Patent GB-172727-A (GB)
Title: Improvements in and relating to electric induction motors
IPC: H02K17/14
Filed: 1920-09-13
Abstract: 172,727. British Thomson - Houston Co., Ltd., (General Electric Co.). Sept. 13, 1920. Alternating - current induction machines; motors, controlling.-Relates to multi-speed induction motor of the general type described in Specification 147,029 used, for example, for ship propulsion, and having a primary winding adapted to set up balanced magnetic fields of n and 3n/2 poles when supplied with 3 and 2-phase power respectively, and consists in grouping the primary windings in the manner described in Specification 22717/10. The primary winding is divided into a number of circuits each containing conductors carrying currents relatively in the same direction when connected together for either phase number, and the circuits are so connected that those which are to be connected to the same phase for 3-phase working are permanently connected together at one end while the opposite ends are so connected that those which are to be connected to the same phase for 2-phase working are also connected together. The resulting terminals for the n and 3n/2 poles form neutral points for the 3n/2 and n poles respectively. The winding is of drum type and may, alternatively, be connected as an n, 3n/2 or 2n-pole winding of six, two, and three phases respectively. Fig. 3 shows motor primary windings, connected as shown in Fig. 2, for giving n or 3n/2 poles on a 3 or 2-phase supply from a generator 16. The vertical lines, Fig. 2, are the centre lines of the motor coil sections, and the top legend a, b, c, a, b, c indicates the n-pole 6-phase sections connected to common terminals A, B, C at one end. The middle legend p, q, p, q shows the sections for 3n/2-poles on 2-phase supply, all sections, in the case shown, having the same number of coils and the phase-belts, therefore, coinciding in the diagram. At the other end, the sections are connected to four terminals P, Q, P, Q. As shown by the bottom legend a, p, b, q, the group a, p is connected to phases A and P for n and 3n/2- pole running respectively, group b, q to phases B (reversed) and Q, and so on for the twelve circuits. The generator 16 has four sections 40 - - 43 sixty electrical degrees apart and four sections 44 - - 47 30‹ apart. With the switch 21 thrown upwards, 2-phase power is supplied to terminals P, Q, P<1>, Q<1> for 3n/2-pole running and with the switch downwards. 3-phase power is supplied to the terminals A. B. C for n-pole running. A reversing switch 22 is provided in the 2-phase leads for reversal on the lower motor speed only. Fig. 8 shows a generator 22 and switches 23, 26 supplying 6, 2, or 3-phase power to a motor wound for 16, 24, and 32 poles and having eleven terminals. The sections are connected at one end to common terminals A, B, C and at the other to terminals P, P, Q, Q, the terminals Q, Q being each made up of three terminals a<1>, b<1>, c<1>. These are connected together in pairs a<1>, a<1>, b<1>, b<1>, c<1>, c<1> when the switch 24 is thrown up and in threes a<1>, b<1>, c<1>, a<1>, b<1>, c<1> when the switch is down. Switch manipulation is described for connecting the motor coils as shown in Figs. 9, 10, and 11, respectively for 24- pole 2-phase, 16-pole 6-phase, and 32-pole 3-phase running. By means of a switch 25, alternate sections embracing 4, 6, or 8 poles, according as the motor is connected for 16. 24, or 32 poles. are disconnected from the supply as described in Specification 173,556. A double secondary winding is used comprising a high-resistance cage 20 and a low-resistance winding 31 connected to rings 33, 34 so that when alternate primary sections are disconnected, as described above, secondary currents will flow in circuits containing bars corresponding to both active and idle primary sections, whereby high-resistance bars adjacent to current-carrying conductors corresponding to idle primary sections are included in the secondary winding by transformer action. The winding 31 is internally short-circuited for a 16 or 32- pole field and effectively open-circuited for 24 poles, but external resistances 35 may be provided. In another arrangement, similar to that shown in Fig. 8, a 5-section generator supplies 2, 3, or 6-phase power through ten leads to a motor wound for n/pole 6-phase, 3n/2-pole 2- phase, and 2n-pole 3-phase running. Double squirrel cage rotors, as described in Specification 15199/13, may be employed for giving increased torques.