MCQs on Electrical Machines

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01․ During no load test, an induction motor draws power
for core loss and copper loss.
for copper loss and windage friction loss.
only for the small copper loss.
for core loss and windage and friction loss.

No load test is similar to the open-circuit test on a transformer. The motor is made run at no load, the input power is measured by the 2-wattmeter method. No load current is measured by an ammeter and the normal rated supply voltage is measured by a voltmeter. Since no load current is 20 - 30 % of full load current, the I2R losses in the primary may be neglected as they vary with the square of the current. Since the motor is running at no load, total input power is equal to constant iron loss, friction and windage losses of the rotor.

02․ Unbalanced supply voltage given to a 3 - φ, delta connected induction motor will cause
negative sequence current.
less heating of the rotor.
less heating of the rotor.
all of these.

If the supply voltage to a 3 - φ IM is unbalanced and the it is delta connected, then a counter rotating magnetic field is formed. This counter rotating field results in the formation of a negative sequence current.

03․ Exciting current of an induction motor is
higher than power transformer.
lesser than power transformer.
equal as power transformer.
none of these.

Unlike that of a power transformer the magnetic circuit of an induction motor has an air gap. Therefore the exciting current of an induction motor ( 30 % - 40 % of full load current) is much higher than that of a power transformer.

04․ If starting current of a 3 - Φ induction motor is 5 times the rated current, while rated slip is 4 %, then ratio of starting torque to full load torque is


05․ In travelling cranes for hoisting and lowering which type of motor is preferred?
Synchronous motor.
DC series motor.
AC slip ring motor.
Single phase induction motor.

For hoisting and lowering of crane requires high starting torque. Hence ac slip ring induction motor, Ward Leonard controlled DC shunt motors are preferred.

06․ In travelling cranes for crane travel, trolley and boom hoist which motor is preferred?
Stepper motor.
Slip ring induction motor.
DC differential compound motor.
Synchronous motor.

For crane travel, trolley travel and boom hoist of traveling crane requires high starting torque and constant speed in operation. Hence slip ring induction motor is preferred.

07․ In travelling cranes for swing action which motor is preferred?
DC series motor.
AC slip ring IM.
Reluctance motor.
Capacitor type induction motors.

AC slip ring or DC shunt motors are preferred for slew and swing action of traveling cranes.

08․ Which of following motor is preferred for belt conveyor?
Slip ring induction motor.
Synchronous motors.
Reluctance motor.
Universal motor.

Normal starting current and high starting torque double cage IM with DOL starter or slip ring ring induction motors are preferred for belt conveyors.

09․ A crusher is equipped with
DC shunt motor.
Synchronous motor.
Slip ring induction motor.
Reluctance motor.

A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Belted slip ring induction motor are employed in jaw crushers because of their high starting torque.

10․ In shapers and slotters which motor are preferred?
Squirrel cage IM.
Slip ring IM.
Double cage motor.
Synchronous motor.

A shaper is another machine tool that is used to plane metal. It is smaller than a planer and has a cutting action opposite to that of the planer. Constant speed squirrel cage induction motors are prefer for shapers and slotters.