Starting of Synchronous Motors

Author: E.E. Kimberly

In order that a synchronous motor may be self-starting as such, the rotor must be so light and its inertia must be so small that it will accelerate to synchronism and its field poles will lock in step with the rotating stator-produced fields as they pass by at synchronous speed. In any but the very smallest synchronous motors used in timing devices, the rotor inertia is too great for such rapid acceleration, and the resulting alternate torque in opposite directions produces no rotation.

In starting an industrial synchronous motor, it must be driven to some speed within about 5 per cent of synchronous speed by some means before the revolving-field structure is magnetized. When the field is then magnetized, it pulls into synchronism with the rotating field produced by the stator winding, and the original driving torque may be discontinued. The rotor may be brought to proper speed by a belted or direct-connected motor or by the more usual method of providing the pole faces with a squirrel-cage winding similar to that of an induction motor. This winding, called an amortisseur winding, is shown occupying the longitudinal slots in the faces of the poles in Fig. 19-1. After the rotor has attained near-synchronous speed by amortisseur-winding drive, the field structure is excited by direct current, and the rotor locks with and runs synchronously with the rotating field.

Synchronous motors are available in both line-start and reduced-voltage-start types. The type chosen is usually determined by the starting torque, synchronizing torque and horsepower required by the load, and also by the capacity of the power system on which it is to be used.