Synchronous motors have their pole-shoes slotted for placing copper bars. The copper bars are placed in these slots and short-circuited at both ends by heavy copper rings (like squirrel cage rotor of induction motors).
This arrangement is known as damper winding in synchronous motor. The damper winding in synchronous motor performs two functions:
- provides starting torque and
- prevents hunting
When the rotor is rotating at synchronous speed, then the relative velocity between the RMF (rotating magnetic field of stator) and the rotor is zero. Hence induced EMF in the damper winding is zero.
Thus, under normal running conditions, damper winding does not carry any current.
Hunting in Synchronous Motor
The stator and the rotor poles of running synchronous motor are magnetically locked and, hence, both run with same synchronous speed. But centerlines of the two poles do not coincide with each other. The rotor slips back behind the stator poles by a small angle δ.
This angle is known as load angle or torque angle. This backward shift of rotor is essential for developing motor torque.
As the load on the motor is increased, the backward shift of rotor poles increases by a larger angle but rotor poles still continue to run synchronously. The value of load angle δ depends on the load carried by the motor. This load angle also controls stator current.
Greater will be the value of δ, higher will be the value of stator armature current. It is so because motor needs more input power to carry the increased load. It too much load is put on a synchronous motor, the rotor will be pulled out of synchronism after which it will stop.
It the load on the motor is increased gradually, then the rotor goes to new position smoothly. But if the motor load is changed suddenly then the rotor cannot its new position corresponding to the new value of δ smoothly. Due to the inertia of the rotor, rotor oscillates around its correct position for some time before adjusting to the correct position. This is known as hunting of synchronous motor.
Due to hunting
- value of δ changes continuously
- stator armature current changes continuously
- rotor swings about its new position due to inertia
- motor may be thrown out of synchronism
The damper winding in synchronous motor plays a very important role in hunting. When rotor oscillates the relative motion between RMF and rotor becomes nonzero. Hence an EMF is induced proportional to relative motion in damper winding.
This induced EMF is in such a direction that it will try to oppose the cause of it (Lenz’s law). Here the cause is relative motion due to hunting. Hence the hunting reduces quickly due to damper winding.
Damper Winding in Synchronous Generator
The hunting can also occur in a synchronous generator. In this case also, due to sudden change in electrical output or mechanical input, oscillations are set up in the rotor called hunting, which can be prevented by providing damper winding in synchronous generator.
- Parts of Synchronous Generator
- Synchronization of Alternators
- Synchronous Motor Working and Construction
- Methods of Starting of Synchronous Motor
- Damper Winding in Synchronous Motor
- Over Excited Synchronous Motor
- Advantages and Disadvantages of Synchronous Motor
- Compare Synchronous Motor and Induction Motor