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 harmonics 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 L-C filter was used to suppress the switching frequency harmonics; however, there is reason to fear resonance in the L-C filter. Accordingly, in order to remove harmonics of the L-C filter resonance frequency, the authors add a voltage feed-back loop.A conventional system can operate without difficulty within 50Hz. However, with accompanying increases of the output frequency, output voltages are largely delayed and reduced by a high-pass filter inserted in the feed-back loop. These problems are caused by a high-pass filter inserted in the feed-back loop. Accompanied by the inverter output frequency, a high-pass filter can not remove the fundamental component perfectly. As the result, a small fundamental component is fed back, which causes a delay and decrease of output voltage.In this paper, the authors propose to apply coordinate transformation to a high-pass filter inserted in the feed-back loop. As a result, the proposed system has realized the ideal filter which can suppress fundamental frequency components perfectly, and characteristics of the inverter with sinusoidal voltage outputs are greatly improved. And theoretical analysis, simulations and experiments showed satisfactory results.