MCQs on Electrical Machines

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01․ Slip speed is the
difference of synchronous speed and actual rotor speed.
difference of actual rotor speed and synchronous speed.
sum of synchronous and rotor speeds.
half of the sum of synchronous and rotor speeds.

Slip speed is defined as the difference of synchronous speed and rotor speed. The difference between the speed of the stator field, known as synchronous speed ( Ns) and the actual speed speed of rotor (N) is known as the slip and denoted by s. \

02․ At slip s = 1 torque developed in machine is
starting torque.
maximum torque.
electromagnetic torque.

The torque developed at unity slip is starting torque or stalling torque.

03․ Capacitor fitted with ceiling / table fans is of the value of
1 - 2 µF.
2 - 3 µF.
5 - 10 F.
0.1 µF.

The capacitor used in ceiling or table fans are of small values and is of the order of 2 - 3.15 µF.

04․ Which 1 - Φ motor does operate at high power factor?
Capacitor start.
Capacitor run.
Split phase.
Shaded pole.

Capacitor run type 1 phase motor operate at high power factor.

05․ Why are capacitors used with large AC induction motor?
Improve voltage level.
Improve power factor.
Improve power level.
Improve current level.

Three phase capacitors banks are used with large AC induction motor to improve the power factor of the circuit and to get the resultant effect of current reduction.

06․ Which single phase motor is used for a refrigerator?
Capacitor run.
Split phase.

Split phase / capacitor start single phase motor is used for a refrigerator. Capacitor run induction motor is used for room cooler, the universal motor is used in drill machine, hooter, mixing machine, repulsion motor is used for the compressor.

07․ What type of dielectric material is used in capacitors used for fans and for power factor correction?
Oil impregnated paper.

Oil impregnated paper is used as dielectric medium for capacitors used in fans / capacitors used for power factor correction and also small size large value capacitors. Air / vacuum is used for precision small value capacitor. Mica / glass is used for standard capacitors used in labs.

08․ A three phase induction motor has a synchronous speed of 1500 rpm the machine runs at 1460 rpm at a particular load. The slip at this load is ___________.
30 %.
1.5 %.
2.67 %.
5 %.


09․ A 3 - Φ, 12 pole, 600 V, 50 Hz star connected IM has rotor resistance and stand still resistance of 0.03 Ω and 0.5 Ω respectively. The speed of maximum torque will be
240 rpm.
450 rpm.
470 rpm.
540 rpm.


10․ A 3 - Φ, 400 / 200 V, Y - Y connected wound rotor induction motor has 0.06 Ω rotor resistance and 0.3 Ω standstill reactance per phase. To make the starting torque equal to maximum torque of the motor the additional resistance required
0.24 Ω.
0.45 Ω.
0.78 Ω.
0.66 Ω.