Patent GB-777278-A (GB)
Title: Improvements in or relating to multi-phase induction motors
IPC: H02P23/10
Filed: 1955-02-09
Abstract: 777,278. Induction motors; phase changing. LANCASHIRE DYNAMO & CRYPTO, Ltd., BUTLER, O. 1., BARTON, T. H., and STERLING, H. Jan. 27, 1956 [Feb. 9, 1955], No. 3845/55. Classes 35 and 38 (2). [Also in Group XXXVII] A three-phase induction motor is provided with a single primary winding having each of its phases divided into a number of parts, displaced from one another by thirty electrical degrees and with means for supplying the polyphase current necessary for running the motor at full speed to the full primary winding and for alternatively supplying a second current incorporating a lower number of phases to all or selected parts of the primary winding, thereby reducing the speed of the motor. The primary winding may be on the stator and star or deltaconnected, each of its three phases being divided into halves, the second current being a single-phase current passing through one half of each phase, either in series or in parallel. When the second current is applied, part of each phase winding may be short-circuited, either directly, or by way of resistance, reactance, or capacitance or a combination of such elements, the single-phase pulsating field having three times as many stationary poles as there are rotating poles due to the three-phase field, and the three half-windings which are short-circuited being displaced by ninety electrical degrees from the single phase excited windings during the braking operation. The short-circuited windings may be connected together in series or in parallel. In another arrangement, the halfphase windings are divided into two groups and the single-phase current is applied in parallel through the two groups, an impedance such as a capacitor being connected in series with one group to provide self-starting characteristics. The second current may be a two-phase current derived from a balanced or unbalanced supply and applied to the similar two groups of half phase-windings, the machine operating as a motor at speeds below one-third of the full speed and being braked regeneratively at speeds above the low speed. By reversing the two-phase supply the motor can be braked to rest and then accelerated to low speed in the opposite direction. This is useful when the motor is frequently reversed. The two-phase current can be obtained from the three-phase supply by means of a three-to-two phase transformer or by the use of a single-phase auto-transformer. The secondary windings may be of the solid iron type or of the single or multiple cage types. A wound arrangement or a combination of cage and wound arrangements may be used, however. For braking to standstill, the equivalent resistance of the secondary winding during operation at the higher pole number is preferably greater than the total leakage reactance per phase. The rotor may have two separate secondary windings of which one is a high-resistance low reactance squirrel cage, whilst the second winding has a low resistance, interacting with the threephase field but not with the field having the higher pole-number. The secondary winding may consist of a single winding having each phase divided into halves, each half being connected through a slip-ring to a control rheostat. In another arrangement of the secondary windings, each phase is divided into three parallel parts, the three ends of the windings being interconnected through resistances. Changing number of phases.-Figs. 18, 19 show alternative methods of supplying the motor from a balanced two-phase three-wire supply. A four-wire input comprises a neutral conductor N and three-phase conductors R, Y, B. In Fig. 18 two of the two-phase output conductors are constituted by conductors Y, B, and the third conductor 37 is supplied by transformer 38 having its primary winding supplied at phase voltage. In Fig. 19, the output conductors 39, 40, B, are supplied at phase voltage by use of transformer 41. Other schemes of threephase to two-phase conversion using singlephase auto transformers, are shown.