Functions of damper winding

A damper winding consists of heavy copper or brass bars set in slots on the rotor pole faces and short-circuited at both ends by end rings, like a partial squirrel cage.

      pole face (one pole)
   _______________________
  |  o    o    o    o    |  o = damper bars
  |______________________|
       \ field coil /
  bars of all poles joined by end rings

At synchronous speed there is no relative motion between the stator field and the rotor, so the damper bars carry no current. They work only when the rotor speed differs from $N_s$.

Functions

1. Starting the motor (self-starting). At start, the rotating stator field induces currents in the bars and produces induction-motor torque. The motor runs up to about 95-98 % of $N_s$; then DC excitation is applied and the rotor pulls into step. Without dampers a synchronous motor has zero starting torque.

2. Damping hunting. When load changes suddenly the rotor swings about its new load angle. Any speed above or below $N_s$ causes slip, induces current in the bars and produces a torque opposing the swing (induction motor and generator action). The oscillation dies out quickly.

3. Suppressing negative-sequence fields. Under unbalanced loads or faults, the backward-rotating field induces currents in the bars which oppose it, reducing rotor heating and voltage distortion.

4. Reducing harmonics and voltage distortion by opposing harmonic flux in the air gap, giving a better waveform.

5. Improving stability. By damping oscillations after faults or switching, they help the machine stay in synchronism.

6. Protecting the field winding at start. Because the damper carries the induced currents, the induced voltage stress on the field winding is reduced (the field is also shorted through a discharge resistor).

Limitation

The cage is small (only on pole faces), so starting torque is low; motors started this way are started at light load.