Comparison of 3- and 9-phase brushless PM drive systems for aerospace actuation

Abstract

Nearly all electric vehicles (EVs), hybrid electric vehicles (HEVs), more electric aircraft, electric ship propulsion and others power conversion industrial applications uses power electronic inverters to convert AC to DC or vis versa, as in the starter/generator machine system and generation/regeneration process of the traction motor. Also, it has become possible to integrate HEVs power electronics component in one package with one set of liquid cooling. Recently, compactness and reliability of the power conversion and generation system design in automotive, in particular, and in aerospace industries are growing with global necessity of shifting from conventional airborne emission vehicles and aircrafts to the green all electric ones. Therefore, an advanced technology in the area of energy storage, conversion and dc-link capacitors, which account for a major fraction of the volume, weight and also cost of the inverter, is demanded by the developer of HEVs and other industrial areas. In three-phase system a dc-link voltage source inverters (VSI) are fed via controlled or uncontrolled rectifier bridges from a main source. Here, a dc-link capacitor is usually used to maintain a stiff dc-link voltage across the VSI, and to perform other tasks, as in [1]. Such that, in this application, the dc-link capacitor deals with the main voltage fluctuation and provides a dc-link current ripple to suppress the generated electromagnetic interface (EMI) caused by pulsating current (ripple current) associated with the inverter switching. It is recognized in the literature [1-7] that for voltage stiff inverter the dc-link capacitor adequate selection, sizing and minimizing presents a major issue for improving electrical system compactness and utilization. Lai et al discusses the use of low-inductance high current film capacitor in a high-power dc-bus inverter for battery-powered traction motor drives [2]. Another discussion that shows merits of an advanced film capacitor technology over conventional electrolytic capacitor for dc-link applications was presented by Grinberg et al [3]. A dc-link capacitor minimization method via direct capacitor current control has been described by Gu and Nam [4]. Such that, the dc-link capacitor can be minimized by making the converter side dc-link current similar to the inverter side dc-link current waveform in a pulse width modulation (PWM) converter-inverter system. However, the dc-link capacitor reduction technique of Gu et al is not for stiff voltage-source inverter. A simple analytical expression for current stress on the dc-link capacitor caused by the load-side inverter of the voltage dc-link converter system is derived by Kolar and Round [1]. Conclusions have been obtained by some of the previous papers, which declare that the dc-link capacitor of practical systems usually determined by the effective capacitor current stress and not, for example, by a given maximum admissible value of the amplitude of the dc-link voltage ripple [1]. In Dahono paper [5], analysis and minimization of output current ripple of multiphase PWM inverters was presented. The minimization of this current ripple is achieved for three-phase PWM inverter by injecting a twenty-five percent third harmonic in the reference sinusoidal signal. Whilst, minimization of the output current ripple of a multiphase PWM inverters can not be attained and it will become unsymmetrical if harmonics are injected into the sinusoidal reference signal [5]. Also the paper discussed by Dahono mentioned that multiphase PWM inverters with number of phases equal to multiple of three will give better results than the one that is not a multiple of three, for example, if the number of phases is a multiple of three, the multiphase PWM inverter can be operated as several three-phase PWM inverters. As for the authors knowledge, no verification nor investigation of this claim have been found in depth in the literature. In the paper, 3-phase and 9-phase PWM inverter modules with different threeand nine-phase brushless permanent magnet (BLPM) machine configurations will be analysed in detail to show the adequate use of this configuration to either eliminate the need or significantly reduce the size of the dc-link capacitor that suppress the generated EMI caused by pulsating current associated with the inverter switching for stiff voltage-source inverter. In addition, different operating points on the general torque speed curve for different BLPM machine drive configurations will be investigated to give decision of the adequate and best machine drive configuration, in terms of improving power and torque capabilities with the minimization of output torque ripple, which can be practically implemented and used in many promising industrial areas. (2 pages)