Approximately 2% of the energy generated in the United States is dissipated in the iron losses of ferromagnetic structures in electric motors in the power range of 1 to 125 horsepower and in ballasts for gas-discharge lamps including fluorescents. 60-80 percent of this loss may be spared with more efficient ferromagnetic structures.
Iron losses in ferromagnetic materials are squared functions of maximum induction, exciting frequency, and material, e.g., lamination, thickness. All may be manipulated to reduce iron loss. Stator lamination iron losses dominate energy dissipation in common alternating current rotating machines as they are driven at line frequency whereas the armature usually experiences only a DC or slip-frequency (1 to 5 hz.) magnetic flux. Iron losses may also be a major loss source in brushless direct current motors as the stators may be driven at high frequencies by phase switching or the even higher frequency components of pulse-width modulation drives. Iron losses in transformer-like apparatus including transformers, ballasts, and inductors are concentrated in laminated cores.
In these products it is desirable to limit iron losses or improve high frequency performance through the use of intrinsically low-loss materials,thin lamination sections, or both. This has been done successfully in contemporary distribution transformers which use wound cores of substantially amorphous ferromagnetic materials, often called metallic glasses, referred to hereinafter generically as amorphous ferromagnetic materials. Modest improvements in other areas have been achieved through reduced flux density, thinner lamination sections, and improved metal alloys and metal processing, but the major potential gains of amorphous ferromagnetic material technology have not been widely realized.
Amorphous ferromagnetic material such as Allied-Signal Metglas.TM. lacks crystalline structure and therefore is isotropic, having the same permeability in all directions, and has about one-tenth the losses of common crystalline iron alloys It is formed by planar flow casting or spraying in a sheet roughly 1 mil thick, obviating costs of subsequent thickness reduction through rolling as is required when producing thin laminations from common 12 and 14 mil sheet steel lamination stock. However its hardness is in the range of 63-70 Rockwell C, like many tool steels, so that it is machined only by grinding, EDM, or lasers, and those at the risk of heat-induced crystal formation and performance degradation. According to the manufacturer it may be folded with a zero internal radius prior to annealing, an extraordinary attribute for a very hard material, because of its lack of crystalline structure. It may be slit and sheared economically, perhaps because maintenance costs on the simple tooling used are bearable. Punching, as for stator laminations, is impractical due to the material hardness and thin sections.
Reduced losses can also be achieved with thin section (under 12 mils) electrical steels, most commonly silicon alloyed, which are much more readily fabricated than are amorphous ferromagnetic materials but add to their base cost the costs of thickness reduction and the handling of the increased parts count if parts are not produced by quasi-continuous processes.
Major components of a permanent-magnet rotor axial-gap motor, shown in FIG. 1, illustrate problems of fabricating motor stator components of laminated materials using prior art. A rotor assembly 51, comprising a hub 52, four axially polarized magnets 53 disposed equally about the perimeter of the hub 52 with polarization directions alternating (this rotor assembly example is called a quadrupole). The hub 52 is fixed to a longitudinal shaft 54 rotatably mounted in bearings (not shown) and a motor frame (not shown) which place the rotor assembly 51 between two stator assemblies 55 and 56 separated from the rotor assembly 51 by small axial airgaps on both sides of the rotor assembly 51. The faces of the stators are shaped to include slots 57, and teeth 58 between said slots 57, which support windings (none are shown in this document) which systematically magnetize the stator assemblies 55 and 56 to produce torque on the rotor assembly 51. The dominant magnetic flux paths pass normally through the rotor magnets 53 into the faces of the stator assemblies 55 and 56, trav.RTM.rse a quarter turn in either direction in this quadrupole example, and exit the stator assembly faces to link with either of the adjacent magnet poles on the rotor assembly 51.
The stator ferromagnetic assemblies 55 and 56, which are toroidal, may be fabricated of molded bonded powdered metal or laminated, as by winding ferromagnetic strip of width equal to the torus axial dimension about a mandrel sized to the torus internal diameter until sufficient material is built up to achieve the desired torus external diameter. A powdered metal stator ferromagnetic assembly has limited permeability, in the 50 to 500 range, but is magnetically isotropic (equal characteristics in all directions), and relatively easy to form. The low permeability limits the permeance which may be achieved in the stator assembly as a whole. Sheet-steel laminations offer much higher permeabilities, in the 5,000 to 100,000 range, but the commonest thin materials, such as grain-oriented silicon steels, are not isotropic, having a low transverse (across the grain) permeability which increases the reluctance of the axial portion of the flux path within the stator. Amorphous ferromagnetic materials are nearly isotropic and of very high permeability, above 100,000offering low path reluctance.
Slots 57 and teeth 58 may be readily formed in the powdered metal stator ferromagnetic assembly as part of a molding process. Slots 57 and teeth 58 may be formed in a wound laminated torus by edge notching prior to winding or machining or grinding after winding and bonding the laminations together.
Powdered metal fabrication is simple if low permeability is acceptable. Edge notching the laminations of silicon steel is possible, but notch spacing must be increased as the torus diameter increases to preserve slot geometry, implying computer control of the notching process and probably post-winding machining or grinding to true the slots 57. Whole-slot machining post winding and bonding without prior notching is possible, but is time consuming and expensive as considerable material must be removed. The thin sections and extraordinary hardness of amorphous ferromagnetic materials have thus far rendered processes like pre-winding notching or post-winding slot machining economically impractical. Heat-induced grain formation at machined or ground surfaces is a further deterrent to mechanical processing.