Slip - Calculation of electric motors

The slip is the speed difference between the stator rotating field (stator) and the rotor, usually specified as a percentage value in relation to the rotating field speed. If the rotor were to rotate at the same speed as the stator rotating field, no magnetic flux change would be possible in the rotor and the rotor would not produce any torque. The rotor speed in motorised operation is therefore always lower than the rotating field speed. Example: In a three-phase asynchronous machine with a stator coil for each of the three phases, the magnetic rotating field rotates at 3000 revolutions per minute at a mains frequency of 50 Hz. According to the rating plate, however, the speed of the armature is only 2700 rpm. The slip of 300 rpm 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 AC motors have poorer efficiencies and consequently higher slip values.

S=(_Ns - N) / _Ns * 100%

Units of measurement used:

Slip S=per cent [%]
Synchronising speed Ns₌ revolutions per minute [rpm]
Rotor speed in N=revolutions per minute [rpm]