Optimal current control of PWM inverters for brushless motor drives

Abstract

Brushless motor drives are widely used in the high-performance servo applications by virtue of their relative superiority to the dc drives, e.g., improved reliability, higher efficiency, smaller size, less environmental restrictions, etc. All of these advantages are due to the replacement of conventional commutator of dc motors by electronic one, which can be easily realized by inverter nowadays. In order to control the motor current with inverters, pulse-width-modulation (PWM) technique is an effective means, and for its realization various schemes have been reported. The current-controlled PWM of voltage-fed inverter has attracted considerable attention by virtue of its fast current response and inherent current limiting characteristics. It is generally required in the current-controlled drive that fast current response be attained and harmonic content be small for low torque ripples and noises, which seems mutually contradictory. In the hysteresis controller with simple configuration switching frequency becomes much higher than necessary, though fast current response is attainable. The predictive controller is to find the best switching pattern at the expense of complicated computations. This thesis suggests another strategy whereby we can generate an optimal switching pattern for the inverter while reducing the complexity of the control algorithm greatly. The controller is of feedback structure, and selection of the optimal output can be achieved by simple logic. It is to minimize either the current error or switching frequency according to the operating mode. In order to investigate the performance, the entire drive has been designed and important features of the design are explained. A 16-bit single-chip microcomputer is utilized for reference-current generation while the current controller is implemented with electronic hardware. Experimental results show good performances as expected.