Comparison of Optimized Control Strategies of a High-Speed Traction Machine with Five Phases and Bi-Harmonic Electromotive Force

Hussein Zahr,Jinlin Gong,Eric Semail,Franck Scuiller

doi:10.3390/en9120952

Abstract

The purpose of the paper is to present the potentialities in terms of the control of a new kind of PM synchronous machine. With five phases and electromotive forces whose first ( E 1 ) and third ( E 3 ) harmonics are of similar amplitude, the studied machine, so-called bi-harmonic, has properties that are interesting for traction machine payload. With three-phase machines, supplied by a mono-harmonic sinusoidal current, the weak number of freedom degrees limits the strategy of control for traction machines especially when voltage saturation occurs at high speeds. As the torque is managed for three-phase machines by a current with only one harmonic, flux weakening is necessary to increase speed when the voltage limitation is reached. The studied five-phase machine, thanks to the increase in the number of freedom degrees for control, aims to alleviate this fact. In this paper, three optimized control strategies are compared in terms of efficiency and associated torque/speed characteristics. These strategies take into account numerous constraints either from the supply (with limited voltage) or from the machine (with limited current densities and maximum acceptable copper, iron and permanent magnet losses). The obtained results prove the wide potentialities of such a kind of five-phase bi-harmonic machine in terms of control under constraints. It is thus shown that the classical Maximum Torque Per Ampere (MTPA) strategy developed for the three-phase machine is clearly not satisfying on the whole range of speed because of the presence of iron losses whose values can no more be neglected at high speeds. Two other strategies have been then proposed to be able to manage the compromises, at high speeds, between the high values of torque and efficiency under the constraints of admissible total losses either in the rotor or in the stator.

Highlights

With more than two independent currents, multiphase machines [1] are logically chosen when fault tolerance is required in critical applications, such as historically marine electric propulsion supplied by a current-source inverter and, since the 21st century, in marine [2,3], aerospace [4,5,6,7] and automotive traction [8,9,10,11,12], with multiphase machines supplied by a voltage source inverter
It can be observed that the saturation of voltage is reached at about 3000 rpm, and 2000 rpm can be considered as a corner speed for the optimization of the torque for the given losses, since the frequency is already 200 Hz for the third harmonic, separating the low and high speed areas and motivating optimal strategies considering all of the losses
A/mm2 for transient operations), in areas of the torque/speed plane where different limitations are found in traction drives: maximum voltage from the supply at high speeds, copper losses in the stator, iron losses separately in the stator and rotor and permanent magnet losses in the rotor

Summary

Introduction

With more than two independent currents, multiphase machines [1] are logically chosen when fault tolerance is required in critical applications, such as historically marine electric propulsion supplied by a current-source inverter and, since the 21st century, in marine [2,3], aerospace [4,5,6,7] and automotive traction [8,9,10,11,12], with multiphase machines supplied by a voltage source inverter. The reason is that, referring to classical wye-coupled three-phase machines, which possess the minimum number of independent currents for achieving a rotating field in normal condition, these machines have. Even in normal working conditions with drives without the need for fault-tolerance, it is possible to take advantage of the numerous degrees of freedoms of a multiphase machine, by considering during the design the number of phases as a parameter. As the freedom degrees number increases, the vector control gives more possibilities: the study of the current harmonic injection for higher torque density keeping a smooth torque [20,21,22,23,24,25,26]; the enlargement, for traction machines working in flux-weakening mode under voltage saturation [27,28,29,30,31], of the area of working. Multi-leg Voltage Source Inverters (VSI) [32,33]

Objectives

Results

Conclusion