Synchronous motor not self-starting; starting methods

Why a synchronous motor is not self-starting

When a three-phase supply is applied to the stator, a field rotating at synchronous speed $N_s = \frac{120f}{P}$ is produced. The rotor poles, excited by DC, are stationary. A rotor pole is attracted by a stator pole, but in the next half cycle (1/100 s at 50 Hz) the stator pole has moved on, an opposite pole has taken its place, and the force reverses. Because of its inertia, the rotor cannot accelerate to $N_s$ in that short time. The torque reverses every half cycle, so the average starting torque is zero. The rotor must first be brought close to synchronous speed by other means.

Starting methods

1. Using a pony (auxiliary) motor: a small induction or DC motor drives the rotor to near synchronous speed. The field is then excited, the machine is synchronised with the supply like an alternator, and the pony motor is disconnected. It is used for large motors that can be started without load.
2. Using damper windings (induction motor method): squirrel-cage type bars are placed in the pole faces and short-circuited by end rings.
   • The field winding is short-circuited through a resistance (to avoid high induced voltages), and the stator is supplied at reduced voltage (by an autotransformer or star-delta starter).
   • The damper winding develops torque like a squirrel-cage rotor, and the motor accelerates to about 95 % of $N_s$.
   • DC excitation is then applied. The rotor pulls into step (synchronises), and the damper winding then carries no current.
3. Using a variable-frequency supply: the frequency is raised gradually from a very low value, so the rotor stays locked to the slowly rotating field as it speeds up.
4. As a slip-ring induction motor (synchronous-induction motor): the rotor carries a three-phase winding brought out to slip rings. It is started like a slip-ring induction motor with external resistance in the rotor circuit, which is cut out as the speed rises. Near full speed, DC excitation is applied through the slip rings and the rotor pulls into synchronism. This gives a high starting torque and is used where the motor must start against load.

  Supply
    |
 [Autotransformer] -> stator
                     rotor: damper bars shorted
                     field winding --[R]-- (shorted)
    after ~95% Ns: remove R, apply DC field

Except in method 4, the motor is started on no load or light load because the pull-in torque is limited.