Correct Answer - Option 4 : starting torque in synchronous motor and to prevent the hunting in generators
Explanation:
In large alternators, damper windings are used to
- Reduce the vibrations (hunting)
- To increase the stability i.e. the voltage fluctuations will get reduced
The damper winding in synchronous motor performs two functions:
- Prevents hunting (damped out oscillations)
-
Provides starting torque (made self-starting)
Note: Under normal running conditions, damper winding does not carry any current.
Hunting is the phenomenon that occurs in synchronous motors due to varying load or supply frequency.
Causes of hunting
- Periodic variation of load
- Sudden changes in load
- Faults occurring in the system when supplied by the generator
- Sudden change in the field current
- Cyclic variations of the load torque
Effects of hunting
- It can lead to loss of synchronism
- It can cause variations of the supply voltage producing undesirable lamp flicker
- The possibility of resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place
- Large mechanical stresses may develop in the rotor shaft
- The machine losses increases and the temperature of the machine rises
