“Hunting is the phenomenon of the synchronous machines which oscillates at about its new equilibrium position” which means the synchronous machines rotate at other than synchronous speed.

Use of Damper winding to prevent hunting

• Hunting can be reduced by using damper winding.
• When hunting occurs, the difference in the speed of stator and rotor poles develops and induces emf in the damper winding, which acts in such a way to suppress the rotor oscillation.
• When the rotation at constant load is uniform, there is no relative motion between the rotor and stator forward rotating fields and hence no current is induced in these windings.
• When the rotor speed is more than synchronous speed, induction generator torque will be produced through damper bars in the opposite direction to synchronous machine torque such that the above synchronous speed becomes nearer to synchronous speed. The relative motion of the rotor sets up eddy currents in these windings which flow such as to suppress the oscillations (as per Lenz’s Law).
• When the rotor speed is less than synchronous speed, induction motor torque will be produced through damper bars in the same direction of synchronous machine torque such that below synchronous speed becomes nearer to synchronous speed.
• The dampers should have low resistance . be more effective. However, this method cannot eliminate – completely.

Use of Damper winding to start synchronous motor

• Along with the main field winding, an additional compensating winding consisting of copper bars is placed in the slots and these bars are short-circuited at its end rings. These windings are called damper windings.
• The short-circuited winding acts as a squirrel-cage rotor winding of an induction motor.
• Upon excitation of the stator by a three-phase supply, the motor starts rotating as an induction motor at sub-synchronous speed and the DC supply is excited to the field winding.
• Once the field winding is excited, the motor is pulled into synchronism and starts rotating at its synchronous speed. During this condition, the relative motion between the damper winding and the rotating magnetic field becomes zero.
• Now, the motor behaves like a synchronous motor and it does not induce an emf in the damper winding.
• Therefore, at the time of starting, the damper winding is used to run the motor as an induction motor, and afterward, it goes out of the circuit.
• While starting a synchronous motor as an induction motor, the field winding is short-circuited by low resistance in this method. The number of poles in the Induction motor is less than the number of poles in the synchronous motor.

For synchronous Motor MCQ Click Here