Patent GB-173556-A (GB)
Title: Improvements in and relating to induction motors
IPC: H02K17/14
Filed: 1920-08-05
Abstract: 173,556. British Thomson - Houston Co., Ltd., (General Electric Co.). Aug. 5, 1920. Alternating - current induction machines; motors, controlling.-An induction motor, with or without a variable pole number, is provided with high and low resistance secondary windings and the connections of its primary windings are so arranged at starting, or other times when increased torque is required, as to produce an irregular magnetic field and cause transformer action between the two secondary windings to give an increased effective secondary resistance. The irregular field is produced by cutting out zones of the primary winding. As shown diagrammatically in Fig. 1, N, S - - indicate the polarity for normal running in an 8-pole stator giving six-phase belts of conductors a, b, c, a, b, o when all belts are energized, as in the upper legend. The rotor has a high resistance squirrel cage 20 and a low resistance multiple wave winding 21. At starting, alternate pole pairs are not energized, as shown by the lower legend a, b, c - -. Under the inactive poles the coils 21 induce currents in the adjacent high resistance bars 20 which give an increased torque. The connections for such a motor taking a three-phase supply at 23 are shown in Fig. 2, the phase belts a, b, c, a, b, c corresponding to those in Fig. 1. When the switch 22 is in its left-hand position, alternate poles are disconnected from the supply, and when it is at the right-hand all the poles are excited. All the phase belts a, a, b, b and c, c of each component winding are connected in series and the two components in parallel, as shown in Fig. 2, or they may be connected in series or in any series-parallel combination. The stator windings are usually of two-layer lap form, giving intermediate zones of half active and half inactive conductors, but other distributions of the active and inactive belts may be used. The winding 21 may be divided into short sections connected to the end rings of the high resistance cage or to separate rings to limit the maximum secondary voltage. A two-layer lap winding having locally closed circuits is also exemplified. The application of a two-phase motor is also exemplified in which the stator coils allow alternate pairs of poles to be de-energized, and the rotor low resistance coils are divided into full pitch local groups between four pairs of rings, two at each end of the winding. The high and low resistance bars are preferably placed in the same slots, and bv omitting bars from the cage the reactance of the low resistance winding may be increased in order to flatten the speed-torque curve of the motor. In this case, other bars of the cage may be given a larger section to prevent undue increase in the effective secondary resistance. The application to a motor of variable pole number is shown in Fig. 9, this machine being wound for sixteen, twenty-four, and thirty-two poles on six, two, and three phase supplies respectively. The high resistance rotor windings 31 have connections to slip-rings 33, 34 (those to rings 33 only being fully shown), so that for twenty-four poles there are twelve groups connected to each set of rings, each group comprising a coil in series with a coil six poles ahead. Transformer action between the windings 31 and cage 20 is obtained by opencircuiting alternate groups of six stator poles for twenty-four pole running only, the field being regular for the other two-pole, numbers. External resistances 35 preferably close the windings 31, which for twenty-four poles are effectively opencircuited and for sixteen and thirty-two poles form, in effect, a short-circuited star winding. Fig. 10 shows the application to a two-phase twospeed motor of the type described in Specification 143,272, having eight circuits supplied with two-phase power at P, Q through main and auxiliary switches 40, 41. Each circuit is in two sections, the right-hand of which Pq+, Qq+ - - is always employed, and the other of which P<1>q<1>+, Q<1>q<1>+ - - is disconnectible from the supply or may have its phases reversed by means of the switch 41. Switch manipulation is described for giving, in general terms, (i) m poles with the disconnectible sections either idle or carrying currents to co-operate with or oppose those of the permanently connected sections, and (ii) n poles, in which case all the sections are used and no provision is made for disconnecting some of the sections to give an irregular field. In the form shown in Fig. 12, two rotors 50, 51 are mounted on a common shaft with a low resistance squirrel cage 55 common to both and a high resistance cage 56 in the rotor 51 only. The stator winding 54 is disconnectible from the supply by means of a switch 57. With the switch in its left-hand position, the stator winding 53 only is energized and the currents in the low resistance bars 55 react with the high resistance cage 56 to give an increased effective secondary resistance. With the switch thrown to the right, both primary coils 53, 54 are energized.