Hunting in Synchronous Motor

WHAT IS HUNTING?

When a Synchronous motor is connected to a varying load that mean load is not fixed, a condition known as HUNTING.  it's produced which may also occur if the supply frequency is pulsating. 

When synchronous motor is on load, its rotor falls back i phase by a coupling angle δ delta. As the load increased progressively this angle also increases so as produce more torque., to follow with the increased load. If load sudden decreased the motor immediately pulled up or advance to a new value corresponding to a new load. But in this process rotor overshoots, Hence it is again pulled back. In this way rotor starts oscillating about its new position of equilibrium corresponding to new load. If the time period of this oscillation happens to the equal natural time period of the machine the mechanical resonance (Mechanical Resonance is the tendency of a mechanical system to respond at greater amplitude) is set up. The amplitude of these oscillations is built up to a large value and may eventually become so great that the machine is thrown out of synchronism. To stop the build up of these oscillation, circuit damper are employed which consist of short copper bars embedded in the faces of the field poles of the motor. The oscillation of the rotor sets up eddy currents in the dampers which flow in such a way so as to suppress the oscillation.