MCQs on Electrical Machines

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01․ In a DC series motor, if the armature current is reduced by 50 %, the torque of the motor will be equal to
25 % of previous value.
50 % of previous value.
100 % of previously value.
None of above.

In case of DC series motor φ ∝ Ia and T ∝ Ia2. /

02․ If the field circuit of a DC shunt motor is opened
DC Shunt Motor
The motor will attain dangerously high speed.
The speed of motor will be reduced.
The armature current will reduce.

If the field circuit is opened, the flux will drop to practically zero and therefore, the speed would to increase to an extremely high value. The centrifugal force developed at the high speed may cause the commutator segments to buckle, resulting in damage to the motor.

03․ In case of DC shunt motor if load is reduced
DC Shunt Motor
The speed will increase abruptly.
The speed will remains almost constant.
The speed will increase in proportion to reduction in load.

In case of DC shunt motor, flux is considered to be constant. If load is reduced then the speed of motor will remain constant.

04․ A DC series motor is that which
Has a poor torque.
Has a almost constant speed.
Can be started easily without load.
Has its field winding consinting of thick wire and less turn.

In series motor, the field coils, consisting of few turns of thick wire are connected in series with the armature. The cross-sectional area of the wire used for field coils has to be fairly large to carry the armature current, nut owing to the higher current, the number of turns of wire in them need not be large.

05․ The type of DC motor used for shears and punches is
Series motor.
Shunt motor.
Cummulative compound motor.
Differential compound motor.

Cummulative compound DC motor develop high starting torque and only fairly constant speed has pulsating load and only flywheel action. So this motor is best suited for shears and punches.

06․ For small value armature current Ia the torque developed by a DC series motor is proportional to

For small value of armature current armature core does not saturated. So upto saturation point flux is proportional to field current and hence to armature because field current is equal to armature current. Therefore small of armature current, torque is proportional square of the armature current.

07․ For higher values of armature, the speed of a DC series motor is
Proportional to armature current.
Proportional to the square of armature current.
Inversely proportional to armature current.

For higher value of armature current, magnetic saturation occured. SO flux tends to become constant hence back emf will be constant here back emf will be constant. Since speed in a motor is directly proportional to back emf and inversely proportional to flux. Hence for higher value of armature current, speed is contant.

08․ A DC series motor is running at rated speed without any additional resistance in series. If an additional resistance is placedin series, the speed of motor
Remains unchanged.
Oscillates around the rated speed.

If an additional resistance is kept in series, then the current and hence the flux are affected by the increase of armature circuit resistance. The voltage drop in additional resistance reduces the voltage applied to the armature and therefore speed is reduced.

09․ A DC series motor must started on no load because it would
Fail to start without load.
Run at dangerously high speed.
Not develop starting torque.
Draw more current without producing sufficient torque.

On no load the series motor would attain tremendoushly high speed and the motor may get damaged due to heavy centrifugal force set up in rotating part.

10․ DC series motor is best suited for electric traction as
It is cheaper in cost.
It consume less power.
It develops high starting torque.
It runs at very high speed at light load.

Because of DC series motor developes large starting torque and slow down with the increase in load and so gets automatically relieved from heavy excessive load.