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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 (UCG) of IPM motors at high speed and the constant power speed range (CPSR) 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 UCG overvoltage. The results of the analysis show a tight relationship between the UCG overvoltage and the CPSR of the IPM drive. In particular, for high UCG voltages the relationship between the motor current and the obtainable CPSR becomes more and more stiff: small variations of the current can produce large reductions of the CPSR. The analysis is validated experimentally for two motors of very different size (1kW ÷ 1MW).