Calculation of skewed squirrel cage induction motors taking interbar currents into account - Part 1: theoretical presentation of the calculation method

Finite element or difference calculations with a time stepping feature and moving boundaries are clearly suitable for two-dimensional problems concerning electrical machines. Its usage for three-dimensional problems is possible, but the application fails often because of the necessary element number and therefore the excessive calculation time. Skewed squirrel cage induction motors require a three-dimensional solution of the field problem in order to take the skewing into account. In this paper a method is presented which regards the skewing, though only two-dimensional finite element calculation is used. In the same way as in Williamson et al. (1995) the machine geometry is divided in the axial direction in several layers. These layers allow a current distribution only in the axial direction. The layers are coupled by electric circuits. Interbar currents are taken into account by interbar resistances for this finite element calculation the first time. A large number of possible electric circuits is needed to stimulate the skin-effect. In a second part the method will be applied on transients in squirrel cage induction machines and compared with measurements.