---
title: "Short note: Starting synchronous motor"
subject: "Electrical Machine"
chapter: 6
key: "semester_4/electrical_machine:short-note-sync-motor-starting"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/short-note-sync-motor-starting
site: BCT Engineering
---

# Short note: Starting synchronous motor

Electrical Machine · Chapter 6: Three Phase Synchronous Machines

- Asked in: 2068 Magh, 4 marks
- Video: [SM40 Starting Methods of Synchronous Motor](https://www.youtube.com/watch?v=iOvlQ5BhZcI) — 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/short-note-sync-motor-starting
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-6

## Question

Write a short note on starting methods of synchronous motor.

## Short answer

A synchronous motor has no starting torque. It uses damper windings to start as an induction motor with reduced stator voltage, then DC field is applied near synchronous speed to pull into synchronism. A pony motor or variable-frequency supply can also be used.

## Answer

A synchronous motor produces torque only at synchronous speed, so it has no starting torque of its own. It must be brought near synchronous speed by an auxiliary means, and then the d.c. field is applied to pull it into step. The main methods are:

1. **Damper (induction-motor) winding.** Short-circuited bars are embedded in the pole faces and joined by end rings, like a squirrel cage. Reduced three-phase voltage is applied to the stator (autotransformer or star-delta starter) while the field winding is closed through a discharge resistance. The motor starts as an induction motor and reaches about 95 % of N_s. The d.c. field is then switched on and the motor pulls into synchronism. This is the most common method.
2. **Using a small auxiliary (pony) motor.** A small induction or d.c. motor is coupled to the shaft and brings the machine to synchronous speed. The synchronous machine is then synchronised to the supply like an alternator and the pony motor is disconnected. It is used where starting must be done off load.
3. **Using a variable-frequency supply.** The stator is fed through an inverter at low frequency, which is raised slowly, so the rotor field remains locked to the rotating field (starting from rest).
4. **Using the d.c. exciter as a motor.** The exciter, if fed from a d.c. source, can run the set up to speed.

```text
 3-ph supply -> [autotransformer] -> stator
 Field: closed through discharge resistance while starting;
        switched to d.c. exciter at ~95% of N_s
```

## Check yourself

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

1. Why does a synchronous motor need a starting method?
   A. It produces torque only at synchronous speed
   B. It has too much torque
   C. Field winding is absent
   D. Supply is d.c.
   Answer: A. It has no starting torque of its own. (Key point: Need for starting method)

2. Which method is most common?
   A. Pony motor
   B. Damper winding with reduced stator voltage
   C. Exciter as motor
   D. Variable frequency
   Answer: B. Damper bars let it start as an induction motor up to about 95 % of Ns. (Key point: Most common method)

3. How does a variable-frequency start work?
   A. Frequency is raised slowly so rotor stays locked to rotating field
   B. Frequency is fixed at 50 Hz
   C. Frequency is reversed
   D. The rotor is short-circuited
   Answer: A. Starting from low frequency keeps the rotor field locked. (Key point: Variable frequency)

## 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×
- [Starting methods of synchronous motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-motor-starting-methods) — Electrical Machines, chapter 6, asked 2×
- [Sync motor not self-starting; starting method](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Async-motor-not-self-starting-method) — Electrical Machine, chapter 6, asked 2×
- [DFRT and making single phase motor self-starting](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adfrt-self-starting-explain) — Electrical Machines, chapter 7, asked 3×
- [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×
- [Starting current and starting resistance of shunt motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-starting-current-80pc) — Electrical Machines I, chapter 4

## 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×
- [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
- [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