Patent GB-190526091-A (GB)
Title: Improvements in and relating to Alternating Current Dynamo Electric Machines.
Filed: 1905-12-14
Granted: 1906-12-13
Abstract: 26,091. British Thomson-Houston Co., [General Electric Co.]. Dec. 14. Field magnets; motors, controlling.-In connecting for a given number of poles and phases a winding which has a number of coils not a common multiple of the number of poles and of the desired number of coils per pole, the proper proportion of the total number of coils is assigned to each phase and distributed as evenly as possible amongst the several poles of that phase. For example, the speed of induction motors is varied by altering the number of the field poles by changing the terminal connexions without the use of special windings. In the case of a two-speed motor, a T-connexion is employed for both speeds, and the invention consists in dividing up the total number of coils of a winding, which when divided by the number of poles gives a number not divisible in the proper ratio for a T-connexion, namely 6 to 7 or 86, into two sets or phases bearing to each other approximately the proper ratio for a T-connexion, and distributing each set as uniformly as possible among the separate poles. The coils which undergo similar changes in terminal connexions in shifting from one speed to another are combined together in a group. In Fig. 1, the stator S of a three-phase motor is shown divided into eight groups of coils, which may be combined to give either four or six poles by means of a controller c. In the four-pole connexion shown in Fig. 3, the coils having the reference letters A are connected in one phase, and those having the letter B in the other phase. In the six-pole connexion, Fig. 2, the coils are assembled according to the index letters a or b, and the relative connexions of some of the coils, e.g. A<a> and B<a>, are reversed. The method of selecting the coils for the different groups is shown in Fig. 4. The winding has fifty-four coils, each represented by an arrow, the direction of which indicates the direction of current-flow therein. The phases A, B are for the four-pole connexion, and the phases a, b for the six pole connexion. To obtain the ratio 86 between the numbers of coils in the two phases, twenty-five coils are alloted to one phase and twenty-nine to the other. The twenty-five coils are divided up uniformly to give the four-pole groups A and the six-pole groups a, the remaining coil in each case being added to any one of the pole-groups. Similarly, the twenty-nine coils are divided up to form the four-pole groups B, the excess-coil pole being displaced a maximum distance from the excess-coil pole in the A-phase. In the case of the six-pole groups b, there is a deficiency of one coil in one group, which is preferably placed on the stator adjacent to the excess-coil pole in the a-phase. To obtain the group A<ax> of Figs. 2 and 3, the current-flow through which is the same for the two speeds, a row A<ax> is formed in Fig. 4, in which is placed a line whenever two corresponding arrows having the same direction are found in the rows A, a. All the coils represented by the lines in the row A<ax> may therefore be connected in series, care being taken to secure the same direction of current in the coils of the group. For example, the first four coils have open-end connexions, while the remaining two coils have crossed or closed connexions, since the direction of current-flow through these latter is opposed to that of the first. The row A<a> is next formed by selecting arrows common to the rows A, a but pointing in opposite directions. The other groups A<bx> &c. are selected in a similar manner. A developed portion of a complete winding is shown with the coils of the two phases superposed instead of being separated as in Fig. 4. The invention is not limited to multiple-speed motors, but may be used to secure a proper balance of the phases with a number of coils, not obtainable by the ordinary arrangement,