Rajesh P. Deodhar

The air-cored resonant induction machine removes the magnetic core so the fields produced by the windings are truly 3-D in nature. The end-windings normally regarded as a non-active and leakage source in conventional iron-cored machines now become an active part and contribute to the torque production. Therefore, the electromagnetic modeling can no longer be reduced to a 2-D analysis and the 3-D inductance calculation becomes a key problem. The 3-D Finite Element Analysis (FEA) can solve the 3-D magnetic field but, firstly, the validity of its solution depends on the precision in geometry modeling. In particular, representing the end-winding region avoiding conductor clashing can be very complicated. Secondly, 3-D FEA solutions are computationally-slow and therefore inefficient as an “internal routine” of an optimization procedure. This paper proposes a fast analytic 3-D winding inductance estimation method for air-cored resonant induction machines. The approach breaks down the real coils into straight conductors and represents them by single filaments located at their centers, then uses closed-form expressions derived from Neumann integrals to calculate the coil self and coil-to-coil mutual inductances which are then collected into winding phase self and mutual inductances. All the independent coil-pair contributions are isolated so as to eliminate redundant calculations. Good accuracy of the calculated results is confirmed by validation against both 3-D FEA and experimental results, including winding inductance breakdown and overall machine tested performance.

In this paper, two dual three-phase winding configurations are compared based on the Toyota Prius 2010 interior permanent magnet (IPM) machine. It is found that the winding configuration with single-layer full-pitched (SF) windings can improve average torque and reduce torque ripple in constant torque range. The winding configuration with double-layer short-pitched (DS) windings has better torque performance in a constant power range. The electromagnetic performances of the two winding configurations when one winding set is excited and the other one is open-circuited are also compared. The DS winding configuration shows much better performance under this condition. Overall, the dual three-phase winding configuration with DS windings is preferred for dual three-phase IPM machines in electric vehicles. A Toyota Prius 2010 IPM machine equipped with DS windings was manufactured to verify the analyses presented in this paper.

The operation of air-cored resonant induction machines is very sensitive to the tuning of the capacitors. By using the equivalent circuit equations, this paper explores in detail the machine performances under four different tuning criteria: (a) both stator and rotor capacitors resonating with the stator and rotor self-inductances; (b) both capacitors resonating with the leakage inductances; (c) stator capacitors tuned to compensate for the overall stator terminal reactance and no rotor capacitors; and (d) stator capacitors only to cancel out the machine ideal input reactance neglecting resistances, which is the new option introduced in this paper. Extensive comparisons of all tuning options in torque, efficiency, power factor, capacitor size and feasibility are presented and their performances are compared with a pure air-cored induction machine without capacitors. Analytic expressions for the maximum efficiency and maximum torque and related slip values are also derived as functions of the machine parameters, which provides a valuable tool for design optimization. A numerical case study based on parameters of a real air-cored induction machine is discussed and the paper includes the first experimental validation to be carried out on a spinning air-cored resonant induction machine prototype. Results show that using only stator capacitors to compensate for the machine equivalent terminal reactance can be a practical choice to achieve both high torque and high efficiency and avoid rotor capacitors.

Variable flux memory (VFM) permanent magnet (PM) machines exhibit an additional degree-of-freedom for control, i.e. PM magnetization state, and thus excellent flux controllability. Moreover, the hybrid PM topologies having variable PM (VPM) with low coercive force and constant PM (CPM) with high coercive force at the same time, are employed to improve the torque density and the flux controllability. The parallel and series connections between the two different kinds of PMs are both feasible. Based on equivalent magnetic circuits, two-dimensional (2-D) finite element (FE) analyses and experiments, the VFM machines with these two connection types are investigated and compared in this paper. The results reveal that the VPM with series connected CPM is beneficial for more stable working point and higher torque density. A pair of VFM prototypes with parallel and series hybrid PMs respectively are manufactured and tested to validate the analyses.

This paper proposes a novel variable flux memory (VFM) machine, in which the “constant” permanent magnet (CPM) with high coercive force and the “variable” PM (VPM) with low coercive force are alternatively located in the interior-PM rotor. Thus, the VPMs and CPMs are magnetically connected in series, with which the CPMs can assist the VPMs to withstand the unintentional demagnetization caused by armature reaction. Therefore, a high armature current can be applied to the machine. Meanwhile, the reluctance torque is retrieved. Thus, a high torque density can be obtained. Based on two-dimensional finite element analysis, first, the electromagnetic performance of the proposed VFM machine in two extreme magnetization states is evaluated in detail. Then, the demagnetization and remagnetization characteristics are investigated, in which the working points of VPMs are illustrated. Furthermore, the advantages of improved efficiency of the proposed VFM machine are demonstrated. A prototype machine is manufactured and tested to validate the predictions.