Slip Calculation for Electric Motors

Slip is the difference in speed between the stator rotating field (stator) and the rotor, usually given as a percentage value, relative to the rotating field speed. If the rotor were to rotate at the same speed as the stator rotating field, there would be no more magnetic flux change in the rotor, and the rotor would not apply any torque. The rotor speed is therefore always lower than the rotating field speed in motor operation.

For example: In a three-phase asynchronous machine with a stator coil for each of the three phases, the magnetic rotating field rotates at 3000 rpm at a line frequency of 50Hz. According to the nameplate, however, the speed of the armature is only 2700/min. The slip of 300/min depends on the load and is almost proportional to the rotor efficiency. At rated motor power, it is between 1.2% and 10% of the rotating field speed, depending on the motor size. Smaller three-phase motors have poorer efficiencies and consequently greater slip values.

To simplify the slip calculations, we have expanded our “Online Calculator” section with the formulas listed below.

S = (Ns - N) / Ns * 100%

Measuring Units:

Slip                                       S = Percent [%]
Synchronous speed          Ns = Revolutions per minute [rpm]
Rotor speed                        N = Revolutions per minute [rpm]