---
title: "Hunting in synchronous motor"
subject: "Electric Machines II"
chapter: 2
key: "semester_5/electric_machines_ii:hunting-synchronous-motor"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/hunting-synchronous-motor
site: BCT Engineering
---

# Hunting in synchronous motor

Electric Machines II · Chapter 2: Three Phase Synchronous Motor

- Asked in: 2083 Baisakh (new course), 3 marks; 2081 Bhadra, 8 marks (asked 2 times)
- Video: [hunting in synchronous motor | hunting in synchronous machine | Industrial automation | machines](https://www.youtube.com/watch?v=u70AgTulUjk) — Mind of Engineer 
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/hunting-synchronous-motor
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2

## Question

What is meant by hunting in a synchronous motor? Explain in details and what is done to minimize it.

## Short answer

Hunting is rotor oscillation about steady load angle due to sudden load/supply changes, caused by inertia and insufficient damping. It produces mechanical stress and can cause loss of synchronism. It is minimised mainly by damper windings on pole faces, which induce torque opposing the swing, and by flywheels to alter natural frequency.

## Answer

**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.

```text
   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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is hunting in a synchronous motor?
   A. Permanent drop in speed under load
   B. Periodic oscillation of the rotor about its equilibrium load angle after a sudden change
   C. Failure of the field winding
   D. Heating of the stator core
   Answer: B. Hunting is phase swinging of the rotor about its steady load angle, while the average speed remains synchronous. (Key point: Definition of hunting)

2. After a sudden load increase, why does the rotor overshoot the new load angle?
   A. Back emf rises suddenly
   B. Rotor inertia carries it past delta2, giving more torque than the load needs
   C. Supply frequency increases
   D. The damper winding pushes it
   Answer: B. Because of inertia the rotor goes to delta3 > delta2 where developed torque exceeds load torque, then swings back, producing oscillation. (Key point: Why it overshoots)

3. Which is the main method of minimising hunting?
   A. Removing the field winding
   B. Damper (amortisseur) windings
   C. Increasing supply voltage
   D. Using a lower pole count
   Answer: B. Swinging induces currents in the damper bars that produce a torque opposing the swing, damping the oscillation. (Key point: Main remedy)

4. How does a flywheel help against hunting?
   A. It adds inertia, smoothing torque and shifting the natural frequency away from the forcing frequency
   B. It reduces the field current
   C. It increases the air gap
   D. It removes the load
   Answer: A. Extra inertia smooths pulsating torque of reciprocating drives and moves the natural frequency away from resonance. (Key point: Flywheel)

## Related in Electric Machines II

- [Synchronous motor drawing lagging and leading current](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-lagging-leading-current) — Electric Machines II, chapter 2, asked 4×
- [Why synchronous motor is not self starting; damper starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-not-self-starting-damper) — Electric Machines II, chapter 2, asked 2×
- [Leading kVAR supplied by synchronous motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Apf-correction-leading-kvar) — Electric Machines II, chapter 2
- [Synchronous motor starting methods and damper winding](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-starting-methods-damper) — Electric Machines II, chapter 2
- [Synchronous motor features, applications and excitation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-applications-excitation) — Electric Machines II, chapter 2
- [Synchronous motor features and lagging/leading operation](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-features-lagging-leading) — Electric Machines II, chapter 2

## Related in other subjects

- [Hunting; loaded operation of synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-hunting-loaded-operation) — Electrical Machines, chapter 6
- [Effect of excitation on synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-motor-excitation-effect) — Electrical Machine, chapter 6, asked 6×
- [Effect of excitation on synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-excitation-effect-pf) — Electrical Machines, chapter 6, asked 5×
- [Synchronous motor not self-starting; starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-not-self-starting-methods) — Electrical Machines, chapter 6, asked 2×