Electromagnetic Space Vectors and General Equivalent Circuits
As pointed out in Chap.1, the simplest model of a three-phase induction motor consists of two magnetically coupled sets of symmetrical, three-phase concentrated windings, one on the stator and the other on the rotor. These two sets of windings were schematically presented in Chap.1, Sec. 1.2, Fig. 1.4. Recall that the actual rotor windings (rotor bars) have been replaced by a short-circuited set of symmetrical three-phase windings that have the same number of turns as those of the stator. Moreover, it is assumed that appropriate currents flow in the new rotor windings in order to create, along with the stator currents, the same flux within the air gap as is present in the actual motor. We have demonstrated that such rotor winding replacement raises no problems, since, for a stator-based observer, the magnetic reaction of the rotor can only be detected through its contribution to the creation of magneto-motive forces within the air gap. It is irrelevant for an observer to know the actual structure of the rotor windings or the actual current flowing within them. Insofar as the magnetic reaction of the rotor sensed by a stator-based observer is concerned, a fictitious set of rotor windings in which appropriate currents flow behaves like actual windings, provided they create the same magneto-motive forces within the air gap.
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