Digital signal processor based controller for an AC traction drive system

The aim of this research was to implement a modern digital control system using Digital Signal Processor (DSP) technology with an existing 10kW prototype AC traction drive system. The system drives an induction motor from a single phase supply, via two IGBT pulse width modulation (PWM) converters. The model was built as a teaching aid to demonstrate the implementation of various control strategies, and also acts as a research tool for the development of new control techniques. Because variable speed AC drives often utilise complex mathematical calculations which must be performed very rapidly, normal microprocessor based systems often present the designer with serious difficulties. Engineers are therefore turning to DSP technology to replace such microprocessors. The two controllers used for this research were obtained from dSpace GmbH. Named 'DS1102', they were independent add-on cards for IBM PC's. Each DS1102 card contained two state-of-the-art Texas Instruments DSP processors; the TMS320C31 and the TMS320P14. The IBM PC was used for both software development, and system interfacing. Through the use of two 'Windows' based software packages known as COCKPIT and TRACE respectively, the user was able to adjust control system parameters, and monitor converter variables in real-time. These packages were designed specifically for research and education purposes. The PWM rectifier system provides a unity power factor utility interface capable of bi-directional power transfer, which also regulates the DC link voltage. The control technique is known as Predictive Current Control (PCC) with a fixed switching frequency. Briefly, the technique calculates the required duty ratio for the switches in order to bring the actual current equal to the demand current within a switching period. For the PWM inverter, semiconductor switching losses were a major concern. Therefore device switching frequency was limited to below 500 Hz. Accordingly a technique known as Direct Self Control (DSC) was chosen for the induction machine. It is based on a single switching command, and produces similar characteristics to a six -step inverter. The typical switching frequency achieved was approximately 300 Hz.