GB208222A - Improvements in and relating to alternating-current dynamoelectric machines - Google Patents

Abstract

208,222. British Thomson-Houston Co., Ltd., (General Electric Co.). Sept. 13, 1922. Alternating - current commutator machines; dampers and shortcircuited screens on cores.-The secondary member of an alternating-current motor has commutator and shortcircuited windings at different radii with a flux path between them such that the normal operating flux passes through this path without excessive saturation and of such cross reluctance that a portion of the flux may be deflected beneath the inner winding. The invention is described in connection with a single-phase repulsion motor having a squirrel-cage winding 10, Fig. 2, in slots below, and preferably in line with, the slots which carry the commutator winding 4. At starting, the flux path is chiefly across the teeth between the two windings without saturation of the iron, and as synchronous speed is reached and the squirrel-cage currents decrease, most of the flux shifts to a path behind the cage winding and through the teeth longitudinally. The machine thus starts and runs respectively with repulsion and induction motor characteristics. The requisite reluctance in the path between the windings may be obtained by connecting the two sets of slots by narrow air slits 12. The degree to which an induction motor characteristic is obtained is proportional to the width of these slits and the load-carrying capacity of the cage winding is approximately proportional to the ratio of the depth of the bridge between tne windings to the slit width. The slit width employed is such that with the rotor locked and full line volts applied to the stator, from one to three times, and preferably twice, the full load current is drawn from the line. This usually leads to a slit width of from 1/20 to 1/40 of the bridge depth, the ratio of the load capacities of the commutator and cage windings being about 3 : 1. The cage bars preferably have their broad sagmental ends directed outwards, as shown, and commutation may be improved by means of flat high-resistance non-magnetic conductors, for instance of phosphor bronze, in the slits to form an eddy-current system for flux pulsations of more than twice the line frequency. Fig. 5 shows eddy-current conductors 15 of bent steel plate anchored at 16 in the inner slots and enclosing an air slit which may be filled with an insulating material. An alternative form, Fig. 7, consist of a bent strip of high-resistance non- magnetic plate 17 filled with magnetic laminae 18. The constructions described may be applied to the stationary element of a machine instead of the rotor. A numerical example of the dimensions for a motor rotor is given in the Specification. Specification 30208/09 is referred to.