Brushless variable-speed induction motors

The first part of the paper describes experiments performed on a spherical machine. These experiments show that speed ranges up to 5.5:1 can be obtained, and that the speed range obtained depends markedly on the ratio of pole width to pole pitch. They also show that the losses in the machine cannot be accounted for by conventional induction-motor theory. It is shown, however, that the extra losses are due to the fact that the stator is `short¿ in that it is not continuous round the machine; they are not due to the variable-speed feature. The second part is devoted to a theoretical analysis of the properties of short-stator machines, in the first place without reference to variable-speed properties. Equations are developed which permit the formulation of the salient external characteristics of such machines, it being shown in particular that high efficiencies can be obtained provided there are four or more poles on the `short¿ stator. This general theory is then applied to the variable-speed case. The theoretical findings are supported by numerous experimental results. Broad design criteria are laid down, but since one of these is that the machine should be large it has not yet been possible to make and test a properly designed machine.