QUICK RESPONSE AND LOW-DISTORTION CURRENT CONTROL FOR MULTIPLE INVERTER-FED INDUCTION-MOTOR DRIVES

This paper describes a PWM control method to balance two unit inverter currents and to reduce the distortion of motor currents in a multiple GTO inverter for large capacity ac variable-speed drive systems. For an 11-kW induction motor driven by a 15-kVA multiple inverter, the imbalance current can be reduced to less than 5\% of the rated motor current, and the motor current waveform can be made very close to sinusoidal. The GTO switching frequency in the multiple inverter can be lowered to one-quarter of that in the single inverter. In a 2750-kVA GTO inverter drive system which was developed based on the 15-kVA multiple inverter, excellent performance is obtained including unity power factor operation at an ac power source, smooth 4-quadrant operation, and quick speed response of 85 rad/s. The quick response is realized by field-oriented control with decoupling control between torque and exciting currents, and direct control of three-phase ac currents. In the future, the multiple GTO inverter is expected to be applied to rolling mill drives instead of cycloconverters.