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Single phase induction motors, Lecture notes of Engineering

Acharya Nagarjuna UniversityEngineering

Stepper motors are motors used to advance the position of a shaft or other mechanical device by a fixed amount each time a control pulse is received. They are used extensively in control systems for positioning objects.

Typology: Lecture notes

2020/2021

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EE 323 Electrical Machines II
1
MODULE 6 SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES
Constructional details of single phase induction motor – Double field revolving theory and operation – Equivalent
circuit – No load and blocked rotor test – Performance analysis – Starting methods of single-phase induction motors
– Capacitor-start capacitor run Induction motor- Shaded pole induction motor - Linear induction motor – Repulsion
motor - Hysteresis motor - AC series motor- Servo motors- Stepper motors - introduction to magnetic levitation
systems.
CONSTRUCTIONAL DETAILS OF SINGLE PHASE INDUCTION MOTOR
Constructionally, single phase induction motor is similar to polyphase induction motor except that (i) its stator
is provided with a single phase winding and (ii) a centrifugal switch in order to cut out a winding used for starting
purposes. It has distributed stator winding and a squirrel cage rotor.
The constructional details of single phase induction motor are shown in figure.
1.
Stator of Single Phase Induction Motor
•
The single-phase motor stator has a
laminated iron core with two windings
arranged perpendicularly
•
One is the main and other is the auxiliary
winding or starting winding.
•
The stator has laminated construction,
made up of stampings. The stampings are
slotted on its periphery to carry the winding
called stator winding or main winding.
•
This is excited by a single phase a.c.
supply. The laminated construction keeps iron losses to minimum, lie stampings are made up of material like
silicon steel which minimizes the hysteresis loss.
•
The stator winding is wound for certain definite number of poles means when excited by single phase a.c. supply,
stator produces the magnetic field which creates the effect of certain definite number of poles.
•
The number of poles for which stator winding is wound, decides the synchronous speed of the motor. The
synchronous speed is denoted as Ns and it has a fixed relation with supply frequency f and number of poles P. The
relation is given by, Ns = 120f/P.
2.
Rotor of Single Phase Induction Motor
•
The rotor of single phase induction
motor is shown in figure.
•
The construction of the rotor of the
single phase induction motor is similar
to the squirrel cage three phase
inductions motor.
•
The rotor is cylindrical in shape and
has slots all over its periphery.
•
The slots are not made parallel to
each other but are bit skewed as the
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16
pf17
pf18
pf19
pf1a
pf1b
pf1c
pf1d
pf1e
pf1f

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MODULE 6 SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES

Constructional details of single phase induction motor – Double field revolving theory and operation – Equivalent circuit – No load and blocked rotor test – Performance analysis – Starting methods of single-phase induction motors

  • Capacitor-start capacitor run Induction motor- Shaded pole induction motor - Linear induction motor – Repulsion motor - Hysteresis motor - AC series motor- Servo motors- Stepper motors - introduction to magnetic levitation systems. CONSTRUCTIONAL DETAILS OF SINGLE PHASE INDUCTION MOTOR Constructionally, single phase induction motor is similar to polyphase induction motor except that (i) its stator is provided with a single phase winding and (ii) a centrifugal switch in order to cut out a winding used for starting purposes. It has distributed stator winding and a squirrel cage rotor. The constructional details of single phase induction motor are shown in figure. 1. Stator of Single Phase Induction Motor
  • The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly
  • One is the main and other is the auxiliary winding or starting winding.
  • The stator has laminated construction, made up of stampings. The stampings are slotted on its periphery to carry the winding called stator winding or main winding.
  • This is excited by a single phase a.c. supply. The laminated construction keeps iron losses to minimum, lie stampings are made up of material like silicon steel which minimizes the hysteresis loss.
  • The stator winding is wound for certain definite number of poles means when excited by single phase a.c. supply, stator produces the magnetic field which creates the effect of certain definite number of poles.
  • The number of poles for which stator winding is wound, decides the synchronous speed of the motor. The synchronous speed is denoted as Ns and it has a fixed relation with supply frequency f and number of poles P. The relation is given by, Ns = 120f/P. 2. Rotor of Single Phase Induction Motor
  • The rotor of single phase induction motor is shown in figure.
  • The construction of the rotor of the single phase induction motor is similar to the squirrel cage three phase inductions motor.
  • The rotor is cylindrical in shape and has slots all over its periphery.
  • The slots are not made parallel to each other but are bit skewed as the

skewing prevents magnetic locking of stator and rotor teeth and makes the working of induction motor more smooth and quieter.

  • The squirrel cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor.
  • The rotor conductors are permanently shorted by the copper or aluminium rings called the end rings.
  • In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as "squirrel cage induction motor".
  • As the bars are permanently shorted by end rings, the rotor electrical resistance is very small and it is not possible to add external resistance as the bars are permanently shorted.
  • The absence of slip ring and brushes make the construction of single phase induction motor very simple and robust. DOUBLE FIELD REVOLVING THEORY When fed from a single-phase supply, its stator winding produces a flux (or field) which is only alternating i.e. one which alternates along one space axis only. It is a synchronously revolving (or rotating) flux, as in the case of a two- or a three-phase stator winding, fed from a 2-or 3-phase supply. Now, alternating or pulsating flux acting on a stationary squirrel-cage rotor cannot produce rotation (only a revolving flux can). That is why a single-phase motor is not self starting. However, if the rotor of such a machine is given an initial start by hand (or small motor) or otherwise, in either direction, then immediately a torque arises and the motor accelerates to its final speed (unless the applied torque is too high). This peculiar behaviour of the motor can be explained in two ways (i) by two - field or double- field revolving theory and (ii) by cross-field theory. According to double field revolving theory, an alternating uniaxial quantity can be represented by two oppositely rotating vectors of half magnitude. Accordingly, an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to half the value of the alternating flux and each rotating synchronously (Ns = 120f/p) in opposite direction. As shown in fig. (a), let the alternating flux have a maximum value of Φm. its component fluxes A and B will each be equal to Φm/2 revolving in anticlockwise and clockwise direction respectively. After some time, when A and B would have rotated through angle +θ and – θ as in fig (b), the resultant flux would be

MAKING SINGLE PHASE INDUCTION MOTOR SELF-STARTING

  • A single phase induction motor is not self starting. To overcome this drawback and make the motor self starting, it is temporarily converted into a two phase motor during starting period.
  • For this purpose, the stator of a single phase motor is provided with an extra winding, known as starting or auxiliary winding, in addition to the main or running winding.
  • The two windings are spaced 90^0 electrically apart and are connected in parallel across the single phase supply.
  • It is so arranged that the phase difference between the currents in the two stator windings is very large (ideal value is 900 ).
  • Hence the motor behaves as a two phase motor. These two currents produce a revolving flux and hence make the motor self starting. Depending on the methods by which the necessary phase difference between the two currents can be created, the single phase induction motors are classified as,
  1. Split phase motor
  2. Capacitor start induction run motors
  3. Capacitor start and run motors
  4. Shaded pole single phase motors
    1. Split Phase Motors (Resistance Start split phase induction motors)
  • In split phase machine, the main winding has low resistance but high reactance whereas the starting winding has a high resistance but low reactance.
  • Hence the current Is drawn by the starting winding lags behind the applied voltage V by a small angle whereas current Im drawn by the main winding lags behind V by a very large angle.
  • Phase angle between Is and Im is made as large as possible because the starting torque of a split phase motor is proportional to sin α.
  • A centrifugal switch S is connected in series with the starting winding and located inside he motor.
  • Its function is to automatically disconnect the starting winding from the supply when the motor has reached 70 to 80 percent of its full load speed.
  • The starting torque is 150 to 200 percent of the full load torque.
  • Starting current is 6 to 8 times the full load current. Applications
  • Fans, blowers, centrifugal pumps and separators, washing machines, small machine tools, duplicating machines, domestic refrigerators, and oil burners etc.
  • Available sizes range from 1/20 to 1/3 h.p. (40 to 250 W) with speeds ranging from 3450 to 865 rpm.

2. Capacitor Start Induction Run Motors - In this motor, the phase difference between Is and Im is produced by connecting a capacitor in series with the starting winding. - The capacitor is electrolytic type and is mounted outside the motor as a separate unit. - When the motor reaches about 75 percent of the full speed, the centrifugal switch S opens and cuts out both the starting winding and capacitor from the supply, thus leaving only the running winding across the lines. - As shown in figure, current Im drawn by the main winding lags the supply voltage V by a large angle whereas Is leads V by a certain angle. - The two currents are out of phase with each other by about 80^0 as compared to nearly 30^0 for a split phase motor. - Torque developed is proportional to sin α (angle between Is and Im), therefore starting torque is as high as 350 to 450 percent. 3. Capacitor Start and Run motor

  • This motor is similar to the capacitor start motor except that the starting winding and capacitor are connected in the circuit at all times.
  • The advantages of leaving the capacitor permanently in the circuit are o Improvement of overload capacity of the motor o A higher power factor o Higher efficiency o Quieter running of the motor Types (a) Single value Capacitor Run motor – start and run with one value of capacitance in the circuit (b) Two Value Capacitor Run motor – start with high value of capacitance but run with low value of capacitance. (a) Single value Capacitor Run motor
  • It has one running winding and one starting winding in series with a capacitor.
  • Since capacitor remains in the circuit permanently, this motor is referred to as permanent split capacitor run motor.
  • Since the same capacitor is used for starting and running, neither optimum starting not optimum running performance can be obtained.
  • Capacitors of 2 to 20 μF are used.
  • The low value capacitor result in small starting torque which is about 50 to 100 % of the rated torque.
  • This type of motor can be easily reversed by an external switch provided its running and starting windings are identical. Applications Fans, blowers, voltage regulators etc.
  • Consider the three instants say t1, t2 and t3 during first half cycle of the flux as shown, in the Fig.
  • At instant t = t 1 , rate of rise of current and hence the flux is very high. Due to the transformer action, large e.m.f. gets induced in the copper shading band.
  • This circulates current through shading band as it is short circuited, producing its own flux.
  • According to Lenz’s law, the direction of this current is so as to oppose the cause i.e. rise in current. Hence shading ring flux is opposing to the main flux.
  • Hence there is crowding of flux in unshaded part while weakening of flux in shaded part. Overall magnetic axis shifts in unshaded part as shown in the Fig.
  • At instant t = t 2 , rate of rise of current and hence the rate of change of flux is almost zero as flux almost reaches to its maximum value. Hence there is very little induced e.m.f. in the shading ring.
  • Hence the shading ring flux is also negligible, hardly affecting the distribution of the main flux. Hence the main flux distribution is uniform and magnetic axis lies at the center of the pole face as shown in the Fig.
  • At instant t = t 3 , the current and the flux is decreasing. The rate of decrease is high which again induces a very large e.m.f. in the shading ring.
  • This circulates current through the ring which produces its own flux. Now direction of the flux produced by the shaded ring current is so as to oppose the cause which is decrease in flux. So it oppose the decrease in flux means its direction is same as that of main flux, strengthening it.
  • So there is crowding of flux in the shaded part as compared to unshaded part. Due to this the magnetic axis shifts to the middle of the shaded part of the pole.
  • This sequence keeps on repeating for negative half cycle too. Consequently this produces an effect of rotating magnetic field, the direction of which is from unshaded part of the pole to the shaded part of the pole.
  • Due to this, motor produces the starting torque and starts rotating. The starting torque is low which is about 40 to 50 % of the full load torque for this type of motor. The torque speed characteristic is shown in the fig.
  • Shaded pole motors are built in very small sizes varying from 1/250 h.p. (3W) to 1/6 h.p. (125 W). Advantages - Simple in construction - Extremely rugged - Reliable - Cheap Disadvantages - Low starting torque - Very little overload capacity - Low efficiency - Direction of rotation cannot be changed, because it is fixed by the position of copper rings. Applications - Used for small fans, toys, hair dryers, ventilators, electric clocks etc.

EQUIVALENT CIRCUIT

Imagine that the single phase induction motor is made up of one stator winding and two imaginary rotor windings. One rotor is rotating in forward direction i.e. in the direction of rotating magnetic field with slip s while other is rotating in backward direction i.e. in direction of oppositely directed rotating magnetic field with slip 2 - s. Without Core Loss Let the stator impedance be Z Ω Z = R 1 +jX 1 Where R 1 = Stator resistance, X 1 = Stator reactance, X 2 = Rotor reactance referred to stator R 2 = Rotor resistance referred to stator Hence the impedance of each rotor is r 2 + j x 2

Where x 2 = X

2

; r 2 =R

2

2 2 The resistance of forward field rotor is r 2 2 while^ the resistance of backward field rotor is r 2

(2-s) As the core loss is neglected, R 0 does not exist in the equivalent circuit. The x 0 is half of the actual magnetising

reactance of the motor. Therefore, x 0 =X

O

2 So the equivalent circuit referred to stator is shown in the Fig. The impedance of the forward field rotor is Zf is parallel combination of (j x 0 ) and (r 2 /s) + j x 2. While the impedance of the backward field rotor is Zb is parallel combination of (j x 0 ) and (r 2 / (2-s)) + j x 2. Under standstill condition, s = 1 and 2 - s = 1. Hence Zf = Zb and Vf = Vb. But in the running condition, Vf becomes almost 90 to 95 % of the applied voltage. Equivalent impedance, Zeq = Z 1 + Zf + Zb Let I2f = Current through forward rotor referred to stator and I2b = Current through backward rotor referred to stator

TESTS ON SINGLE PHASE INDUCTION MOTOR

  1. No load test or open circuit test
  2. Blocked rotor test or short circuit test No Load Test The test is conducted by rotating the motor without load. The input current, voltage and power are measured by connecting the ammeter, voltmeter and wattmeter in the circuit. These readings are denoted as V 0 , I 0 and W 0. W 0 = V 0 I 0 cosΦ 0 Therefore, No load power factor, cosΦ 0 = WO VO (^1) O The motor speed on no load is almost equal to its synchronous speed hence for practical purposes, the slip can be assumed zero. Hence r 22 becomes ∞ and acts as open circuit in the equivalent circuit. Hence for forward rotor circuit, the branch r 2 /s + j x 2 gets eliminated. While for a backward rotor circuit, the term r 2 / (2-s) tends to r 2 /2. Thus x 0 is much higher than the impedance r 2
    • j x2. Hence it can be assumed that no current can flow through xm and that branch can be eliminated. 2 So circuit reduces to as shown in the Fig. The voltage across x 0 is VAB VAB = V 0 – I 0 [(R 1 + r^2 ) + j(x 1 + x 2 ) 2 Also VAB = I 0 x 0 Therefore x 0 = VAB (^1) O But x 0 = XO 2 Therefore, magnetizing reactance, X 0 = 2 x 0 = 2VAB (^1) O No load power W 0 = rotational losses. Blocked Rotor Test In blocked rotor test, the rotor is held fixed so that it will not rotate. A reduced voltage is applied to limit the short circuit current. This voltage is adjusted with the help of autotransformer so that the rated current flows through main winding. The input voltage, current and power is measured by connecting voltmeter, ammeter and wattmeter respectively. These readings are denoted as Vsc, Isc and Wsc. As rotor is blocked, the slip s = 1. Hence the magnetising reactance x 0 is much higher than the rotor impedance and hence it can be neglected as connected in parallel with the rotor. Thus the equivalent circuit for blocked rotor test is as shown in the Fig. Wsc = Vsc Isc CosΦsc Short circuit power factor, CosΦsc = Wsc Vsc (^1) sc Zeq = Vsc^ ; Req = Wsc^ ; but Req = R 1 + R 2 ; therefore Rotor resistance referred to stator, R 2 = Req – R 1 ; (^1) sc 1 sc^2

Xeq = JZ^2 - R^2 ; eq eq Assume, X = X ; therefore rotor reactance referred to stator, X (^) = Xeq (^1 2 2 ) The stator resistance R 1 is measured by voltmeter-ammeter method, by disconnecting the auxiliary winding and capacitors present if any. Due, to skin effect, the a.c. resistance is 1.2 to 1.5 times more than the d.c resistance. Thus with these two tests, all the parameters of single phase induction motor can be obtained. RELUCTANCE MOTOR

  • A single phase synchronous Reluctance Motor is basically the same as the single cage type induction motor.
  • The stator of the motor has the main and auxiliary winding. The stator of the single phase reluctance and induction motor are same.
  • The rotor of a reluctance motor is a squirrel cage with some rotor teeth removed in the certain places to provide the desired number of salient rotor poles. The figure shows the 4 pole reluctance type synchronous motor.
  • In the figure the teeth have been removed in four locations to produce a 4 pole structure. The two end rings are short circuited.
  • When the stator is connected to a single phase supply, the motor starts as a single phase induction motor.
  • A centrifugal switch disconnects the auxiliary winding as soon as the speed of the motor reaches about 75% of the synchronous speed.
  • The motor continues to speed up as a single phase motor with the main winding in operation.
  • A reluctance motor torque is produced due to the tendency of the rotor to align itself in the minimum reluctance position, when the speed of the motor is close to the synchronous speed. Thus, the rotor pulls in synchronism.
  • The load inertia should be within the limits, for proper effectiveness.
  • At synchronism, the induction torque disappears, but the rotor remains in synchronism due to synchronous reluctance torque. The Torque Speed Characteristic of a single phase Reluctance Motor is shown below.
  • The starting torque depends upon the rotor position.
  • The value of the starting torque varies between 300 to 400 % of its full load torque.
  • As motor attains speed nearly of synchronous speed the auxiliary winding is disconnected and the rotor continues to rotate at the synchronous speed.
  • The motor operates at a constant speed up to a little over than 200% of its full load torque.
  • If the loading of the motor is increased above the value of the pull out torque, the motor loose synchronism but continues to run as a single phase induction motor up to over 500% of its rated torque.
  • At the starting the motor is subjected to Cogging. This can be reduced by skewing the rotor bars and by having the rotor slots not exact multiples of the

1 Operation ofa Hysteresis Motor

  • When supply is given applied to the stator, a rotating magnetic field is produced. This magnetic field magnetises the rotor ring and induces pole within it.
  • Due to the hysteresis loss in the rotor, the induced rotor flux lags behind the rotating stator flux.
  • The angle δ between the stator magnetic field BS and the rotor magnetic field BR is responsible for the production of the torque. The angle δ depends on the shape of the hysteresis loop and not on the frequency.
  • Thus, the value of Coercive force and residual flux density of the magnetic material should be large.
  • The ideal material would have a rectangular hysteresis loop as shown by loop 1 in the hysteresis loop figure. The stator magnetic field produces Eddy currents in the rotor. As a result, they produce their own magnetic field.

The eddy current loss is given by the equation, Pe = Kef 22 B^2

Where, ke is Eddy current constant, f 2 is the eddy current frequency, B is the flux density The relation between rotor frequency f 2 and supply frequency f 1 is f 2 = sf 1 where s is the slip.

Therefore, Pe = Ke s^2 f 12 B^2

The torque due to eddy current is Te =^

Pe sws or Te = K’s ............... (1)

Where K’ =

Kef^2 B^2 ws

The hysteresis loss is Ph = Khf 2 B1.6^ or Kh sf 1 B1.6^ .......................^ (2)

The Torque due to hysteresis is Th Kh f 1 B1.

Ph sws

= k” ................... (3)

Where k” = ws

  • From the equation (1) it is clear that the torque is proportional to the slip. Therefore, as the speed of the rotor increases the value of Ʈe decreases.
  • As the speed of the motor reaches synchronous speed, the slip becomes zero and torque also becomes zero.
  • As the electromagnet torque is developed by the motor is because of the hysteresis loss and remains constant at all rotor speed until the breakdown torque.
  • The nature of the torque will be pulsating, and the frequency will be twice that of line frequency as shown in the waveform. Thus, a Universal motor can work on both AC and DC.
  • However, a series motor which is mainly designed for DC operation if works on single phase AC supply suffers from the following drawbacks. ✓ The efficiency becomes low because of hysteresis and eddy current losses. ✓ The power factor is low due to the large reactance of the field and the armature windings. ✓ The sparking at the brushes is in excess. In order to overcome the above following drawbacks, certain modifications are made in a DC series motor so that it can work even on the AC current. They are as follows:- ✓ The field core is made up of the material having a low hysteresis loss. It is laminated to reduce the eddy current loss. ✓ The area of the field poles is increased to reduce the flux density. As a result, the iron loss and the reactive voltage drop are reduced. ✓ To get the required torque the number of conductors in the armature is increased.
  • A compensating winding is used for reducing the effect of the armature reaction and improving the commutation process. The winding is placed in the stator slots as shown in the figure below.
  • The winding is put in the stator slot. The axis of compensating winding is 90 degrees with the main field axis. The compensating winding is connected in series with both the armature and the field; hence, it is called conductively compensated.
  • If the compensating winding is short circuited, the motor is said to be inductively compensated. The connection diagram is shown below.
  • The construction of the universal motor is same as that of the series motor.
  • In order to minimize the problem of commutation, high resistance brushes with increased brush area are used.
  • To reduce Eddy current losses the stator core and yoke are laminated.
  • The Universal motor is simple and less costly. It is used usually for rating not greater than 750 W.
  • The characteristic of Universal motor is similar to that of the DC series motor.
  • When operating from an AC supply, the series motor develops less torque.
  • By interchanging connections of the fields with respect to the armature, the direction of rotation can be altered.
  • Speed control of the universal motors is obtained by solid state devices.
  • This motor is most suitable for applications requiring high speeds.
  • Since the speed of these motors is not limited by the supply frequency and is as high as 20000 rpm. Applications of Universal Motor The Universal motor is used for the purposes where speed control and high values of the speed are necessary. The various applications of the Universal Motor are as follows:- Portable drill machine. Used in hair dryers, grinders and table fans. used in blowers, polishers and kitchen appliances. **REPULSION MOTOR Repulsion Type Motors
  1. Repulsion Motor.** It consists of ( a ) one stator winding ( b ) one rotor which is wound like a d.c. armature ( c ) commutator and ( d ) a set of brushes, which are short-circuited and remain in contact with the commutator at all times. It operates continuously on the ‘repulsion’ principle. No short-circuiting mechanism is required for this type. 2. Compensated Repulsion Motor. It is identical with repulsion motor in all respects, except that (a) It carries an additional stator winding, called compensating winding (b) There is another set of two brushes which are placed midway between the usual short- circuited brush set. The compensating winding and this added set are connected in series. 3. Repulsion-start Induction-run Motor. This motor starts as a repulsion motor, but normally runs as an induction motor, with constant speed characteristics. It consists of ( a ) one stator winding ( b ) one rotor which is similar to the wire- wound d.c. armature ( c ) a commutator and ( d ) a centrifugal mechanism which short-circuits the commutator bars all the way round (with the help of a short-circuiting necklace) when the motor has reached nearly 75 per cent of full speed. 4. Repulsion Induction Motor. It works on the combined principle of repulsion and induction. It consists of ( a ) stator winding ( b ) two rotor windings: one squirrel cage and the other usual d.c. winding connected to the commutator and ( c ) a short-circuited set of two brushes. Repulsion Motor Constructionally, it consists of the following: 1. Stator winding of the distributed non-salient pole type housed in the slots of a smooth-cored stator (just as in the case of split-phase motors). The stator is generally wound for four, six or eight poles. 2. A rotor (slotted core type) carrying a distributed winding (either lap or wave) which is connected to the commutator. The rotor is identical in construction to the d.c. armature. 3. A commutator, which may be one of the two types : an axial commutator with bars parallel to the shaft or a radial or vertical commutator having radial bars on which brushes press horizontally. These can be divided into the following four distinct categories:

magnetic axis of main poles.

  • Motor speed can also be controlled by means of brush shift. Variation of starting torque of a repulsion motor with brush-shift is shown in Fig. A straight repulsion type motor has high starting torque (about 350 per cent) and moderate starting current (about 3 to 4 times full-load value). Principal shortcomings of such a motor are: 1. speed varies with changing load, becoming dangerously high at no load. 2. low power factor, except at high speeds. 3. tendency to spark at brushes. Compensated repulsion motor
  • In this type of motor an additional stator winding called compensation winding is provided. This is the modified form of the basic repulsion motor.
  • The compensation winding serves for two purposes: ·
    • To Improve the Power factor ·
    • For better speed regulation
  • This type of motor is used whenever there is a need for motor to run at constant speed and at higher power factor so an additional stator winding called compensating winding is used.
  • The additional winding which is connected in series with the armature, is smaller than stator winding and wounded to the inner slots of main pole.
  • It also consists of additional set of brushes which are placed mid-way between the short-circuited brushes.
  • Such a type of modification reduces the quadrature drop and improves the power factor. And speed regulation also improves due to this compensation.
  • Quadrature drop occurs in salient pole types due to non-uniform air gap length. Due to quadrature drop cross magnetizing effect occurs which opposes the mmf waves.
  • By providing such a type of compensation, this effect can be reduced which increases power factor. Further the leakage between armature and field is reduced Repulsion start induction run motors
  • As the name suggests this motor starts as a repulsion motor and runs as an Induction motor.
  • This type of motor starts as a normal Repulsion motor and after achieving three-fourths of its full speed, it runs as an Induction motor.
  • For this purpose a centrifugal force-operated device is used. This centrifugal device short circuits the commutator segments and this aids in running the motor as a squirrel cage motor.
  • As soon as the commutator is short circuited, the brushes present do not carry any current.
  • So the brushes can be removed to avoid the wear and tear.
  • The advantage in running the motor as a squirrel cage one is that, it provides high starting torque, 350 percent without excessive current. Also constant speed is ensured for wide range of torque.
  • There are two different designs in repulsion start motors:

BRUSH LIFTING TYPE- In this type the brush is lifted as soon as the commutator is short circuited to avoid unnecessary wear and tear and losses due to friction. So in this type the brush is present only when the motor is started as a repulsion one. BRUSH RIDING TYPE- In this type of motors the brushes ride along with the commutator at all times. So the brushes are present even after the commutator is short circuited. Applications Compressors, Hoists, Pumps, Machine tools, Floor-polishing Repulsion induction motor

  • This type of motor is a combination of repulsion motor and induction motor. It is also referred as squirrel cage repulsion motor.
  • This motor possesses the characteristic of both induction motor and repulsion motor. It combines the desirable starting characteristics of repulsion motor and constant speed characteristics of an induction motor.
  • Here the stator winding is same as every other repulsion motor but there are two separate rotor windings

o A squirrel cage winding ·

o A Commutator winding

  • The commutator winding lies on the outer slots while the squirrel cage winding is located in the inner slots. Both the windings operate during the entire period of operation of motors. The brushes are in contact with the commutator all the time.
  • The biggest advantage in such type of motors is that they don’t need a separate centrifugal short-circuit system as in Repulsion-start Induction-run motors.
  • As soon as the motor is started, the squirrel cage winding is practically inactive for a small period of time due to high reluctance.
  • Only the commutator winding supplies most of the torque.
  • But during normal running condition, the squirrel cage winding supplies most of the torque and commutator winding supplies relatively lower torque when compared to Squirrel cage winding.
  • So the squirrel cage winding takes up most of the load as the rotor accelerates
  • The starting torque is very high, 300 percent with better speed regulation. Applications Petrol pumps, Compressors, Refrigerators, Mixing machines, Lifts and Hoists LINEAR INDUCTION MOTOR
  • If the stator is laid out flat and a flat squirrel cage winding is brought near to it, we get a linear induction motor.
  • In practice, instead of a flat squirrel cage winding, an aluminium or copper or iron plate is used as a ‘rotor’.
  • The flat stator produces a flux that moves in a straight line from its one end to the other at a linear synchronous speed given by

vs = 2.w.f

where vs = linear synchronous speed (m/s)

w = width of one pole pitch (m)

f = supply frequency (Hz)