Why are 1,800 and 1,200 revolutions per minute common synchronous speeds for 60-hertz motors?

The synchronous speed of an electric motor is determined by the frequency of the power supply and the number of magnetic poles built into the machine. The formula for calculating this speed in revolutions per minute (RPM) is:

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

In this equation, $f$ represents the frequency in hertz (60 Hz in this case) and $p$ represents the number of magnetic poles, which must be an even number.1

The common synchronous speeds of 1,800 and 1,200 RPM occur because of the specific number of poles used in the motor's construction:

• 1,800 RPM: This speed is achieved by a 4-pole motor ($\frac{120\times 60}{4}=1,800$).
• 1,200 RPM: This speed is achieved by a 6-pole motor ($\frac{120\times 60}{6}=1,200$).

By increasing the number of pole pairs, engineers can design motors to operate at these standard, lower synchronous speeds while maintaining the same frequency of the electric supply.1