Mathematical Modeling And Analysis Of High Torque Density Multiphase Squirrel Cage Induction Machines

Multiphase machines have many advantages over their three phase counterparts due to more phases and a lower content of magnetomotive force(MMF). These merits include higher efficiency and system reliability, pulsating torque exhibiting lower amplitude but increasing frequency. The ability of fault tolerance operation also excels at three phase machines. Nevertheless, poles changed by altering phase sequence set of phase currents is the unique characteristic that mul-tiphase machines possess, which can even be employed to enhance torque density combined with lower current harmonics injection. In view of their many strengths and it is practicable to feed them by power converters, Research on them has been still proceeding by researchers and engi-neers from 1980s.This paper focused on the operation principles of multiphase squirrel-cage induction ma-chines(MSCIMs) with high torque density, mathematical modeling and performance analysis un-der non-sinusoidal excitation. The main content of this paper was as follows:Based on winding function method and circular diagram of winding structures of multiphase induction machines(MIMs), a comprehensive MMF analysis of symmetrical and unsymmetrical MIMs with arbitrary phase number was exhibited. It demonstrated a lower MMF content is present in MIMs and poles can be changed by altering phase sequence set of phase currents. Because the improvement of flux density waveform will lead to full utilization of iron core and torque density enhancement further, study of the characteristic of arbitrary traveling waveform was implemented. After describing the method to synthesize arbitrary traveling waveform by making use of funda-mental component and harmonics of phase currents in MIMs, case study of square traveling wave-form was conducted and spikes occurred in the wave shape of magnetizing current was clarified in theory.Predicated on winding function method and multi-loop theory, the mathematical model of MSCIMs with arbitrary phase number was established in natural reference frame. Expressions for calculations of parameters in this model were yielded. Multiphase transformation matrix was shown so as to simplify this complicated model.For purpose of showing the characteristic of pole-changing by altering phase sequence set of phase currents in the model, the criterion of selecting the maximum value of space harmonic in MIMs with full-pitch concentrated winding was proposed and parameters of the mathematical model in dq rotating reference frame with arbitrary velocity was given. Meantime, space vector defined in three phase induction machines was extended to MIMs.In order to verify the model proposed, A state space model of nine phase full-pitch squirrel-cage induction machine with 44 rotor bars and concentrated windings was constructed in Mat-lab\Simulink, which took the 1st,3rd,5th,7th space harmonics into consideration. In terms of design parameters and experimental data of a 450kW machine prototype, a comparative analysis of performance including the response of stator phase current and torque speed curve when loaded between simulating and experimental results was implemented under square and trapezoidal power supplies. At the same time, the status of torque pulsation was exposed in this nine phase prototype under square and SPWM excitation. To further illustrate the advantages of full-pitch MIMs with concentrated windings, simulation of one three phase induction machine with medium power un-der custom PWM strategies was carried out. At last, according to the electrical parameters of nine phase induction machines in the dq rotating reference frame and the definition of space vectors, turns ratio transformation in complex vector equivalent circuits was preliminarily analyzed.