Q1. The basic principle of operation of a three-phase induction motor is based on:
A) Electromagnetic induction
B) Electrostatic induction
C) Lorentz force
D) Piezoelectric effect
Correct Option: A
Explanation: A three-phase induction motor works on electromagnetic induction. A three-phase supply produces a rotating magnetic field which induces current in the rotor conductors, producing torque.
Q2. In a three-phase induction motor, the rotating magnetic field is produced due to:
A) DC excitation in stator
B) Single-phase AC supply
C) Three-phase currents displaced by 120°
D) Rotor current interaction
Correct Option: C
Explanation: Three-phase currents displaced by 120° electrically create a constant magnitude rotating magnetic field.
Q3. The speed of rotating magnetic field in a three-phase induction motor is called:
A) Rotor speed
B) Mechanical speed
C) Synchronous speed
D) Slip speed
Correct Option: C
Explanation: The speed of the rotating magnetic field is known as synchronous speed Ns = 120f/P.
Q4. Which part of an induction motor carries the armature winding?
A) Rotor
B) Stator
C) Shaft
D) Air-gap
Correct Option: B
Explanation: In an induction motor, the stator carries the armature winding which produces the rotating magnetic field.
Q5. The squirrel-cage rotor bars are usually made of:
A) Copper
B) Aluminium
C) Steel
D) Brass
Correct Option: B
Explanation: Aluminium is commonly used due to low cost, light weight, good conductivity, and ease of die-casting.
Q6. Which rotor provides high starting torque and low starting current?
A) Squirrel-cage rotor
B) Solid rotor
C) Phase-wound rotor
D) Permanent magnet rotor
Correct Option: C
Explanation: Phase-wound rotor allows external resistance during starting, giving high starting torque with limited current.
Q7. Why does the rotor of an induction motor never run at synchronous speed?
A) Due to stator losses
B) Due to rotor copper losses
C) Relative speed required to induce rotor current
D) Due to mechanical friction only
Correct Option: C
Explanation: Rotor current is induced only when there is relative speed between rotor and RMF. At synchronous speed, induced current and torque become zero.
Q8. Slip of an induction motor at synchronous speed is:
A) 0
B) 1
C) Less than 1
D) Greater than 1
Correct Option: A
Explanation: Slip s = (Ns − Nr)/Ns. At synchronous speed, Nr = Ns, so slip is zero.
Q9. Slip of an induction motor at standstill is:
A) 0
B) 0.5
C) 1
D) Infinity
Correct Option: C
Explanation: At standstill, rotor speed is zero, hence slip is unity.
Q10. The frequency of rotor current at standstill is:
A) Zero
B) Equal to supply frequency
C) Half of supply frequency
D) Twice supply frequency
Correct Option: B
Explanation: Rotor frequency fr = s f. At standstill, s = 1, so fr = supply frequency.
Q11. Rotor current frequency at running condition is given by:
A) fr = s f
B) fr = (1 − s) f
C) fr = f / s
D) fr = s² f
Correct Option: A
Explanation: Rotor current frequency is proportional to slip and equals fr = s f.
Q12. Which of the following is NOT a part of squirrel-cage rotor?
A) Rotor bars
B) End rings
C) Slip rings
D) Rotor core
Correct Option: C
Explanation: Slip rings are used only in phase-wound motors, not in squirrel-cage motors.
Q13. In a phase-wound induction motor, slip rings are used to:
A) Reduce rotor speed
B) Increase rotor resistance during starting
C) Supply DC to rotor
D) Reduce stator current
Correct Option: B
Explanation: Slip rings allow external resistance in the rotor circuit to improve starting performance.
Q14. Which motor has better efficiency and rugged construction?
A) Phase-wound motor
B) DC motor
C) Squirrel-cage motor
D) Universal motor
Correct Option: C
Explanation: Squirrel-cage motors are simple, robust, efficient, and require minimal maintenance.
Q15. If slip of an induction motor is zero, then:
A) Rotor current is maximum
B) Rotor current is zero
C) Rotor frequency is maximum
D) Torque is maximum
Correct Option: B
Explanation: At zero slip, there is no induced EMF in rotor, hence rotor current becomes zero.
Q16. A 4-pole, 50 Hz induction motor runs at 1440 rpm. Find slip.
A) 0.02
B) 0.04
C) 0.06
D) 0.08
Correct Option: B
Explanation: Ns = 1500 rpm. Slip = (1500 − 1440)/1500 = 0.04.
Q17. Synchronous speed of a 6-pole, 50 Hz motor is:
A) 1000 rpm
B) 1200 rpm
C) 1500 rpm
D) 3000 rpm
Correct Option: A
Explanation: Ns = 120f/P = 120×50/6 = 1000 rpm.
Q18. The stator of a 3-phase induction motor has 3 slots per pole per phase at 50 Hz. Speed of rotating magnetic field is:
A) 1000 rpm
B) 1200 rpm
C) 1500 rpm
D) 3000 rpm
Correct Option: A
Explanation: Slots per pole per phase = 3 ⇒ Poles = 6. Ns = 1000 rpm.
Q19. A 3-phase induction motor has synchronous speed 1500 rpm and rotor speed 1440 rpm. Rotor current frequency is:
A) 1 Hz
B) 2 Hz
C) 3 Hz
D) 4 Hz
Correct Option: B
Explanation: Slip = 0.04, rotor frequency fr = 0.04×50 = 2 Hz.
Q20. A 4-pole, 50 Hz induction motor has rotor speed 600 rpm. Rotor current frequency is:
A) 50 Hz
B) 20 Hz
C) 30 Hz
D) 60 Hz
Correct Option: C
Explanation: Slip = (1500 − 600)/1500 = 0.6, fr = 0.6×50 = 30 Hz.