Adjustment of Rated Current and Power Factor in a Synchronous Reluctance Motor Optimally Designed for Maximum Saliency Ratio

This article analyzes an engineering problem of adjusting the rated current of a synchronous reluctance motor to match the rated current of a commercially available inverter. This situation may occur when a synchronous reluctance motor is optimally designed for maximum torque per ampere (MTPA) operation using standard IEC stator frames for induction motors. When the motor has a maximized saliency ratio along with MTPA control at rated speed and available inverter voltage, it often happens that rated current of the motor exceeds the rated current of a commercially available inverter. This article shows that the motor current can be reduced and power factor increased by choosing different number of turns per coil and even changing the stack length. In other words, rewinding and axial scaling of the motor is performed in order to retain the same rated voltage and shaft power and hence to operate with better power factor which means lower current and lower kVA requirement. This comes with a certain increase in slot current density and therefore reduction of motor efficiency. All the design tradeoffs, which emerge in this situation and their effects on the motor performance are explained using a linear model of the motor based on a phasor diagram with constant inductances. The predictions of the simple analytical model were confirmed by utilizing flux linkage versus current maps obtained from nonlinear finite element analysis and afterward by conducting measurements on two identical motors wound with different number of turns per coil.