Fig: Parts of a synchronous motor
- When a synchronous motor is used for driving a varying load, then a condition known as hunting is produced. Hunting may also be caused if supply frequency is pulsating (as in the case of generators driven by reciprocating internal combustion engines).
- 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 α. 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 motor load, the motor is immediately pulled up or advanced to a new value of α corresponding to the new load. But in this process, the rotor overshoots and hence is again pulled back. In this way, the rotor starts oscillating (like a 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 as great as to throw the machine out of synchronism.
- To stop the build-up of these oscillations, dampers or damping grids (also known as squirrel-cage winding) are employed. These dampers consist of short circuited Cu bars embedded in the faces of the field poles of the motor (Fig).
- 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.