MCQs on Electrical Machines

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01․ Rotating part of DC motor is known as
Carbone brush.

Armature is rotating part of a DC motor and is built up in a cylindrical or drum shape.

02․ Speed of DC motor can be controlled by varying
Applied voltage.
Its flux oer pole.
Resistance of armature culture.
All of these.

Speed of a DC motor is expressed as \/ So, speed can be controlled by adjusting any one of the three factors appearing on right-hand side of expression - (i) applied voltage to the armature circuit R and (ii) flux per pole φ.

03․ Eb/V ratio of a DC motor is an indication of
Speed regulation.
Starting torque.
Running Torque.

Electrical motor efficiency is defined that the ratio of mechanical power developed to the total electrical power input. In case of DC motor total electrical power input = V X IL. Where V is supply voltage and IL = Load current. The mechanical power developed = Ta X φ. Where Ta = Torque in armature into mechanical power in armature = Eb X Ia. So efficiency = EbIa/VIL.

04․ DC Motor
DC Shunt Motor
Reduce slightly.
Remains constant.
Increase proportional.

In case of DC shunt motor, the flux per pole is considered to be constant, torque increase with the increase in load current. If the load current increase then the armature current also be increase then the armature current the speed slightly falls due to increase in voltage drop in armature.

05․ A large series motor is never started without some mechanical on it because otherwise it will
Produce sparking at brushes.
Open fuse or circuit breaker.
Draw too much current.
Develop excessive speed and damage itself.

When the motor is connected across the supply mains without load, it draws small current from the supply mains flowing through the series field and armature, the speed tends to increase so that back emf may approach the applied voltage in magnitude. The increase in back emf weakens the armature current and hence the field current. This cause again increase in speed so in back emf. Thus the field continues to weaken and speed continues increase until the armature gets damaged.

06․ Which of following winding is connected in series with armature winding of DC machine?
Auxiliary winding.
Compensating winding.
Commutating pole winding.
Series winding.

Commutating pole or interpole winding is connected service with armature winding. So that they carry full armature current and they induce an emf in the coil (under commutation) which helps the reversal of current. This emf is proportional to the armature current. This ensure automatic neutralize of reactance voltage which is also due to armature current.

07․ Direction of rotation of DC motor is reversed by
Reversing supply connection.
Adding resistance to field circuit.
Interchanging armature and field connection.
Reversing armature connection.

The direction of rotation of a motor can be reversed by reversing the current through either the armature winding or the field coils. If the current through both is reversed, the motor will continue to rotate in same direction as before.

08․ If the field winding connection of DC shunt motor is changed
motor will become slow.
motor will not run.
It will run in opposite direction.
It will run in same direction.

If the field winding connection of DC shunt motor is changed then the motor will run in opposite direction.

09․ With the increase in speed of DC motor
Back emf increase but line current falls.
Back emf falls and line current increase.
Both back emf as well as line current increase.
Both back emf as well as line current fall.

In case of DC motor, the speed is proportional to the back emf. So with the increase in speed, the back emf also be increased. Therefore armature current is also small. In case of motor, armature current is equal to the line or load current.

10․ If the back emf in DC motor vanishes suddenly the motor will
Run at very high speed.
Run at very low speed.
Start haunting.

If the back emf in DC motor vanishes suddenly then the armature current is larger flow through the armature. The larger armature current produces high electromagnetic torque which is not equal to load torque.