Harmonic filter reactors for low-voltage applications are typically dry-type iron-core. No existing standard addresses harmonic filter reactors, but most manufacturers use IEEE Std C57.12.01-1998 as a guideline.

Cores are constructed from silicon sheet steel (such as M-6). The number 6 corresponds to the approximate power loss per pound of steel at a magnetic flux density of 1.5 T, i.e., M-6 has a loss of “0.6 W/lb” or 1.5 W/kg. M-6 is the typical grade of silicon steel used, but both lower and higher grades of steel are available.

A manufacturer may choose to use a lower grade steel and either let the harmonic filter reactor operate with a higher temperature rise or use more steel. Conversely, a higher grade of steel can be used and either the harmonic filter reactor may operate with a lower temperature rise or the harmonic filter reactor could be made smaller.

The construction may be from individual pieces of cut strip stock or E-I laminations. To create a harmonic filter reactor, it is necessary to have gaps in the core. These gaps are known as air gaps, but for physical integrity and rigidity the gaps are filled with hard insulation.

This insulation will have permeability similar to air. These gaps can be distributed (many small gaps) or a single larger gap. A single gap will use E-I laminations whereas a distributed gap will be made up of individual cut strips. The E-I construction requires less labor and can be clamped and wedged better than a distributed gap core.

However, the distributed gap core will significantly reduce fringing. This reduction in fringing helps control the effective cross-sectional area as well as stray fields that may result in localized heating of the coil. A “C” core may be used, but it will not offer an advantage for harmonic filtering.

Reactor coils may be constructed from sheet conductor or magnet wire. Sheet conductor may be more economical and easier for construction, but reactors with significant harmonic currents can incur heating problems.

If not properly designed, sheet copper windings can become annealed due to large localized current densities. Sheet conductor windings should be used only on harmonic filter reactors that have a distributed gap well within the boundaries of the coil. Magnet wire, which is less susceptible to localized heating, is commonly used for harmonic filter reactors.

In some cases, it may be desirable to have parallel strands of smaller gauge magnet wire to reduce heating. Although using parallel strands of such wire significantly complicates winding construction, the increase in winding complexity can be justified because coil losses may be significantly reduced.

Clamping is also very important. If a reactor is not properly clamped, the harmonic current can cause laminations to vibrate. Lack of proper clamping could result in a loud audible noise and the breakdown of the insulation coating on the laminations.

Laminations are clamped with insulated through-bolts, or bolts that go through clamps, and are external to the laminations. Clamps that bridge air gaps must be of nonferrous construction. Such nonmagnetic clamps are used to avoid shunting the air gap with a magnetic path.

Furthermore, ferrous components need to be as far from air gaps as possible to prevent inductive heating of the ferrous material. Coils are held in place with spacers and wedges. The harmonic filter reactor should be vacuum impregnated, preferably vacuum-pressure impregnated with a suitable varnish. Impregnation, however, should not be relied upon as the only mechanical means of support.

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