How does a damper winding help a synchronous motor start and remain stable?

A synchronous motor is not inherently self-starting because, without assistance, its rotor experiences only an oscillating torque with a zero average value as the stator's magnetic field passes it. To address this, a short-circuited winding—known as a damper winding or amortisseur—is added to the rotor.12

Starting the Motor

The damper winding functions similarly to the rotor of an induction motor. When the stator is energized, currents are induced in the damper winding, creating the torque necessary to accelerate the rotor to approximately 95 percent of synchronous speed. Once the rotor reaches this speed, direct current is applied to the field winding, allowing the rotor to pull into synchronism with the revolving stator field.1

Maintaining Stability

Beyond starting, the damper winding serves to stabilize the motor during operation. When sudden load changes occur, the rotor experiences short-term fluctuations in speed as it adjusts its lag angle relative to the stator field. These speed variations induce currents in the damper winding, which generate a torque that opposes the change, effectively damping out oscillations and maintaining steady performance.12