The Effects of Nonlinear Loads on EMI/RFI Filters

Power feeder electromagnetic interference/radio frequency interference (EMI/RFI) filters are widely used in the Army and industry for filtering unwanted signals on power lines supplying sensitive loads such as computers and communication equipment. Typically, these low pass filters are passive circuits designed to attenuate frequencies above 10 kHz. Under 60-Hz sinusoidal conditions, the filters dissipate little power. But when nonlinear loads are present (such as those associated with computers, adjustable speed drives, electronic power supplies, and communication equipment), significant power dissipation can occur within the filters. This can shorten filter lifetime or even lead to complete filter failure. Laboratory testing and mathematical analyses were performed on a selected EMI/RFI filter to: 1) determine its frequency response in the 60 Hz–3 kHz range, 2) identify the filter components that dissipate the most power when subjected to harmonic currents, and 3) measure filter component temperature rises when harmonic line currents are present. It is shown that power line harmonics, not considered by EMI/RFI filter manufacturers, interact with filter component parasitics and cause considerable power dissipation in the filters.