The oscillation or vibrating of the rotor in the synchronous machine due to sudden changes in the load or other factors is known as Hunting or Phase Swinging or Surging.

Hunting happens in some electrical synchronous machines such as Synchronous Generators and Synchronous Motors. The concept of hunting comes when we studying about three-phase synchronous machines and their operation. In this article, we are going to know HUNTING in detail explanation with its causes, effects, and prevention.

## Understand Hunting in Detail

In a synchronous motor, when the electromagnetic torque developed by the stator and the load torque developed by the rotor are equal is known as the "condition of equilibrium" or "steady state condition". In this equilibrium or steady state condition, the rotor rotates at the same speed as the stator synchronous speed. During this time the synchronous motor can maintain a constant value of torque angle. The torque angle is denoted by Î´.

But if there are any sudden changes in the load connected with the rotor this steady state or equilibrium condition will be got disturbed. For example, if the load suddenly increased the rotor speed will decrease from the stator synchronous speed, and if the load is suddenly decreased the rotor speed will increase from the stator synchronous speed. So there will be a difference between the stator electromagnetic torque and the rotor load torque as well as the value of the torque angle also be changed.

So when the load on the motor suddenly increased the speed of the motor will decrease temporarily and the torque angle will be increased. Now, as the torque angle is increased, the motor will draw more power and increase the stator electromagnetic torque to restore its steady state or equilibrium state.

The rotor cannot achieve the steady state condition because when it reaches the synchronous speed the torque angle will be increased above the required torque angle this will cause to increase in the rotor speed above the synchronous speed. So when actually the stator torque becomes equal to the rotor torque, then already the rotor speed goes beyond the synchronous speed. This will prevent the rotor to achieve the steady state condition. So the rotor will always try to achieve the steady state condition but it will be not able to achieve instead of it will oscillate or vibrate like a pendulum clock. This is called Hunting.

## Causes of Hunting

Hunting happens in both synchronous motor and generator due to different reasons such as,
1. Hunting happens in the synchronous motor due to sudden changes in the mechanical load connected to the rotor, changes in field current, fluctuating power sources, cyclic variations or harmonics in load torque, etc.
2. Hunting happens in a synchronous generator due to changes in the electrical load, speed of the prime mover, synchronization, etc.

## Hunting Effects

Some important effects of hunting are,
1. It affects proper synchronism.
2. It may create an external vibration and noise.
3. Create mechanical stress on the rotor that can destroy the physical structure.
4. It can produce excessive unwanted heat that can increase the temperature.
5. Make power fluctuation to the power source circuit.
6. It increases the machine losses and reduces the efficiency and life span.

## Hunting Prevention

Here are some important methods that can be used to prevent or reduce the hunting synchronous machine,
1. Using of Damper Winding: Damper winding can be used in salient pole machines to place them in the pole face. So the damper winding will create the torque opposite to the produced slip due to the hunting and the magnitude of the damping torque will be proportional to the slip speed.
2. Using Flywheel: To maintain a constant rotor speed and to increase its inertia a large and heavy flywheel can be connected to the rotor of the synchronous motor.
3. Proper Design: Proper designing of a synchronous machine with a suitable synchronizing power coefficient can reduce or decrease the Hunting.