MCQs on Electrical Machines

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01․ If an additional resistance is connected in series with a DC motor operating at rated speed, the speed of the motor will
Remain unaltered.
Oscillate around the normal speed.

In DC series motor, with increase in resistance of armature circuit increases the voltage drop in armature circuit and series field increases and therefore back emf dcreases. Hence speed of DC series motor will reduce to insert resistance in armature circuit.

02․ Field divertor method of speed control of a DC series motor gives speed above rated one due to reduction of
Line current.
Field current.
Armature current.
None of above.

Field divertor reduces the current flowing through the field winding in series motor. Hence reduces the flux and increases the speed.

03․ The field divertor resistance for a DC series motor is of order of
100 Ω.
50 Ω.
10 Ω.
0.1 Ω.

In series motor, lesser the divertor resistance, less the field current, less flux and therefore more the speed.

04․ For a given torque, reducing divertor resistance of a DC series motor
Decrease in speed demanding less armature current.
Decrease in speed but armature current remains the same.
Increase in speed but armature currnet remains the same.
Increase in speed demanding more armature current.

In DC series motor, reducing divertor resistnace decrease the current and flux and also increse the armature current and therefore more speed.

05․ The slip of an induction motor during DC rheostatic braking is
2 - s.
1 - s.
2 + s.

The rheostatic braking with a poly phase induction motor can be obtained by disconnecting the stator winding from the AC supply and exciting it from a DC source to produce a stationary DC field. In rheostatic braking, the stator winding is employed as a DC field winding and rotor winding as an armature winding. While the machine is operating normally as a motor, its stator magnetic field is rotating at a synchronous speed in the same direction as that of the rotor, but slightly faster than the rotor conductors. When the stator windings are disconnected from the AC supply and excited with DC, the magnetic field produced will be stationary in space, thus making the rotor conductors move past the field with a speed ( 1 - s )Ns or sNs. Here slip is,

06․ A 3 - Φ induction motor draws an current of 50 A from mains when starts by direct switching. If an auto transformer with 60 % tapping is used for starting the current drawn from the mains will be


07․ The power input to a 3 - φ induction motor is 60 kW and stator loss is 1 kW the rotor cu loss per phase is
slip X 50 / 3.
slip X 59 / 3.
slip X 69 / 3.

Given stator input = 60 kW, stator loss = 1 kW. Rotor input = 60 - 1 = 59 kW. Rotor loss = slip X 59. ⇒ Rotor loss / phase = slip X 59 / 3.

08․ Hunting in a synchronous motor takes place
when load varies.
when supply voltage fluctuates.
when power factor is unity.
motor is under loaded.

Under construction.

09․ The oscillation in a synchronous motor can be damped out by
running the motor on leading power factors.
providing damper bars in the rotor pole faces.
oscillation cannot be damped.
maintaining constant excitation.

Under construction.

10․ When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop
synchronous motor torque.
induction motor torque.
induction generator torque.
DC motor torque.

Under construction.