Oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in synchronous motor.
Causes for Hunting:
i) Sudden change in load.
ii) Sudden change in supply system
iii) Sudden change in field system (excitation system).
iv) Load containing harmonic torque.
Elimination of Hunting:
1. By using the fly wheel.
2. By designing the machine having suitable synchronizing power coefficient.
3. By using damper winding.
When the rotor speed is less than synchronous speed, induction motor torque is developed in the same direction of rotor rotation then the rotor will accelerate to reach synchronous speed.
When the rotor speed is more than synchronous speed, induction generator torque is developed in the opposite direction of rotor rotation then the rotor will decelerate to reach synchronous speed.
When rotor rotates at synchronous speed, then there is no relative speed between armature flux and damper winding. Therefore no induced emf and no current in damper winding and hence no torque. The machine is said to be under steady state condition.
The damping torque will comes into picture when the machine is subjected to disturbance. It exist as long as disturbance exist. Once the disturbance is cleared the damping torque will disappear (reduces to zero).