MCQs on Electrical Machines

Page 34 of 114. Go to page
01․ If the full load speed of 3-phase, 50Hz induction motor is 950 rpm, what is its half load speed nearly equals to?
975 rpm
950 rpm
940 rpm
1000 rpm

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

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

03․ Which of the following motor is expensive for same Kw output rates?
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. DC machines has more losses, so for same output the size is also greater.

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

Shunt dc motor has constant speed characteristics so it can provide zero speed regulation.

05․ 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.

As they are loss less and connected in series they have same current through them

06․ A 4-pole DC generator runs at 1500 rpm. The frequency of current in the armature winding is:
25 Hz.
50 Hz.
100 Hz.
Zero Hz.

In a DC generator, the current developed in the armature is AC. The frequency of this current is given by:

07․ A separately excited DC motor has an armature resistance of 0.5 Ohm. The supply voltage is 250V. Armature current is 20A and the speed of rotation is 1500 rpm. Find the torque developed?
15.6 Nm
15.28 Nm
16.55 Nm

We know torque developed in separately excited DC motor is given as:

08․ Find the developed torque DC Series Motor having armature resistance of 0.2 Ω and taking armature current of 60 A is connected to 250 V supply. Considered brush drop of 1 V each.
133.22 N-m
234.76 N-m.
266.20 N-m.
cant be determined.

09․ 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 %.
3.76 %.
4.76 %.

10․ On what factors the speed of dc motor depends?
Applied voltage.
Field flux.
Armature current Ia.
All of these.