Motors and transformers
Induction-motor slip
- where
- slip (%)
- synchronous field speed (rpm)
- rotor speed (rpm)
Explain this equation
Gives an induction motor's slip: how far the rotor lags behind the rotating field, as a percentage. Use it to compare nameplate speed with synchronous speed or to judge how heavily a motor is loaded.
n_s is the synchronous field speed in rpm and n_r the actual rotor speed in rpm. s% is the shortfall expressed as a percentage of n_s.
An induction motor only makes torque if its rotor turns slower than the field, because the field has to sweep past the rotor bars to induce current in them. More load means more slip. Dividing by n_s expresses the shortfall as a fraction of field speed, and the 100 makes it a percentage.
A 4-pole 50 Hz motor running at 1440 rpm has 4% slip. Full-load slip is typically a few per cent, lower on larger motors. Slip near zero means the motor is almost unloaded.
Watch out: divide by synchronous speed, not rotor speed. A negative result means the rotor is above synchronous speed and the machine is generating.
Slip measures how far rotor speed falls below field speed, relative to field speed. In normal induction motoring it is positive.
To work backwards, use n_r = n_s × (1 − s). Convert a stated percentage such as 4% into 0.04 before using the reverse equation.
Enter your values to see the result.
Common mistake
Divide by synchronous speed, not rotor speed.
Limits
Steady operating point of an induction machine. Slip varies with load; generating and braking conditions can have different signs or ranges.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.