Electric motors operating from induction can be widely specified and considered single operating devices, by the various, relatively decisive factors, among them, for example, by the fact that only polyphase alternating voltages can be applied to their own stator.
The induction machines can be evaluated as double excitation, where an alternating current of alternating current, should be applied to the two windings of the stator, which may also be called the armature, as well as the rotor itself. MB-3
The voltage applied to the armature winding is a frequency and potential excitation voltage, most of which is stable, provided by a multi-phase or even single-phase bus, in the same way as the synchronous type machines. Consequently, the concentrated voltage to the rotor is an induced voltage, of variable frequency and potential, caused as a consequence of the speed of the rotor in relation to the synchronous speed.
Thus, it is very important to stress that an alternating voltage of variable frequency can be induced in its rotor, in the same way as it induces an alternating voltage, through a transforming action in a secondary one of a transformer.