Power Factor, Capacitors, Harmonic Filters, and Resonance - Part 2 - How adverse effects of unintended resonance impact the use of capacitors and harmonic filters

摘要

What do harmonic filters look like? The Photo shows the input harmonic filter of a large UPS. The green arrow highlights one of the three separate inductors, and the yellow arrow highlights the associated capacitors. Why use three separate inductors rather than a single common core? The answer lies in flux density developed in the steel of the core. With a common core (EI lamination), it's difficult to achieve consistent flux density in all three legs. Because inductance in a steel core inductor varies with the permeability of the steel, the rate of change of the current flowing through the inductor, and the resulting flux density, it's easier to achieve and maintain consistent filter performance with three separate components.