The nature of the magnetic field produced by the stator of a three-phase induction motor, with special reference to pole-changing motors

In dealing with the subject of stator and rotor windings it has been usual to describe a coil in terms of its slot pitch, that is, in terms of the number of slots spanned by the coil: The author feels that a certain amount of confusion arises when it is attempted to describte-windings in terms of ¿pitch,¿ and it is proposed to use the terms ¿Normal Distribution¿ and ¿Super-Distribution¿ which are definite and lend themselves to very simple mathematical representation. A case of normal distribution of the winding is one in which the windings of one phase are put into a number of consecutive slots per pole equal to the actual number of stator slots per pole per phase, and where no other phase windings are interleaved with them. It is hoped that this innovation will prove useful and convenient. It is shown that with one coil-side per slot there is little benefit to be derived from a departure from a normal distribution of the winding, but that with two coil-sides per slot super-distribution is useful. In Table 6 it is shown how, by means of simple mathematical expressions, to decide upon the style of winding most likely to suppress a tendency to ¿crawl¿ in any given case. The changes in the flux density and magnetizing current, with changes in the number of poles, are discussed, together with their bearing upon the phenomenon of crawling. The influence of the type of slot employed and its proximity to the bore is also examined.