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