MCQs on Electrical Machines

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01․ In a DC motor under constant terminal voltage what is the relation between torque (Te) and power (P)
T∝ P
T∝ P²
T∝ P³
T is independent of P

where they have their usual meaning.

02․ Slot wedges in a dc machine is made up of
cast iron.
silicon steel.
mild steel.

Slot wedges are used to reduce the air gap flux to reduce armature reaction. So, made up of silicon steel.

03․ For P pole machine relation between electrical degrees and mechanical degrees is
θelec = 2/P*θmech
θelec = P/2*θmech
θelec = θmech
θelec = 4/P*θmech

One mechanical rotation covers P/2 electrical cycles.

04․ The air gap between the yoke and armature is dc machine is kept very small
to avoid locking of the armature.
to avoid over heating.
to achieve a stronger magnetic field.
all of the above.

To achieve stronger magnetic field created by the field poles, the air gap between the yoke and armature is kept small in rotating machines. Air gap is minimum at the center of the yoke section.

05․ A 4 pole dc generator is running at 1500 rpm the frequency of current in the armature winding is

The current produced in the armature winding of a DC machine is alternating in nature.

06․ A triangular mmf wave is produced in the air gap of an electric machine. Such wave is produced by
rotor of an synchronous machine.
stator of a induction machine.
rotor of a dc machine.
stator of a dc machine.

In dc machine, armature winding is placed in rotor and it has triangular mmf wave.

07․ Which of the following motor has negative speed regulation?
Shunt motor
Series motor
Cumulative compound motors
Differential compound motor

In Differential compound motor the series flux opposes the shunt field flux. As load increases, flux will decrease for differential compound motor. Therefore speed will increase and the speed regulation will be negative.

08․ A commutator in dc machine provide ----------------- rectification?
half wave
full wave
half wave controlled
full wave controlled

Commutator in DC machine is used to convert AC voltages into DC voltages.

09․ In dc machine armature windings are placed on rotor because of the necessity for
electromechanical energy conversion.
generation of voltage.
development of torque.

The armature winding of a DC machine is placed on the rotor to facilitate commutation.

10․ Dynamic braking can be used for
shunt motors.
series motors.
compound motors.
all of the above.

Dynamic braking can be used for all types of motors.