Formula for the speed of the magnetic field in an induction motor is Ns = (60 × f) ÷ P. What does P represent?

• A. Number of poles per phase
• B. Number of poles total
• C. Number of pairs of poles per phase
• D. Number of rotor slots

Answer: (B)

The speed of the rotating magnetic field depends on the supply frequency and how many poles the stator has. More poles means the magnetic field must complete more individual pole transitions in one mechanical revolution, so the field turns more slowly at the same frequency.

In this formula, P represents the total number of poles in the stator. For a given frequency, increasing the pole count reduces the synchronous speed, while fewer poles raise it. For example, at 60 Hz, a machine with two poles would have a faster rotating field than one with four poles, illustrating the inverse relationship.

If you come across a form like 60 f divided by P, you can think of P as the total number of poles; the key idea is that the pole count sets how many times the field must pass by in one electrical cycle, directly shaping the field’s rotational speed.