Design criteria for pole-changing windings

A set of rational criteria are identified for assessing the merits of pole-changing windings for cage induction motors, in terms of capacity and performance of the two speed settings. It is shown that a change-pole setting is simply a nonstandard design that differs from the standard design essentially in respect of flux density and winding factor. In terms of these two quantities, relative performance indexes are derived that are pure numerics, with unity for the standard machine. Optimum coil spans are obtained for the familiar 1:2 pole, changing winding. Other criteria are the quality of the winding in terms of balance and m.m.f.harmonics, and the size of the controller. The uses of the main criteria are illustrated by application to some existing designs. Earlier criteria are surveyed and shown to be either incomplete or incorrect. The new criteria form a logical basis on which meaningful comparison can be made, not only between pole. changing machines and standard machines, but also between different designs of two-speed machine for a given pole combination.