---
title: "Sync motor not self-starting; starting method"
subject: "Electrical Machine"
chapter: 6
key: "semester_4/electrical_machine:sync-motor-not-self-starting-method"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/sync-motor-not-self-starting-method
site: BCT Engineering
---

# Sync motor not self-starting; starting method

Electrical Machine · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2072 Magh; 2070 Magh, 8 marks (asked 2 times)
- Video: [SM39 Why Synchronous Motor is Not Self Starting ?](https://www.youtube.com/watch?v=k5CEMtQzAsY) — Lectures in Electrical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/sync-motor-not-self-starting-method
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-6

## Question

Explain why a 3 phase synchronous motor is not self starting. Explain a method of starting a 3 phase synchronous motor (using damper winding).

## Short answer

A synchronous motor is not self-starting because the stator rotating field reverses torque every half-cycle on the stationary rotor, yielding zero average torque. Starting is achieved with damper windings on the rotor poles, which let the machine accelerate as a squirrel-cage induction motor; then dc field is applied to pull into synchronism.

## Answer

#### Why a synchronous motor is not self-starting
When the stator of a synchronous motor is connected to a three-phase supply, a rotating magnetic field is produced at synchronous speed N_s = 120f/P. The rotor, fed with d.c. excitation, has fixed N and S poles.

- At the instant of starting the rotor is at rest and the stator poles sweep past it at N_s (for 50 Hz and 4 poles, 25 revolutions per second).
- In one half-cycle (0.01 s) a stator N pole attracts the rotor S pole, and in the next half-cycle the stator poles have moved on by one pole pitch, so the rotor S pole is repelled. Because of its inertia, the rotor cannot accelerate fast enough to follow this rapidly alternating pull.
- The torque on the rotor therefore reverses every half-cycle, and the average torque over a cycle is zero.

```text
 t = 0:       Stator  N  S  N  S   ->   moving at Ns
              Rotor    S  N        (at rest)   torque to right

 t = half cycle: stator poles moved one pitch:
              Stator  S  N  S  N
              Rotor    S  N        torque to left
 Net = 0, so the rotor does not start.
```
The rotor must be brought to near synchronous speed (within about 2 to 5 %) by some other means before the field is excited. Then the stator and rotor poles lock and the motor pulls into step.

#### Starting using damper (amortisseur) winding
The rotor pole faces carry short-circuited copper or brass bars, embedded in slots and joined together at both ends by end rings, like a squirrel cage.

```text
   Rotor pole face with damper bars
       __________________
      | o  o  o  o  o  o |   o = damper bars
      |__________________|   end rings join them
```
Starting steps:
1. Disconnect the d.c. field winding from the exciter and short-circuit it through a discharge resistance (about 7 to 10 times the field resistance). This prevents a dangerously high voltage being induced in the many-turn field winding at starting; it must not be left open.
2. Apply reduced three-phase voltage to the stator (by autotransformer or star-delta starter). The rotating field cuts the damper bars and induces currents in them, and the machine accelerates as a squirrel-cage induction motor.
3. When the speed reaches about 95 % of N_s, the discharge resistance is removed and the d.c. field is applied. The stator and rotor poles lock magnetically and the motor is pulled into synchronism (by the pull-in torque). Full voltage is then applied.
4. At synchronous speed there is no relative motion between the rotating field and the damper bars, so no current flows in them and they play no part. The same bars also damp rotor oscillations (hunting) when the load changes.

Other methods are a small auxiliary (pony) motor or a d.c. machine on the shaft, or a variable-frequency supply from an inverter.

## Check yourself

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

1. Why does a synchronous motor develop no average starting torque?
   A. The rotor has no poles
   B. Torque reverses every half-cycle as stator poles sweep past, averaging zero
   C. The supply frequency is too low
   D. Stator field is stationary
   Answer: B. The inertia-laden rotor cannot follow the alternating attraction and repulsion. (Key point: Why not self-starting)

2. What is the damper winding?
   A. A d.c. coil on the stator
   B. Short-circuited bars in pole faces joined by end rings, like a squirrel cage
   C. A resistor in series with the supply
   D. A permanent magnet
   Answer: B. Induced currents in these bars give induction-motor starting torque. (Key point: Damper winding)

3. How is the d.c. field treated during starting?
   A. Fully excited
   B. Open or closed through a discharge resistance
   C. Reversed
   D. Connected to the stator
   Answer: B. This prevents high induced voltage in the field winding. (Key point: Field during starting)

4. When is the d.c. field applied?
   A. At standstill
   B. At about 95 % of Ns so poles lock in
   C. Only after reaching 150 % of Ns
   D. When damper bars carry maximum current
   Answer: B. Near synchronous speed the rotor is pulled into step. (Key point: When to excite)

## Related in Electrical Machine

- [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×
- [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×
- [Single phase motor not self-starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-starting-methods) — Electrical Machines, chapter 7
- [Reason: single phase motor not self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-not-self-starting-reason) — Electrical Machines, chapter 7
- [1-phase IM not self-starting; two methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asingle-phase-im-not-self-starting-methods) — Electrical Machine, chapter 7, asked 2×
- [Reason: 1-phase IM not self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Areason-single-phase-im-not-self-starting) — Electrical Machine, chapter 7

## Related in other subjects

- [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×
- [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
- [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
- [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