the [Pg 1518]current passes through the battery J, through one-half of the main reactance coil E, and back to the negative terminal G of the transformer. When the impressed voltage falls below a value sufficient to maintain the arc against the reverse pressure of the arc and load, the reactance E, which heretofore has been charging, now discharges, the discharge current being in the same direction as formerly. This serves to maintain the arc in the rectifier tube until the pressure of the supply has passed through zero, reversed, and built up such a value as to cause the anode A to have a sufficiently positive value to start the arc between it and the cathode B. The discharge circuit of the reactance coil E is now through the arc A'B instead of through its former circuit. Consequently the arc A'B is now supplied with current, partly from the transformer, and partly from the reactance coil E. The new circuit from the transformer is indicated by the arrows enclosed in circles.
Ques. How is a mercury arc rectifier started?
Ans. A rectifier outfit with its starting devices, etc., is shown in figs. 2,114 to 2,116. To start the rectifier, close in order named line switch and circuit breaker; hold the starting switch in opposite position from normal; rock the tube gently by rectifier shaker. When the tube starts, as shown by greenish blue light, release[Pg 1522] starting switch and see that it goes back to normal position. Adjust the charging current by means of fine regulation switch on the left; or, if not sufficient, by one button of coarse regulation switch on the right. The regulating switch may have to be adjusted occasionally during charge, if it be desired to maintain charging amperes approximately constant.
Ques. In the manufacture of rectifiers, could other metals be used for the cathode in place of mercury?
Ans. Yes.
Ques. Why are they not used?
Ans. Because, on account of the arc produced, they would gradually wear away and could not be replaced conveniently.
In the case of mercury, the excess vapor is condensed to liquid form in the large glass bulb or condensing chamber of the tube and gravitates back to the cathode, where it is used over and over again.
Ques. In the operation of rectifiers, how is the heat generated in the bulb dissipated?
Ans. In small rectifier sets the heat generated is dissipated through the tube to the air, and in large tubes such as used in supplying 40 to 60 kw. for constant current flaming arc lights operating at 4 or 6.6 amperes, the tubes are immersed in a[Pg 1525] tank of oil, and cooled similar to the arrangement used for oil insulated water cooled transformers.
Ques. What results are obtained with oil cooled tubes?
Ans. In practice it is found that the life of oil cooled tubes is greatly increased and temperature changes do not affect the ability to start up as in the air cooled tubes.
Ques. In the operation of a rectifier, name an inherent feature of the mercury arc.
Ans. A reverse pressure of approximately 14 volts is produced, which remains nearly constant through changes of load,[Pg 1526] frequency, and voltage. Its effect is to decrease the commercial efficiency slightly on light loads.
Ques. What is the advantage of a rectifier set over a motor generator set?
Ans. Higher efficiency and lower first cost.
Ques. What is the capacity of a rectifier tube?
Ans. 40 to 50 amperes.
Ques. How is greater capacity obtained?
Ans. When a greater ampere capacity is required, two or more rectifier sets can be joined to one circuit.
The rectifier may be joined in series for producing an increased voltage or two tubes can be connected in series in a single set.
Ques. For what service is a rotary converter better adapted than a rectifier?
Ans. For power distribution and other cases where a great amount of alternating current is to be converted into direct current, the rotary converter or large motor generator sets are more practical.
Ques. For what service is a rectifier especially adapted?
Ans. It is very desirable for charging storage batteries for automobiles from the local alternating current lighting circuit.
When the consumer installs and operates the apparatus for his own use and wear, there is considerable saving over motor generator sets because a small one to two horse power motor generator outfit has an efficiency of only 40 to 50 per cent. while mercury vapor rectifiers will have from 75 to 80 per cent.
Ques. What precautions should be taken in installing a rectifier?
Ans. It should be installed in a dry place and care should be taken to avoid dangling wires near the tube to prevent[Pg 1527] puncturing. If the apparatus be installed in a room of uniform moderate temperature very little trouble will be experienced in starting, while extreme cold will make starting more difficult.
[12]Electro-magnetic Rectifiers.—Devices of this class consist essentially of a double contact rocker which rocks on pivot (midway between the contacts), in synchronism with the frequency of the alternating current, so changing the connections at the instants of reversals of the alternating current that a direct current is obtained.
[12] NOTE.—The Edison electromagnetic rectifier is described in detail in Guide No. 4, pages 942 to 945.
Fig. 2,121 is a combined sketch and diagram of connections of a type of electromagnetic rectifier that has been introduced for changing alternating into direct current. The actual apparatus consists of a box, with perforated metal sides, about ten inches square and six inches deep. This box contains the step down transformer P,S,S', and the condensers K and K', the magnets and contact making device about to be described being fixed on the polished slate top of the box, exactly as shown in the figure. The transformer primary winding P may be connected through a switch s with a pair of ways on the nearest distribution box, or to a plug connection or lamp-holder, and the apparatus will give a rectified current of 6 or 12 amperes at 20 volts, according to the size.
S and S' is the secondary winding of the transformer, with a tapping t midway, joining it to a series circuit containing two alternating current electromagnets E and E', whose cores are connected by the long soft iron yoke Y. Pivoted at P' is a steel bar SB, which is polarized by the two coils C and C' the current being supplied by a cell A. Fixed[Pg 1528] rigidly to SB, and moving with it, is a double contact piece CP with platinum contacts opposite similar ones on the fixed studs CS, CS'.
CP is flexibly connected through F to one of the direct current terminals T, to which also are joined up one coating of each condenser K and K'.
The other direct current terminal T' is connected to the center of the transformer secondary at t; and CS and CS' are respectively joined up to either end of the secondary winding and to the other coatings of the condensers.
When the alternating current circuit is broken, the springs SP, SP, carried by SB and bearing against the adjustable studs, keep SB, CS and CS'. The apparatus thus acts also as a no voltage circuit breaker, for should the supply fail, the storage battery A' under charge will be left on open circuit.
The action of the device is briefly as follows:
Owing to the direct current in the magnetizing coils C and C' one end of SB will be permanently of north and the other of south polarity; and since the polarities of the poles E and E' will alternate with the alternations of the transformer secondary current, SB will rock rapidly on its pivot, and contact will be made by turns with CS and CS'.
The purpose of the condensers K and K' is to reduce the sparking at these points. When contact is made at CS, the direct current terminals T and T' are connected to the S half of the secondary winding; and when contact is made at CS', they are connected to the S' half. Thus a rectified uni-directional current will flow from T and T', and it may be used to charge the battery A', work a small motor or for various other purposes requiring direct current.
When the rectifier is used for charging storage batteries, the separate cell A may sometimes be dispensed with, the winding C,C' being connected to one of the cells under charge.
The rectifier is adjusted to suit the frequency of the supply circuit by altering the distance of the poles of E and E' from the ends of the polarized armature SB; and also by changing the tension of SP, SP by means of the screw studs against which they bear.
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ELECTRICAL GUIDE, NO. 1
Containing the principles of Elementary Electricity, Magnetism, Induction, Experiments, Dynamos, Electric Machinery.
ELECTRICAL GUIDE, NO. 2
The construction of Dynamos, Motors, Armatures, Armature Windings, Installing of Dynamos.
ELECTRICAL GUIDE, NO. 3
Electrical Instruments, Testing, Practical Management of Dynamos and Motors.
ELECTRICAL GUIDE, NO. 4
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage Batteries.
ELECTRICAL GUIDE, NO. 5
Principles of Alternating Currents and Alternators.
ELECTRICAL GUIDE, NO. 6
Alternating Current Motors, Transformers, Converters, Rectifiers.
ELECTRICAL GUIDE, NO. 7
Alternating Current Systems, Circuit Breakers, Measuring Instruments.
ELECTRICAL GUIDE, NO. 8
Alternating Current Switch Boards, Wiring, Power Stations, Installation and Operation.
ELECTRICAL GUIDE, NO. 9
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
ELECTRICAL GUIDE, NO. 10
Modern Practical Applications of Electricity and Ready Reference Index of the 10 Numbers.
Silently corrected simple spelling, grammar, and typographical errors.
Retained anachronistic and non-standard spellings as printed.