Modeling and Control of Low Switching Frequency High-Performance Induction Motor Drives

Complex state variables are used to study the dynamic behavior of induction motors considering the propagation in space of the distributed magnetic field inside the machine. The objective of this paper is to improve the dynamics of pulsewidth modulation inverters in medium-voltage drive systems. To keep the dynamic losses of the power devices at a tolerable level, the switching frequency must be below 1 kHz. The sampling rate of the digital signal processing system is then low which introduces considerable signal delay. The delay has an adverse influence on the dynamic behavior of the current control system. It introduces undesired cross coupling between the current components id and iq. The degree of cross coupling is described by a cross-frequency transfer function. It is shown that the mechanism of cross coupling is different and more adverse than the conventional theory discloses. A current controller structure having poles and zeroes of the single-complex type is synthesized. Cross coupling is completely eliminated at any low switching frequency. Experimental results demonstrate that high dynamic performance and zero cross coupling is achieved even at very low switching frequency.