In an AC machine, what is synchronism in the context of synchronous generators?

• A. The rotor speed is synchronized with the supply frequency; rotor speed constant at n_s = 120 f / P.
• B. The rotor speed is synchronized with the supply frequency during startup only.
• C. The rotor speed varies with load to maintain voltage.
• D. Synchronism means the stator current remains in phase with the rotor current.

Answer: (B)

Synchronism means the rotor of a synchronous generator runs at the same speed as the rotating magnetic field produced by the stator. Because the electrical grid fixes the frequency f, the mechanical speed must match the synchronous speed given by n_s = 120 f / P, where P is the number of poles. When the machine is synchronized, the rotor’s field stays locked to the stator field, so the generator produces power in step with the grid and speed remains essentially constant despite changes in load, within the machine’s pull-out limits. Startup is the process used to bring the machine into that locked condition, after which it stays synchronized as long as it’s connected and operating properly. If synchronism is lost, the machine can slip out of step and stop contributing to the grid.