CHAPTER LI
ALTERNATING CURRENT MOTORS
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
_a. Single phase; b. Polyphase;_
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:
1. An armature, 2. A field.
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 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 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
_s_. Revolutions per minute of the motor; _p_. Number of poles of the motor; S. Revolutions per minute of the alternator; P. Number of poles of the alternator.
~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 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.
[1] NOTE.--See Hunting of synchronous motors, page 1,280.
~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.
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 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:
1. Single phase; 2. Polyphase.
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
1. Oscillating[3], or 2. Rotating,
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 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 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:
1. _The production of a rotating field;_ 2. _Induction of current in the armature;_ 3. _Reaction between the revolving field and the induced currents._
~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 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 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 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
[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 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.) = S_f_ - Sₐ
or, expressed as a percentage of synchronism, that is, of the synchronous speed,
(S_f_ - Sₐ) × 100 Slip (%) = ----------------- S_f_
where
S_f_ = Synchronous speed, or R.P.M. of the rotatory magnetic field; Sₐ = Speed of the armature.
The synchronous speed is determined the same as for synchronous motor by use of the following formula:
2_f_ S_f_ = ---- P
where
S_f_ = Synchronous speed or R.P.M. of the rotating magnetic field; P = Number of poles; _f_ = frequency.
The following table gives the synchronous speed for various frequencies and different numbers of poles:
~Table of Synchronous Speeds~
+------------------------------------------------------+ | | R.P.M. of the rotating magnetic field, | | | when number of poles is | |Frequency|--------------------------------------------| | | 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:
~SLIP OF INDUCTION MOTORS~
+----------+------------------------+----------+------------------------+ | | Slip at full load | | Slip at full load | | Capacity | per cent. | Capacity | per cent. | | of motor |------------------------| of motor |------------------------| | H. P. | Usual limits | Average | H. P. | 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 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.
~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 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:
1. Resistances in the field; 2. Auto-transformer or compensator; 3. Resistance in armature.
~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 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.
[7] NOTE.--The construction of starting devices for induction motors is fully explained later, the accompanying cuts serving merely to illustrate the principles involved.
~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 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 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 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 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 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 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:
1. Series; 2. Shunt; 3. Compensated; 4. Repulsion.
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:
1. The transformer pressure; 2. The generated pressure; 3. The self-induction pressure.
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 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
~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 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
1. Conductively; or 2. Inductively.
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 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 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 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 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 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 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.