Manuscript received October 25;
1999. This work was supported in part by the German railway company Deutsche Bahn AG. The authors are with the Institute for Electrical Energy Conversion;
Darm-stadt University of Technology;
64283;
Darmstadt;
Germany (e-mail: {mhof-mann;
twerle;
rpfeiff;
abinder}@ew.tu-darmstadt.de). Publisher Item Identifier S 0018-9464(00)06742-X;