Variable frequency inverter with sinusoidal voltage outputs using rotating coordinate transformation

Abstract

An ideal inverter should have sinusoidal voltage and current outputs. Generally, output voltages of a voltage‐source PWM inverter contain high‐level switching frequency harmonies due to the PWM operation, while output currents are kept nearly sinusoidal. High‐level harmonics contained in output voltages of a voltage‐source inverter cause acoustic noises, iron losses and electromagnetic interferences. An LC filter was used to suppress the switching frequency harmonics; however, there is a danger of resonance in the LC filter. Accordingly, to remove harmonics of the LC filter resonance frequency, the authors add a voltage feedback loop. A conventional system can operate without difficulty within 50 Hz. However, with accompanying increases in the output frequency, output voltages are largely delayed and reduced by a high‐pass filter inserted in the feedback loop. These problems are caused by a high‐pass filter inserted in the feedback loop. Accompanied by the inverter output frequency, a high‐pass filter cannot remove the fundamental component perfectly. As a result, a small fundamental component is fed back, which causes a delay and decrease in output voltage. This paper proposes the application of coordinate transformation to a high‐pass filter inserted in the feedback loop. As a result, the proposed system realizes an ideal filter which can suppress fundamental frequency components perfectly, and improves the characteristics of the inverter with sinusoidal voltage outputs greatly. Theoretical analyses, simulations and experiments showed satisfactory results. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (4): 94–102, 1997