Such filters are constructed from specially designed reactor (L) and three phase power capacitor(C). One of the characteristic of a series L-C filter is that its impedance is extremely low at the tuning, or resonant, frequency. If we select the tuning frequency to be major harmonic drawn by a non-linear load, the load will only its fundamental frequency current from the distribution system. Most of the selected harmonic current will be drawn from trap filter. Since much of harmonic current demanded by non-linear load now comes from the trap filter and less harmonic current is drawn through distribution system.

A series line reactor (Ls) placed ahead of the tuned filter increases the impedance toward the system to harmonic frequencies. The result is diversion of a greater portion of harmonic current to the trap. Harmonic current in the system are further reduced, and the added impedance prevents other harmonic producing loads from over loading the tuned filter. The 5th harmonic tuned filter will absorb significant amount of 7th harmonic current as well. The series impedance will significantly attenuate the higher harmonic frequencies. A harmonic Tuned filter can easily offer guaranteed harmonic current levels, right at the drive / filter input, as low as to 12% THID. The harmonic tuned filter capacity selection is based on the VFD or non-linear load KW/KVA rating. Choose model based on the VFD HP rating or Non-linear load 1KVA = 1HP rating.