Damper winding to make motor self starting

A damper (amortisseur) winding is a set of copper or brass bars embedded in slots on the rotor pole faces and short-circuited at both ends by end rings. It forms a partial squirrel cage, so the synchronous motor can start as an induction motor and then pull into synchronism.

Construction

     end ring                     end ring
        ||==========bar============||
        ||==========bar============||   pole face
        ||==========bar============||
   (bars of all poles joined -> squirrel cage)

• Bars are placed in semi-closed slots on each pole shoe.
• End rings (or segments) join the bars of all poles, giving a closed cage.
• Bar resistance is chosen to give reasonable starting torque.

Starting procedure

1. Field kept unexcited: the DC field winding is disconnected from the exciter and shorted through a discharge resistance. This avoids a dangerously high voltage being induced in the many-turn field winding at standstill, and the induced field current adds some starting torque.
2. Supply applied: 3-phase supply is given to the stator, usually at reduced voltage (autotransformer or star-delta) to limit starting current.
3. Induction motor action: the rotating field cuts the damper bars, induces emf and current in them, and produces torque exactly as in a squirrel-cage induction motor. The rotor accelerates.
4. Near synchronous speed: the rotor reaches about 95-98 % of $N_s$ (slip of 2-5 %). It cannot reach $N_s$ by induction action alone.
5. Field excited: the discharge resistor is removed and DC is applied. The rotor poles are now magnetised; because slip is small, the stator poles drag the rotor poles into step (pull-in torque), and the motor runs at $N_s$.
6. Full voltage is then applied to the stator.

Behaviour after synchronising

• At $N_s$ there is no relative motion between the stator field and the damper bars, so no emf, no current and no torque in them; they do not affect normal running.
• If the rotor speed swings above or below $N_s$ (hunting), currents are induced that oppose the swing, so the same winding damps hunting.

Limitations

• Starting torque is low (cage is small and fits only on pole faces), so the motor must be started at light load.
• Pull-in is possible only if load and inertia are not too large.