How to Determine the Number of Poles in an Electric Motor Based on Its Speed?

To determine the number of poles in an electric motor based on its speed, you can use the following formula, which relates the motor's synchronous speed, the number of poles, and the frequency of the AC power supply:

Where:

• $N_s$= Synchronous speed of the motor in revolutions per minute (RPM)
• $f$f = Frequency of the AC supply in Hertz (Hz)
• $P$P = Number of poles in the motor

Steps to Find the Number of Poles:

1. Identify the motor's synchronous speed (or approximate speed) in RPM. If you only have the operating speed of the motor, note that asynchronous (induction) motors typically operate at speeds slightly less than the synchronous speed.
2. Know the supply frequency. Most motors in the U.S. operate on a 60 Hz supply, while many countries outside the U.S. use 50 Hz.
3. Rearrange the formula to solve for the number of poles:

4. Plug in the known values for $f$f (frequency) and $N_s$Ns​ (synchronous speed), and calculate $P$P.

Example Calculation:

Let's say you're working with a motor running on a 60 Hz supply and you measure its synchronous speed to be 1800 RPM:

So, this motor has 4 poles.

Important Notes:

• Synchronous speed: This is the speed at which the magnetic field of the stator rotates. In an induction motor, the actual rotor speed will be slightly less than the synchronous speed, depending on the load.
• For motors with more poles, the synchronous speed will be lower for the same frequency.