Hunting in Synchronous Motor: Causes, Effects and Reduction of Hunting

Quiz

Hunting in Synchronous Motor

Hunting is the phenomenon of oscillation of the rotor about its steady state position or equilibrium state in a synchronous motor. Hence, hunting means a momentary fluctuation in the rotor speed of a synchronous motor.

In a synchronous motor, when the electromagnetic torque developed is equal and opposite to the load torque, such a condition is known as "condition of equilibrium" or "steady state condition".

In the steady-state, the rotor of the synchronous motor runs at synchronous speed, thereby maintaining a constant value of torque angle (δ). If there is a sudden change in the load torque, then the equilibrium of the motor is disturbed and there is a difference between the electromagnetic torque $(\tau_{e})$ and load torque $(\tau_{l})$ which changes the speed of the motor. This difference torque is given by,

$$\mathrm{\tau_{e} \:-\: \tau_{l} \:=\: J\frac{d\omega}{dt} \:\:\:\:\dotso\: (1)}$$

Where,

J is the moment of inertia of the rotor, and
ω is the angular velocity of the rotor.

Because of the sudden change (increase) in the load torque of the motor, the speed of the motor decreases temporarily and the torque angle (δ) is increased to restore the condition of equilibrium and the synchronous speed.

The electromagnetic torque of synchronous motor is given by,

$$\mathrm{\tau_{e} \:=\: \frac{3VE_{f}}{\omega_{s}X_{S}}\:Sin\delta \:\:\:\:\dotso\: (2)}$$

As the torque angle (δ) is increased, hence the electromagnetic torque increases as seen from Equation (2). As a result, the motor is accelerated. When the rotor reaches synchronous speed, the torque angle (δ) is greater than the new required value (δ)' for the new steady state condition. Consequently, the torque angle (δ) decrease due to the acceleration of the rotor above synchronous speed. At the point where the electromagnetic torque becomes equal to the load torque, the steady-state condition is not restored, because at this point the speed of the rotor is more than the synchronous speed. Therefore, the rotor continues to swing backwards and the torque angle goes on decreasing. When the torque angle (δ) becomes less than the new required value (δ)', the load torque becomes greater than the electromagnetic torque. Therefore, the motor starts to slow down. The torque angle (δ) is increased again. Hence, the rotor oscillates around the synchronous speed and the new required value (δ)'of the torque angle before reaching the new state of equilibrium.

This phenomenon of oscillation of the rotor of a synchronous motor about its final steady-state position is known as hunting.

Since during the rotor oscillations, the phase of the phasor E_f varies with respect to the phasor V, therefore, hunting is also known phase swinging.

Causes of Hunting

Following may be the reasons of hunting in a synchronous motor −

Sudden changes of the mechanical load on the motor.
Sudden changes in the field current.
Cyclic variations of the load torque.
Faults occurring in the power system to which the motor is connected.

Effects of Hunting

The effects of hunting in a synchronous machine are given below −

It may lead to loss of synchronism.
Hunting increases the probability of resonance. When the frequency of the torque component becomes equal to that of the oscillations of the synchronous machine, resonance may take place.
Large mechanical stresses may develop in the rotor shaft of the synchronous machine.
Hunting increases the losses of the machine.
It increases temperature of the synchronous machine.
It can disturb the supply system to which the synchronous machine is connected.

Reduction of Hunting

By adapting the following techniques, the hunting in synchronous machines can be reduced −

Hunting may be reduced by using damper windings.
It can be decreased by using flywheel. A large and heavy flywheel is to be connected to the rotor. This increases the inertia of the rotor and helps in maintaining the rotor speed constant.
Hunting can also be decreased by designing the synchronous machine with suitable synchronising power coefficients.

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