damper winding in synchronous motor

 
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:

damper winding in synchronous machine

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.

hunting in synchronous motor

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”.
 
 

Synchronous Motor Objective Type question Answers | MCQ

 

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#1 The maximum power developed in the synchronous motor will depend on

rotor excitation, maximum value of coupling angle and supply voltage.

#2 A synchronous motor switched on to supply with its field winding shorted on themselves. It will

start as an induction motor

#3 The back EMF set up in the stator of a synchronous motor will depend upon

rotor excitation only

#4 With the increase in the excitation current of synchronous motor the power factor of the motor will

improve

#5 The armature current of a synchronus motor has large values for

both low and high excitation

#6 A 3 phse 400 V, 50 Hz salient pole synchronus motor is fed from an infinite bus bars and is running at no load. Now if the field current of the motor is reduced to zero

the motor will run at synchronous speed

#7 A 3 phase, 400 V, 50 Hz 4 pole synchronous motor has a load angle of 10 degrees electrical. The equivallent mechnical degrees will be

5 degrees

#8 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly

#9 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly

#10 The break down torque of synchronous motor varies as

applied voltage

#11 The name plate of an induction motor reads 3 phase, 400 V, pf 0.8 lagging, 1440 RPM. On similar lines a name plate of synchronous motor should read

3 Phase, 400 V, 50 Hz, 0.8 pf leading, 1500 RPM

#12 In a 3 phase synchronous motor, the magnitude of field flux

remains constant at all loads

#13 A four pole synchronous machine has 48 slots. A coil having one coil side in slot number 1 and the other coil side in slot number 13 will be termed as

full pitch coil

#14 A three phase synchronous motor is running clockwise. In case, the direction of its field current is reversed

the motor will continue to run in the same direction

#15 In a synchronous motor out of the following losses, which one will have the highest proportion ?

Iron losses

#16 When a synchronous motor is connected to an infinite bus, while operating on leading power factor

the excitation voltage will be more than the supply voltage

#17 In a synchronous motor hunting can be minimised

by any of the above method

#18 While starting a synchronus motor by induction motor action, very high EMF is induced in the field. This induced EMF may damage the insulation of the field winding and of the slip rings. This insulation damage can be prevented by

either short-circuiting the field winding by field discharge resistance or splitting the field winding into several sections

#19 Synchronous motors are gererally of

salient pole type machines

#20 The fact that a synchronous motor with salient poles will operate, even if the field current is reduced to zero, can be explained by

magnetization of rotor poles by stator magnetic field

#21 Hunting of a synchronous motor may be due to

any of the above.

finish

Synchronous Machines | All Posts

 

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