Design tradeoffs between constant power speed range, uncontrolled generator operation and rated current of IPM motor drives

A closed-form approximate formulation is proposed to establish a relationship between the uncontrolled generator operation of IPM motors at high speed and the constant power speed range and drive current capabilities. High saliency IPM motors of the PM-assisted synchronous reluctance type are mainly considered in the analysis, since they are proved to have the most favorable ratio between flux weakening capability and uncontrolled generator voltage. The results of the analysis show the tight relationship between the uncontrolled overvoltage and the constant power speed range of the drive. Moreover, where the uncontrolled voltage is higher, the relationship between the constant power speed range and the motor current amplitude becomes stiffer: for small current variations a large reduction of the speed range can occur. The analysis is validated experimentally on two motors of very different size (500 W and 1 MW).