Patent GB-344447-A (GB)
Title: Improvements in or relating to alternating current electric motors
IPC: H02K17/16
Filed: 1929-11-29
Abstract: 344,447. Alternating current motors. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, and SAUNDERS, N. F. T., of General Electric Co., Witton, Birming- ham. Nov. 29, 1929, No. 36718. [Class 35.] In order to avoid a depression in the speed torque characteristic of an induction motor and to reduce starting and running noises, the secondary winding comprises one or more permanently short-circuited phase windings the conductors of which are so disposed that the sum of the third harmonic electromotive forces induced in a circuit is substantially zero on all numbers of primary poles on which the motor is designed to operate. One method of obtaining this result is to wind the secondary member with coils having a pitch equal to or substantially equal to an even number of thirds of the pole pitch. The Figure shows a three-phase two-pole secondary winding, the bars of which are uninsulated or covered merely with a layer of oxide. The phases are short-circuited by an end connection 49. The winding may be modified to operate on either two or four poles by introducing a short-circuit connection 50. The winding pitch chosen is 15 slots, i.e. approximately two-thirds of the pole pitch for two poles and four-thirds of the pole pitch for the four pole number. The third harmonic may be eliminated also by forming the winding with two coils spaced apart by one-third of a pole pitch. In the winding shown, inductors 1, 5 and also conductors 3, 7 are separated by one-sixth of the pole pitch of the two pole number and conductors 1, 3 and 5, 7 are separated by one-sixth of the pole pitch of the four pole number so that the fifth and seventh harmonics will be reduced. The effects of any particular harmonic may be reduced by arranging that a group of bars of one phase has a spread of 2 n/q of a pole pitch where n is an integer and q is the harmonic. Thus in the Figure, the group of conductors 16, 18, 20; 22, 25, 27, 29, 31 has a spread of two-thirds of the pole pitch of the two pole number i.e. n = 1 and q = 3 so that in the particular winding shown additional means are provided for eliminating the third harmonic. The third harmonic may be eliminated also by employing a symmetrical three-phase star connection, different short-circuit connections being employed for the star point and phase ends. The winding may be formed by inserting a number of U-shaped bars in the slots the arms of which are separated by the required pitch and the projecting ends of which are bent over and brazed to form end connections. The ends of a U-bar may be connected together in which case the winding comprises n/2 separate phases where n is the number of slots. An additional high resistance winding may be provided for starting purposes. Double layer windings may be employed. The Provisional Specification describes also a number of lap windings including one in which the conductor bars are provided with two sets of end connections of different pitches to enable the motor to work on two pole numbers. The conductors may have split ends to facilitate connection with the two sets of end windings or each conductor may comprise two bars welded or soldered together in the slot or joined outside the slot or at the ventilating ducts by clips.