Analysis of bundle losses in high speed machines

Elevated frequencies in high-speed ac machines increase the skin effect in stator windings, making it necessary in many cases to divide each phase winding into high numbers of small-diameter strands connected in parallel. Unfortunately, circulating currents among the strands in a single bundle can significantly increase the total copper losses depending on several factors including the twisting (i.e., transposition) of the strands in the bundles. An analytical model for these bundle proximity losses is presented for individual bundles as well as slot-bound bundles configured for high-speed machines. Loss predictions provided by this model match well with finite element results for high-speed operation of a 55 kW (peak) PM machine with concentrated windings. Stators with litz and non-transposed windings are compared to highlight the large differences in their bundle proximity losses. Analytical and finite element results are shown to match well with experimental results at frequencies of 700 and 800 Hz.