50 Most Important Objective Type Question|MCQ Of Single Phase Induction Motor

 

Ques 1.  In a split phase motor, the running winding should have

  1. High resistance and low inductance
  2. High resistance and High inductance
  3. Low resistance and high inductance
  4. Low resistance and Low inductance

Answer 3. Low resistance and high inductance

Explanation:

  • In order to self-start a single phase induction motor one must create a rotating magnetic field on the stator. Therefore split phase motor has two winding one is starting winding and other is main winding or running winding.
  • The starting winding has fewer turns and small diameter, therefore, it has high resistance and low inductance
  • The running winding has the heavier wire of more turns, therefore, it has low resistance and high inductance.
  • As we know that for high inductance current lag behind voltage by a large angle and for high resistance the current is almost in phase with the voltage.
  • Therefore for starting winding the current lags behind the supply voltage by an angle of 15 degrees while for running winding the current lags behind voltage about an angle of 40 degrees.
  • This difference in phase angle creates a phase delay in the rotating magnetic field that allows the motor to start otherwise without the phase difference the motor will oscillate in the standstill position.

 

Ques 2. If the capacitor of a single-phase motor is short-circuited

  1. The motor will not start
  2. The motor will run in the same direction at reduced speed
  3. The motor will run in reverse direction
  4. None of the above

Answer 1. The motor will not start

Explanation:

  • When the capacitor is short-circuited high inrush current will flow in the winding which may burn the starting winding of a single phase induction motor.

 

Ques 3. In a split phase motor

  1. Both starting and running windings are connected through a centrifugal switch
  2. Centrifugal switch is used to control supply voltage
  3. The running winding is connected through a centrifugal switch
  4. The starting winding is connected through a centrifugal switch

Answer 4. The starting winding is connected through a centrifugal switch

Explanation:

  • In split phase motor, the starting or auxiliary winding is connected through the centrifugal switch the purpose of the switch is to disconnect the starting winding of the motor once the motor approaches its normal operating speed.

 

Ques 4. The torque developed by a single-phase motor at starting is

  1. less than the rated torque
  2. More than the rated torque
  3. zero
  4. None of the above
Answer 3. Zero

Explanation:

  • The field produces by the single phase induction motor can be explained by double revolving field theory.
  • The basic idea behind the double field revolving theory is that any alternating quantity can be resolved into 2 equal components rotating component of same strength which is revolving at the same speed but in opposite direction.
  • The interaction with two rotating fluxes produces torque in opposite direction, leading net torque to zero.

Torque slip characteristic of single phase induction motor

 

Ques 5. In a capacitor start and run motors the function of the running capacitor in series with the auxiliary winding is to

  1. Improve power factor
  2. Reduce fluctuations in torque
  3. Increase overload capacity
  4. To improve torque

Answer 1. Improve Power Factor

Explanation:

  • Inductive Loads consume Reactive Power and Capacitive Loads deliver Reactive Power.
  • The Power Factor Angle is inversely proportional to the Power Factor( reducing the angle gap between the Voltage vector and the Current Vector improved power factor).
  • So in a single phase Induction Motor, a Capacitor Bank is connected, than the Reactive power demand by the Inductive load (Induction Motor) is supplied by the Capacitor Bank, not from the Source.
  • Therefore the system reactive power demand is reduced and the Power factor is improved.

 

Ques 6. Which of the following motor will have relatively higher power factor?

  1. Capacitor start motor
  2. Shaded pole motor
  3. Capacitor run motor
  4. Split phase motor

Answer 3. Capacitor run motor

Explanation:

  • In capacitor run motor the starting winding remains connected all the times. There is no centrifugal switch in the capacitor run motor.
  • Because the capacitor remains in the circuit all the time a lower value capacitor is used the lower value capacitor has more phase shift as compared to the high-value capacitor, therefore, winding current is low.
  • Therefore the capacitor run motor has high running torque and improved power factor.

 

Ques 7. A centrifugal switch is used to disconnect ‘starting winding when motor has

  1. Picked up 10% speed
  2. Picked up 20% speed
  3. Picked up 5 – 10% speed
  4. Picked up 50 – 70% speed

Answer 4. Picked up 50% – 70% speed

Explanation:

  • The centrifugal switch is used to disconnect the starting winding of the motor once the motor approaches its normal operating speed i.e 50% – 70% speed.

 

Ques 8. In repulsion motor direction of rotation of motor

  1. Same as that of brush shift
  2. Independent of brush shift
  3. Opposite to that of brush shift
  4. None of the above

Answer 1. Same as that of brush shift

Explanation:

  • The direction of rotation is determined by the position of the brushes with respect to the magnetic field of the stator.
  • If the brushes are shifted clockwise from the main magnetic axis, the motor will rotate in a clockwise direction.
  • If the brushes are shifted counterclockwise from the main magnetic axis, the motor will rotate in a counter-clockwise direction

 

Ques 9. If a particular application needs high-speed and high starting torque, then which of the following motor will be preferred?

  1. Shaded pole motor
  2. Capacitor start motor
  3. Capacitor run motor
  4. Universal Motor

Answer 4. Universal Motor

Explanation:

  • The universal motor is very similar to a DC series motor in construction but is modified slightly to allow the motor to operate properly on AC power.
  • It is a commuted series-wound motor where the stator’s field coils are connected in series with the rotor windings through a commutator.Therefore they produce high starting torque.
  • Universal motors can be and are generally run at high speeds, 4000-16000 rpm, and can go over 20,000 rpm.

 

Ques 10. Which of the following motor is used in the mixer?

  1. Repulsion Motor
  2. Reluctance Motor
  3. Hysteresis Motor
  4. Universal Motor

Answer 4. Universal motor

Explanation:

  • We use universal motors in mixer grinders. Universal motors can run both on a.c. and d.c.
  • The universal motor works on the same principle that DC series motor works. DC series motor has the characteristics of operating at high-speed when there is no load and operating at low-speed when the load is applied. It has high starting torque characteristics.
  • So it is used in mixers, where initially we put some load at starting.