MCQs on Electrical Machines

Page 86 of 114. Go to page
01․ A 3-phase, alternator has generated emf per phase of 230 V with 10% 3rd harmonic and 6% 5th harmonic content. Calculate rms line voltage for star connection.
50 V.
100 V.
230.2 V.
399 V.

E3 = 230 V E5 = 0.6X230 = 13.8 V \ [math] herefore Emf \; per \; phsae = \sqrt{E_3^2 + E_5^2} = \sqrt{230^2 + 13.8^1} = 230.2 \; V [/math].

02․ In above question, find rms line voltage for delta connection?
50 V.
100 V.
230.2 V.
399 V.

Since for delta connection, line emf is same as phase emf. So the rms value of line emf = 230.2 V.

03․ Type of single phase motor having highest power factor at full load is
shaded pole type.
capacitor run.
capacitor start.
split phase.

Capacitor run motor has better running power factor and efficiency and smoother operation. Modern application is ceiling fan.

04․ Induction motors have the advantage of
all of below.
less cost.
simple in construction.
less maintenance.

As compared with DC motors the IM does not have any commutators and brushes. Hence it requires less maintenance. Poly-phase induction motors are used because of 1. Less cost. 2. Simple and robust construction. 3. Less maintenance required.

05․ For a single phase capacitor start induction motor which of the following is valid?
Rotation can be changed by reversing main winding terminals.
Direction of rotation cannot be changed.
Rotation can be changed by interchanging supply terminals.
None of these.

For changing direction of rotation, we have to change main winding terminal and direction of magnetic flux will be reversed. The single-phase capacitor start induction motor is an AC single phase motor having two windings as in split phase motor. One heavy duty electrolyte capacitor is connected in series with the starting winding in order to produce the phase difference between the currents in the two windings and thereby producing starting torque. The direction of rotation of capacitor-start, induction run motor is reversed by reversing either the starting or running winding leads, not both.

06․ Which motors are preferred for refrigeration and air conditioning in smaller units?
Induction motors.
Universal motors.
Reluctance motors.
Stepper motors.

Under construction.

07․ A 230 V, 50 Hz, 4 pole single phase induction motor is rotating in the clockwise ( forward ) direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8 Ω. Then the effective rotor resistance in the backward branch of the equivalent circuit will be
2 Ω.
4 Ω.
8 Ω.
10 Ω.


08․ Which of the following motor used in hair dryers?
Synchronous motors.
Shaded pole IM.
Split phase IM.
Cage induction motor.

Shaded pole induction motor used in hair dryers, small fans etc.

09․ Which of the following motor is used in grinders?
Capacitor start IM.
Capacitor start capacitor run IM.
Split phase IM.
Synchronous motor.

The capacitor start capacitor run IM is applied for blowers, refrigerators, drilling machines, grinders etc.

10․ Capacitor start capacitor run provides
high staring torque and high power factor.
high staring torque only.
high power factor only.

Two capacitors are used in this type of motor. One capacitor is used at the starting and another capacitor is used at the time of normal running for providing high staring torque and high power factor.