High power density AC to DC conversion with reduced input current harmonics

Abstract

This thesis investigates the bene ts and challenges arising from the use of minimal capacitance in AC to DC converters. The purpose of the research is to ultimately improve the power density and power factor of electrical systems connected to the grid. This is carried out in the con- text of a low cost brushless DC drive system operating from an o ine power supply. The work begins with a review of existing applications where it is prac- tical to use a limited amount of DC link capacitance. The vast majority of these have a load which is insensitive to supply power variations at twice the line frequency. Low performance motor drives are found to be the most prevalent, with the inertia of the rotor mitigating the e ect of torque ripple. Further research is carried out on active power factor cor- rection techniques suitable for this application, leading to the conclusion that no appropriate systems exist. A power supply is developed to enable a 24V, 200W brushless motor drive to operate from the mains. The system runs successfully using only 1µF of DC link capacitance, which causes the motor supply volt- age to have 100% ripple. It is noted that whilst this drastically reduces the low frequency input current harmonics, those occurring at the load switching frequency are greatly increased. To combat this, a novel active power factor correction system is proposed using a notch lter to detect the input current error. The common problem of voltage feedback ripple is avoided by eliminating the voltage control loop altogether. The main limitations are identi ed as a high sensitivity to load step changes and variations in line frequency. Despite this, a high power factor is maintained in all operating conditions, as well as compliance with the relevant harmonic standards.