Patent GB-204843-A (GB)
Title: Improvements relating to dynamo electric machines adapted for synchronous working
IPC: H02K17/26
Filed: 1922-08-22
Abstract: 204,843. Hunt, L. J., and Sandycroft, Ltd. Aug. 22, 1922. Exciting; motors, controlling.-A cascade-connected motor adapted for synchronous working is made without any rubbing contacts on the rotor and is provided with means associated with the stator windings for controlling the currents induced therein by the rotor field of the second basal pole number during starting. Fig. 1 shows the use of resistances o, p, q bridged across the mid-points of a double-paralleled star-stator wind. ing of the type described in Specification 24838/14, which is supplied with alternating current at A, B, C, and at synchronism, with continuous current at the neutral points n, n<1>. At starting, a resistance e is included and the resistances o, p, q are cut out, after which the resistance e is cut out, the mid-point tappings are opened, and the exciter x is connected in. The rotor R is of known star-mesh type without slip-rings. With no heavy load at starting, the resistances o, p, q may be omitted, and in cases when a low voltage excitation is not desired a second and separate exciting winding may be placed on the stator in series with the neutral points and the exciter x. Alternatively, the continuous-current winding may consist of a second double-paralleled star winding connected in series with the neutral points n, n<1> and the exciter and having adjustable resistances bridged across the mid-points of the star arms, similar to those at o, p, q for controlling the second-field currents and so producing a high starting-torque. An alternative method for use when a separate continuous-current coil is employed consists in regrouping the stator coils so that the second-field voltages cancel out during starting. Fig. 5 shows the starting connections for the coils of a stator which, when running, are grouped as a star with two paralleled groups, each of four coils in each arm. The coils are numbered in their numerical order around the stator. The mid-points of the starting groups are bridged by resistances o, p, q, which provide paths for the induced currents and are reduced down to short-circuit at synchronism. The separate continuous-current excitation winding is shown at f. This winding, as shown in Fig. 6, may consist of coils c<1>, a<2>, b<1>, c<2> and a<1>, b<2> connected in star to the neutral points n, n<1>. Fig. 6 shows the starting condition, when the neutral points are connected by the switch wand each group of coils is split centrally and bridged by resistances o<1>, q<1> by means of which the second-field currents are controlled. For synchronous running, the switch w is thrown over, the resistances o<1>, q are cut out and the points c, a are directly connected in each limb. A continuous-current winding of this nature may be connected in series with the exciter x and neutral points n, n<1>, Fig. 1, on a stator in which the resistances o, p, q are not employed. In an alternative form, the groups c<1>, b<1>, a<1> and a<2>, c<2>, b<2> may each be star grouped on to a 3-phase starting resistance; or the two neutral points of the star groups may be connected together and the other ends of the limbs connected to a 6-phase starting resistance so that the coils comprise a 6-phase system for the induced currents during starting. Fig. 11 shows the starting grouping of coils, numbered as in Fig. 5, suitable for continuous-current excitation in a motor having a separate alternating-current winding. Branches such as b, c<1> comprising coils in adjacent zones in an 8-pole winding are connected in series and resistances s<1>, s, s<2> are connected to the terminals of the resulting 3-phase 4-pole winding. The resistances are cut down during starting and at synchronism are cut out, the star limbs a, a<1>, b, b<1>, and c, c<1> being then respectively paralleled and the continuous current being supplied to the neutral points. An arrangement is also described in which the coils of an alternating-current stator winding for use with a separate continuous-current winding are grouped at starting as a single mesh or star, in which the induced second-field voltages cancel out, and are regrouped for running as two paralleled stars. Fig. 15 shows two star groups which, during starting, are paralleled through inductances o, p, q, non-inductive to the main supply current fed to their centre points A, B, C, but inductive to the induced currents. At synchronism, the inductances are short-circuited. The use of " eddy current " starters is shown in Fig. 17, these having solid cores so that their windings oppose the flow of high-frequency currents. Corresponding phases are connected to the supply A, B, C at one end through starters o, p, q and to the neutral point n at the other end through starters o<1>, p<1>, q<1>. The mid-points are bridged by switches m<1>, m<2>, m<3> during starting, whereby each pair of branches forms a 2-phase system with the currents out of phase to give a high starting-torque. At synchronism the effective resistance of the starters falls and they are short-circuited by switches u<1>, v<1>. The inner ends of each phase are then opened by switches w<1>, z<1> and connected to the neutral points n<1>, n<2> to which the exciter x is connected. At the same moment, the bridges m<1>, m<2>, m<3> are opened. The bridges and switches w<1>, z<1> may be omitted so that the winding is a 3-phase instead of a 6-phase system. Another form is described in which " eddy current " starters replace the resistances o, p, q of Fig. 1 and s, s<1>, s<2> of Fig. 11. The rotor winding R, Fig. 1, which gives a ratio of ampere-bars per slot between a 4-pole and a 2- pole field of 1.73: 1, may be used with any of the stator windings, or it may be replaced by one giving a ratio of 2: 1. Winding diagrams are given showing how such ratio may be obtained for a star-mesh grouping of the rotor bars giving one coil in each mesh portion and two in each star portion; also by placing three bars of one in certain slots. Specifications 92,61/13, 15113/15, 124,018, and 167,242 also are referred to.