A synchronous motor is not self-starting because it develops torque only when the rotor locks with the stator’s rotating magnetic field at synchronous speed. To overcome this limitation, the motor is often started as an induction motor using damper windings.
Table of Contents
Basic Idea
During starting:
- The rotor field winding is not excited (no DC supply)
- The motor behaves like a squirrel cage induction motor
- Once near synchronous speed, DC excitation is applied
Construction Feature
- Copper/aluminum bars embedded in rotor pole faces
- Short-circuited by end rings
- Similar to squirrel cage rotor
These windings called Damper Bars enable induction motor action during starting.
Step-by-Step Starting Process
Step 1: Supply Given to Stator
- 3-phase AC supply is applied
- Rotating magnetic field is produced
Step 2: Induction Motor Action
- Rotor is stationary → relative motion exists
- EMF is induced in damper windings
- Current flows in damper bars
Produces starting torque (like induction motor).
Step 3: Acceleration
- Motor accelerates gradually
- Slip reduces as speed increases
- Rotor approaches synchronous speed
Step 4: Field Excitation Applied
- At about 95–98% of synchronous speed:
- DC supply is given to rotor field winding
- Rotor develops a constant magnetic field
Step 5: Pull into Synchronism
- Rotor locks with stator rotating field
- Motor runs at synchronous speed (zero slip)
Why Field is Not Applied Initially?
If DC excitation is applied at standstill:
- Rotor field is stationary
- No average torque is produced
- Motor will not start
Hence, excitation is applied only after acceleration