MCQs on Electrical Machines

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01․ Total core loss is also called as -------------?
Eddy current loss
Hysteresis loss
Magnetic loss
Copper loss

As iron loss is proportional to flux density or flux, it is also called magnetic loss. The total core loss or magnetic loss consists of eddy current loss and hysteresis loss.

02․ Which of the following are variable losses?
eddy current loss
hysteresis loss
shunt field copper loss
armature copper loss

Armature copper loss is directly proportional to square of armature current. Therefore as the load varies it will also vary.

03․ Maximum efficiency will occur, when copper loss is_______to iron loss?
greater than
less than
equal to
any of the above

Condition for maximum efficiency is, copper loss = iron loss or variable losses is equal to fixed losses.

04․ Swinburne test is conducted under which of the following condition?
no load
full load
half load
any of the above

Swinburne test is conducted under no load condition to find the no load loss or constant loss. It can't be done on DC series machine.

05․ Hopkinson test is conducted under which of the following condition?
no load
full load
half load
any of the above

This test is also called as back to back test or regenerative test. It requires two identical DC shunt or compound machines. One acts as a motor and another acts a generator. Both the machines are coupled mechanically as well as electrically. Therefore this test is conducted at full load condition. The main power supplies compensate the internal losses of the two machines.

06․ The current drawn by the a 230 V DC motor of armature resistance 0.5 Ω and back emf 200 V is

07․ Which of the following methods are used to control the speed of DC motors is
field current control
armature circuit resistance control
supply voltage control
all of the above

Field flux control, armature circuit resistance or rheostat control, supply voltage or Ward Leonard control are the speed control methods of DC motors.

08․ A DC series motor is accidentally connected to single phase ac supply voltage. The torque produced will be
pulsating and unidirectional
steady and unidirectional
none of the above

In DC series motor, the AC currents through the field and armature winding will be unidirectional by commutator and brushes. So torque will be unidirectional but pulsating due to AC.

09․ What is the increase in the torque expressed as percentage of initial torque, if current drawn by the dc series motor is increased from 10A to 11A?

10․ A flat compound generator is obtained from compound generator by connecting a variable resistor
in series with the series field
in parallel with the series field
in series with the shunt field
in parallel with the shunt field

Variable resistor in parallel with the series field winding is called diverter. A flat compound generator has constant terminal voltage characteristics.