Power Quality and Drives LLC

Calculation of Harmonic Reduction
Copyright © 2006 Francis J. Martino

Can you direct me to resources that describe how to calculate the harmonic reduction resulting from a reactor? Info from one manufacturer shows a 50% reduction of 5th harmonics with a 3% reactor. My calculation results in a much lower reduction.


The impedance ratings of reactors are nominal values.

TCI has several technical papers available on their equipment.

The cross reference list shows which reactor to use for each of several drive manufacturers. Thus, impedance values will change with the design characteristics of the manufacturer's product.

The ratings of 3% and 5% impedance is based on the fundamental frequency of 60Hz. On their new line of reactors, TCI rates the reactors as either low impedance or high impedance.

The new ratings are an admission that the ratings are nominal and not precise.

The overall impedance which a drive is "looking" at includes not only the reactor but also the impedance of the upstream transformer and, more importantly for long cable length, the impedance presented by the length and size of the feeder cables. When specifying equipment, a manufacturer may be assuming a certain nominal feeder length and cable size. Therefore, it is possible for manufacturers to have differing reduction values of harmonic current.

When selecting drives for a new application, first measure the harmonics that exist on the system. Then select either a drive for use with an input reactor or a drive that has the capability of adding a DC link choke rather than an input reactor. Drives with accessible DC bus links will allow the use of DC link chokes which attenuate the fifth harmonic much more than AC line reactors. The DC chokes will give less attenuation to the seventh and eleventh harmonics.

When measuring the harmonics, be sure that the meter will record the interharmonics as well as the integral harmonics. The interharmonics are the result of interaction on the system of several harmonic frequencies. Thus you may find harmonics of orders other than the 5th, 7th and 11th. They may appear as order 4.3, 5.7, 6.6, etc. The interharmonics are of importance if your system is continually troubled by harmonic currents.

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