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.
Function of Damper Winding in Synchronous Motor
The damper winding in synchronous machine 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 in synchronous machine 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 in synchronous motor.
Due to hunting
- Due to hunting, the value of δ changes continuously.
- Due to change in δ, the back EMF Ebchanges which forces the armature current Ia to change continuously.
- This continuously varying armature current will cause problems to the other appliances connected on the same AC line.
- If the load changes are frequent then the rotor swings about its new position due to inertial.
- If the frequency of osculations match with the natural frequency of the motor then the amplitude of rotor swings increases and the motor may be thrown out of synchronization.
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.
The time taken by the rotor to reach its final equilibrium position after hunting is known as ‘settling time’. It should be as short as possible. The use of damper winding in synchronous motor reduces its settling time considerably.
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.
Thanks for reading about damper winding in synchronous machine.
1. In a synchronous motor, damper winding is provided in order to
2. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
3. An electric motor in which both the rotor and stator fields rotates with the same speed is called a/an ........motor.
4. While running, a synchronous motor is compelled to run at synchronous speed because of
5. The direction of rotation of a synchronous motor can be reversed by reversing
6. When running under no-load condition and with normal excitation, armature current Ia drawn by a synchronous motor
7. The angle between the synchronously-rotating stator flux and rotor poles of a synchronous motor is called........ angle.
8. If load angle of a 4-pole synchronous motor is 8º (elect), its value in mechanical degrees is
9. The maximum value of torque angle a in a synchronous motor is ........degrees electrical.
10. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in its
11. When load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because
12. When load on a normally-excited synchronous motor is increased, its power factor tends to
13. The effect of increasing load on a synchronous motor running with normal excitation is to
14. Synchronous capacitor is
15. If the field of a synchronous motor is underexcited, the power factor will be
16. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is decreased from its normal value, it leads to
17. A synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity p.f. On changing the excitation only, the armature current will have
18. The V-curves of a synchronous motor show relationship between
19. When load on a synchronous motor is increased, its armature currents is increased provided it is
20. If main field current of a salient-pole synchronous motor fed from an infinite bus and running at no-load is reduced to zero, it would
21. In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop
22. In a synchronous motor, the rotor Cu losses are met by
23. A synchronous machine is called a doubly-excited machine because
- Parts of Synchronous Generator
- Synchronization of Alternators
- Synchronous Motor Working and Construction
- Methods of Starting of Synchronous Motor
- Damper Winding & Hunting in Synchronous Motor
- Over Excited Synchronous Motor
- Advantage and Disadvantage of Synchronous Motor
- Compare Synchronous Motor and Induction Motor