Top Electrical Interview Questions On Induction Motor for Steel Plant/Power plant/Cement Plant/Construction
INDUCTION MOTORS
1. State the condition for maximum torque of a three phase induction motor. What is the maximum torque equal to?
Ans: When the slip is maximum, the torque of a three phase IM is maximum.
The maximum torque is equal to Sm E22 R2
R22+ Sm2 X22
2. A 3 ph, 4 pole, 50 Hz induction motor is running at 1440 rpm. Determine the slip speed and slip.
Ans: Slip speed = Ns – N; slip =Ns-N/Ns*100
Ns = 120f / p = (120) (50) / 4 = 1500 rpm.
Slip speed = Ns – N = 1500 – 1440 = 60 rpm.
Slip = (60/1500) *100 = 4%
3. Mention the losses occur in induction motor.
Ans: 1. Constant loss which includes hysteresis, eddy current loss and mechanical losses.
2. Variable loss which includes the losses in the stator and rotor winding due
to the current flowing in the winding.
4. Give the relationship between the following in a 3 phase induction motor:
1. Rotor input and rotor output
2. Starting torque and applied voltage
Ans: Pm = P2 (1-s) which indicates the relationship between the rotor input and rotor output where Pm denotes the gross mechanical power developed and P2
denotes the rotor input.
Tst = 3 V22 R2 / ( 2πNs ( R22+ X22) which indicates the relation ship between the starting torque and applied voltage.
5. What are the purposes that could be served by external resistors connected in the rotor circuit of phase would induction motor.
Ans: 1. To limit the starting current
2. To increase the starting torque
2. To control the speed.
6. How can the reversal of rotation of poly phase induction motor be attained?
Ans: The reversal of rotation of poly phase induction motor is obtained by interchanging any two terminals of the three phase windings when connecting to
the supply.
7. A 3 ph, 50 Hz induction motor runs at 960 rpm on full load. Find the number of poles and slip speed.
Ans: Induction motor always runs nearer to synchronous speed. So, assume
Ns = 1000rpm.
Ns = 120f / p = 1000 p = (120) (50) / 1000 = 4.
Slip speed = Ns – N = 1000 – 960 = 40 rpm.
8. Enlist four application of wound rotor induction motor.
Ans: 1. Cranes
2. Hoists
3. Pumps
4. Fans and blowers
5. Chippers
6. Conveyors
7. Banbury mixers
8. Ball and sag mill
9. Explain the conditions for maximum torque under running condition.
Ans: The torque depends on slip at which motor is running. So the only parameter which controls the torque is slip. So by differentiating the torque
equation with respect to slip the condition for maximum torque can be achieved.
That is slip is equal to the ratio between the rotor resistance and reactance.
R2 = SX2
10. Point out the disadvantages of rotor rheostat control to obtain variable speed of induction motor.
Ans: 1. Large power loss due to the increase in resistance value.
2. Due to large power loss efficiency is low.
3. The speed above normal speed can not be obtained.
4. Bulky and expensive
5. Wide speed range is not possible. Because it needs large resistance which will be the cause to increase the losses.
➡️Read Now: Top Electrical Interview Questions On Transformer
11. What is the function of slip ring in 3 phase induction motor?
Ans: The function of slip ring is used to connect the external stationary circuit to the rotor circuit of the induction motor. Here it is used to connect the external
rheostat to the rotor of IM.
12. Write the advantages of slip ring induction motor.
Ans: 1. External resistance can be added in to the rotor circuit.
2. High starting torque can be obtained
3. Speed control by the rotor rheostat is possible.
4. Rotor resistance starter can be used.
13. Define slip.
Ans: The ratio between difference in synchronous speed Ns and the actual speed N of the rotor to the synchronous speed Ns is known as slip.
S = Ns –N / Ns
14. Why the squirrel cage rotor is named so?
Ans: The rotor bars are brazed or electrically welded to two heavy and short circuiting end rings which give the pictorial representation of the squirrel’s cage.
That’s why this motor is called as squirrel cage motor.
15. What are the principles involved in the starting of the rotor to rotate?
Ans: 1. Faraday’s laws of electromagnetic induction.- By this principle an emf will be induced in the rotor conductors .
2. Lenz’s law – By this law the rotor current is produced since the rotor
conductors are short circuited.
16. Why the induction motor is called as rotating transformer.
Ans: The induction motor is same as transformer in the principle of operation, i.e. mutual induction principle except the rotation. Similar to primary and
secondary here stator and rotor parts are available which involve in that principle of operation. That’s why IM is called as rotating transformer.
17. Compare the squirrel cage and wound rotor IM.
Ans:
| Sl.No | Characteristics | Slip Ring IM | Squirrel Cage IM |
| 1 | Stator winding | Three phase winding | Three phase winding |
| 2 | Construction | Complicated | Very simple |
| 3 | Rotor | Three phase winding | Bars shorted with end rings |
| 4 | External | resistance Can be added | Can not be added |
| 5 | Slip rings and brushes | Present | Absent |
| 6 | Cost | High | Low |
| 7 | Starting torque | High | Moderate |
| 8 | Speed control | By rotor resistance | By rotor res. Not possible |
| 9 | Losses | High | Low |
| 10 | Efficiency | Low | High |
| 11 | Applications | Lifts, hoists, cranes | Laths, blowers, pumps |
| 12 | Usage | Only 5% | 95% |
➡️Read Now: Top Electrical Interview Questions On DC Machines
18. Why the IM is called as asynchronous motor?
Ans: The speed of the rotor is not same as the speed of the synchronously revolving flux of the stator. That’s why it is called as asynchronous motor.
19. Define synchronous speed.
Ans: The speed of rotating magnetic field in the stator is called as synchronous speed. It is denoted as Ns .
Ns=120f/P
where f denotes the supply frequency and p denotes the number of poles.
20. List the types of starters used for an IM.
Ans: DOL starter
For SCIM:
1. Primary resistor started
2. Auto transformer starter
3. Star delta starter
For SRIM:
1. Rotor rheostat starter.
21. What is the necessity of starter?
Ans: If normal supply voltage is applied to the stationary motor, then as in the case of transformer very large initial current is taken atleast for a short while
which is 5 to 7 times of their full load current. Hence to avoid this high value of initial current the starters are needed.
22. What are the merits and demerits of injecting emf method of speed control?
Ans: Advantages:
1. Wide speed range is possible
2. Improved power factor.
Demerit:
1. It can be only applied for slip ring induction motors.
23. What is the name of the starter used for starting the slip ring IM?
Ans: Rotor resistance starter is used for the starting of slip ring IM. There is a provision to change the value of rotor resistance in slip ring IM alone. This type
of starter also improves the starting torque.
24. Why capacitor start and run induction motors advantageous?
Ans: 1. Improvement of over load capacity of the motor
2. Higher power factor
3. High efficiency
4. Quite running.
25. Is single phase induction motor is self starting? Why?
Ans: No. It is not self starting. This is explained by double revolving field theory. It states that, when single phase ac supply is given to the stator will
produce pulsating (forward & backward toque which cancel each other) torque and hence continuous rotation is not achieved.
26. List the methods adopted to control the speed of an IM.
Ans: From stator side:
1. By changing the supply voltage
2. By changing the supply frequency
3. By changing the number of stator poles
From rotor side:
1. Rotor rheostat control
2. Cascade operation
3. By injecting an emf in the rotor circuit.
27. List the types of single phase IM.
Ans: 1. Split phase
2. Shaded pole
3. Capacitor start and run induction motor
4. Capacitor start induction motor
Read More:
