Torque maximizing control of permanent magnet synchronous motor under voltage and current limitations of PWM inverter

A method to improve torque capability of surface mounted permanent magnet synchronous motors (SPMSM) in a higher speed region, where the back EMF of the motor is approaching to the voltage limit of a PWM inverter, is proposed. In this speed region, a voltage vector laid on the boundary of the hexagon limit of the voltage of a PWM inverter is selected to optimally use the capability of the inverter and motor. The voltage vector, which gives maximum torque under the limitation of voltage and current, is calculated at the given operating condition in real time. Experimental results show that the operation speed, by using the proposed method, can be extended to 45% compared to that of not using flux weakening scheme. Of flux weakening schemes, the speed by using this proposed method can be extended 5% to 13%.