How does the number of magnetic poles affect an induction motor’s speed?

In an induction motor, the number of magnetic poles is a key factor in determining the motor's synchronous speed. The synchronous speed of the rotating magnetic field, measured in revolutions per minute (rpm), is calculated using the formula:

$$\text{Speed}=\frac{120f}{p}$$

In this equation, $f$ represents the frequency of the electric supply in hertz, and $p$ is the number of magnetic poles, which must be an even number.1

Because the number of poles appears in the denominator, increasing the number of poles results in a lower synchronous speed for a given supply frequency. Conversely, fewer poles result in a higher speed. For example, while a two-pole motor operating on a 60-hertz supply has a synchronous speed of 3,600 rpm, a motor with more poles will have a lower synchronous speed, such as 1,800 or 1,200 rpm.1

While the rotor speed is always slightly lower than the synchronous speed due to slip, the number of poles remains the fundamental design parameter for setting the motor's operating speed range.12