Total Power Factor by John Houdek

Total Power Factor Tutorial 

What harmonic current distortion should I expect from a VFD? PQM-105

An 18-pulse drive to typically will achieve about 4% to 6% THD-i when operating at full load (assumes balanced and non distorted voltage source). Distortion will increase as load is reduced and when voltages are not balanced.

 

A 12-pulse drive typically achieves 12 – 15% THD-i when operating at full load, (assuming balanced and non distorted voltage source). Distortion will increase as load is reduced and when voltages are not balanced. Filters are available for 12-pulse drives that will reduce current distortion to 5% or less.

A 6-pulse drive without a DC or AC inductor may be as high as 60-100% THD-i.  However a 6-pulse drive with 5% line reactor (or equivalent DC choke) achieves about 35% THD-i. at full load (45% at 60% load) and with 3% line reactor (or equivalent dc choke) achieves about 45% THD-i at full load (about 60% THDi at 60% load).  Filters are available for 6-pulse drives that will reduce current distortion to 5%, 8%, 10%, 12%.