What is Hunting in Synchronous Motor ?

Hunting ( or ) Surging ( or ) Phase Swinging :

     When synchronous motor is loaded, there exists a phase angle between rotor and field ' α '. When load increases slowly, angle ' α ' also increase so that torque produced is more. 

     When this load on the motor suddenly falls then angle ' α ' also changes accordingly to new load. But in this process, rotor overshoots. Due to this sudden change in load rotor starts oscillating like a pendulum about its new equilibrium position. This oscillations make the  motor to cause 'hunting' for the right position. This is also called as phase swinging ( or ) surging. 

     If this frequency of oscillations is equal to the natural frequency then mechanical resonance is setup and the amplitude of the swing of the rotor poles relative to the poles of rotating field becomes so great that the machine thrown out of synchronism.

     Hunting is an objectionable characteristics of synchronous motors, as it produces severe mechanical stresses as well as large variations in current and power drawn by the motor.

Causes of Hunting :

(i) Driving a motor for varying load.

(ii) If the frequency is pulsating.

(iii) If there is any change in field current.

(iv) Faults in supply given.

Prevention of Hunting :

Use of Damper Winding :

     Hunting is prevented by providing the damper winding also known as squirrel cage winding ( or ) amortissure winding ( or ) damper grids.

     The damper winding consists of copper bars embedded in the faces of the field poles and short circuited at both ends by means of end rings.

Damper winding

      When the motor is operating at exactly synchronous speed. there is no relative motion between the rotor and the stator rotating field and so no current is induced in this winding. But when hunting takes place, there is a relative motion exists and therefore eddy currents are set up in this winding which flows in such a way to suppress or damps out the oscillations.

     The dampers can not prevent hunting completely because their operation depends upon the presence of oscillating motion. However, the damper winding also serves to make the synchronous motor self starting.

Use of Flywheels : 

     In this the prime mover is provided with a weighted device known as flywheel. This weight increases the inertia and stability in the position of prime mover which helps in maintaining the rotor speed constant.

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