III. Synchronous Motors（三世。）.pdf

Synchronous Motors Dr. Suad Ibrahim Shahl Synchronous Motors Suad Ibrahim Shahl III. Synchronous Motors Synchronous Motors are three-phase AC motors which run at synchronous speed, Synchronous motors have the following characteristics: 飦禔 three-phase stator similar to that of an induction motor. Medium voltage stators are often used. 飦禔 wound rotor (rotating field) which has the same number of poles as the stator, and is supplied by an external source of direct current (DC). Both brush-type and brushless exciters are used to supply the DC field current to the rotor. The rotor current establishes a north/south magnetic pole relationship in the rotor poles enabling the rotor to 鈥渓ock-in-step鈥 with the rotating stator flux. 飦 Starts as an induction motor. The synchronous motor rotor also has a squirrel-cage winding, known as an Amortisseur winding, which produces torque for motor starting. 飦 Synchronous motors will run at synchronous speed in accordance with the formula: Example: the speed of a 24 -Pole Synchronous Motor operating at 60 Hz would be: 120 x 60 / 24 = 7200 / 24 = 300 RPM Synchronous Motor Operation 飦 The squirrel-cage Amortisseur winding in the rotor produces Starting Torque and Accelerating Torque to bring the synchronous motor up to speed. 飦禬hen the motor speed reaches approximately 97% of nameplate RPM, the DC field current is applied to the rotor producing Pull-in Torque and the rotor will pull-in -step and 鈥渟ynchronize鈥 with the rotating flux field in the stator. The motor will run at synchronous speed and produce Synchronous Torque. 1 Synchronous Motors Dr. Suad Ibrahim Shahl Synchronous Motors