1. An induction motor works with;
(A) DC only
(B) None of these
(C) Both AC & DC
(D) AC only
2. The stator frame in an induction motor is used to;
(A) Provide return path for the flux
(B) Ventilate the armature
(C) Protect the whole machine
(D) Hold the armature stampings in position
3. Regarding skewing of motor bars in a squirrel cage induction motor (SCIM), which statement is false?
(A) It prevents cogging
(B) It increases starting torque
(C) It produces more uniform torque
(D) It reduces motor ‘hum’ during its operation
4. The principle of operation of a 3-phase induction motor is most similar to that of a;
(A) Synchronous motor
(B) Repulsion-start induction motor
(C) Transformer with a shorted secondary
(D) Capacitor-start, induction-run motor
5. The magnetizing current drawn by transformers and induction motors are the cause of their .........power factor.
(A) Lagging
(B) Unity
(C) Zero
(D) Leading
6. The effect of increasing the length of air-gap in an induction motor will be to increase the;
(A) Power factor
(B) Magnetizing current
(C) Speed
(D) Air-gap flux
7. In a 3-phase induction motor, the relative speed of stator flux with respect to ..........is zero.
(A) Rotor flux
(B) Rotor
(C) Stator winding
(D) Space
8. An eight-pole wound rotor induction motor operating on 60 Hz supply is driven at 1800 r.p.m. by a prime mover in the opposite direction of revolving magnetic field. The frequency of rotor current is;
(A) 180 Hz
(B) 120 Hz
(C) 60 Hz
(D) None of these
9. A 3-phase, 4-pole, 50-Hz induction motor runs at a speed of 1440 r.p.m. The rotating field produced by the rotor rotates at a speed of .......r.p.m. with respect to the rotor.
(A) 1500
(B) 1440
(C) 0
(D) 60
10. In a 3-φ induction motor, the rotor field rotates at synchronous speed with respect to;
(A) Stator flux
(B) Rotor
(C) Stator
(D) None of the above
11. Irrespective of the supply frequency, the torque developed by a SCIM is the same whenever ........ is the same.
(A) Slip speed
(B) External load
(C) Rotor resistance
(D) Supply voltage
12. The number of stator poles produced in the rotating magnetic field of a 3-φ induction motor having 3 slots per pole per phase is;
(A) 2
(B) 6
(C) 3
(D) 12
13. The power factor of a squirrel-cage induction motor is;
(A) Low at heavy loads only
(B) Low at light loads only
(C) Low at light and heavy loads both
(D) Low at rated load only
14. Which of the following rotor quantities in a SCIM does NOT depend on its slip?
(A) Reactance
(B) Speed
(C) Induced emf
(D) Frequency
15. A 6-pole, 50-Hz, 3-φ induction motor is running at 950 rpm and has rotor Cu loss of 5 kW. Its rotor input is ......kW.
(A) 5.3
(B) 10
(C) 95
(D) 100
16. The efficiency of a 3-phase induction motor is approximately proportional to;
(A) S
(B) (1− S)
(C) N
(D) Ns
17. A 6-pole, 50-Hz, 3-φ induction motor has a full load speed of 950 rpm. At half-load, its speed would be ......rpm.
(A) 475
(B) 500
(C) 975
(D) 1000
18. If rotor input of a SCIM running with a slip of 10% is 100 kW, gross power developed by its rotor is ...... kW.
(A) 10
(B) 99
(C) 90
(D) 80
19. Pull-out torque of a SCIM occurs at that value of slip where rotor power factor equals;
(A) Unity
(B) 0.866
(C) 0.707
(D) 0.5
20. When applied rated voltage per phase is reduced by one-half, the starting torque of a SCIM becomes ...... of the starting torque with full voltage.
(A) 1/2
(B) 3/2
(C) 1/2
(D) 1/4
21. The torque developed by a 3-phase induction motor depends on the following three factors:
(A) Rotor e.m.f, rotor current and rotor p.f.
(B) Voltage, current and stator impedance
(C) Synchronous speed, rotor speed and frequency
(D) Speed, frequency, number of poles
22. The efficiency and p.f. of a SCIM increases in proportion to its;
(A) Speed
(B) Rotor torque
(C) Voltage
(D) Mechanical load
23. A SCIM runs at constant speed only so long as;
(A) Its torque exactly equals the mechanical load
(B) Its supply voltage remains constant
(C) Torque developed by it remains constant
(D) Stator flux remains constant
24. The number of slip rings on a squirrel cage induction motor is usually;
(A) 2
(B) None
(C) 4
(D) 3
25. The shaft of an induction motor is;
(A) Flexible
(B) Stiff
(C) Hollow
(D) All of these
26. The frame of an induction motor is usually made of;
(A) Silicon steel
(B) Bronze
(C) Aluminium
(D) Cast iron
27. The crawling in an induction motor is caused by;
(A) Improper design of the machine
(B) Low voltage supply
(C) Harmonics developed in the motor
(D) High loads