Q1. A shaded-pole motor produces starting torque due to:
A. Repulsion torque
B. Capacitor action
C. Phase difference created by shading coil
D. Centrifugal switch
Correct Option: 3
Explanation:
In a shaded-pole motor, a copper shading coil delays the flux in the shaded portion of the pole. This creates a phase difference between shaded and unshaded flux, producing a weak rotating magnetic field that generates starting torque.
Q2. The direction of rotation of a shaded-pole motor is from:
A. Shaded to unshaded part
B. Unshaded to shaded part
C. Depends on supply polarity
D. Random direction
Correct Option: 2
Explanation:
Flux in the unshaded part reaches maximum earlier than the shaded part. Hence, the apparent motion of flux is from unshaded to shaded portion, fixing the direction of rotation permanently.
Q3. Which motor has lowest starting torque among single-phase motors?
A. Split-phase motor
B. Capacitor start motor
C. Repulsion motor
D. Shaded-pole motor
Correct Option: 4
Explanation:
Shaded-pole motors produce very low starting torque (about 25–50% of rated torque), which is the lowest among single-phase motors.
Q4. In a repulsion motor, rotor current is induced by:
A. Transformer action
B. Slip rings
C. Commutator action
D. Capacitor action
Correct Option: 1
Explanation:
The stator flux induces current in the rotor winding by transformer action since the rotor is not electrically connected to the supply.
Q5. The starting torque of a repulsion motor is maximum when brush axis is:
A. Parallel to stator field
B. At 90° electrical
C. At 45° electrical
D. At zero degree
Correct Option: 3
Explanation:
Torque in a repulsion motor is proportional to sin(2α). Maximum torque occurs when α = 45°, where sin(2α) is maximum.
Q6. Repulsion motor operates on the principle of:
A. Electromagnetic induction
B. Lorentz force
C. Repulsion between magnetic fields
D. Reluctance variation
Correct Option: 3
Explanation:
Torque is produced due to repulsion between stator magnetic field and rotor magnetic field created by induced currents.
Q7. In a compensated repulsion motor, compensating winding is placed on:
A. Stator
B. Rotor
C. Both stator and rotor
D. Air-gap
Correct Option: 2
Explanation:
The compensating winding is embedded in the rotor slots and connected in series with the armature to neutralize armature reaction.
Q8. Main purpose of compensating winding in repulsion motor is to:
A. Increase speed
B. Reduce armature reaction
C. Increase power factor
D. Reduce losses
Correct Option: 2
Explanation:
Compensating winding produces flux opposing armature reaction, reducing flux distortion and improving commutation.
Q9. In repulsion-start induction-run motor, repulsion winding is disconnected at:
A. Starting
B. 25% speed
C. 70–80% of synchronous speed
D. Full load
Correct Option: 3
Explanation:
At about 70–80% of synchronous speed, a centrifugal switch lifts the brushes and the motor continues to run as an induction motor.
Q10. Which motor combines high starting torque and good running efficiency?
A. Shaded-pole motor
B. Repulsion motor
C. Repulsion-start induction-run motor
D. Split-phase motor
Correct Option: 3
Explanation:
This motor combines the high starting torque of a repulsion motor with the good efficiency and speed regulation of an induction motor.
Q11. In repulsion induction motor, rotor winding is:
A. Short-circuited
B. Open-circuited
C. Connected to external resistance
D. Connected through capacitor
Correct Option: 1
Explanation:
The rotor winding is short-circuited through the commutator, enabling induction motor action during running condition.
Q12. Direction of rotation of repulsion motor is determined by:
A. Supply frequency
B. Brush position
C. Rotor resistance
D. Stator slots
Correct Option: 2
Explanation:
Changing the brush position changes the direction of repulsive torque, thereby reversing the direction of rotation.
Q13. At synchronous speed, torque of repulsion motor becomes:
A. Maximum
B. Zero
C. Negative
D. Infinite
Correct Option: 2
Explanation:
At synchronous speed, relative speed between stator flux and rotor becomes zero, hence no induced current and zero torque.
Q14. Which motor is most suitable for record players and toys?
A. Repulsion motor
B. Capacitor start motor
C. Shaded-pole motor
D. Repulsion induction motor
Correct Option: 3
Explanation:
Shaded-pole motors are cheap, quiet, simple in construction, and suitable for low-power applications like toys and record players.
Q15. A shaded-pole motor has slip of 0.15 at rated load. If synchronous speed is 1500 rpm, actual speed is:
A. 1275 rpm
B. 1350 rpm
C. 1400 rpm
D. 1450 rpm
Correct Option: 1
Explanation:
Actual speed N = Ns(1 − s) = 1500 × 0.85 = 1275 rpm.
Q16. In a repulsion motor, maximum torque occurs when brush shift angle is:
A. 0°
B. 30°
C. 45°
D. 90°
Correct Option: 3
Explanation:
Maximum torque condition occurs at a brush shift of 45°, where magnetic repulsion effect is strongest.
Q17. If synchronous speed is 1800 rpm and slip is 0.2, rotor speed is:
A. 1440 rpm
B. 1500 rpm
C. 1600 rpm
D. 1700 rpm
Correct Option: 1
Explanation:
Rotor speed N = 1800 × (1 − 0.2) = 1440 rpm.
Q18. A repulsion motor draws 10 A at 230 V during starting. Input power (approx.) is:
A. 1.5 kW
B. 2.0 kW
C. 2.3 kW
D. 3.0 kW
Correct Option: 3
Explanation:
Input power ≈ VI = 230 × 10 = 2300 W ≈ 2.3 kW.
Q19. If frequency is doubled, synchronous speed of shaded-pole motor will:
A. Remain same
B. Become half
C. Become double
D. Become zero
Correct Option: 3
Explanation:
Synchronous speed Ns = 120f / P. Hence, doubling frequency doubles synchronous speed.
Q20. Which motor has commutator but no external electrical connection to rotor?
A. DC motor
B. Universal motor
C. Repulsion motor
D. Induction motor
Correct Option: 3
Explanation:
Repulsion motor has a commutator and brushes, but rotor current is induced by stator flux with no external electrical supply.