Motor Speed Calculator

About the Motor Speed Calculator

Calculates the synchronous speed of an AC induction motor from the supply frequency and number of poles, plus the actual speed given a slip percentage.

How to use this calculator

1. Enter the supply frequency in hertz, such as 120 Hz or 100 Hz.
2. Enter the number of poles on the motor, such as 2, 4, or 6.
3. Use the calculator to compute the synchronous speed in revolutions per minute.
4. Compare the result with the motor nameplate speed if you want to estimate slip.

The formula explained

The formula N = \frac{120 f}{P} computes synchronous speed in revolutions per minute, where frequency and pole count determine how fast the rotating magnetic field turns. This is the ideal speed for an AC motor, not the exact loaded speed.

• N = synchronous speed in revolutions per minute
• f = supply frequency in hertz
• P = number of motor poles

Step by step method

1. Write down the frequency f and the number of poles P.
2. Multiply 120 by f.
3. Divide the result by P to get N in revolutions per minute.

Worked example

Problem. Find the synchronous speed of a 4 pole motor on a 60 Hz supply.

1. Use the formula N = \frac{120 f}{P}.
2. Substitute the values: N = \frac{120 \times 60}{4}.
3. Compute N = 1800 revolutions per minute.

Tips and common mistakes

• If the motor is under load, its actual speed is usually a little less than the synchronous speed because of slip.
• Make sure you use the correct pole count, because doubling the poles halves the synchronous speed.

Frequently asked questions