- Hunting Defined: Hunting in a synchronous motor is the phenomenon where the rotor oscillates around a new equilibrium position due to sudden changes in load.
- Causes of Hunting: Hunting can be caused by abrupt load changes, sudden field current adjustments, harmonic torque loads, or faults in the supply system.
- Effects of Hunting: This instability may cause the motor to lose synchronism, induce mechanical stresses, increase losses, and elevate temperatures.
- Reduction Techniques: To reduce hunting, use damper windings to oppose rotor slip and install flywheels to stabilize rotor speed.
- Types of Synchronous Motor: Understanding different types of synchronous motors helps in selecting the right motor design to minimize hunting effects.
The term HUNTING describes an electromechanical oscillation that can occur during three phase synchronous motor operations. After a disturbance, the rotor angle and speed swing about a new steady operating point. This transient behavior is called hunting in a synchronous motor.
During steady-state operation of synchronous motor, electromagnetic torque balances the shaft-load torque and losses. The rotor runs at synchronous speed with a constant torque angle (δ). A sudden mismatch between those torques produces accelerating or decelerating torque, so rotor speed and angle change temporarily.
What is Hunting?
At any stable operating point, the rotor has a load angle set by its shaft load, excitation and terminal conditions. If the load steps to a higher value, P1, the rotor momentarily slows relative to the rotating stator field and the load angle increases.
The new torque balance corresponds to an angle δ1. Rotor inertia carries the angle past that point toward δ2. In this part of the swing, the developed electrical power and electromagnetic torque exceed the new shaft-load demand.
The excess electromagnetic torque restores rotor speed, but inertia can carry the rotor past its equilibrium angle. The rotor may then run momentarily above synchronous speed, causing the load angle to decrease. Without enough damping, the rotor crosses the equilibrium point repeatedly.
This rotor-angle oscillation around an operating point is called hunting or phase swinging. It can follow a sudden mechanical or electrical disturbance in both synchronous motors and synchronous generators.
Causes of Hunting in Synchronous Motor
- A sudden change in shaft-load torque.
- A sudden change in field current or excitation control.
- A cyclic or pulsating load containing harmonic torque.
- A supply-voltage, frequency or phase disturbance, including a fault and its clearing.
Effects of Hunting in Synchronous Motor
- A large rotor-angle swing may cause loss of synchronism.
- Oscillating torque applies cyclic stress to the shaft and couplings.
- Oscillating currents increase machine losses and temperature.
- The disturbance can produce surges in current and oscillations in power flow.
- A periodic torque near a natural oscillation frequency can cause resonance.
Reduction of Hunting in Synchronous Motor
The appropriate control depends on the machine and connected system. Common measures include:
- Use of Damper Winding: Low-electrical resistance copper or aluminium bars are embedded in salient-pole faces and short-circuited at their ends. A speed difference between the rotor and rotating field induces bar currents. Their induction torque opposes that difference and damps the oscillation.
- Use of Flywheels: A correctly engineered flywheel adds shaft inertia and reduces the initial rate of speed and angle change after a torque disturbance. It changes the oscillation dynamics but does not replace electrical or mechanical damping.
- Design the synchronous machine and its excitation controls with adequate synchronizing torque and damping for the connected load.
