Patent GB-123395-A (GB)
Title: Improvements in and connected with Dynamo Electric Machinery of Variable Pole Number.
Inventor: CREEDY FREDERICK
IPC: H02K17/14
Filed: 1918-02-20
Abstract: 123,395. Creedy, F. Feb. 20, 1918. Alternating - current induction machines; dynamos, regulating; motors, controlling. - In Specification 18887/14, arrangements are described for varying the pole-number of induction motors and other machines by the use of windings adapted to give a large number of phases in combination with a phase transformer for producing the number of phases necessary from a two-phase or three-phase supply. The present invention consists in simplifying the phase transformer and reducing the number of motor &c. leads by a suitable choice of the number of phases in relation to the various pole-numbers required and by interconnexion of the sections of the motor winding. The transformation of the number of phases of the supply to the large number of phases required may be done in two stages, as shown vectorially for 36 phases in Fig. 2, in which the transformer P first converts the three-phase supply into three threephase systems differing in phase by 10 degrees, and, for the second stage, each of three transformers S1, S2, S3 of the kind described in Specification 18887/14 converts one of the three-phase systems into a twelve-phase system. In the case of an even number of phases and winding sections, those winding sections requiring opposite phases may bo connected in pairs relatively reversed, thereby halving the number of phases necessary from the phase converter; the number of phases may be again halved by connecting the winding sections in a star-mesh arrangement, as shown, for example, in Fig. 5. Fig. 3 shows the diagram of a phase-transformer P giving six phases in one stage for supplying a machine Q having twelve sections requiring twelve distinct phases ; three-phase current is supplied to the terminals L1, L2, L3 and gives six-phase current at its terminals a - - f; winding sections requiring opposite phases are relatively reversed, as shown. Machines with winding-sections relatively reversed may be supplied with current from a phase-transformer which distorts the three-phase vectors of the supply into two-phase vectors by employing the T-connexion, or the L-connexion, or both. Fig. 16 illustrates the relative connexions of the winding-sections of a variable-pole motor for operation at speeds corresponding to 8, 10, 12, 14, and 16 poles and supplied with current from a phase-transformer fed from a threephase supply L1, L2, L, and having eighteen terminals; the motor winding has 24 sections as shown in the circular key diagram. The phasetransformer has a three-phase star-connected primary winding with secondary windings tapped on each phase, as shown; each secondary winding is wound on one of the remaining two limbs of the transformer, being co-phasal with one of the two other primary windings to which it is parallel. Vector diagrams are given in the Specification for determining the necessary tapping points &c. of the secondary windings. Further reduction in the complexity of the phase converter may be obtained by joining adjacent winding sections in series and treating such .sections as a single unit; or, proceeding round the periphery of the winding, the first section may be left intact, the second and third connected in series, the fourth left intact, and so on; or sets of two and three or more sections in series mav be used alternately. To maintain the flux density in the air-gap approximately constant for all pole-numbers and to prevent current rushes in changing the pole-number, the primary turns of the phase-transformer may have adjustable tappings for changing the connexions to the supply leads L1, L2, L3. The controller may be constructed in the manner described with reference to Fig. 15 of the said prior Specification; the number of controller brushes may be reduced by connecting some of the winding-sections together and joining one end only of these sections to a brush.