What is synchronous speed?

Synchronous speed is the theoretical no-load speed of an AC motor's rotating magnetic field, equal to (120 × frequency) / number of poles. A real induction motor always runs slightly slower due to slip.

Why must the pole count be even?

AC motor poles always come in North–South pairs, so the total pole count is always an even number (2, 4, 6, 8, etc.). An odd number is physically impossible.

What is slip and how much is normal?

Slip is the difference between synchronous speed and actual rotor speed, expressed as a percentage. Typical induction motors have 1–5% slip at full load. Higher slip means more torque is available but less efficiency.

Does this tool work for three-phase and single-phase motors?

Yes. The synchronous speed formula applies equally to single-phase and three-phase AC induction motors. Only the frequency and pole count matter for this calculation.

How do I find the pole count if it is not on the nameplate?

Divide 120 by the supply frequency and multiply by the stated synchronous speed in the formula rearranged: poles = (120 × frequency) / RPM_sync. You can also physically count the stator poles inside the motor.

Motor RPM From Frequency

Name: Motor RPM From Frequency
Availability: InStock
Author: Nham Vu

Calculate AC motor synchronous speed and actual induction motor RPM from supply frequency, number of poles, and slip percentage.

Motor Parameters

Supply Frequency

Number of Poles

Poles are always even (come in N–S pairs).

Include slip (induction motor)

Results

Synchronous Speed

1,800 RPM

Formula: (120 × frequency) / poles

Parameter	Value
Supply frequency	60 Hz
Number of poles	4
Pole pairs	2
Slip	3%
Speed loss due to slip	54 RPM

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Summary

Calculate AC motor synchronous speed and actual induction motor RPM from supply frequency, number of poles, and slip percentage.

How it works

Enter the AC supply frequency in Hz (e.g. 50 or 60).
Enter the total number of poles on the motor (must be an even number: 2, 4, 6, 8…).
The synchronous speed is computed as RPM = (120 × frequency) / poles.
For induction motors, enter the slip percentage (typically 1–5%) to get actual shaft RPM.
Results update instantly as you type.

Use cases

Determine the synchronous speed of a new motor before installation.
Verify nameplate RPM against supply frequency and pole count.
Estimate shaft speed of an induction motor given measured slip.
Compare 50 Hz vs 60 Hz motor performance when changing supply regions.
Size belt drives and gearboxes based on expected motor output speed.
Troubleshoot over-speed or under-speed conditions in variable-frequency drive (VFD) systems.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu