Active filtering of multiple non-integer harmonic currents

Cycloconverters are widely used to provide variable speed, AC motor drives, the main advantage of the cycloconverter being that it can provide direct frequency conversion without the requirement for a DC link. The frequency and voltage of the cycloconverter output are controlled by a reference waveform and in simple terms the power converter can be considered to act as an amplifier with the output following this reference signal. Unfortunately, any distortion of the output voltage waveform will also be reflected back into the supply, resulting in harmonic contamination of the input current. This occurs as a direct consequence of the inherent switching operation of these types of power converters and it is therefore a problem common to all cycloconverter drives. A method of harmonic reduction that can be applied to this problem involves the use of an active filter. This type of filter needs to have an extremely narrow bandwidth, ideally at one distinct frequency only. This paper details the authors´ development of such a system