Mechanism of Polarization
Hunting in Synchronous Motor
What is Hunting?Unloaded synchronous machine has zero degree load angle. On increasing the shaft load gradually load angle will increase. Let us consider that load P1 is applied suddenly to unloaded machine shaft so machine will slow down momentarily.
Also load angle (δ) increases from zero degree and becomes δ1. During the first swing electrical power developed is equal to mechanical load P1. Equilibrium is not established so rotor swings further. Load angle exceeds δ1 and becomes δ2. Now electrical power generated is greater than the previous one. Rotor attains synchronous speed. But it does not stay in synchronous speed and it will continue to increase beyond synchronous speed. As a result of rotor acceleration above synchronous speed the load angle decreases. So once again no equilibrium is attained. Thus rotor swings or oscillates about new equilibrium position. This phenomenon is known as hunting or phase swinging. Hunting occurs not only in synchronous motors but also in synchronous generators upon abrupt change in load.
Causes of Hunting in Synchronous Motor
- Sudden change in load.
- Sudden change in field current.
- A load containing harmonic torque.
- Fault in supply system.
Effects of Hunting in Synchronous Motor
- It may lead to loss of synchronism.
- Produces mechanical stresses in the rotor shaft.
- Increases machine losses and cause temperature rise.
- Cause greater surges in current and power flow.
- It increases possibility of resonance.