Characterization of the brushless, doubly-fed machine by magnetic field analysis

The magnetic fields produced by the stator windings and induced via the rotor of a composite, cascade induction machine are investigated with simplified operating assumptions. The rotor used is the single-layer, modified cage developed by A. R. W. Broadway and L. Burbridge (see Proc. IEE, vol.117, p.1277-90, 1970). The input and control circuits share the same stator coils. The operation of the machine is dependent upon spatial harmonics of the stator and rotor conductor structures; therefore, the spatial harmonic contents of the excitation and induced fields are determined. A design formula for minimizing low-order, undesirable harmonics produced by the stator coils is presented. The form of the field harmonics and their direction of travel with respect to the rotor are established. Based upon the physical interactions of the rotor fields, a novel steady-state equivalent circuit with two excitation sources is established, along with the form of the requisite coupling factors. Contributions of the harmonic fields to the instantaneous and average torques are considered.<>