Skin-effect bars of squirrel-cage rotors [includes discussion]

BAR SHAPES are used in rotors of squirrel-cage motors for which a straightforward mathematical determination of the skin-effect is not possible. The solutions obtained for bar shapes, for which a straightforward mathematical solution is possible, indicate that the skin-effect is not affected by the actual width of the bar. This leads to the assumption that the “depth of penetration” is independent of the shape of the bar. The depth of penetration must be defined differently for the ratio of a-c to d-c resistance: r ac /rdc, and for the ratio of a-c to d-c inductance: l ac /l dc . Denoting the depth of penetration for r ac /r ac by h pr and that for l ac /l dc by h px , the assumption of independent depth of penetration means that for the bars of the same height but of different shapes, h pr is the same and h px is also the same. This is indicated in Fig. 1 for three bars of different shapes.