# MCQs on Electrical Machines

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01․ Which slots are used in the rotor of an induction motor?
Open slots.
Semi closed slots.
Closed slots.
None of above.

Usually semi closed slots are used in the rotor of an IM, since open slots increase effective length of air gap and closed slots increase leakage reactance.

02․ In double cage induction motor, the inner cage offers
high resistance and low reactance.
low resistance and high reactance.
low resistance and low reactance.
high resistance and high reactance.

Double cage induction motor consists of 2 stages. 1. Inner cage. 2. Outer cage. Inner cage is designed in such way that having low resistance and high reactance. As if frequency is less then impedance is less in the inner cage. Hence, current flows through inner cage.

03․ In double cage induction motor, the outer cage offers
high resistance and low reactance.
low resistance and high reactance.
low resistance and low reactance.
high resistance and high reactance.

Double cage induction motor consists of 2 stages. 1. Inner cage. 2. Outer cage. Outer cage offers high resistance and low reactance. At high frequencies inner cage offers high impedance and outer cage offers low impedance. Hence, current flows through outer cage because it offers lower impedance.

04․ Which of the following induction motor has maximum speed?
8 pole.
6 pole.
4 pole.
2 pole.

Speed Ns = (120f / P), where P is the number of poles. Therefore speed is inversely proportional with number of poles in the machine. So the answer will be 2 pole induction motor.

05․ High starting torque and low starting current is the advantage of
squirrel cage induction motor.
hysteresis motor.
slip ring induction motor.
stepper motor.

In the slip ring induction motor slip rings are mounted on the shaft with brushes resting on them. The brushes are connected to three variable resistors connected in star. The purpose of slip rings and brushes is to provide external resistors in the rotor circuit. Varying the resistors we can vary each rotor phase resistance. As the starting torque and starting current can be controlled by rotor resistance, thus slip ring motor has that advantage. Hence the correct option is slip ring induction motor.

06․ Which method is less recommendable for induction motor starting in the industries?
Stator resistance starting.
Rotor resistance starting.
Direct online starting.
Auto transformer starting.

Stator resistance starting method will not be recommendable. This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances. Therefore, this method is used for starting small motors only.

07․ The air gap between stator and the rotor of the 3 - φ induction motor is kept
as wide as possible.
as small as possible.
depends on the material of stator and the rotor.
air gap does not matter.

The air gap between the rotor and stator of a 3 - φ induction motor is made as small as possible in order to produce sufficient mutual flux with a minimum exciting current and thereby making leakage reactance are as small as possible.

08․ What is power factor of a 3 - φ induction motor at no load?
High.
Low.
Equal to 1.

Because of air-gap, the reluctance of the magnetic circuit of 3 - φ induction motor is very high. Consequently, the current drawn by the motor on no load is largely magnetizing current, the no load current lagging the applied voltage by a large angle. For this reason, the power factor of a lightly loaded 3-phase induction motor is very low.

09․ For a 3 - ÃŽÂ¦, 4 pole, 50 Hz IM rotating at full load speed of 1440 rpm, calculate speed of rotor field with respect to rotor structure and stator structure respectively?
60, 1500.
1440, 1500.
60, 1440.
50, 1500.

10․ Crawling in an induction motor is due to
time harmonics in supply.
slip ring rotor.
space harmonics produced by winding currents.
insufficient starting torque.

Whenever rotor slots are integral multiple of stator slots then space harmonic waves are introduced developed in the gap along with the main flux on giving supply. These harmonics create additional torque fields. A common problem is with the seventh harmonic wave where it creates a forward rotating torque field at one seventh of the synchronous speed. There will be a maximum torque at a speed just below ( 1 / 7)Ns, and if this is high enough, the net torque produced can be higher than the torque due to the line frequency even though speed is ( 1 / 7)Ns. The slip all this while remaining high. This can cause the motor to crawl at a speed just below (1 / 7) of synchronous speed. Thus crawling of IM is due to space harmonics.

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