Sync motor not self-starting; starting method

Why a synchronous motor is not self-starting

When the stator of a synchronous motor is connected to a three-phase supply, a rotating magnetic field is produced at synchronous speed N_s = 120f/P. The rotor, fed with d.c. excitation, has fixed N and S poles.

• At the instant of starting the rotor is at rest and the stator poles sweep past it at N_s (for 50 Hz and 4 poles, 25 revolutions per second).
• In one half-cycle (0.01 s) a stator N pole attracts the rotor S pole, and in the next half-cycle the stator poles have moved on by one pole pitch, so the rotor S pole is repelled. Because of its inertia, the rotor cannot accelerate fast enough to follow this rapidly alternating pull.
• The torque on the rotor therefore reverses every half-cycle, and the average torque over a cycle is zero.

 t = 0:       Stator  N  S  N  S   ->   moving at Ns
              Rotor    S  N        (at rest)   torque to right

 t = half cycle: stator poles moved one pitch:
              Stator  S  N  S  N
              Rotor    S  N        torque to left
 Net = 0, so the rotor does not start.

The rotor must be brought to near synchronous speed (within about 2 to 5 %) by some other means before the field is excited. Then the stator and rotor poles lock and the motor pulls into step.

Starting using damper (amortisseur) winding

The rotor pole faces carry short-circuited copper or brass bars, embedded in slots and joined together at both ends by end rings, like a squirrel cage.

   Rotor pole face with damper bars
       __________________
      | o  o  o  o  o  o |   o = damper bars
      |__________________|   end rings join them

Starting steps:

1. Disconnect the d.c. field winding from the exciter and short-circuit it through a discharge resistance (about 7 to 10 times the field resistance). This prevents a dangerously high voltage being induced in the many-turn field winding at starting; it must not be left open.
2. Apply reduced three-phase voltage to the stator (by autotransformer or star-delta starter). The rotating field cuts the damper bars and induces currents in them, and the machine accelerates as a squirrel-cage induction motor.
3. When the speed reaches about 95 % of N_s, the discharge resistance is removed and the d.c. field is applied. The stator and rotor poles lock magnetically and the motor is pulled into synchronism (by the pull-in torque). Full voltage is then applied.
4. At synchronous speed there is no relative motion between the rotating field and the damper bars, so no current flows in them and they play no part. The same bars also damp rotor oscillations (hunting) when the load changes.

Other methods are a small auxiliary (pony) motor or a d.c. machine on the shaft, or a variable-frequency supply from an inverter.