MCQs on Electrical Machines

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01․ It is advisable to start a DC series motor with some load to
Limit the speed.
Limit the current.
Limit the flux.
Avoid sparking.

The minimum load on a DC series motor should be great enough to keep the speed within limits. If speed becomes dangerously high, then motor must be disconnected from the supply.

02․ In the DC motor the iron losses occur in
The field.
The armature.
The cummotator.
The brushes

Iron or core losses are influenced by the magnetitude of flux. It consist of hyterisis and eddy losses caused by changing flux densities in the iron core of the machine. In case of DC motor iron or core losses occur in armature core and pole faces.

03․ When the armature of a DC motor rotates emf induced is
Self induced.
Back emf.
Mutually induced emf.
None of above.

When the armature coils rotates under the influence of electro magnetic torque in the magnetic field the coil cuts the magnetic flux thereby inducing an emf in it which oppose the applied voltage. This induced emf is called back emf.

04․ Which of the following DC motors has almost constant speed over their full load range?
Compound (cummolative).
none of these.

The flux φ and back emf is Eb in a shunt motor are almost constant under normal condition. Therefore, the speed of shunt motor will remain constant as the armature current varies. However, there is the slight change in speed of shunt motor from no load to full load. Hence, it is essentially a constant speed motor.

05․ On switching a DC motor to DC supply mains, it is found to rotate in the direction opposite to that for which it is designed. The DC motor is
Differentical compound.
Series shunt.
Cummulative compound.
have an equal number of electrons and protons.

In differentical compound motor, in very heavy over load as the field may be weakend too much and the motor may ultimately result in the service result in the series field winding over powering the shunt causing the motor to reverse. Hence series field is risen faster than that in high inductance shunt field, the series field coil will overpower the shunt and motor starts up in reverse direction.

06․ Which of the following DC motors has got maximum relieving property?
Differentical compound.
Cummulative compound.

DC series motor develops large starting torque and slow down with increase inload and so get automatically relived from heavy excessive load.

07․ When a DC cummulative compound wound motor is loaded
Torque increase and speed decrease.
Torque and speed both increase.
Torque decrease and speed increases.
Both torque and speed decrease.

In DC cummulative compound wound motor, when it is loaded, the flux due to series field winding increase and cause the torque grater that it would have been with shunt field winding alobe for a given machine and for a given given current. The increase in flux due to series field winding on account of increase inload cause the speed to fall more rapidly.

08․ DC shunt motors are suitable for
Centrifugal pumps
Blowers and fans.
All of this.

DC shunt motors are almost constant speed motors, develop medium starting torque and their speed can be adjusted by using speed control methods. So they are best suited for driving centrifugal pumps, lathes, blowers and fans where constant speed is required.

09․ In applied voltage is reduced in a dc shunt motor
Full load torque.
Full load current.
Full load speed.
None of these.

In case of DC shunt motor, the shunt field current is equally affected by variation in applied voltage to the armature. As it result full load current will reduced.

10․ Which of following DC motors will have percentage increase of input current for the same percentage increase in torque?
Cummulative compound.
Differntical compound.

In case of DC shunt motors, the flux remain almost constant for all condition. So torque developed is directly proportional to input current. Hence least percentage increase of input current increase of least percentage in torque.