Synchronous Speed

This formula gives the synchronous speed: the exact rotational speed of the magnetic field produced by a three-phase AC supply in a motor or generator. It comes from the fact that one electrical cycle advances the rotating field by one pole pair, so the field completes one mechanical revolution after a number of cycles determined by the pole count; converting cycles per second into revolutions per minute gives the factor 120. In practice, more poles mean the field must move through more magnetic zones per revolution, so the rotational speed drops as P increases.

Use this when you need the theoretical speed of an AC machine before slip is considered, especially for induction motors, synchronous motors, and generator speed selection. It is also used in matching motor speed to frequency in drives and in checking whether a machine is running near its intended operating speed.

• Using the actual rotor speed of an induction motor instead of the synchronous speed, which is slightly higher because of slip
• Forgetting that P is the total number of poles, not the number of pole pairs
• Mixing units and using frequency in rpm instead of Hz