a method and apparatus control a power converter of an ac motor drive system . the method and apparatus : generate a field current regulating signal to control a field current component flowing from the power converter to the ac motor , thereby achieving field current regulation ; generate a torque current regulating signal to control a torque current component flowing from the power converter to the ac motor , thereby achieving torque current regulation , the torque current regulation having lower priority than the field current regulation ; and execute a close - loop field weakening control scheme , which generates a field weakening control command as a function of the difference between a torque current regulation voltage demand and voltage available for torque current regulation . the field current regulating signal is adjusted in accordance with the field weakening control signal to selectively reduce back emf of the ac motor , thereby enabling the step of generating a toque current regulating signal to achieve a toque current component needed to drive the ac motor at a desired speed despite a limitation on dc voltage available to the power converter .