| Abstract |
Paralleling litz wires is often necessary in medium-frequency transformers rated for several hundred Ampères, due to unavailability of wire of large cross-section and the difficulties to bend such wire. Without careful winding design, significant losses may be incurred by circular currents induced between parallel-connected wires. These losses add to the AC losses due to skin and proximity effect on the level of individual strands or bundles within a litz wire. To mitigate circular currents between parallel wires, different transposition schemes are experimentally investigated using windings prepared for a 10 kHz, 240 kVA medium-frequency transformer. The effectiveness of continuous transposition is demonstrated for frequencies from 5 to 20 kHz. Continuous transposition is also shown to be a means of reducing the sensitivity of the AC losses to axial misalignment of the windings, enhancing the robustness of the design. An analytical model is set up for comparison and shown to capture the key phenomena observed experimentally. The practical arrangement of wire transposition is discussed. |
