Induction Machine MCQ Quiz
1. An induction motor works on the principle of:
a) Electromagnetic induction
b) Self-induction
c) Mutual induction
d) Static induction
2. The rotating magnetic field (RMF) in a 3-phase induction motor is produced in the:
a) Rotor windings
b) Stator windings
c) Air gap
d) Frame
3. The speed of the rotating magnetic field in an induction motor is called:
a) Rotor speed
b) Slip speed
c) Synchronous speed
d) Shaft speed
4. Slip (s) in an induction motor is defined as:
a) (Ns - Nr) / Nr
b) (Nr - Ns) / Ns
c) (Ns - Nr) / Ns
d) Ns / Nr
(Where Ns = Synchronous speed, Nr = Rotor speed)
5. At standstill, the slip of an induction motor is:
a) 0
b) 1
c) Infinity
d) 0.5
6. When an induction motor runs at synchronous speed, the slip is:
a) 1
b) 0
c) -1
d) 0.05
7. The rotor frequency (fr) of an induction motor is related to the stator frequency (fs) and slip (s) by:
a) fr = s / fs
b) fr = fs / s
c) fr = s * fs
d) fr = fs * (1-s)
8. A squirrel cage induction motor has:
a) Slip rings and brushes
b) A wound rotor with external resistance connections
c) Rotor bars short-circuited by end rings
d) A commutator
9. A wound rotor (slip-ring) induction motor is preferred over a squirrel cage motor when:
a) Low starting torque is required
b) High starting torque and speed control are required
c) Simpler construction is desired
d) Lower maintenance is a priority
10. The direction of rotation of a 3-phase induction motor can be reversed by:
a) Increasing the supply voltage
b) Decreasing the supply frequency
c) Interchanging any two of the three stator supply leads
d) Adding resistance to the rotor circuit
11. The synchronous speed of a 4-pole, 50 Hz induction motor is:
a) 3000 rpm
b) 1500 rpm
c) 1000 rpm
d) 750 rpm
12. If the rotor of an induction motor is locked (blocked rotor test), the slip is:
a) 0
b) 1
c) Infinity
d) Depends on applied voltage
13. The no-load test on an induction motor is used to determine:
a) Copper losses
b) Total losses
c) Core losses and friction & windage losses
d) Rotor resistance
14. The blocked rotor test on an induction motor is analogous to which test on a transformer?
a) Open circuit test
b) Short circuit test
c) Polarity test
d) Sumpner's test
15. In the equivalent circuit of an induction motor, Rr'/s represents:
a) Stator copper loss
b) Rotor copper loss
c) Mechanical power developed
d) Total rotor input power (electrical equivalent of mechanical power + rotor copper loss)
16. The torque developed by an induction motor is approximately proportional to:
a) Slip
b) Square of slip (at low slip)
c) Voltage
d) Square of voltage (V²)
17. The starting torque of a squirrel cage induction motor is generally:
a) Very high
b) Low to moderate
c) Zero
d) Equal to full load torque
18. Star-delta starting method is used in induction motors to:
a) Increase starting torque
b) Reduce starting current
c) Improve power factor
d) Increase running speed
19. When star-delta starting is used, the starting current is reduced to:
a) 1/sqrt(3) times the DOL current
b) 1/3 times the DOL current
c) sqrt(3) times the DOL current
d) 3 times the DOL current
20. Autotransformer starting for an induction motor provides:
a) Only one starting voltage tap
b) Multiple starting voltage taps
c) Increased starting torque compared to DOL
d) Increased starting current
21. The purpose of adding external resistance to the rotor circuit of a wound rotor induction motor at starting is:
a) To reduce starting current and increase starting torque
b) To increase starting current
c) To reduce slip
d) To improve power factor at starting
22. The phenomenon of an induction motor running at a very low speed (1/7th of synchronous speed) is known as:
a) Cogging
b) Crawling
c) Plugging
d) Hunting
23. Cogging in induction motors is caused by:
a) Harmonics in the supply voltage
b) Magnetic locking between stator and rotor teeth at standstill
c) Uneven air gap
d) High rotor resistance
24. Crawling is primarily due to:
a) Space harmonics in the air gap flux
b) Time harmonics in the supply voltage
c) Low supply voltage
d) High inertia load
25. The power factor of an induction motor at no-load is typically:
a) High (0.8-0.9 lagging)
b) Low (0.1-0.2 lagging)
c) Unity
d) Leading
26. As the load on an induction motor increases, its power factor generally:
a) Decreases
b) Increases
c) Remains constant
d) Becomes leading
27. The efficiency of an induction motor is highest at:
a) No load
b) Full load or near full load
c) Half load
d) Overload conditions
28. Which of the following losses in an induction motor is variable with load?
a) Core loss
b) Friction and windage loss
c) Stator copper loss
d) All of the above
29. The magnetizing reactance (Xm) in the equivalent circuit of an induction motor represents:
a) Leakage flux
b) Mutual flux or magnetizing flux path
c) Rotor reactance
d) Stator winding resistance
30. The parameter Rc in the equivalent circuit of an induction motor represents:
a) Rotor resistance
b) Stator resistance
c) Core loss component
d) Mechanical load
31. If the air gap of an induction motor is increased:
a) Magnetizing current will decrease
b) Magnetizing current will increase
c) Power factor will improve
d) Efficiency will increase
32. The maximum torque of an induction motor is:
a) Directly proportional to slip
b) Inversely proportional to rotor resistance
c) Independent of rotor resistance
d) Directly proportional to rotor resistance
33. The slip at which maximum torque occurs in an induction motor is:
a) Rr' / Xs (where Xs is total stator reactance)
b) Rr' / Xlr' (where Xlr' is rotor leakage reactance referred to stator)
c) Rr' / (Xs + Xr')
d) Rr' / Xm
34. Speed control of a squirrel cage induction motor by varying supply voltage results in:
a) Constant torque operation
b) Constant power operation
c) Reduced torque capability at lower speeds
d) Increased efficiency at lower speeds
35. The V/f control method for induction motors aims to keep:
a) Rotor current constant
b) Air gap flux approximately constant
c) Slip constant
d) Stator voltage constant
36. Pole changing method of speed control for induction motors allows for:
a) Smooth, continuous speed variation
b) Only discrete speed steps
c) Speed control above synchronous speed only
d) Increased starting torque
37. Which type of single-phase induction motor has the highest starting torque?
a) Shaded pole motor
b) Split-phase motor
c) Capacitor-start motor
d) Capacitor-start capacitor-run motor
38. A capacitor-start single-phase induction motor has its starting capacitor connected:
a) In series with the main winding
b) In series with the auxiliary winding
c) In parallel with the main winding
d) Across the supply lines
39. In a capacitor-start capacitor-run motor, the run capacitor is designed to:
a) Improve starting torque only
b) Improve running performance (power factor, efficiency)
c) Be disconnected after starting
d) Handle high starting current
40. The auxiliary winding of a split-phase induction motor has:
a) Low resistance and high reactance
b) High resistance and low reactance
c) Equal resistance and reactance to the main winding
d) No resistance
41. A shaded pole motor is characterized by:
a) High starting torque and high efficiency
b) Low starting torque and low efficiency
c) Reversible direction of rotation easily
d) Use in high power applications
42. The direction of rotation of a shaded pole motor depends on:
a) Supply polarity
b) The position of the shading coil (from unshaded to shaded part)
c) Number of poles
d) Load on the motor
43. Plugging of an induction motor means:
a) Starting the motor with full voltage
b) Stopping the motor by applying mechanical brake
c) Reversing the phase sequence of supply to brake the motor
d) Running the motor at very low speed
44. Dynamic braking of an induction motor is achieved by:
a) Feeding AC supply to the stator
b) Feeding DC supply to the stator windings after disconnecting AC
c) Short-circuiting the rotor
d) Increasing the rotor resistance
45. Regenerative braking in an induction motor occurs when:
a) The motor speed is less than synchronous speed
b) The motor is stationary
c) The motor speed is greater than synchronous speed (e.g., hoist lowering a load)
d) Supply voltage is suddenly reduced
46. The number of slip rings on a standard wound rotor induction motor is usually:
a) 2
b) 3
c) 4
d) 6
47. If a 6-pole, 50 Hz induction motor runs at 950 rpm, the slip is:
a) 0.05 or 5%
b) 0.10 or 10%
c) 0.02 or 2%
d) 0.95 or 95%
48. The frame of an induction motor is usually made of:
a) Silicon steel
b) Cast iron or fabricated steel
c) Copper
d) Aluminum
49. The shaft of an induction motor is made of:
a) Mild steel
b) Cast iron
c) Stainless steel
d) Aluminum
50. Double cage induction motors are used for:
a) High starting torque and good running performance
b) Very low starting torque
c) Speed control
d) High power factor at no load
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