A Differential Strand-Slot Inductance Model for Improved Compensation of Circulating Currents in the Core Part of Large AC Machines

The Roebel bar conventional design of large AC machines uses the classical strand-slot inductance model (CSSIM). Suitable alternatives are missing as the CSSIM is favored for its inherent simplicity based on the ideally permeable iron core hypothesis. However, saturated armature slots can lead to high variations of the slot inductance, where the CSSIM cannot represent this precisely. An accurate prediction of the strand inductances is crucial when optimizing the transpositions of large Roebel bars to be competitive on efficiency and low measurement tolerances. This fact is crucial in under-roebeling, having less than a 360-degree transposition over the active part. In the end, the goal is to compensate the winding overhang parasitic field with the slot-parasitic field. This paper proposes a differential strand-slot inductance model (DSSIM) based on the concept of differential inductance (DI). It is compatible with a circuital lumped-element model (LEM) that considers the strand topology, geometrical dimensions, saturation level, and small-scale effects. Numerical simulations showcase the performance improvement of the DSSIM against known models. Finally, a 20-strand prototype of a slot model with actual Robel bar strands corresponding to a simplified bar cross-section in a large AC machine's slot demonstrates the presented DSSIM's precision.