Abstract
This paper describes an operating method for a PWM rectifier suitable for air-cooled heat pump chillers. The efficiency of chillers have to be improved in the light to middle load ranges, and the harmonic suppression should be reduced in heavy load ranges. To satisfy this requirement, sinusoidal modulation is adopted in the PWM rectifier by applying over-modulation and two-step boost voltage control. Although over-modulation can reduce switching loss, current distortion remains. In this study, the input current distortion characteristic is derived in the over-modulation, and it is clearly shown that the distortion has a local minimum around a voltage boost ratio of 1.0. Using this characteristic, the efficiency is improved by the operating voltage boost ratio of 1.0 from light to mid-range load. At heavy loads, harmonic restraint and compressor motor high-speed rotation are made compatible by boosting the DC bus voltage to the second level. It is confirmed that ac to dc power conversion efficiency is equivalent at the voltage boost ratio of 1.0, and the input current harmonic is reduced by 40% at the boost ratio of 1.14, compared with the test results of 120° discontinuous pulse width modulation.
