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

Conventional AC induction motor drives use 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 of rated speed, a synchronous rectifier is used in this paper to boost the DC bus voltage slightly. 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 to ensure unity power factor operation. Furthermore, optimized PWM patterns and pattern synchronization are used to reduce the switching frequency of the rectifier. The paper presents an analysis and design guidelines for the complete drive system. Simulation and experimental results on a 5 kVA induction motor drive confirm the feasibility and advantages of the proposed structure