Slip designates the difference between synchronous rotating magnetic field

und and the rotor in a non-synchronous induction motor:

The rotor attempts to follow the stator rotating magnetic field but can never reach it due to its velocity of circulation, which would then be equivalent to a rotor frequency of f_L = f∙s = 0 and would no longer induce any voltage. The slip results from the ohmic loss in the rotor and from friction, air gap and iron losses. For smaller motors it ranges between 10… 5% and drops to 1% for motors from 1000 kW. Smaller motors with a built-in fan and high speed exhibit the highest slip.