| Abstract |
In foil windings of medium-frequency transformers rated for several hundred Ampères and operating at ten or several tens of kHz, parallel connection of foils is necessary to provide sufficient conductor cross-section. In this case, careful winding design is required to keep circulating currents among the foils under control. Such currents were investigated by means of an analytical model, a 2-d finite-element model, and measurements on a reduced-scale transformer for the case of two parallel connected foils. Simulations and measurements yield a consistent picture and show the potential of high extra losses in foil windings with unmitigated circulating currents. In particular, spiral windings of many turns may incur a circulating current that exceeds the useful net current by far if the inductance of the foil connections is small. Practically, it can be expected that the inductance of the foil connections leads to a noticeable reduction of the circulating current. This reduction, however, is not sufficient to bring the AC losses down to acceptable levels, such that it is recommended to transpose the foils between series-connected winding portions. Transposition largely cancels out the axial magnetic flux in the radial gaps between the parallel connected foils. The effectiveness of transposition in balancing the foil currents at frequencies up to 40 kHz is shown both theoretically and experimentally for aluminum foils of 0.2 mm thickness. With proper transposition, the extra AC losses due to circulating currents between foils can be reduced to practically negligible levels. |
