Permanent magnet motor drives with switched stator windings

Author: Eckart Nipp; Kth; []

Keywords: ;

Abstract: Permanent magnet (PM) motors aretoday regarded as aninteresting solution for a wide range ofinverter-fedvariable-speed drives. The generally increasedinterest in these motors hasled to many investigations on theirfeasibility for vehicle propulsion.Consequently, they are alsoof interest for traction applications which led tothe researchproject that is presented in this thesis. The mostimportantadvantages that are expected in comparison to thestate of the artasynchronous motors are lower losses and ahigher torque density. Often thefield weakening speed range isimportant, but difficult to obtain withPM machines because theinductance in the direction of the magnetization tendsto below. An alternative can be to switch different coils groups ofthestator winding into different configurations. This is thecentral topic ofthis thesis.Various aspects of the design of PM motor drives areconsidered withspecial attention to the requirements for theapplication of the switchedwinding concept. The studies werethereby limited to the inner rotor, radialflux topology. It wasfound that two winding parts per phase, implying fourdifferentwinding connections, is the only interesting solution. Anadvantageof switched windings is that the internal voltage ofthe machine will neverexceed the maximum inverter output, whichincreases the operation safety.Furthermore the machine designcan uncompromisingly be optimized for operationbelow basespeed, which means low inductances implying a large airgaplengthand thick magnets. A problem with switched windings isthat circulatingcurrents can occur. To diminish them, a 2/3magnet covering of the polesurface must be chosen incombination with a non-salient rotor. Moreover itwas found thatthe eddy current losses in the magnets can reachnon-negligiblelevels and must be considered when designing adrive system.The major drawback of switched stator windings is probablythe occurrenceof torque interruptions when changing theconfiguration. The durations ofthese zero torque periods andtheir avoidance have been examined thoroughly inthis thesis bymeans of simulations and experiments on a specially designedandbuilt 50kW test machine. It was found that semi-conductorwinding switchesshould be used to avoid noticeable jerks. Anovel method is suggested on howto perform windingreconfigurations so that torque interruptions are avoided.Theprinciple is based on temporary operation of the machine withits windingsin asymmetric configurations which is analyzedanalytically and validated byexperiments. It is proven thatsuccessive winding switching is an appropriatemeans to entirelyomit torque interruptions.Finally, the application of PM motors with switched statorwindings in asubway propulsion drive is briefly considered.Different aspects of theirfeasibility are taken intoconsideration and the great potential of suchdrives is pointedout.Keywords:Permanent magnet synchronous motors, tractionpropulsion, inverter supply, circulating currents, variablespeed, torque interruptions, switched stator windings,successive winding reconfigurations, fieldweakening, contactorsvs. thyristor switches, permanent magnet motor design, windingreconfiguration tests, magnet losses