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 motor performs two functions:
- provides starting torque and
- prevents hunting in synchronous motor
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 motor” and “hunting in synchronous motor”.
Alternators – 4 | Objective Type Question Answers
#1 Overheating of generator’s winding
reduces life of the machine
#2 The maximum current that can be supplied by an alternator depends on
speed of the exciter
#3 The regulation of an alternator is likely to be negative in case of
leading power factor of the load
#4 The regulation of an alternator is
the increase in terminal voltage when load is thrown off
#5 An alternator driven by a Francis hydraulic turbine is a ………..alternator.
low or medium speed
#6 If two alternators are running in proper synchronism and the voltage of one machine is suddenly increased
synchronizing torque will be produced to restore further synchronism
#7 If the steam supply of an alternator running in parallel with another identical alternator is increased keeping its excitation constant, then
it will supply greater portion of the load
#8 An exciter for a generator is a
#9 Two alternators ‘1’ and ‘2’ sharing an inductive load equally. If the excitation of alternator ‘1’ is increased
the alternator ‘2’ will deliver less current and alternator ‘1’ will deliver more current
#10 Which of the following coils in an alternator will have e.m.f. closer to sine waveform ?
Distributed winding in short pitch coils
#11 The distribution factor, in alternators, is defined as the ratio of e.m.fs. of
distributed winding to concentrated winding
#12 As load power factor of an alternator becomes more leading the value of generated voltage required to give rated terminal voltage
#13 In an alternator,theflux created by the armature m.m.f. subtracts directly from the main flux for the following conditions of the load
load power factor is zero lagging
#14 Synchronous impedance method of finding voltage regulation of an alternator is called pessimistic method because
it gives regulation value higher than is actually found by direct loading
#15 Two alternators are running in parallel. If the field of one of the alternators is adjusted it will
change its power factor
#16 In a synchronous machine. If the field flux axis is ahead of the armature field axis, in the direction of rotation, the machine is working as
#17 The advantage of salient poles in an alternator is
adaptability of low and medium speed operation
#18 . For parallel operation of the two alternators, desirable feature is that both should have
less resistance as compared to synchronous reactance
#19 If two alternators are running in parallel and the excitation of one of the alternators is increased, then
wattless component will change
#20 When an alternator is supplying unity power factor load, the armature reaction will produce
distortion of the main field
Do not forget to click the “finish” button to see the correct answers and result.
- 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