---
title: "Justify synchronous motor is not self starting"
subject: "Electric Machines II"
chapter: 2
key: "semester_5/electric_machines_ii:motor-not-self-starting-justify"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-not-self-starting-justify
site: BCT Engineering
---

# Justify synchronous motor is not self starting

Electric Machines II · Chapter 2: Three Phase Synchronous Motor

- Asked in: 2074 Asoj, 6 marks
- Video: [SM39 Why Synchronous Motor is Not Self Starting ?](https://www.youtube.com/watch?v=k5CEMtQzAsY) — Lectures in Electrical Engineering
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/motor-not-self-starting-justify
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-2

## Question

"Synchronous motor is not self starting". Explain it with proper justification.

## Short answer

The stator field rotates at synchronous speed while the DC-excited rotor is stationary. Torque direction reverses every half cycle; the heavy rotor cannot follow within 0.01 s, so average torque is zero. The motor must be brought near synchronous speed by auxiliary means.

## Answer

A 3-phase synchronous motor with its field excited **cannot start by itself** because the average torque on the stationary rotor is zero.

#### Justification

1. When 3-phase supply is switched on, the stator produces a magnetic field that immediately rotates at synchronous speed, $N_s = 120f/P$ (e.g. 1500 rpm for a 4-pole, 50 Hz machine).
2. The rotor is excited by DC, so it has fixed N and S poles, and it is at rest with large inertia.
3. **Instant 1:** suppose stator poles are placed so that stator N is just ahead of rotor S. Unlike poles attract, so the rotor gets a torque in, say, the clockwise direction.
4. **Half a cycle later** (0.01 s at 50 Hz): the stator field has moved by one pole pitch; now a stator S pole faces the rotor S pole. Like poles repel and the torque is anticlockwise.
5. The torque thus reverses every half cycle. The heavy rotor cannot pick up speed in 0.01 s to follow the field, so it only vibrates.
6. The **average torque over one cycle is zero**, and the motor does not start.

```text
 Instant 1                 Instant 2 (T/2 later)
  stator: N      S          stator: S      N
          |      |                  |      |
  rotor : S      N          rotor : S      N
  attract -> clockwise       repel -> anticlockwise
         average torque = 0
```

#### Condition for running

Continuous torque in one direction is possible only when the rotor poles move at the same speed as the stator field, so that they stay locked to opposite stator poles. Therefore the rotor must first be brought close to $N_s$ by another means.

#### How it is started

- **Damper winding** (induction motor starting), then DC field is switched on.
- **Pony motor** on the same shaft.
- **Slip-ring induction motor** method.
- **Variable frequency supply** (VFD) raising frequency from zero.

## Check yourself

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

1. Why is a synchronous motor with excited field not self-starting?
   A. The rotor has no poles
   B. The torque on the stationary rotor reverses every half cycle, so the average is zero
   C. The stator field does not rotate at start
   D. The supply voltage is too low
   Answer: B. The stator field rotates at once, attracting then repelling the rotor poles every half cycle; the heavy rotor only vibrates. (Key point: Zero average torque)

2. Why can't the rotor follow the stator field during the first half cycle?
   A. It has a large inertia and half a cycle is only 0.01 s at 50 Hz
   B. It is not magnetised
   C. The stator field is stationary
   D. The DC supply is absent
   Answer: A. The torque reverses every 0.01 s, too fast for a heavy rotor to pick up speed. (Key point: Role of rotor inertia)

3. When can continuous unidirectional torque exist?
   A. When the rotor is at rest
   B. When the rotor poles run at the same speed as the stator field
   C. When the field current is zero
   D. When the rotor speed is double Ns
   Answer: B. Only at synchronous speed do rotor poles stay locked to opposite stator poles, so the rotor must first be brought near Ns. (Key point: Condition for steady torque)

4. Which is a valid way to start a synchronous motor?
   A. Switch on the DC field first with full voltage
   B. Damper winding, pony motor or variable-frequency supply
   C. Reverse the field polarity continuously
   D. Reduce the number of poles at start
   Answer: B. The rotor must be brought near synchronous speed by another means such as dampers, a pony motor or a VFD. (Key point: Starting methods)

## Related in Electric Machines II

- [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 not self starting; two starting methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Amotor-starting-two-methods) — Electric Machines II, chapter 2
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- [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×
- [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

## Related in other subjects

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- [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×
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