Question

Which of the following is true about hunting in synchronous machines?

• A. Occurs at low speed only
• B. Occurs when mechanical load fluctuates
• C. Is due to sudden change in field current
• D. Is due to interaction between rotor inertia and synchronizing torque

Hint:

To prevent hunting in synchronous motors:
1. Use damper windings embedded in the pole faces (damper windings provide damping torque to damp out the oscillations).
2. Use flywheels to increase rotor inertia (though this can affect transient stability).

Answer 1

The Correct Option is B

Step 1: Understanding the Problem:
The question asks for the correct statement regarding the phenomenon of "hunting" in synchronous machines.
Hunting is a periodic oscillation of the rotor around its equilibrium position (synchronous speed) in a synchronous machine.

Step 2: Detailed Explanation:

• Under normal operating conditions, the rotor of a synchronous machine runs at synchronous speed, and the load angle \(\delta\) is constant.

• If there is a sudden change or fluctuation in the mechanical load on the shaft, the rotor speed temporarily deviates from the synchronous speed to adjust to the new load angle.

• Because of the mechanical inertia of the rotor, the rotor overshoots its new equilibrium position. This leads to oscillations of the rotor about the synchronous speed.

• These periodic oscillations of the rotor are called hunting (or phase swinging).

• Therefore, the primary cause of hunting is the fluctuation of the mechanical load.

• Hunting is undesirable as it causes mechanical stress, fluctuations in terminal voltage and current, and can even lead to the machine falling out of synchronism. It is mitigated by using damper windings.

Step 3: Final Answer:
Hunting occurs when the mechanical load fluctuates.