MCQs on Electrical Machines

01․ No load test of 3-phase induction motor gives which of the following?

No-load test is performed to find out the constant losses or magnetic losses only.

02․ which of the following losses are negligible in blocked rotor test?

As the applied voltage is low in blocked rotor test, iron losses are negligible (as rotor frequency is low) and mechanical losses are zero as the rotor is blocked.

03․ The speed of stator rotating magnetic field (RMF) with respect to rotor is

When the supply is given to the stator, stator will produce RMF with synchronous speed. This will make the rotor to rotate at speed N. Therefore the speed of stator RMF with respect to rotor is S.N_s. The relative speed of stator RMF with respect to rotor = N_s - N_r ⇒S.N_s

04․ Which of the following motors has low ratings only?

In shaded pole motor, the pole has slit and one portion is shade through a copper strip or winding known as shading coil. This will produce a moving field under the pole which can be considered as rotating magnetic field but very weak and also not at constant magnitude. Therefore, the torque and speed are non uniform. But these are popular, because their compact design required for small ratings. These are most popular only for small ratings starting from 20 watts. In fact all the motors are fractional horse power motors but among this shaded poles are of smaller rating with constant load torque.

05․ Reversing the direction of rotation of split phase motors is done by

Reversing the direction of rotation of split phase motors can be done by,

1. Reversing the terminals of main winding
2. Reversing the terminals of starting winding
3. Interchanging the capacitor from starting winding to main winding or vice versa.

Any of the above methods can be used but not both at the same time.

06․ In shaded pole induction motors, the flux moves from ------------- region to ----------------- region respectively?

When an alternating voltage is applied across the stator, rate of change of flux is positive and it will induce emf in the shading coil, produce its current and flux which opposes the main flux due to stator supply. Therefore all the flux is crowded under unshaded region. After that, rate of change of flux is negative and the emf induced in the shaded coil will not oppose but aid the main flux. As it encourages all the flux is crowded in the shaded region. Therefore, the flux moves from unshaded region to shaded region and flux difference between them is almost 90^o to produce torque.

07․ Which of the following motor has the highest power factor at full load?

In capacitor run type induction motor, the capacitor is permanently connected with the auxiliary winding. In this type of motor, capacitor is not disconnected throughout the operation. Therefore the power factor is improved in starting and running. In capacitor start motor the capacitor improves p.f. only at the time of starting.

08․ In a resistance split phase motor, what is the range of torque angle?

The stator contains two windings which are essentially in parallel, uniformly distributed in the stator slots. The two windings are main winding and starting winding. Main winding containing large number of turns, naturally inductive in nature. Starting winding containing less number of turns with thin wire, intentionally designed for high resistance and low inductance. Starting winding is used for self starting of 1-phase motor. The angle between main winding current and starting winding current is called torque angle, generally it lies between 25° to 30°. Because main winding is inductive in nature and starting winding has high resistance and low reactance.

09․ Which of the following represents the operating region of slip in a 1-phase induction motor?

In a 1-phase induction motor, due to both forward and backward torque slips, the operating region lies between 0 and 2. In motoring mode slip is 0 to 1 and breaking mode 1 to 2.

10․ In a 1-phase induction motor, if the forward torque slip is S, what is the backward torque slip?

In induction motor when the rotor and the magnetic field rotates in reverse direction or at the time of plugging then slip becomes 2-S.