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Mechanical Calculators

Motor Speed Calculator

Calculate an AC induction motor's synchronous speed from supply frequency and pole count.

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About this calculator

This calculator finds the synchronous speed of an AC induction motor from the supply frequency and the number of poles it is wound for. Actual running speed under load (nameplate RPM) is slightly lower than synchronous speed due to slip.

Formula

N_sync = (120 × f) ÷ P
  • fSupply frequency — Hz
  • PNumber of poles — count
  • N_syncSynchronous speed — RPM

Worked Example

Given: A 4-pole motor runs on a 50Hz supply.

Result: N_sync = (120 × 50) ÷ 4 = 1500 RPM. A typical nameplate full-load speed for such a motor is about 1440–1460 RPM due to slip.

How to Use

  1. Enter the supply frequency (50Hz in India/Europe, 60Hz in the US).
  2. Select the motor's pole count (from the nameplate — a 4-pole 50Hz motor is the most common industrial motor).
  3. Click Calculate to get synchronous speed.

Important Assumptions

  • This is synchronous (no-load, theoretical) speed. Actual full-load speed is lower due to slip — typically 2–5% for induction motors.

Common Mistakes

  • Comparing this synchronous speed directly to a motor's nameplate RPM and assuming they should match exactly — the nameplate value already includes slip.

Frequently Asked Questions

Why does my 4-pole motor nameplate say 1440 RPM, not 1500?

1500 RPM is the synchronous (no-slip) speed; induction motors always run slightly slower than synchronous speed under load — this difference is called slip.

How do I know how many poles my motor has?

It's usually printed on the nameplate; otherwise it can be inferred from the rated RPM being just below one of the standard synchronous speeds (3000, 1500, 1000, 750 RPM at 50Hz).

These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.