Patent GB-213725-A (GB)
Title: Improvements in multi-speed induction motors
IPC: H02K17/14
Filed: 1923-02-05
Abstract: 213,725. Hunt, L. J., and Sandycroft, Ltd. Feb. 5,1923. Alternating - c u r r e n t motors; motors, controlling. -A multi-speed induction motor which developes at high speed a considerably greater power than at low speed, as required, for instance, for drawing the rods and for normal working respectively in oil well drilling, has a stator winding adapted to be grouped in mesh connection with two or more parallel paths in each phase to give a low pole number for the high speed and in star connection to give a high pole number for the low speed. At low speeds, an increased torque may be obtained by using only a part of each branch of the star and a high starting torque may be obtained by connecting the stator coils in star with two or more paths in parallel in each phase on the high pole number. Alternatively, by connecting the coils in mesh to give the high pole' number a high starting torque may be obtained and after the initial static friction of the plant has been overcome, the coils may be re-grouped in plain star to give the same pole number for continuous running. Fig. 7 shows a 12-coil stator s connected to switchgear for giving the three groupings shown in Figs. 1, 2 and 3, The controller terminals e<1>, f<1>, g<1> - - t<1> are connected to the similarly lettered fifteen terminals of the stator coils 1, 2, 3 - - 12 and M<1>, M<2>, M<3> are contacts connected to the line terminals A, B, C. In Figs. 1 to 3, a, b, and c indicates the phases into which the several coils are connected. With the controller in the position " mesh " the grouping is as in Fig. 1, giving a 2-pole mesh system with two parallel paths in each phase for high speed. With the controller at " star," the grouping is as in Fig. 2 viz., a plain 4-pole star system for low speed. Fig. 3 shows a 4-pole mesh grouping for giving a high torque, for which purpose the star point r, Figs. 1 and 2, is broken and an additional terminal u is used. The change of grouping from Fig. 2 to Fig. 3 is effected automatically by a switch D when the line current in the magnet coil d, reaches a limiting value. The switch D is shown as positioned during normal working at low speed and the terminals h, r, u, are in 4-pole star, as in Fig. 2. When the load increases sufficiently, the switch D is raised so as to connect the pairs of contacts h t, k u, and r s, giving the 4-pole mesh grouping of Fig. 3 by which a higher torque is obtained although the efficiency and power factor are decreased. A phase-changing switch RS enables the rotation of the motor to be reversed. Fig. 8 shows the grouping for obtaining a high torque, as for starting, by the alternative method of connecting the coils as a star system with parallel paths giving 4-poles. The Specification also describes groupings of a 36- coil winding which gives a 6-pole mesh with parallel paths or a 12-pole star system. Also a 12-coil winding and switch-gear such as to give a 2-pole parallel mesh or a 4-pole star grouping suitable for use with cascade rotors, as described in Specification 9260/13, which have three, sliprings and do not operate in cascade but run at the two requisite speeds on their basal pole numbers. In this case, the stator coils are so grouped that the voltages induced by the second field of the rotor cancel out and circulating currents are thus avoided. As an example of the use of tapping points in the star arrangement to give an increased torque at the lower speed, the connection M<1>, Fig. 2. supplying phase a would be changed from the terminal s to o and the mains M<2>, M<3> to terminals g and n. Specification 175,306 is referred to.