Answer / iamdon

In synchronous machines, the damper winding is placed on
pole face of rotor which is short circuited at end.

When load on machine is suddenly changed, the rotor starts
oscillating (due to its inertia )about the new equilibrium
point.However the oscillation finally dies out because of
presence of damper winding.The oscillation is called
hunting.

During hunting,there is a relative motion between rotor
(damper winding)and synchronously rotating magnetic field.
So emf induces in the damper winding as per Faraday's law
and current flows in accordance with Lenz's law as long as
the relative motion exists.As we know,the effect will
always oppose the cause.Here effect is induced current in
damper winding and cause is relative motion.So induced
torque in damper winding acts in opposition to
instantaneous relative motion.

Author:arbelectbit@yahoo.com

Answer / nidhi sharma

When a synchronous motor is used for driving a varying load, then a condition known as hunting may occur. Hunting may also be caused if supply frequency is pulsating (as in the case of generators driven by reciprocating internal combustion engines).

We know that when a synchronous motor is loaded (such as punch presses, shears, compressors and pumps etc.), its rotor falls back in phase by the coupling angle \alpha. As load is progressively increased, this angle also increases so as to produce more torque for coping with the increased load. If now, there is sudden decrease in the load, the motor is immediately pulled up or advanced to a new value of \alpha corresponding to the new load. But in this process, the rotor overshoots and hence hence is again pulled back. In this way, the rotor starts oscillating (like pendulum) about its new position of equilibrium corresponding to the new load. If the time period of these oscillations happens to be equal to the natural time period of the machine, then mechanical resonance is set up. The amplitude of these oscillations is built up to a large value and may eventually become so great as to throw the machine out of synchronization. To stop the build up of these oscillations, dampers or damping grids are employed. These dampers consists of short circuited Copper bars embedded in the faces of the field poles of the motor. The oscillatory motion of the rotor sets up eddy currents in the dampers which flow in such a way as to suppress these oscillations.

But it should be clearly understood that dampers do not completely prevent hunting because their operation depends upon the presence of some oscillatory motion. However, they serve the additional purpose of making the synchronous motor self-starting.

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