---
title: "Functions of damper winding"
subject: "Electric Machines II"
chapter: 2
key: "semester_5/electric_machines_ii:damper-winding-functions"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/damper-winding-functions
site: BCT Engineering
---

# Functions of damper winding

Electric Machines II · Chapter 2: Three Phase Synchronous Motor

- Asked in: 2073 Chaitra, 6 marks
- Video: [Lecture 21: Synchronous motor types and role of damper winding](https://www.youtube.com/watch?v=zFSnAJzgZMM) — Electrical Engineering
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/damper-winding-functions
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2

## Question

Explain the functions of damper winding provided on pole face of rotor of a synchronous motor.

## Short answer

Damper windings are short-circuited bars on pole faces. They provide induction-motor starting torque, damp hunting oscillations by inducing opposing torque during speed swings, suppress negative-sequence and harmonic fields, and protect the field winding during starting.

## Answer

A **damper winding** consists of heavy copper or brass bars set in slots on the rotor pole faces and short-circuited at both ends by end rings, like a partial squirrel cage.

```text
      pole face (one pole)
   _______________________
  |  o    o    o    o    |  o = damper bars
  |______________________|
       \ field coil /
  bars of all poles joined by end rings
```

At synchronous speed there is no relative motion between the stator field and the rotor, so the damper bars carry no current. They work only when the rotor speed differs from $N_s$.

#### Functions

1. **Starting the motor (self-starting).** At start, the rotating stator field induces currents in the bars and produces induction-motor torque. The motor runs up to about 95-98 % of $N_s$; then DC excitation is applied and the rotor pulls into step. Without dampers a synchronous motor has zero starting torque.

2. **Damping hunting.** When load changes suddenly the rotor swings about its new load angle. Any speed above or below $N_s$ causes slip, induces current in the bars and produces a torque opposing the swing (induction motor and generator action). The oscillation dies out quickly.

3. **Suppressing negative-sequence fields.** Under unbalanced loads or faults, the backward-rotating field induces currents in the bars which oppose it, reducing rotor heating and voltage distortion.

4. **Reducing harmonics and voltage distortion** by opposing harmonic flux in the air gap, giving a better waveform.

5. **Improving stability.** By damping oscillations after faults or switching, they help the machine stay in synchronism.

6. **Protecting the field winding at start.** Because the damper carries the induced currents, the induced voltage stress on the field winding is reduced (the field is also shorted through a discharge resistor).

#### Limitation

The cage is small (only on pole faces), so starting torque is low; motors started this way are started at light load.

## Check yourself

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

1. What is a damper winding?
   A. A DC winding on the stator
   B. Copper or brass bars in pole-face slots shorted by end rings, like a partial squirrel cage
   C. A capacitor across the field
   D. A resistor in series with the armature
   Answer: B. The bars are joined at both ends by end rings, forming a partial cage. (Key point: Damper construction)

2. When do the damper bars carry current?
   A. Always, at all speeds
   B. Only when the rotor speed differs from Ns
   C. Only at synchronous speed
   D. Only when the field is off
   Answer: B. At Ns there is no relative motion between the field and the rotor, so no emf is induced in the bars. (Key point: No current at synchronous speed)

3. How do dampers help starting?
   A. They give induction-motor torque from the rotating stator field, bringing the rotor to about 95-98 percent of Ns before DC excitation is applied
   B. They supply DC to the field
   C. They raise the supply frequency
   D. They reduce the number of stator poles
   Answer: A. Currents induced in the bars produce induction-motor action; then DC excitation pulls the rotor into step. (Key point: Starting torque)

4. How do dampers reduce hunting after a sudden load change?
   A. They increase Ef
   B. Any speed above or below Ns causes slip and induced current, giving a torque that opposes the swing
   C. They lock the rotor at delta = 0
   D. They increase the load angle
   Answer: B. Induction motor and generator action opposes the oscillation, so it dies out quickly. (Key point: Damping hunting)

## Related in Electric Machines II

- [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
- [Damper winding to make motor self starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adamper-winding-self-starting) — Electric Machines II, chapter 2
- [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×
- [Construction of synchronous machine and rotor types](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asynchronous-machine-construction-rotors) — Electric Machines II, chapter 1, asked 2×
- [25 MVA salient pole motor maximum power](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asalient-motor-25mva-max-power) — Electric Machines II, chapter 2, asked 3×
- [30 MVA salient pole motor maximum power](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asalient-motor-30mva-max-power) — Electric Machines II, chapter 2, asked 3×

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