Correct Answer - Option 1 : Prevent hunting
Oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in synchronous machine.
Causes for Hunting:
- Sudden change in load.
- Sudden change in the supply system or in the field system.
- Load containing harmonic torque.
Methods for eliminating hunting:
- By designing the machine with a suitable synchronizing power coefficient.
- By using the fly wheel.
- By using damper winding.
- Damper winding is made with low resistance copper, aluminum, or brass.
- They are inserted in the slots made under the pole shoes.
With respect to damper winding, the rotor behaves like a squirrel cage rotor of an induction motor.
Functions of damper winding:
- In alternator to eliminate hunting and to suppress the negative sequence field.
- In synchronous motor to eliminate hunting and for starting purpose.
- When the rotor speed is less than synchronous speed, the inductor motor torque is developed in the same direction of the rotor rotation then the rotor will accelerate to reach synchronous speed.
- When the rotor speed is more than synchronous speed (super synchronous speed), the induction generator torque is developed in the opposite direction of rotor rotation then the rotor will decelerate to reach synchronous speed.
- When the rotor rotates at the synchronous speed, then there is no relative speed between armature flux and damper winding. Therefore no induced emf and no current in the damper winding and hence no torque. The machine is said to be under steady-state condition.