synchronousspeed

Synchronous speed is the speed at which the rotating magnetic field produced by a stator in an alternating-current machine would rotate, and the speed at which the rotor would turn if it stayed locked in step with that field. It is a theoretical reference speed for both synchronous motors and synchronous generators.

The synchronous speed n_s is determined by the supply frequency f and the number of poles P of the stator, and is given by n_s = 120 f / P, with n_s expressed in revolutions per minute (rpm). For example, a 60 Hz supply with a 4-pole stator yields n_s = 120 × 60 / 4 = 1800 rpm. In a 50 Hz system with 6 poles, n_s would be 1000 rpm, and so on.

In a synchronous motor, the rotor carries a direct-current excitation to establish a constant magnetic field. The motor then runs at the synchronous speed, provided the load torque does not exceed the machine’s pull-out torque. If the load increases beyond capability, the motor may lose synchronism. A synchronous motor typically cannot start from rest on its own and requires a starting method such as damper windings, a pony motor, or a variable-frequency drive.

In a synchronous generator (or alternator), the rotor is driven mechanically at the synchronous speed, and the electrical frequency of the output is fixed by the grid or connected system. The rotor’s field excitation controls voltage and reactive power; governors adjust input to maintain constant frequency under varying load.

Synchronous speed is distinct from the speed of induction motors, where the rotor typically runs below n_s due to slip. The concept serves as a foundational reference for design, control, and analysis of both synchronous machines.