What is synchronous speed?

What is synchronous speed?

• A. The speed of the rotor under load.
• B. The speed of the rotating magnetic field in the stator, Ns = 120F/P.
• C. The speed of the stator windings.
• D. The speed at which the rotor reaches slip zero.

Answer: (B)

Synchronous speed is the speed at which the stator’s magnetic field rotates. It’s determined by the supply frequency and the machine’s number of poles, and it’s given by Ns = 120 f / P (rpm). This speed is a property of the magnetic field, not the actual rotor motion under load. The rotor tends to chase that field, but under load it runs slightly slower due to slip, with slip defined as s = (Ns − Nr)/Ns, where Nr is the rotor speed. For example, at 60 Hz with a four-pole machine, the synchronous speed is 1800 rpm. The stator windings stay stationary themselves; they create the rotating field that drives the rotor.

Synchronous speed is the speed at which the stator’s magnetic field rotates. It’s determined by the supply frequency and the machine’s number of poles, and it’s given by Ns = 120 f / P (rpm). This speed is a property of the magnetic field, not the actual rotor motion under load. The rotor tends to chase that field, but under load it runs slightly slower due to slip, with slip defined as s = (Ns − Nr)/Ns, where Nr is the rotor speed. For example, at 60 Hz with a four-pole machine, the synchronous speed is 1800 rpm. The stator windings stay stationary themselves; they create the rotating field that drives the rotor.