%% This LaTeX-file was created by Thu Feb 17 13:52:25 2000 %% LyX 1.0 (C) 1995-1999 by Matthias Ettrich and the LyX Team %% Do not edit this file unless you know what you are doing. \documentclass{IEEEtran} \usepackage[T1]{fontenc} \usepackage{geometry} \geometry{verbose,tmargin=20mm,bmargin=20mm,lmargin=20mm,rmargin=20mm} \makeatletter %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands. \providecommand{\LyX}{L\kern-.1667em\lower.25em\hbox{Y}\kern-.125emX\@} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands. \newcommand{\lyxaddress}[1]{ \par {\raggedright #1 \vspace{1.4em} \noindent\par} } %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands. IEEEtran \makeatother \begin{document} \title{Performance comparison of reluctance synchronous and induction traction drives for electrical multiple units} \author{J. J. Germishuizen, F. S. Van der Merwe, K. Van der Westhuizen, M. J. Kamper } \maketitle \lyxaddress{Department of Electrical and Electronic Engineering\\ University of Stellenbosch, Banghoek Road\\ 7600 Stellenbosch, South Africa} \begin{abstract} In this paper Reluctance Synchronous Machine (RSM) and Induction Machine traction drives are compared. The investigation is done at a rated power level of 110 kW. An optimised 110 kW traction IM for electrical multiple units (EMUs) was obtained for this investigation. For a first comparison the same stator is used for both the RSM and the IM. For the RSM the rotor of the IM is removed and replaced by an optimum designed, normal laminated flux-barrier rotor. The focus is on the comparison between the two motors and the optimum design of the rotor for the RSM. In the comparison the torque, efficiency, power factor and temperature rise as funtions of speed are investigated. \end{abstract} \section{Introduction} Internationally there is renewed interest in the design of Reluctance Synchronous Machine (RSM) aimed at obtaining the same or better performance than its Induction Machine (IM) counterpart. Despite the lower motor power factor and consequently a probably higher kVA rating of the RSM inverter, the RSM drive has attractive advantages from a machine point of view, including, amongst other things, a good torque density and efficiency, a very simple current vector controller, and comparatively low manufacturing costs. Previous research on RSMs at the University of Stellenbosch is outlined in this section. First, a RSM rotor \cite{Hosinger(1971)} was used in a 5.5 kW IM stator. This was followed by an optimum designed 9 kW RSM, using a standard 5.5 kW IM frame, and a 42 kW RSM using a 37 kW IM frame. Lastly, the 42 kW RSM was used to investigate the effect of the number of flux barriers per pole. A brief summary for each of the four cases and the outcome are as follows. \section{Reluctance synchronous motor} The torque equation of the RSM is the starting point for designing the rotor lamination and is given by \begin{equation} T_{RSM}=\frac{3}{2}p\left( \lambda _{ds}I_{qs}-\lambda _{qs}I_{ds}\right) \end{equation} where \( p \) is the number of pole pairs, \( L_{d} \) and \( L_{q} \) the d- and q-axis inductances respectively and \( I_{d} \) and \( I_{q} \) the d- and q-axis fundamental stator current components respectively. \section{Performance Indices} For the comparison the same stator \emph{rms} current for both machines, at the same supply frequency, is used. The reason for this is that the IM is used as reference in the comparison, and that both machines will be operated to obtain the maximum torque per ampere for a given stator current. Further, with the same stator \emph{rms} current the stator copper losses and the \emph{mmf} for both machines will be the same. The points on the torque-speed plain of Fig. \ref{fig: torque-speed characteristic} are the operating points used at which the capabilities of both motors are determined. Note that the frequency and not rotor speed is used for the independent axis. This is because the IM operates asynchronous and the RSM synchronous. Also note that the operating points are defined in terms of the IM rated power. From the input, three-phase and output power at the points on the torque-speed plain, the performance indices can be measured. portant. Safety factors on voltage and current have to be taken into account when rating an inverter. \section{Optimum operating point} The conduction losses will be influenced in a different way by an increase in the terminal voltage. Assuming that the motor runs at a more or less constant speed, the load torque and hence the mechanical power developed will be roughly constant. This implies that the power in the rotating field will be roughly constant. Hence, if the voltage increases, the current must vary in inverse proportion. The stator and rotor currents will then drop as the voltage increases, and the conduction losses will drop with the square of the current. The relationship between \( P_{3} \) and the voltage will hence be as shown in Fig. \ref{fig: optimum operation point}. From this it is evident that the total losses will reach a minimum and the efficiency will become a maximum at a certain voltage as shown in Fig. \ref{fig: optimum operation point}. \section{Measurement setup} The measurement setup is shown in Fig. \ref{fig: measurement setup}. It consists of an inverter, a three-phase motor and a mechanical load. The 630 kVA inverter is supplied by a 0-900 V/ 500 A thyristor controlled rectifier. The motors used in the setup are a 110 kW traction IM and a traction RSM, the latter consisting of the IM stator and an optimally designed normal laminated flux-barrier rotor. The load (see Appendix \ref{app: model DT-2000}) is a momentun exchange waterbrake which operates on a water waste principle. The dc current is measured using a hall effect current sensor and the dc bus voltage is measured directly. On the ac side of the inverter only two phase currents are measured using hall effect sensors and the two line voltages are measured directly. The stator windings have four embedded RTDs to measure the winding temperature. \section{Torque comparison} In order to illustrate the torque producing capability of a traction RSM, it is compared with that of a traction IM, assuming that the stator is identical in both cases. The goal is to compare the torque per ampere of the IM and the RSM. \section{RSM calculations and measured results} In this section the finite element program is evaluated at a 110 kW power level. \section{Measurements} The measurements conducted on both motors are outlined in this section. First the measurements that were done to detemine the optimum operating point, as explained in section \ref{sec: determination of optimum point for each motor}, for each motor are given. This is followed by the optimum measurements at these points. The performance capabilities were measured in the speed range 800-3200 rpm. Below the rated supply frequency of 80 Hz these measurements are done at different p.u. values of the stator current. Above rated speed only 1 p.u. stator current is used \section{Comparison} In this section the RSM is compared with the IM with both motors at maximum torque per ampere. The performance indices torque, efficiency, power factor and steady-state temperature rise are to be compared. RSM, this project was aimed at investigating and evaluating the RSM compared with the IM. Given that the stator of the RSM was the same as for the IM and that only the RSM rotor was optimised, the evaluation was carried out to the fullest. The RSM compares favourably with the IM below the rated speed. The evaluation results of the RSM correspond well to their designed values. \section{Conclusions} From the results the conclusions drawn are summarised as follows: The design of the RSM at the 110 kW power level using the finite element analysis is verified. The measured results are accurate within 5\% of the calculated results. This means that an overall design of the RSM at this power level can be carried out. Further, the finite element program proved to be successful at power levels of 9 kW \cite{Kamper(1996)} and 42 kW \cite{Bomela(1998)}. However, this was for four pole machines while the RSM used in this project was a six pole machine. \section{Future work} Based on the results from the RSM design and the comparative study between the RSM and IM drives the following are recommended: \begin{thebibliography}{1} \bibitem{Bomela(1999)}Bomela, X.B.: ``Some design aspects of the multi-flux barrier rotor reluctance synchronous machine'', Incomplete M.Sc.Eng. dissertation, University of Stellenbosch. \bibitem{Bomela(1998)}Bomela, X.B., Jackson S.K., Kamper M.J.: ``Performance of small and medium power flux barrier rotor reluctance synchronous machine drives'', ICEM, September 1998, Vol. 1, pp. 95-99. \bibitem{Fratta(1994)}Fratta, A., Vagati, A., Villata, F., Franceschini, G., Petrache, C.: ``Design comparison between induction and synchronous reluctance motors'', ICEM September 1994, Vol. 3, pp. 329-334. \end{thebibliography} \end{document}