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Motor Slip Calculator

Enter the synchronous and actual rotor speeds to find the induction motor slip.

How it is calculated

You enter the synchronous speed Ns and the actual rotor speed Nr, both in rpm. The tool calculates slip speed Ns − Nr and slip percent (Ns − Nr) ÷ Ns × 100. The synchronous speed is not worked out for you: it is 120 × f ÷ p with supply frequency f in Hz and pole count p, for example 1500 rpm for 4 poles at 50 Hz and 1800 rpm at 60 Hz. The slip in rpm is rounded to a whole number for display, while the percentage is computed from the unrounded speeds. Ns must be above zero. A rotor speed above Ns is not rejected; it gives negative slip, which is the generating region, not a normal motoring reading.

Worked example

A 4-pole induction motor on a 50 Hz supply has Ns = 120 × 50 ÷ 4 = 1500 rpm. A tachometer reads 1455 rpm on the shaft. Slip speed: 1500 − 1455 = 45 rpm. Slip percent: 45 ÷ 1500 = 0.03, times 100 gives 3.00%. The tool reports a slip speed of 45 rpm. The same motor at a lighter load, say 1485 rpm, would show 1.00% slip, because slip grows as the mechanical load grows.

Try this example →

Common mistakes

  • Entering the nameplate speed as the synchronous speed: the nameplate shows the full-load rotor speed (such as 1455), while Ns is the field speed (1500) for that pole count and frequency.
  • Using the wrong pole count: a 6-pole motor on 50 Hz has Ns = 1000 rpm, so putting 1500 in would give a large negative slip.
  • Mixing 50 Hz and 60 Hz: the same motor has a different Ns on each supply, so the slip must be computed with the matching one.
  • Entering speeds in rad/s or Hz: both fields are rpm.
  • Reading the slip percent as lost power: slip is a speed ratio, and a motor running at 3% slip is normal, not 3% inefficient.

About this calculator

An induction motor's rotor never quite catches up to the rotating magnetic field set up by the stator — that gap is called slip, and it's precisely what induces the current in the rotor that produces torque, so a small amount of slip is inherent to how the motor works at all. Slip is calculated as (synchronous speed − rotor speed) ÷ synchronous speed × 100%, where synchronous speed itself comes from Ns = 120f/p, based on the supply frequency f and the number of motor poles p.

This calculator takes the synchronous and actual rotor speeds and returns both the slip percentage and the slip speed in RPM. Electrical engineers and motor maintenance technicians use slip as a quick health indicator — it typically rises under heavier mechanical load and abnormally high slip readings can point to problems like worn bearings, rotor bar damage, or an undersized motor for its application.

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