Why synchronous motor is not self starting; damper starting

Why a synchronous motor is not self-starting

1. When balanced 3-phase supply is given to the stator, a magnetic field rotating at synchronous speed $N_s = 120f/P$ is set up.
2. The rotor poles are excited by DC and are stationary at start.
3. At one instant, a stator N pole is in front of a rotor S pole: attraction tends to turn the rotor in the direction of the field (say clockwise).
4. Half a cycle later (1/100 s at 50 Hz), the stator poles have moved by one pole pitch; now a stator N is in front of the rotor N: repulsion tends to turn the rotor anticlockwise.
5. Because of its inertia, the rotor cannot move in such a short time; the torque reverses every half cycle, so the average starting torque is zero. The rotor only vibrates.

 t = 0:   stator N  -> attracts rotor S  -> torque CW
 t = T/2: stator S  -> repels  rotor S   -> torque CCW
          average torque = 0  (rotor stays at rest)

Hence the rotor must first be brought close to synchronous speed by some other means.

Damper winding method (starting as induction motor)

Construction: copper or brass bars are placed in slots in the rotor pole faces and short-circuited at both ends by end rings, forming a squirrel-cage (damper / amortisseur) winding.

Procedure

1. Do not connect the DC supply to the field winding. Short-circuit it through a discharge resistance (never leave it open, because the rotating field would induce a very high voltage in the many-turn field winding).
2. Switch on the 3-phase supply (often at reduced voltage through an auto-transformer or star–delta starter to limit current).
3. The rotating field induces currents in the damper bars, and the motor starts as an induction motor, accelerating to about 95–98% of synchronous speed.
4. Now apply DC excitation to the field. The rotor poles lock with the stator field and are pulled into synchronism.
5. At synchronous speed there is no relative motion, so no current flows in the damper bars; they carry current only during speed changes, when they also damp hunting.

Advantages: simple, no separate starting motor, also prevents hunting. Limitation: low starting torque, so it suits motors that start at light load.