Hunting in Synchronous Motor

What is Hunting in Synchronous Motor?

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What is hunting in 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.

We come across the term HUNTING when we study about three phase synchronous motor operations. The word hunting is used because after sudden application of load the rotor has to search or hunt for its new equilibrium position. That phenomenon is referred to as hunting in synchronous motor.

What is Hunting in Synchronous Motor?

Now let us know what is the condition of equilibrium in synchronous motor. A steady state operation of synchronous motor is a condition of equilibrium in which the electromagnetic torque is equal and opposite to load torque. In steady state, rotor runs at synchronous speed thereby maintaining constant value of torque angle (δ).

If there is sudden change in load torque, the equilibrium is disturbed and there is resulting torque which changes speed of the 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.

State or Stable Operation of a Synchronous Motor

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 ,ωM 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 if 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. 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 backwards 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 rotate around the synchronous speed. Thus, this process of rotation of the rotor speed equal 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 in Synchronous Motor

  1. Sudden change in load.
  2. Sudden change in field current.
  3. A load containing harmonic torque.
  4. Fault in supply system.

Effects of Hunting in Synchronous Motor

  1. It may lead to loss of synchronism.
  2. Produces mechanical stresses in the rotor shaft.
  3. Increases machine losses and cause temperature rise.
  4. Cause greater surges in current and power flow.
  5. It increases possibility of resonance.

Reduction of Hunting in Synchronous Motor

Two techniques should be used to reduce hunting. These are:

  • Use of Damper Winding: It consists of low electrical resistance copper / aluminum brush embedded in slots of pole faces in salient pole machine. Damper winding damps out hunting by producing torque opposite to slip of rotor. The magnitude of damping torque is proportional to the slip speed.
  • Use of Flywheels: The prime mover is provided with a large and heavy flywheel. This increases the inertia of prime mover and helps in maintaining the rotor speed constant.
  • Designing synchronous machine with suitable synchronizing power coefficients.

Hunting in Synchronous Motor Conclusion

After going through the above portion of synchronous motor we can now establish what hunting in synchronous motor is. I hope you enjoy when reading this article, thank you.