General Characteristics of the Synchronous Motor

Author: E.E. Kimberly

In contrast to the induction motor, the polyphase synchronous motor runs at a constant speed at any load for which the torque is less than its "pullout" or "breakdown" torque. Such a motor has the advantage also of usefulness in power-factor correction when operated in conjunction with other load of low power factor characteristics. Prior to 1922, the poor starting torque obtainable from the starting amortisseur winding, as developed to that time, discouraged widespread adoption of the synchronous motor. However, improvements in design have vastly broadened its field of application until it has become even more popular than the polyphase induction motor in the larger sizes.

Fig. 19-1. Salient-Pole Rotor for Synchronous Motor

In the polyphase induction motor the rotor field, which reacts upon the stator field to produce torque, is dependent on slip for its existence. In the synchronous motor the rotor field is produced by electromagnets mounted radially on the shaft, as shown in Fig. 19-1, and excited (magnetized) by direct current. The direct current is supplied from some external source through slip-rings similar to those of a slip-ring induction motor. In Fig. 19-2 is shown a small direct-current exciting generator directly connected to the motor. The stator of the polyphase synchronous motor is exactly similar to that of the polyphase induction motor. The mutual attraction of the rotating fields of the stator and the fields of the magnetized salient-pole revolving rotor causes the two sets of fields to "lock together" and to rotate in synchronism after they have been synchronized.

Fig. 19-2. Synchronous Motor With Direct-Connected Exciter