MCQs on Electrical Machines

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01․ For 250 volt DC Shunt Motor the armature and field resistance are 0.5 ohm and 250 ohm respectively. What will be the back e.m.f. produced when it takes a load current of 21A.
220 V.
150 V.
240 V.
280 V.

02․ On which of the following factor/factors the speed of a DC motor depends upon?
Applied voltage.
Field flux.
Armature current.
All of these.

From e.m.f. equation, Speed N proportional to . So speed of the DC motor depends upon all the above factors.

03․ The speed of a motor falls from 1100 r.p.m at no-load to 1050 r.p.m at rated load. The speed regulation of motor is
2.56 %.
2.57 %.

04․ Swinburne's test is applicable to
DC compound motor.
DC shunt and level Compound motor.
DC series motor.
DC Shunt motor.

Swinburne's test is an indirect method of testing which measures the no-load losses. DC series motor gives dangerous high speed at no-load, so this test can’t be applied to this machine. It is applicable to those machines which has constant flux and speed. So this test is applicable to both shunt and level compound motor.

05․ 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.

06․ A 250 volt dc series motor having an armature resistance of 0.2 Ω takes a load current of 60 A. What will be the torque produced at the shaft of the motor? Consider brush drop of 1V.
133.22 N-m
234.76 N-m.
266.20 N-m.
can not be determined.

07․ 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.

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

DC shunt motor has constant speed characteristics as the flux practically remains constant from no-load to full-load.

09․ Which of the following motor is more expensive for same KW output rating ?
DC Shunt Motor
AC single phase motor.
Induction motor.
Synchronous motor.

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

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

AC slip ring or DC series motors are preferred for lifts. 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.