Power capability of salient pole permanent magnet synchronous motors in variable speed drive applications

A constant-parameter equivalent circuit model that neglects motor losses is used to determine the effects of direct and quadrature reactances and open-circuit voltage on the power capability of salient-pole permanent-magnet motors in variable-speed drive applications. It is shown that peak power capability over a range of speeds can be obtained by proper control of the magnitude and phase of the armature current. Due to the voltage constraint imposed at the motor terminals, the power capability will fall to zero for most motor designs at a given high speed. A simple relationship between the motor open-circuit voltage and the direct axis reactance is derived to obtain motor designs that, even with this voltage constraint in place, extend their theoretical power capability, neglecting losses, to infinite speed. All results are presented in normalized curves using a per-unit system