The effects of laminations on steep fronted surge propagation in large AC motor coils

To date, the modeling of machine windings for steep fronted surge propagation has been based on an assumption that the slot walls act as flux barriers at the frequencies of interest (1 to 10 MHz). This assumption led to the use of a line end coil structure embedded in a solid slot core for experiments, and excellent agreement between measured and computed waveforms has been reported by P.G. McLaren et al. (1988). The assumption is valid for a solid slot environment as the magnetic flux cannot penetrate beyond the skin depth, δ, which is on the order of 10^-6 m for the frequencies concerned. However, this is not the case for a laminated slot environment as it can allow magnetic flux penetration deeper into the core that the skin depth, δ. In the present study, the effects of such a flux penetration on steep fronted surge propagation in large AC motor coils are studied. The results obtained demonstrate that the solid slot models cannot accurately replace the actual laminated environments in the study of steep fronted surge propagation in machine windings. Maximum interturn voltages observed for the solid slot model are smaller (by a factor of about 2/3) than those for the laminated slot environment