MCQs on Electrical Machines

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01․ In a DC Shunt Motor the speed can be increased by
Increasing the field current.
Decreasing the field current.
Increasing the field current.
Increasing the armature current.

In shunt motor the field current is practically constant. Speed can be increased by reduced the field flux hence by field current.

02․ The type of DC motor generally used for shears and punches is
Series motor.
Shunt motor.
Cummulative compound motor.
Differential compound motor.

Cummulative compound DC motor develop high starting torque and only fairly constant speed has pulsating load and only flywheel action. So this motor is best suited for shears and punches.

03․ If in DC Shunt Motor the flux is reduced suddenly to zero, then
the motor will come to stop.
the motor will attain dangerously high speed.
the speed of motor will be reduced.
the armature current will reduce.

If the field circuit is opened, the flux will drop to practically zero and therefore, the speed would increase to an extremely high value. The centrifugal force developed at the high speed may cause the commutator segments to buckle, resulting in damage to the motor.

04․ In case of a DC Shunt motor if the load is decreased to some extent, what will happen to its speed?
The speed will increase gradually.
The speed will increase abruptly.
The speed will remains almost constant.
The speed will increase in proportion to reduction in load.

In case of DC shunt motor, flux is practically constant as field is parallel to the armature. If load is reduced then the speed of motor will remain almost constant because the field current remains almost constant. Field current is Vt/Rsh.

05․ A DC series motor has which of the following characteristics?
Has a poor torque.
Has a almost constant speed.
Can be started easily without load.
Has its field winding consinting of thick wire and less no of turns.

DC series motor has high starting torque, it has wide range of speed. It can’t be started with no-load because then a heavy centrifugal force will be developed and the motor will get damaged. Its series field resistance has very low value which means series field consists of thick wire and less no. of turns.

06․ The speed of a DC series motor is
Proportional to the armature current.
Proportional to the square of the armature current.
Proportional to field current.
Inversely proportional to the armature current.

In case of DC series motor, . It is obvious that speed is proportional to back emf Eb and inversely proportional to flux per pole φ. Now flux per pole is directly proportional to armature current i.e. φ ∝ Ia because the armature and field are carrying same amount of current. So, speed is inversely proportional to the armature current.

07․ In a DC series motor, if the armature current is reduced by 50 %, the torque of the motor will be equal to
25 % of previous value.
50 % of previous value.
100 % of previously value.
None of above.

In case of DC series motor before magnetic saturation

08․ If a DC Series motor is connected across an AC supply it will
not run.
run at normal speed.
run at lower speed.

If a DC series motor is connected to an AC supply same current will flow through the series field and armature field after being rectified by the commutator. So, the machine will run but with high sparking at brush, poor pf, low speed and low efficiency.

09․ Consider a separately excited DC motor. If the field current is increased while the armature terminal voltage is held constant, then speed of the motor will
remains same.
none of these.

In case of separately excited DC motor, the field flux depends upon the field current, . Where, N = Speed and If = field current. If the field current increases, speed will decrease.

10․ A 220 V DC machine has an armature resistance 1 Ω. If the full load current is 20 A, the difference of induced voltage between generator and motor is
20 V.
0 V.
40 V.
60 V.

In case of generator, induced emf is . In case of motor, the induced emf or back emf is . The difference between induced voltage when the machine is running as a motor and generator will be .