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Single Phase Induction Motors Interview Questions :-
1. Please Name some Single-phase induction motors?
- split-phase type
- capacitor type
- shaded-pole type
2. Please name two types of Repulsion motors?
- Repulsion-start induction-run motor
- Repulsion-induction motor
3. What are different types of single phase Synchronous motors?
- Reluctance motor
- Hysteresis motor
4. How a single phase motor is similar to 3 phase motor?
A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has
- a squirrel-cage rotor identical to a 3-phase motor
- a single-phase winding on the stator
5. What is major difference between a 3-phase induction motor and a single-phase induction motor?
Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means.
6. Why single phase induction motor does not self starts?
The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor.
7. What is basic method to start single phase induction motor and why it is not preferred?
If the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed fur a
motor located at some inaccessible spot.
8. Why single phase induction motor does not start in the light of Double-Field Revolving Theory?
The double-field revolving theory is proposed to explain this dilemma of no torque at start and yet torque once rotated. This theory is based on the fact that an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., m/2) and each rotating at synchronous speed in opposite directions.
Therefore, an alternating field can be replaced by two relating fields of half its amplitude rotating in opposite directions at synchronous speed. Note that the resultant vector of two revolving ?ux vectors is a stationary vector that oscillates in length with time along X-axis.
9. What is the position of rotor at standstill?
Consider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. The alternating ux produced by the stator winding can be presented as the sum of two rotating fluxes 1 and 2, each equal to one half of the maximum value of alternating ux and each rotating at synchronous speed (Ns = 120 f/P) in opposite directions.
At standstill, these two torques are equal and opposite and the net torque developed is zero. Therefore, single-phase induction motor is not self-starting.
10. What is the value of slip at standstill?
Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Thus the point of zero slip for one field corresponds to 200% slip for the other . The value of 100% slip (standstill condition) is the same for both the fields.
11. Explain motor action from standstill to running condition?
At standstill, s = 1 so that impedances of the two circuits are equal. Therefore, rotor currents are equal i.e., I2f = I2b. However, when the rotor rotates, the impedances of the two rotor circuits are unequal and the rotor current I2b is higher (and also at a lower power factor) than the rotor current I2f. Their m.m.f.s, which oppose the stator m.m.f.s, will result in a reduction of the backward rotating ux. Consequently, as speed increases, the forward flux increases, increasing the driving torque while the backward ux decreases, reducing the opposing torque. The motor-quickly accelerates to the final speed.
12. How to make Single-Phase Induction Motor Self-Starting?
The single-phase induction motor is not self- starting and it is undesirable to resort to mechanical spinning of the shaft or pulling a belt to start it. To make a singlephase induction motor self-starting, we should somehow produce a revolving stator magnetic field. This may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding. When the motor attains suficient speed, the starting means (i.e., additional winding) may be removed depending upon the type of the motor.
13. Name the types of single phase motors with respect to the method employed to make them self-starting?
- Split-phase motors-started by two phase motor action through the use of an auxiliary or starting winding.
- Capacitor motors-started by two-phase motor action through the use of an auxiliary winding and a capacitor.
- Shaded-pole motors-started by the motion of the magnetic ?eld produced by means of a shading coil around a portion of the pole structure.
14. How to produce Rotating Magnetic Field From 2-Phase Supply?
As with a 3-phase supply, a 2-phase balanced supply also produces a rotating magnetic field of constant magnitude. With the exception of the shaded-pole motor, all single-phase induction motors are started as 2-phase machine. Once so started, the motor will continue to run on single-phase supply.
15. What is the major cause of noisy operation of the single phase induction motor?
If the two windings arc displaced 90° electrical but produce fields that are not equal and that are not 90° apart in time, the resultant field is still rotating but is not constant in magnitude. One effect of this non uniform rotating field is the production of a torque that is non-uniform and that, therefore, causes noisy operation of the motor. Since 2-phase operation ceases once the motor is started, the operation of the motor then becomes smooth.
16. Describe the principal of operation of Split-Phase Induction Motor?
The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M. The starting winding is located 90° electrical from the main winding and operates only during the brief period when the motor starts up. The two windings are so resigned that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance as shown in the schematic
connections in Fig. Consequently, the currents flowing in the two windings have reasonable phase difference c (25° to 30°) as shown in the phasor diagram in Fig.
When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them as shown in Fig. Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. where k is a constant whose magnitude depends upon the design of the motor.
When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a singlephase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.
17. What is starting torque and starting current of split phase induction motor?
The spinning torque is 15 to 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current.
18. Why split-phase induction motors are most popular single- phase motors in the market?
Due to their low cost, split-phase induction motors are most popular single- phase motors in the market.
19. Why built-in-thermal relay is necessary in split phase induction motor and what is its purpose?
Since the starting winding is made of one wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not frequent
20. What is speed variation of split phase induction motor from no load to full load?
An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full- load.
21. Usually where split phase induction motors are used?
These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive:
(a) fans
(b) washing machines
(c) oil burners
(d) small machine tools etc.
The power rating of such motors generally lies between 60 W and 250 W.
22. Where do we require single-phase induction motors?
Single-phase induction motors are required where (i) 3 phase supply is not available (ii) efficiency is of lesser importance (iii) Rating is less than one H.P. (iv) Equipment is portable
23. Why is the power factor of a single-phase induction motor low?
It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature.
24. What is the function of centrifugal starting switch in a single-phase induction motor?
The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.
25. What happens when the centrifugal starting switch fails to open?
If the starting switch fails to open when needed, then the starting winding will overheat and burn out and motor will not start next time.
26. What happens when the centrifugal switch fails to close when needed?
If the centrifugal starting switch fails to close, the motor will overheat the main winding without any failure of the main winding.
27. Why are resistance split-phase inductions motors most popular?
These motors are most popular due to their low cost. They are used where moderate starting torque is required and where the starting periods are not frequent. They drive fans, pumps, washing machines, small machine tools etc. They have power ranting between 60 watts and 250 watts.
28. What is the draw back of the resistance split-phase induction motor?
The starting winding has a relatively small number of turns of fine wire and its resistance is higher than that of the main winding. Therefore the current density is high and the winding heats up quickly. If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in-thermal relay.
29. Why is the starting torque of a resistance split-phase induction motor not high?
The starting torque is given as, Ts = K Im Is Sin Ф Where K = constant whose magnitude depends upon the design of the motor (i) The angle between Is and Im is small (approximately 25 degree) in a resistance split-phase induction motor, so the starting torque is small. (ii) Since currents Is and Im are not equal in magnitude, the rotating magnetic field is not uniform and the starting torque produced is small.
30. Why is the starting torque of a capacitor start induction motor high?
The capacitor C in the starting winding is so chosen that Is leads Im by 75 degree. Since the starting torque is directly proportional to Sin Ф, and it is quite high in capacitor-start induction motor.
31. Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?
Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.
32. What is the principle of operation of shaded-pole induction motor?
A shaded-pole motor is basically a small single-phase squirrel cage motor in which the starting winding is composed of short-circuited copper ring (called shading coil) surrounding one-third of each pole. The effect of the shading coil is to cause a flux to sweep across the pole faces, from unshaded to shaded portion of the pole, producing a weak rotating magnetic field. As a result, the rotor is set in motion due to induction principle.
33. Which type of torque is developed in single phase motors?
Pulsating torque is produced.
34. If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?
Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.
35. What is a fractional H.P. motor?
A small motor having H.P. less than unit is called fractional H.P. motor.
36. How the starting winding produce rotation in a single phase resistance start induction motor?
The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.
37. How the starting winding is made resistive?
It consists of only few turns of smaller diameter.
38. How the speed of rotation of a split phase induction motor is reversed?
The terminal connections of the starting windings are reversed with respect to main running windings.
39. What will happen if the centrifugal switch fails to open the starting winding?
Excessive heat will be produced due to high resistance of the starting winding due to which stator temperature will rise and eventually both windings will burn.
40. How speed control is made in single phase motors?
It is usually controlled by applying a variable voltage from tapped transformers, variacs, potentio meters, and tapped reactors.
41. Is there any relation between the capacitances of two capacitors used in two value capacitor motor?
Starting capacitor has about 10 – 15 times high capacity than the value of running capacitor.
42. What is size of shaded-pole motor?
These are usually built in small fractional H.P, not exceed 1/4 H.P.
43. Why shaded-pole single phase induction motor does not need any special starting technique like capacitors and auxiliary winding etc.
Because it is inherently self started motor. The construction of the poles is such that they give a sweep to the magnetic flux and motor starts rotating.
44. How can a universal motor be reversed?
By reversing either the field leads or armature leads but not both.
45. What are applications of Stepper motors?
- Paper feed motors in typewriters and printers
- Positioning of print heads
- Pens in XY-plotters
- Recording heads in computer disc drives etc.
46. Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?
Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.
47. If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?
Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.
48. What is a fractional H.P. motor?
A small motor having H.P. less than unit is called fractional H.P. motor.
49. Which type of rotor is used in single phase motors?
Squirrel cage type
50. How the starting winding produce rotation in a single phase resistance start induction motor?
The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.