GB940997A - Improvements in or relating to electric motors - Google Patents

Abstract

940,997. Squirrel cage constructions; core constructions. LOUIS ALLIS CO. Dec. 8, 1959, No. 41715/59. Heading H2A. In a salient pole synchronous induction motor magnetic " barriers " formed in the circular rotor laminations reduce crossaxis flux, i.e. flux that enters or leaves the rotor other than through the poles. The " barriers " are interruptions in the core which may be filled with non- magnetic metal, e.g. aluminium cast with the conductor bars. The conductor bars 14 of a squirrel-cage rotor are located in slots provided in the laminated magnetic core 12 which is fixed to a magnetic shaft 13. Six salient poles 17 are symmetrically spaced round the rotor by peripheral interruptions 18, the arcuate length of which is between 20% and 40% the arcuate distance between adjacent polar axes 28. The peripheral interruptions 18 which may also be filled with non-magnetic metal, are shallower than the winding slots 16 and are flat to enable the use of the same punching tool for different sized rotors. "Barriers" 21 are arranged so that there are three distinct low reluctance flux paths linking adjacent poles; an inner path 22 which includes the ferromagnetic shaft 13; an intermediate path 23<SP>1</SP> ; and an outer path 23. The inner barrier 21a has two radial legs 24 connected by an arcuate segment 25, whilst the outer barrier 21b is arcuate as shown and is spaced from the bottom of the oonductor slots 16 by a neck 26 of ferromagnetic material which saturates at a low percentage of the maximum flux. In larger machines additional strength is provided by a few non-magnetic, e.g. stainless steel, laminations 30 with holes 31 to allow the molten metal to fill all the slots (Fig. 3). In a further embodiment, the mechanical strength is improved by replacing the continuous slots with smaller slots of the same configuration (Figs. 4 and 5, not shown). In another embodiment, the inner " barriers " having radial legs are shaped so that the cast metal keys with the slot edge to resist the centrifugal force acting on the rotor (Fig. 6, not shown). The same punching tool may be used to make the barrier slots for rotors having different numbers of poles, the slots comprising radial and arcuate parts (Figs. 8, 9, not shown). Specification 771,669 and U.S.A. Specification 2,769,108 are referred to.