A rotating magnetic field is at the base of the operating principle of the induction electric motor. This field is produced as follows: a set of three independent coils, 120 ° out of phase in the space, is placed in the stator slots, and three-phase lagged current coils of 120 ° are circulated over time.

The rotating or stationary character of the field of rotating electric machines depends in reality on the adopted reference system. For an observer located in the induction of a synchronous machine with rotating inductor, the field of this machine is rotating. For an observer located in his rotor, the field is stationary. The simplest usual ways of producing rotating fields can be summarized in the use of rotating monophasic windings, powered by direct current.

The synchronous speed is the product of 120 times the frequency in Hz, divided by the number of poles of the motor. From this formula it becomes clear that the higher the frequency that comes to the engine, the greater the speed of its work, and the reverse also influences at a lower speed in an electric motor. And it is this change that the frequency inverter makes: it performs this intervention before the electric motor enters.