High frequency resonant link inverter induction motor drives using microcomputer control

Resonant link inverters show tremendous promise for next generation adjustable speed ac machine drives. A bidirectional initial current control concept in the resonant dc link circuit which solves the voltage overshoot problem and establishes reliable zero voltage crossing for smooth inverter operation is presented here. The resonant circuit has been systematically analyzed to establish the criteria for initial current selection, and a new circuit has been proposed to establish the bidirectional initial current. The improved inverter permits successful operation at higher than 50KHz resonant frequency and even promises up to 100 KHz. The control of the inverter which involves prediction of the inverter input current has been formulated.

In addition to the improved resonant dc link inverter, a quasi-resonant voltage notching scheme has been developed. By a high frequency (100 KHz) resonant circuit, this type of inverter creates zero voltage notches at defined positions so as to permit soft switching of power semiconductor devices. The precise resolution of the voltage notch position in the dc link greatly enhances the inverter power capability and reduces the load current ripple in comparison with the resonant link inverter.

Along with the inverter-fed induction motor speed control system, the two proposed schemes have been simulated on a computer using PC-SIMNON. For experimental setup, two power inverter circuits at 25 KHz and 60 KHz have been constructed using the insulated gate bipolar transitor (IGBT), and the bidirectional initial current control circuit has been fabricated on wire wrapping boards. The induction motor field-oriented vector control system has been implemented using a TMS320C25 digital signal processor with two-channel A/D inputs and two-channel D/A outputs. The software development and user interface for the speed control system were supported by a PC-AT compatible computer. Both resonant and quasi-resonant link inverters have been tested successfully with series resistance-inductance and induction motor loads.