---
title: "Damper winding to make motor self starting"
subject: "Electric Machines II"
chapter: 2
key: "semester_5/electric_machines_ii:damper-winding-self-starting"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/damper-winding-self-starting
site: BCT Engineering
---

# Damper winding to make motor self starting

Electric Machines II · Chapter 2: Three Phase Synchronous Motor

- Asked in: 2076 Asoj, 6 marks
- Video: [Self starting of Synchronous motor by Damper Winding](https://www.youtube.com/watch?v=5Cvqi5t39to) — ABHISEK DE
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/damper-winding-self-starting
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2

## Question

Explain how the damper winding on the rotor pole of a 3-phase synchronous motor can be used to make the motor self starting.

## Short answer

Copper/bass bars embeded in rotor pole faces and shorted by end rings form a squirrel-cage damper winding. During start, the field winding is shorted through a discharge resistor and reduced stator voltage is applied; the rotating field induces currents in the bars producing induction motor torque, bringing the rotor to near synchronous speed. Then DC excitation is switched on, pulling the rotor into synchronism.

## Answer

A **damper (amortisseur) winding** is a set of copper or brass bars embedded in slots on the rotor pole faces and short-circuited at both ends by end rings. It forms a partial squirrel cage, so the synchronous motor can start as an **induction motor** and then pull into synchronism.

#### Construction

```text
     end ring                     end ring
        ||==========bar============||
        ||==========bar============||   pole face
        ||==========bar============||
   (bars of all poles joined -> squirrel cage)
```

- Bars are placed in semi-closed slots on each pole shoe.
- End rings (or segments) join the bars of all poles, giving a closed cage.
- Bar resistance is chosen to give reasonable starting torque.

#### Starting procedure

1. **Field kept unexcited:** the DC field winding is disconnected from the exciter and shorted through a discharge resistance. This avoids a dangerously high voltage being induced in the many-turn field winding at standstill, and the induced field current adds some starting torque.
2. **Supply applied:** 3-phase supply is given to the stator, usually at reduced voltage (autotransformer or star-delta) to limit starting current.
3. **Induction motor action:** the rotating field cuts the damper bars, induces emf and current in them, and produces torque exactly as in a squirrel-cage induction motor. The rotor accelerates.
4. **Near synchronous speed:** the rotor reaches about 95-98 % of $N_s$ (slip of 2-5 %). It cannot reach $N_s$ by induction action alone.
5. **Field excited:** the discharge resistor is removed and DC is applied. The rotor poles are now magnetised; because slip is small, the stator poles drag the rotor poles into step (**pull-in torque**), and the motor runs at $N_s$.
6. **Full voltage** is then applied to the stator.

#### Behaviour after synchronising

- At $N_s$ there is no relative motion between the stator field and the damper bars, so no emf, no current and no torque in them; they do not affect normal running.
- If the rotor speed swings above or below $N_s$ (hunting), currents are induced that oppose the swing, so the same winding **damps hunting**.

#### Limitations

- Starting torque is low (cage is small and fits only on pole faces), so the motor must be started at light load.
- Pull-in is possible only if load and inertia are not too large.

## Check yourself

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

1. What is a damper winding?
   A. Copper or brass bars in pole-face slots shorted by end rings, forming a partial squirrel cage
   B. A DC winding on the stator
   C. A series resistor
   D. A wound rotor with slip rings
   Answer: A. It allows induction-motor starting. (Key point: Construction)

2. Why is the field winding shorted through a discharge resistance at start?
   A. To avoid dangerous induced voltage in the many-turn field winding; it also adds some starting torque
   B. To raise the speed beyond Ns
   C. To excite the rotor
   D. To save current
   Answer: A. At standstill the rotating field induces high emf in the field winding if open. (Key point: Why shorted field)

3. Why can induction action alone not bring the rotor to Ns?
   A. Induced current in the bars vanishes at Ns (no relative motion), so it stops at 95-98% of Ns
   B. The supply is too low
   C. The cage is open
   D. Friction is too high
   Answer: A. Torque needs slip; at Ns there is none, so DC excitation pulls the rotor into step from about 2-5% slip. (Key point: Why not full speed)

4. What does the damper winding do after synchronising?
   A. Carries large current to provide torque
   B. Carries no current in steady state but damps hunting
   C. Is disconnected
   D. Doubles speed
   Answer: B. Speed swings induce opposing currents, damping hunting; otherwise it does nothing. (Key point: After synchronising)

## 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
- [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×
- [Justify synchronous motor is not self starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-not-self-starting-justify) — Electric Machines II, chapter 2
- [Split-phase motor winding currents at starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asplit-phase-starting-currents) — Electric Machines II, chapter 3
- [Functions of damper winding](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adamper-winding-functions) — Electric Machines II, chapter 2
- [Why single-phase IM is not self starting; methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-not-self-starting-methods) — Electric Machines II, chapter 3, asked 2×

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- [Single phase motor not self-starting; DFRT](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-dfrt) — Electrical Machines, chapter 7, asked 3×