Multiple Layer Parallel Connected Air-Core Inductor Design

Abstract

In power line carrier communications, large aircore inductors are used as "line-" or "wave-traps." The line-trap establishes an impedance mismatch to the carrier signal, while allowing 60 Hz power to pass. These inductors may consist of several coaxial windings of untransposed conductor, connected in parallel rather than in series to reduce shunt capacitance between layers. The design of these coils must focus on maximizing inductance, while minimizing losses due to large 60 Hz currents. In this report, the author employs a classical one-dimensional eddy current solution in rectangular coordinates to estimate the power dissipation in each layer of a coil. The dissipation within a given layer depends on the current distribution in the other layers, as well as its thickness. By choosing the thickness (radial build) of conductor correctly, the power dissipation within that layer can be minimized. To illustrate the use of the eddy current calculation, a design example is presented for a coil with two layers of windings, using appropriate approximations. It is found that tightly coupled coils exhibit greater relative inductance than weakly coupled coils. For coupling coefficients of 0.6 and 0.8, a maximum current rating increase of about 60 percent is obtained by adding a second (inner) layer. Finally, a general method for designing coils with an arbitrary number of layers is discussed.