Single Phase Induction Motor Objective Questions With Explanation Part-1

1. In a split phase motor, the running winding should have
A. high resistance and low inductance
B. low resistance and high inductance
C. high resistance as well as high inductance
D. low resistance as well as low inductance

Answer:  B. low resistance and high inductance

Explanation: 
  • The starting winding is wound with fewer turns of thin wire than the running winding, so it has a lower inductance (L) and higher resistance (R).
  • The running winding is wound with large turns of thick wire than the starting winding, so it has a higher inductance (L) and lower resistance (R).
  • The lower L/R ratio creates a small phase shift, not more than about 30 degrees between the flux due to the running winding and the flux of the starting winding.
2. If the capacitor of a single-phase motor is short-circuited
A. the motor will not start
B. the motor will run
C.  the motor will run in reverse direction
D. the motor will run in the same direction at reduced r.p.m.

Answer: A. the motor will not start

Explanation:
  • Capacitor motors are split-phase motors with a capacitor added to the auxiliary start winding, main run winding, or both to improve the operating torque characteristics of the motor by causing a greater phase shift in the current between the main and auxiliary start windings.
  • The capacitor limits the starting surge current to a lower value than is developed by the standard split-phase motor. 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.
  • If the capacitor is short-circuited, there will be zero starting torque, and as a single-phase induction motor is n0t self-starting, Therefore the motor will not start.
3. In capacitor start single-phase motors
A. current in the starting winding leads the voltage
B. current in the starting winding lags the voltage
C. current in the starting winding is in phase with voltage in running winding
D. none of the above 

Answer: A. current in the starting winding leads the voltage

Explanation:
  • A capacitor-start motor is a single-phase induction motor that employs a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and the auxiliary windings.
  • The capacitor start motor has a cage rotor and has two windings on the stator. They are known as the main winding and the auxiliary or the starting winding. The two windings are placed 90 degrees apart.
  • A capacitor CS is connected in series with the starting winding. A centrifugal switch SC is also connected in the circuit.
  • IM is the current in the main winding which is lagging the auxiliary current IA by 90 degrees as shown in the phasor diagram above. The auxiliary current IA leads the voltage.
  • As the motor approaches its rated speed, the auxiliary winding and the starting capacitor is disconnected automatically by the centrifugal switch provided on the shaft of the motor.
4. In a capacitor start and run motors the function of the running capacitor in series with the auxiliary winding is to
A. improve power factor
B. increase overload capacity
C. reduce fluctuations in torque
D. to improve torque

Answer: A. improve power factor

Explanation:
  • In capacitor-start, induction motor has good starting condition but poor running conditions because the capacitor is disconnected in running conditions.
  • Hence in a capacitor-start-and-run motor is used to improve the running conditions hence to improve power factor. Starting torque is more in the capacitor start induction motor.
5. In a capacitor start motor, the phase displacement between starting and running winding can be nearly
A. 10°
B. 30°
C. 60°
D. 90°

Answer: D. 90°

Explanation:
  • To start single phase motors auxiliary winding or starting winding is used. The auxiliary winding is placed with a phase shift of 90° to the running winding and physically connected in parallel to the running winding.
6. In a split phase motor
A. the starting winding is connected through a centrifugal switch
B. the running winding is connected through a centrifugal switch
C. both starting and running windings are connected through a centrifugal switch
D. centrifugal switch is used to control supply voltage

Answer: A. the starting winding is connected through a centrifugal switch

Explanation:
  • The Split Phase Motor is also known as a Resistance Start Motor.
  • It has a single cage rotor, and its stator has two windings known as main winding and starting winding.
  • Both the windings are displaced 90 degrees in space.
  • The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance. 
  • A resistor is connected in series with the auxiliary winding.
  • The current in the two windings is not equal as a result the rotating field is not uniform.
  • Hence, the starting torque is small, of the order of 1.5 to 2 times of the start, running torque.
  • At the starting of the motor both the windings are connected in parallel.
  • The current in the main winding (IM) lag behind the supply voltage V almost by the 90∘  
  • The current in the auxiliary winding IA is approximately in phase with the line voltage.
  • Thus, there exists a time difference between the currents of the two windings.
  • The time phase difference ϕ is not 90 degrees, but of the order of 30 degrees.
  • This phase difference is enough to produce a rotating magnetic field.
  • In a split phase motor the starting winding is connected through a centrifugal switch
  • he main components of the split-phase motor include main winding, auxiliary winding, and a centrifugal switch.
  • This is the simplest way to establish a rotating magnetic field with two windings in the same stator core.
  • The auxiliary or starting winding has a series of resistance that makes its impedance highly resistant.
  • It is not the same winding as the main winding but has fewer turns of a much smaller diameter than the main winding.
  • This will reduce the voltage lagging of the starting current.
  • The main winding is inductive in nature to lag the voltage by a certain angle. This winding is designed to operate at synchronous speeds of 75% and above.
  • Because of the inductive nature, the supply voltage lags at a large angle, with the current induced by the starting winding almost being in phase with the voltage due to its resistive nature. Thus, there is a phase difference between these currents. The resulting current produces a rotating magnetic field and thus the starting torque.
  • The centrifugal switch is connected to the starting winding in series.
  • When the motor is 75-80% of the synchronous speed, the centrifugal switch is opened mechanically, thereby removing the auxiliary winding. The motor, therefore, only works with the main winding.

7. The torque developed by a single-phase motor at starting is
A. more than the rated torque
B. rated torque
C. less than the rated torque
D. zero

Answer: D. zero

Explanation:
  • In the case of single-phase induction motors we have only one winding (main winding). 
  • Due to which we have a pulsating magnetic field which first increases to a maximum value then become zero after that increase in the negative cycle to get maximum then become Zero.
  • It became zero twice in a cycle, so net torque produced is zero. Hence the torque developed by a single-phase motor at starting is zero. That by single-phase induction motor are not self-started.
8. Which of the following motor will give relatively high starting torque ?
A. Capacitor start motor
B. Capacitor run motor
C. Split phase motor
D. Shaded pole motor

Answer: A. Capacitor start motor

Explanation:
  • A capacitor-start motor is a single-phase induction motor that employs a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and the auxiliary windings.
  • The capacitor start motor develops a much higher starting torque of about 3 to 4.5 times of the full load torque.

Key Notes:
  • The order of starting torque in the increasing order is:
  • Shaded-pole motor < Split phase motor < Capacitor start capacitor run motor < Capacitor start motor

9. Which of the following motor will have relatively higher power factor ?
A. Capacitor run motor
B. Shaded pole motor
C. Capacitor start motor
D. Split phase motor

Answer: A. Capacitor run motor

Explanation:
Capacitor Run Motor
  • In this motor, the capacitor that is connected in series with the auxiliary winding is not cut out after starting and is left in the circuit all the time. That is the capacitor is used for starting and remains in the circuit during the running conditions.
  • This motor is similar to the capacitor start motor except that there is no centrifugal switch.
  • The capacitor value is chosen to obtain a nearly 90° phase shift between the currents of main and auxiliary windings around full-load speed.
  • This simplifies the construction and decreases the cost because the centrifugal switch is not needed. The power factor, torque pulsation, and efficiency are also improved because the motor runs as a two-phase motor. The motor will run more quickly.
  • Capacitor Run Motor has relatively higher power factor.
  • Performance and characteristics:- Starting torque is lower about 50 to 100% of full load torque. Power factor is improved may be about unity. Efficiency is improved to about 75%. It is usually used in fans, room coolers, portable tools, and other domestic and commercial electrical appliances.
The main advantages of capacitor run motor are:
  • Higher power factor,
  • Increased overload capacity of the motor,
  • Higher efficiency, and
  • Noiseless running of motor.

10. In a shaded pole motor, the shading coil usually consist of
A. a single turn of heavy wire which is  in parallel with running winding
B. single turn of heavy copper wire which is short-circuited and carries only induced current
C. a multilayer fine gauge copper wire  in parallel with running winding
D. none of the above

Answer: B. single turn of heavy copper wire which is short-circuited and carries only induced current

Explanation:
  • Shaded pole induction motor is a self-starting 1ϕ induction motor.
  • The stator poles of a shaded pole motor are divided into two parts.
  • One part of the pole consists of a short-circuited coil made up of copper which is known as the shading coil (shaded part) and the remaining part is known as the unshaded part of the pole.
  • When a 1ϕ is applied an alternating flux is generated. This flux links with the shaded coil.
  • Then voltage gets induced in the coil due to the variation in the flux linking.
  • Hence, the shaded portion is short-circuited due to which it produces the circulating current in it.
  • The main core flux is opposed by the flux in the ring that is developed by the circulating current.
  • Hence, flux is induced in the shaded portion of the motor along with the unshaded portion with a phase difference, which is lagging behind the unshaded pole flux.
  • There is also a space displacement that is less than 90°  between a shaded ring flux and the main motor flux.
  • Due to this space displacement, a rotating magnetic field is produced which leads rotation of the motor.
  • he stator of the shaded pole motor has a salient pole
  • The salient pole means the poles of the magnet are projected towards the armature of the motor; Each pole of the motor is excited by its exciting coil
  • The copper rings shade the loops; These loops are known as the shading coils
  • The slot is constructed at some distance apart from the edge of the poles
  • The short-circuited heavy copper coil is placed in this slot
  • The part which is covered with the copper ring is called the shaded part and which are not covered by the rings are called unshaded part
  • This shading coil carries only induced current










Previous Post Next Post