Double-Stator Switched Reluctance Machines (DSSRM): Fundamentals and Magnetic Force Analysis
Abstract
In this paper, a new switched reluctancemachine witha double-stator configuration (DSSRM) is introduced. The proposeddesign is based on optimization of themotional forces, whichleads to a high-grade electromechanical energy conversion process.A local examination of the force densities within and throughouta conventional switched reluctance machine (SRM) shows that themajority of the force produced is in the radial direction and doesnot contribute to motion. If the normal forces happen to be inthe direction of motion, a larger motional force profile for SRM isyield. Based on these guidelines, a new SRM (DSSRM) is proposed.To compare energy conversion efficiency of DSSRM with that ofthe conventional SRM, a finite element model is constructed. Anexperimental prototype of the proposed machine is developed, andthe phase inductance is measured. The results of our investigationsindicate that the proposed geometry offers superior performancein terms of higher power density and higher percentage of themotional forces.
Index Terms—Electromagnetic field analysis, energy conversion,finite element methods, reluctance machines.
I. INTRODUCTION
SWITCHED reluctance machines (SRMs) have attractedconsiderable attention for a variety of industrial applications.A rugged structure, robust performance in the presenceof harsh ambient conditions, and a relatively easy control, andlow-cost manufacturing have contributed to this trend [1]. Performanceof SRM drives can be improved by means of optimalexcitation as well as magnetic design. For many motoring applications,maximum torque capability and maximum efficiency ofSRM are viewed as a performance index [2], [3].Despite considerable progress in development of powerelectronics-based control of SRM drives, efforts on optimalmagnetic design of the SRM are far from exhausted. An improvementin the power density significantly changes the sit uation and opens the possibility to consider this machine forsurvivable high-performance applications.The concept of design optimization of the SRM has beenaddressed by several researchers. In [4], a comparison of theefficiency improvements that can be obtained in an SRM usingdifferent materials is presented. Desai et al. [5] introduce a newfamily of SRMs which have higher number of rotor poles thanstator poles and can produce higher torque per volume. In [6],a novel SRM for low-cost production and high power applicationsis presented. Mecrow et al. [7] examine the performanceof SRM, which employs a segmental rotor construction in preferenceto the usual toothed structure, and demonstrates that thismachine delivers over 40% more torque per volume than doesa conventional SRM.Increasing power density of conventional SRM requires arelatively smaller size air gap. This will also force the machineinto a highly saturated operation, accompanied by high radialforces, causing mechanical noise and vibration. In [8], radialforce is calculated using Maxwell stress method and the rotoreccentricity effects on the motor, and reduction of these effectsby winding method is investigated. In [9], a sinusoidal currentexcitation scheme is proposed for the radial force control of a12/8 pole SRM.Double salient configuration of SRM along with the absenceof magnetic sources on the rotor dictates a very limited participationof the air gap in energy conversion. Furthermore, themotional forces are shown to be concentrated in very limited areas.This has forced SRM to operate mostly under saturation toachieve comparable force densities with respect to other singlyexcited candidates. Given this fundamental boundary condition,there is no significant room for force optimization by the virtueof altering the geometry [8].A physically insightful analysis of the electromechanical energyconversion process, including origins and spatial distributionof electromagnetic field quantities, has shed light on theprocess of SRM magnetic performance and design [10]. Thisstudy has unveiled new opportunities for substantial improvementin SRM performance such as energy conversion efficiency(ECE) and torque density.In order to verify the effectiveness of any new magnetic design,development of an accurate and comprehensive force calculationtechnique is essential. Although computation of theelectromagnetic force in SRM can be performed using differentmethods, in this paper theMaxwell stress tensor (MST) methodis selected to accommodate the necessary precision and accessto local force densities.


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