A near unity power factor input stage with minimum control requirements for AC drive applications

Conventional AC induction motor drives use a PWM voltage source inverter (VSI) fed from a diode rectifier. In retrofit applications, due to voltage drops and the inverter voltage gain, six-step operation of the inverter is required at rated speed, resulting in large low order harmonic current and torque components. To allow the inverter to operate in the PWM mode at rated speed, a synchronous rectifier is proposed in this paper to slightly boost the DC bus voltage. The resulting converter system presents all the advantages associated with active front-end rectifiers. However, since the DC bus control requirements are not stringent, a simple control structure is used, the main purpose being to ensure unity power factor operation. Furthermore, optimized PWM patterns and pattern synchronization can be used to reduce the switching frequency of the rectifier. The paper presents a complete analysis and design guidelines for the DC bus supply and inverter systems. Simulation and experimental results confirm the feasibility and advantages of the proposed structure