Synchronous speed is the speed of the rotating magnetic field internally formed in the electric motor. Through it one can know the value of the motor rotation.
Slightly lower than the synchronous speed, the asynchronous speed is the rotation
Measured on the motor shaft. In summary, it is the true rotation of the engine, discounting the losses; Hence the name of asynchronous motor (in Portuguese asynchronous means out of sync, in the case between the speed of the magnetic field is the speed of the motor axis). The value read on the motor nameplate, so the nominal value is the value of the asynchronous speed.
Slippage is the difference between the speed of the magnetic field (synchronous speed) and the rotation of the motor, also called slip. The slipping of an engine normally varies depending on the load: when the load is zero (no-load motor) the slip will be practically zero; When it is the nominal, the slip will also be the nominal.
Slipping can be given in rpm or%. Another feature that we must take into account is that slippage decreases as the nominal power of the electric motor increases.