MCQs on Electrical Machines

Page 66 of 114. Go to page
01․ Swinburnes test is applicable to
DC compound motor.
DC shunt motor.
DC series motor.
none of the above.

Swinburne's test is applicable to those machine in which the flux is practically constant, that is shunt machine.

02․ Which of these types of motor is used in elevators?
DC series motor.
DC compound motor.
DC shunt motor.
None of these.

In DC compound motor having high starting torque up to 450 % depending upon the degree of compounding. Speed regulation is varying up to 25 ~ 30 %. This is why those motors are used in elevators .

03․ Two identical loss less series motors connected in series across a dc supply voltage, runs at speed of N1 and N2, then ratio of their output power is
N12 : N22.
N1 : N2.
1 : 1.

Ia1 = Ia2, Ea1 ∝ N1, Ea2 ∝ N2. ⇒ P1 / P2 = Ea1Ia1 / Ea2Ia2 = N1 / N2.

04․ The dc motor which can provide zero speed regulation at full load without any controller, is
differential compound.
cumulative compound.

Differential compound dc motor has constant speed characteristics.

05․ DC Motor
DC Shunt Motor
AC single phase motor.
Induction motor.
Synchronous motor.

Because the low speed motors are much more expensive than higher speed motors of the same KW output rates.

06․ Which type of motors are preferred in lifts?
DC compound motors.
DC series motors.
DC shunt motors.
None of the above.

AC slip ring or DC compound motors are prefer for lifts. Shunt type commutator motors are preferred in case of single phase installation. The latest lift designs use 3-phase induction motors with variable frequency drive electronic controls. Some may also use a similar system with permanent magnet (brush less) DC motors.

07․ DC Motor
DC shunt motors.
Cage induction motor.
Synchronous motor.
Either A or B.

Lathes machines requires uniform torque. Hence squirrel cage induction motor or dc shunt motors are preferred.

08․ Counter emf of a DC motor
Exceeds supply voltage
Aids applied voltage.
Helps in energy conversion.
Regulates its armature voltage.

When the motor armature continue to rotate due to motor action, the armature conductors cut the magnetism flux and therefore emf are induced in them. This emf is known as counter emf or back emf. This back emf or counter emf makes the DC a self regulating machine i.e. it makes the DC motor to draw as much armature current as is just sufficient to develop the required load torque. Thus it helps in energy conversion.

09․ The armature of DC motor is laminated
To reduce the mass.
To reduce the inductor.
To reduce the eddy current loss.
To reduce the hysteresis loss.

The armature is built up in a cylindrical or drum shape high grade silicon steel in form of stamping or lamination. By using laminations, the path of eddy currents is cut into several units . So the laminations are reduced the eddy current loss.

10․ Width of a carbon brush should be equal to
Less than the width of commutator segment.
The width of the 1 to 2 commutator segment.
The width of the 2 to 3 commutator segment.
The width more than 3 commutator segment.

Width of carbon brush is equal to the width of the 2 to 3 commutator segment.