- When the synchronous motor is driven for variable load such as shears, punching press, compressors etc., its rotor falls back or advance by some angle a .
- When the load on the motor increases, its rotor falls back by some angle a.
- Similarly when the load on the motor decreases, its rotor advances by some angle a .
- The rotor falls back or advances according to nature of the load. The
oscillation of rotor about its main axis is called as Hunting.
- The rotor starts overshoot and pulled back due to variation of load.
- The rotor starts oscillating about its new position corresponding to new load.
- If the time period of the oscillation is equal to natural time period of the machine, the mechanical resonance set up.
- The amplitude of these oscillations is built up such that machine may out of synchronism.
Function of
Damper winding
- The rotor oscillation may dampen out by employing damper winding in the faces of field poles of the motor.
- The damper winding is nothing but copper bar embedded in the rotor faces. The copper bars are short circuited at both ends.
- The motion of rotor sets up eddy current in the damper winding.
- The direction of this eddy current is such that it suppresses the rotor oscillations.
- The damper winding does not prevent completely oscillation but reduce to some extent.
- The function of the damper winding in the synchronous generator is to suppress the negative sequence field and to dampen oscillation whereas it is used to provide starting torque and reduce effect of hunting to some extent in the synchronous motor.
Causes of
hunting
- Sudden change of load
- Sudden change in field current
- Variation of load torque
Loss of Synchronism
- Produces mechanical stress on the shaft
- Cause temperature rise
- Produces resonance condition
Reduction of hunting
- Using flywheel
- Using damper winding