At first, induction machines can also be considered as single excitation machines, because only their polyphase alternating voltages are applied to their stator. However, let us know that an alternating voltage of variable frequency is induced in its rotor, in the same way as it induces an alternating voltage, by transforming action, in a secondary one of a transformer. The RF151DTR electric induction motor, therefore, is a dual excitation machine, where an alternating voltage is applied to both the stator (armature) and rotor windings.
The voltage applied to the winding of the armature is a frequency voltage excitation of generally constant frequency and also constant potential, provided by a multi-phase or single-phase bus, in the same way as the synchronous machines. The voltage applied to the rotor is an induced voltage, of frequency and alterable potential, produced by the consequence of the speed of the rotor in relation to the synchronous speed. The three-phase induction motor generally operates at a constant speed that varies slightly with the mechanical load applied to the shaft, due to its robustness and simplicity, it is a very applied motor and is suitable for almost all types of driven machines, found in practice.