Some factors cause the actual speed at the motor axis to cease to be precisely the speed of the rotating magnetic field, for example, this speed varies smoothly with the mechanical load applied to its axis. As the engine load increases, its slip increases and the rotor speed decreases. Since the rotor speed is smaller, there is more relative movement between the magnetic fields and the rotor of the machine. A larger relative movement produces a greater tension in the rotor, which in turn produces a higher current in the rotor. If the current in the rotor is larger, gates belts it increases its own magnetic field in the rotor.
By its construction, the three-phase electric motor has a difference between the speed of the rotating magnetic field and the actual speed in its rotor. This fact is due to a phenomenon called slippage and is supplied by the motor manufacturer and can vary from machine to machine. Most of the time this phenomenon is described by the manufacturer in percentage (%), varying at full load between 1 to 5% depending on the size and type of the motor.