Hunting in a Synchronous Motor

The phenomenon of oscillation of the rotor about its final equilibrium position is called Hunting. On the sudden application of load, the rotor search for its new equilibrium position, and this process is known as Hunting. The Hunting process occurs in a synchronous motor as well as in synchronous generators if an abrupt change in load occurs.

The steady-state or stable operation of a synchronous motor is a condition of equilibrium. In it, the load torque is equal as well as opposite to the electromagnetic torque. The rotor of the motor runs at synchronous speed in the steady-state condition, maintain a constant value of the torque angle δ. The equilibrium gets disturbed if a sudden change occurs in the load torque. Thus, a resulting torque takes place which changes the speed of the motor. It is given by the equation shown below.

Where J is the moment of inertia

ω_M is the angular velocity of the rotor in mechanical units.

The speed of the motor slows down temporarily, and the torque angle δ is sufficiently increased. This is done to restore the torque equilibrium and the synchronous speed when there is a sudden increase in the load torque.

The electromagnetic torque is given by the equation shown below:

If the value of δ is increased, the electromagnetic torque is also increased. As a result, the motor is accelerated. As the rotor reaches the synchronous speed, the torque angle δ is larger than the required value. Here the rotor speed continues to increase beyond the synchronous speed.

As the rotor accelerates above synchronous speed, the torque angle δ decreases. At the point where the motor torque becomes equal to the load torque, the equilibrium is not restored because now the rotor speed is greater than the synchronous speed. Therefore, the rotor continues to swing backward and as a result, the torque angle goes on decreasing.

When the load angle δ becomes less than the required value, the mechanical load becomes greater than the developed power. Therefore, the motor starts to slow down. The load angle starts increasing again. Thus, the rotor starts to swing or oscillates around the synchronous speed.

The motor responds to a decreasing load torque by a temporary increase in speed and a reduction of the torque angle δ. Thus, the rotor swings and rotates around the synchronous speed. Hence, this process of rotation of the rotor speed equal to or around the synchronous speed is known as Hunting. Since, during the rotor oscillation, the phase of the phasor Ef changes about phasor V. Thus, hunting is known as Phase Swinging.

Causes of Hunting

The various causes of hunting are as follows:

• Sudden changes of load.
• Faults were occurring in the system which the generator supplies.
• Sudden change in the field current.
• Cyclic variations of the load torque.

Effect of Hunting

The various effects of hunting are as follows:-

• It can lead to loss of synchronism.
• It can cause variations of the supply voltage producing undesirable lamp flicker.
• The possibility of Resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place.
• Large mechanical stresses may develop in the rotor shaft.
• The machine losses increase and the temperature of the machine rises.

Reduction of Hunting

The following technique given below is used to reduce the phenomenon of hunting.

• Use of damper windings
• Uses of flywheels

The prime mover is provided with a large and heavy flywheel. This increases the inertia of the prime mover and helps in maintaining the rotor speed constant.

• By designing synchronous machines with suitable synchronising power coefficients.

This is all for hunting in synchronous motors.