Q1. Which of the following motors is not a single-phase induction motor?
A. Capacitor-start motor
B. Shaded pole motor
C. Repulsion motor
D. Split-phase motor
Correct Option: 3
Explanation:
Repulsion motor is an AC commutator motor operating on the principle of repulsion using brushes and commutator. It does not work on induction principle, whereas capacitor-start, shaded pole, and split-phase motors are single-phase induction motors.
Q2. The basic reason why a single-phase induction motor is not self-starting is because:
A. Rotor resistance is high
B. No rotating magnetic field at standstill
C. Slip is unity
D. Power factor is poor
Correct Option: 2
Explanation:
Single-phase supply produces a pulsating magnetic field, not a rotating magnetic field. At standstill, equal forward and backward torques are produced, resulting in zero net starting torque.
Q3. According to double-field revolving theory, a pulsating flux can be resolved into:
A. Two unequal rotating fields
B. One rotating and one stationary field
C. Two equal rotating fields in opposite directions
D. One alternating field
Correct Option: 3
Explanation:
Double-field revolving theory states that a pulsating magnetic field can be resolved into two equal rotating fields of half magnitude, rotating in opposite directions at synchronous speed.
Q4. At standstill, the net torque in a single-phase induction motor is zero because:
A. Forward torque equals backward torque
B. Rotor current is zero
C. Slip is zero
D. Back emf is zero
Correct Option: 1
Explanation:
At standstill, slips for both forward and backward fields are equal (s = 1), producing equal and opposite torques. Hence, net torque becomes zero.
Q5. Which motor has lowest starting torque?
A. Capacitor start motor
B. Split-phase motor
C. Shaded pole motor
D. Capacitor start-capacitor run motor
Correct Option: 3
Explanation:
Shaded pole motor has weak rotating field, no auxiliary winding or capacitor. Hence, it produces the lowest starting torque and is used for very light loads.
Q6. A single-phase induction motor has forward field torque of 8 N·m and backward field torque of 5 N·m. Net torque is:
A. 3 N·m
B. 13 N·m
C. 40 N·m
D. 0 N·m
Correct Option: 1
Explanation:
Net torque in a single-phase induction motor is given by T = Tf − Tb. Hence, T = 8 − 5 = 3 N·m.
Q7. In double-field revolving theory, at standstill:
A. Forward slip = 1, Backward slip = 1
B. Forward slip = 0, Backward slip = 2
C. Forward slip = 2, Backward slip = 0
D. Forward slip = Backward slip = 0
Correct Option: 1
Explanation:
At standstill, rotor speed is zero. Slip for forward field sf = 1 and slip for backward field sb = 1.
Q8. Which component helps a single-phase induction motor to become self-starting?
A. Rotor resistance
B. Auxiliary winding
C. Slip rings
D. Damper winding
Correct Option: 2
Explanation:
Auxiliary winding creates phase difference between currents, producing an artificial rotating magnetic field and hence starting torque.
Q9. The phase difference between main and auxiliary winding currents in a split-phase motor is approximately:
A. 10°
B. 25°
C. 90°
D. 180°
Correct Option: 2
Explanation:
In split-phase motors, auxiliary winding has high resistance, producing a phase difference of about 25°–30°.
Q10. If synchronous speed is 1500 rpm and rotor speed is 1425 rpm, slip of forward field is:
A. 0.05
B. 0.95
C. 0.025
D. 0.075
Correct Option: 1
Explanation:
Slip s = (Ns − N)/Ns = (1500 − 1425)/1500 = 0.05.
Q11. The backward field slip when motor runs at slip ‘s’ is given by:
A. s
B. 1 − s
C. 2 − s
D. 1 + s
Correct Option: 3
Explanation:
Backward field slip is given by sb = 2 − s, since backward field rotates opposite to rotor direction.
Q12. Which equivalent circuit parameter represents core loss?
A. Rotor resistance
B. Stator reactance
C. Shunt resistance Rc
D. Magnetizing reactance Xm
Correct Option: 3
Explanation:
Core (iron) losses are represented by shunt resistance Rc in the equivalent circuit.
Q13. At running condition, forward slip is 0.04. Backward slip will be:
A. 0.04
B. 0.96
C. 1.96
D. 2.04
Correct Option: 3
Explanation:
Backward slip sb = 2 − sf = 2 − 0.04 = 1.96.
Q14. Why is backward field torque small during normal running?
A. Backward slip is small
B. Rotor emf is zero
C. Backward slip is high
D. Flux is zero
Correct Option: 3
Explanation:
During running, backward slip is close to 2, resulting in high rotor impedance and very small rotor current, hence backward torque is negligible.
Q15. Which motor is most suitable for ceiling fans?
A. Capacitor start
B. Shaded pole
C. Capacitor run
D. Repulsion motor
Correct Option: 3
Explanation:
Capacitor run motors provide good efficiency, better power factor, and smooth operation, hence widely used for ceiling fans.
Q16. A 1-phase induction motor operates at slip 0.03. Find backward field slip.
A. 0.03
B. 0.97
C. 1.97
D. 2.03
Correct Option: 3
Explanation:
Backward slip sb = 2 − 0.03 = 1.97.
Q17. Which statement is true regarding equivalent circuit of 1-phase induction motor?
A. Same as 3-phase IM
B. Has only one rotor circuit
C. Has two rotor circuits (forward & backward)
D. No magnetizing branch
Correct Option: 3
Explanation:
Due to double-field revolving theory, the equivalent circuit consists of two rotor circuits corresponding to forward and backward rotating fields.
Q18. If synchronous speed is Ns, backward field speed relative to rotor is:
A. sNs
B. (2 − s)Ns
C. (1 − s)Ns
D. Ns
Correct Option: 2
Explanation:
Speed of backward field relative to rotor is given by (2 − s)Ns.
Q19. In equivalent circuit without core loss, which branch is removed?
A. Xm
B. Rc
C. Rotor resistance
D. Rotor reactance
Correct Option: 2
Explanation:
When core losses are neglected, shunt resistance Rc representing iron loss is removed from the equivalent circuit.
Q20. At standstill, forward and backward field torques are Tf and Tb. Net torque is:
A. Tf − Tb
B. Tf + Tb
C. Zero
D. Depends on rotor resistance
Correct Option: 3
Explanation:
At standstill, Tf = Tb. Hence, net torque is zero, confirming that single-phase induction motors are not self-starting.