MCQs on Electrical Machines

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01․ If field current is decreased in shunt dc motor, the speed of the motor
remains same.
none of the above.

As a shunt field current If decreases, φ also decreases and the speed rises as speed is inversely proportional to flux.

02․ What is the mechanical power developed by a DC series motor is maximum?
Back emf is equal to half the applied voltage.
Back emf is equal to applied voltage.
Back emf is equal to zero.
None of above.

03․ What is the shunt resistance component equivalent circuit obtained by no load test of an induction motor representative of ?
windage and friction loss
core loss only
both 1 and 2
Copper loss

No load loss consists of core loss, friction and windage loss and no load copper loss. But shunt branch resistance Rc in the induction motor equivalent circuit does not represent the no load losses, it represents only the core loss of the induction motor.

04․ In Ward-Leonard system, the lower limit of the speed imposed by
Field resistance.
Armature resistance.
Residual magnetism of the generator.
None of above.

In Ward-Leonard method of speed control, the lower limit of speed is imposed by residual magnetism of the generator.

05․ Ward-Leonard control is basically a ___________ control method.
Field control.
Armature resistance control.
Armature voltage control.
Field diverter control.

Ward-Leonard speed control method consists a motor with a constant excitation and applying a variable voltage to its armature to provide the required speed. Hence it is armature voltage control method.

06․ For very sensitive and wide speed control, the preferable control method is
Armature control.
Ward-Leonard control.
Multiple voltage control.
Field control.

In ward-Leonard method, very fine speed control over the whole range from zero to normal speed in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction.

07․ Eddy current loss will depends on
Flux density
All of the above

. So eddy current loss will depend upon frequency, flux density and the area of the eddy current loop.

08․ Hysteresis loss will depends on

. So, hysteresis loss will depends on frequency.

09․ Thin laminations are used in a machine in order to reduce
Eddy current losses
Hysteresis losses
Both 1 and 2
Copper losses

Thin laminations are used in order to reduce the eddy current losses only. Due to laminations the area of the eddy currents loops are minimized and the losses due to eddy current losses are minimized.

10․ Hysteresis loop represents the area of
copper loss
eddy current loss
hysteresis loss
total iron losses

Hysteresis loop will represents only hysteresis losses. It is found out by area of B-H loop curve of a magnetic material.