Induction Motor Slip & Speed

Compare measured rotor speed with synchronous speed for an induction motor.

How this calculation works

Synchronous speed is the rotational speed of an induction motor's magnetic field for a given supply frequency and pole count. Enter the electrical frequency, the total number of poles and the measured rotor speed. The pole count must be an even integer; enter poles rather than pole pairs. Slip is the difference between synchronous and rotor speeds divided by synchronous speed. Multiplying that fractional slip by supply frequency gives the rotor electrical slip frequency. The model covers forward motoring, so speeds above synchronous speed are rejected as outside its scope. A high slip warning calls attention to the operating condition but does not diagnose its cause. Slip alone is not a reliable motor load or efficiency measurement, nor a protection assessment.

Inputs and units

Electrical supply frequency (Hz)
Total motor pole count (poles): An even integer, not pole pairs
Measured rotor speed (r/min)

Method and formula

Ns = 120f/P in r/min; slip s = (Ns − Nr)/Ns; rotor electrical frequency = sf.

Worked example

Example inputs

Electrical supply frequency: 50 Hz
Total motor pole count: 4 poles
Measured rotor speed: 1440 r/min

Calculation steps

Calculate synchronous speed from 50 Hz and four poles: Ns = 120 × 50/4 = 1500 r/min.
Calculate the speed difference: 1500 − 1440 = 60 r/min.
Divide by synchronous speed: slip = 60/1500 = 0.04, or 4%.
Calculate rotor electrical slip frequency: 0.04 × 50 = 2 Hz.

Example results

Synchronous speed: 1500 r/min
Slip: 4 %
Rotor electrical slip frequency: 2 Hz

Assumptions

Conventional induction motor in forward motoring operation
Pole input is the total number of poles

Limitations

Slip is not a reliable standalone motor load/efficiency measurement
Excludes generating mode and variable transient behavior

Sources

U.S. DOE Fundamentals Handbook: Electrical Science, Volume 4: ES-12 pp. 5–6, synchronous speed and percent slip