The synchronous speed is the result of 120 times the frequency in Hz, divided by the number of poles of the motor. From this formula it becomes evident that the higher the frequency that reaches the engine, the greater the speed of its work, and the reverse also results in a lower speed in an electric motor. And it is this change that the frequency inverter makes, it performs this intervention before the engine input. A rotating magnetic field lies at the basis of the operating principle of the induction machine and this field is caused as follows: a set of three independent coils, 120 ° out of phase in the space, is placed in the stator slots and circular by these three-phase lagged current coils of 120 ° in time.

The rotating or stationary mode of the field of rotating electric machines depends in reality of the adopted reference system. For an observer located in the inductor 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 common ways of producing rotating fields can be summed up in the use of single-phase, rotating, DC-fed windings.