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\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}
