Induction Motor Slip and Synchronous Speed Calculator
By Santaji GadepatilinUpdated October 27, 2026
An induction motor always turns a little slower than its rotating magnetic field — that difference is called slip, and it is what produces torque. Read how an AC induction motor works and see induction motor slip explained in more detail. This free induction motor slip and synchronous speed calculator solves for whichever value you leave blank and shows the formula and steps.
How to use the induction motor slip and synchronous speed calculator
Enter the supply frequency, choose the number of poles and enter the motor’s rated speed from the nameplate. You get the synchronous speed, slip and rotor current frequency.
For more background, read Motor Nameplate Details Explained and Synchronous Motor Working Principle.
Induction motor slip and synchronous speed formulas
Ns = 120 × f ÷ P · s = (Ns − Nr) ÷ Ns · fr = s × f
Synchronous speeds
| Poles | 50 Hz | 60 Hz | Typical full-load speed (50 Hz) |
|---|---|---|---|
| 2 | 3,000 rpm | 3,600 rpm | 2,880–2,970 rpm |
| 4 | 1,500 rpm | 1,800 rpm | 1,420–1,485 rpm |
| 6 | 1,000 rpm | 1,200 rpm | 940–990 rpm |
| 8 | 750 rpm | 900 rpm | 710–740 rpm |
| 10 | 600 rpm | 720 rpm | 570–590 rpm |
| 12 | 500 rpm | 600 rpm | 480–495 rpm |
Example
A 4-pole motor on 50 Hz with a nameplate speed of 1,460 rpm:
- Ns = 120 × 50 ÷ 4 = 1,500 rpm
- Slip = (1,500 − 1,460) ÷ 1,500 = 2.67%
- Rotor frequency = 0.0267 × 50 = 1.33 Hz
What slip tells you
- Load: slip rises roughly in proportion to load. A motor running at half its rated slip is lightly loaded — a quick field check of loading.
- Size: large, high-efficiency motors have lower slip (around 0.5–1.5%); small motors may have 4–6%.
- Rotor faults: broken rotor bars show up as current sidebands at (1 ± 2s) × f in motor current signature analysis.
Speed control with a VFD
Changing frequency changes synchronous speed: at 25 Hz, a 4-pole motor runs near 750 rpm. Slip in rpm stays roughly constant at a given torque.
Frequently asked questions
What is synchronous speed?
The speed of the rotating magnetic field, Ns = 120 × f ÷ P. A 4-pole motor on 50 Hz has a synchronous speed of 1,500 rpm.
What is slip in an induction motor?
The difference between synchronous speed and rotor speed, as a percentage of synchronous speed. Typical full-load slip is 1–5%.
Why can't an induction motor run at synchronous speed?
At synchronous speed, the rotor would not cut any magnetic flux, so no current would be induced and no torque produced.
How do I find the number of poles from the nameplate speed?
Pick the synchronous speed just above the nameplate speed. A 2,940 rpm motor on 50 Hz is 2-pole (3,000 rpm); a 960 rpm motor is 6-pole (1,000 rpm).