Interview Questions - Electrical Machines

When a slip ring induction motor will run at super synchronous speed?
A slip ring induction motor run at super synchronous speed when an emf is injected in the rotor circuit in phase with the rotor induced emf.

How starting torque can be obtained in the case of a single-phase induction motor with identical main and auxiliary windings?
Starting torque can be obtained in the case of a single-phase induction motor with identical main and auxiliary windings by connecting a capacitor in series with the auxiliary winding.

What will be the power factor if the field of a synchronous motor is under excited?
The power factor will be lagging.

Why damper bars in case of salient pole rotors of hydro-alternators are usually interested in pole faces?
Damper bars in case of salient pole rotors of hydro-alternators are usually interested in pole faces to damp out the rotor oscillations during transient state owing to a sudden change in load conditions. The damper winding comes into play only during rotor hunting, when rotor speed departs from synchronous speed.

What do you mean by switched reluctance motor?
Switched reluctance motor means a stepper motor with closed-loop control and with a rotor position sensor.

What is the use of a capacitor in a single-phase induction motor?
The capacitor is used to introduce a phase difference of 90 degrees between the auxiliary and main winding to start the motor. Single-phase induction motor is not self-started.

What is the main advantage of distributing the winding in slots?
The main advantage of distributing the winding in slots is to reduce the harmonics in the generated emf wave, thus making it approach a sine wave, the other two advantages are -

  • Full utilization of the armature iron and copper.
  • Adding rigidity and mechanical strength to the winding.

What will be the effect on starting torque and efficiency of a slip ring induction motors, if the rotor resistance is increased?
For a slip ring induction motor, if the rotor resistance is increased, then starting torque increases but efficiency decreases.

Which type of alternator is used in hydro-electric power stations?
Salient pole alternator is used in hydro-electric power stations.

Where capacitor run motors are generally used?
These motors are used where the required starting torque is low such as air- moving equipment i.e. fan, blower and voltage regulators and also oil burners where quite an operation is particularly desirable.

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