In general, induction machines can also be specified and considered as single operating machines, for some decisive factors, among them, for the reason that only polyphase alternating voltages can be applied to their own stator.
Thus, it is important to note that an alternating voltage of variable frequency may be induced in its rotor, in the same way as it induces an alternating voltage, by means of a transforming action on a secondary one of a transformer.
The induction machine is consequently considered as a dual excitation machine in which an alternating current voltage can be applied to both stator windings, also known as armature, as well as the rotor itself.
The voltage applied to the armature winding is a frequency and potential excitation voltage, most of which is constant, supplied by a multi-phase or even single-phase bus, in the same way as the synchronous type machines. The voltage applied to the rotor is an induced voltage, of variable frequency and potential, produced as a consequence of the speed of the rotor in relation to the synchronous speed.