Analytical Design Framework for Torque and Back-EMF Optimization, and Inductance Calculation in Double-Rotor Radial-Flux Air-Cored Permanent-Magnet Synchronous Machines

Analytical models have been demonstrated to be effective tools in the analysis of electromagnetic devices by providing fast, yet accurate solutions. In this paper, a comprehensive analytical framework for double-rotor radial-flux air-cored permanent-magnet (PM) machines is developed whereby the average as well as the torque ripples, back-electromotive force (EMF) waveforms, air-gap flux density distribution, and a number of other characteristics regarding the design of the machine are predicted. Also, closed-form expressions for inductances of the stator coils are analytically derived. All machine parameters and material properties including iron saturation are considered in the model. Moreover, an algorithm in order to optimally design the thicknesses of the rotor yokes based on the utilized steel is presented. Finally, genetic optimizations regarding the enhancement of the produced torque and back-EMF are performed. It is also shown that the results obtained from the proposed model match well with those issued from finite element method.

I. Introduction

Air-cored permanent-magnet (PM) machines enjoy many advantages such as slotless structure, zero cogging torque, no stator core losses, high torque density, and ease of manufacturing. However, they suffer from some drawbacks such as large effective air-gap, excessive use of PM materials, and low developed torque and torque ripples, all of which may be the reason for relatively few works in the area and restricted applications in higher power levels. Therefore, satisfactory optimizations and performance estimations toward enhancing their overall performance would be necessary.