Hunting in synchronous motor

Hunting is the periodic oscillation (swinging) of the rotor of a synchronous machine about its steady equilibrium position (load angle $\delta$) when the load or supply changes suddenly. The rotor speed swings above and below synchronous speed while the average speed stays synchronous. It is also called phase swinging.

How it occurs

1. Suppose the motor runs at load angle $\delta_1$ with load torque $T_1$.
2. The load is suddenly increased to $T_2$. The rotor slows down momentarily, and $\delta$ increases to give more torque.
3. Due to its inertia, the rotor does not stop at the new angle $\delta_2$; it overshoots to $\delta_3 > \delta_2$, where the developed torque is more than the load torque.
4. The excess torque accelerates the rotor; it swings back past $\delta_2$ towards a smaller angle; then it is retarded again.
5. The rotor thus oscillates about $\delta_2$ at the natural frequency of the machine–system. Small oscillations die out if there is enough damping.

   delta
    |     .-.
 d3 |    /   \     .-.
 d2 |---/-----\---/---\---.--------  new steady value
    |  /       \_/     '-'
 d1 |_/
    +--------------------------------> time
       load step

Causes

• Sudden change in load (especially periodic loads such as compressors and pumps).
• Sudden change in supply voltage or frequency, or faults in the system.
• Sudden change in field current.
• Pulsating torque of the prime mover (generators driven by diesel engines).
• Resonance between the natural frequency of the rotor and the frequency of load pulsations.

Effects

• Large mechanical stresses and fatigue in the shaft.
• Large current and power swings, voltage fluctuations and increased losses.
• If the swing exceeds the stability limit, the machine falls out of step (loses synchronism).
• Can cause resonance and damage.

Methods to minimise hunting

1. Damper (amortisseur) windings: short-circuited copper bars in the pole faces. When the rotor swings, relative motion induces currents in them which produce a torque opposing the swing (like an induction motor), damping the oscillation. This is the main method.
2. Flywheel: adds inertia and smooths the torque of reciprocating prime movers or loads; it changes the natural frequency away from the forcing frequency.
3. Design the machine with suitable synchronising power (proper $X_s$ and excitation), so the natural frequency is not near the frequency of load pulsations.
4. Avoid sudden large changes in load, and use fast-acting excitation systems with power system stabilisers.