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A brushless electrical machine, usable as a motor, generator, or alternator, has a rotor that is comprised of a rim portion and a substantially open center portion. The rim portion has a partially hollow core in which a stationary field coil is supported. Current to the field coil generates magnetic

A brushless electrical machine, usable as a motor, generator, or alternator, has a rotor that is comprised of a rim portion and a substantially open center portion. The rim portion has a partially hollow core in which a stationary field coil is supported. Current to the field coil generates magnetic flux that circulates in a poloidal flux path in the rim, crossing a single magnetic air gap formed by the rim. Protrusions in the rim located around the circumference form poles all having the same polarity. As the rotor rotates, the flux exiting the poles passes through multiple stationary armature windings around the circumference that are located in the single air gap. An AC voltage is induced in the armature windings from rotation.

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1. A brushless electrical machine, comprising:a rotor for rotation about an axis, said rotor being constructed of a rim portion and a substantially hollow a center portion; said rim portion being constructed from ferromagnetic material and having a partially hollow core at an inner end of a slot int

1. A brushless electrical machine, comprising:a rotor for rotation about an axis, said rotor being constructed of a rim portion and a substantially hollow a center portion; said rim portion being constructed from ferromagnetic material and having a partially hollow core at an inner end of a slot into said hollow core, said slot being defined by opposing surfaces, one of which has multiple, circumferentially spaced protrusions around said rotor; a stationary field coil mounted coaxial with said rotor and located in said partially hollow core of said rim portion, said field coil having windings that can be electrically energized to produce homopolar magnetic flux in said protrusions, and said flux circulates in a poloidal flux path in said rim, said poloidal flux path having a single magnetic air gap across said slot; a plurality of stationary armature windings located within said magnetic air gap such that said homopolar flux from said protrusions induces an alternating voltage in said armature windings when said rotor rotates. 2. A brushless electrical machine as described in claim 1, wherein:the moment of inertia of said rim portion has a value greater than 70% of the value of the moment of inertia of said rotor. 3. A brushless electrical machine as described in claim 2, wherein:the moment of inertia of said rim portion has a value greater than 90% of the value of the moment of inertia of said rotor. 4. A brushless electrical machine as described in claim 1, wherein:the cross sectional area of said magnetic air gap definied by said protrusions is greater than 50% of the minimum cross sectional area of said poloidal flux path in said rim portion. 5. A brushless electrical machine as described in claim 1, wherein:said rim has a height, H in meters, and an inner diameter, ID in meters, wherein said substantially open center portion has a mass, M in kilograms, such that: M<500πID2H. 6. A brushless electrical machine as described in claim 1, wherein:said rotor rotates inside an enclosed chamber within a container that is maintained at an internal vacuum, and said electrical machine functions as a flywheel energy storage system. 7. A brushless electrical machine as described in claim 1, wherein:said rotor is used in an electrically driven vehicle in one or more applications selected from a group consisting of electric power generation by coupling to an engine, energy storage in a flywheel system, and motive power for said electrically driven vehicle. 8. A brushless electrical machine as described in claim 1, wherein:said rotor is mechanically driven by a wind turbine blade, such that said electrical machine converts wind energy to electrical energy. 9. A brushless electrical machine as described in claim 1, wherein:said electrical machine is used in a continuous power system such that said rotor is attached to an engine through a clutch; during normal functioning of utility power, said rotor is rotated by applying synchronous AC electrical power to said armature windings; during an interruption of utility power, said rotor continues to rotate because of rotational inertia thereof and provides power through said armature windings; during an extended interruption of utility power, said engine is started, said clutch engages, and said engine rotates said rotor for producing power throughout the duration of the interruption. 10. A brushless electrical machine, comprising:a rotor for rotation about an axis, said rotor being comprised of a ferromagnetic rim with multiple, circumferentially spaced protrusions around said rotor, and having a slot therein that forms a single magnetic air gap, and an internal cavity in said rim communicating through said slot with an exterior of said rim; said rim having an inner diameter and an outer diameter, said inner diameter of said rim being spaced from said axis by an open space; a stationary field coil mounted in said cavity coaxial with said rotor, wherein said field coil has exterior surfaces that are surrounded on three sides by interior surfaces of said cavity such that said field coil produces homopolar flux in said protrusions, and said flux circulates in a poloidal flux path in said rim; more than one stationary armature windings located within said magnetic air gap such that said homopolar flux from said protrusions crosses said air gap and links said armature windings, inducing an alternating voltage therein when said rotor rotates. 11. A brushless electrical machine, comprising:a rotor mounted on bearings for rotation about an axis, said rotor having a central supporting structure and a rim formed from at least two ferromagnetic pieces having opposed facing surfaces that, when assembled, define therebetween a hollow core and a slot communicating from said core to an exterior surface of said rim, said opposed facing surfaces in said slot defining a single magnetic air gap between said two pieces; said rim having an inner diameter and an outer diameter, said inner diameter being spaced apart from said axis by an open space spanned by a hub that connects said rim to said central supporting structure; multiple circumferentially spaced protrusions around at least at one of said opposed surfaces of said rim in said single magnetic air gap; a stationary field coil mounted coaxial with said rotor in said hollow core such that said field coil produces homopolar flux in said protrusions; a plurality of stationary armature windings located within said single magnetic air gap such that said homopolar flux from said protrusions induces an alternating voltage in said armature windings when said rotor rotates. 12. A brushless electrical machine as described in claim 11, wherein:the moment of inertia of said rim has a value greater than 90% of the total value of the moment of inertia of said rotor. 13. A brushless electrical machine as described in claim 11, wherein:said rim is constructed from material at least at one surface of said magnetic air gap that provides reduced eddy current loss. 14. A brushless electrical machine as described in claim 11, wherein:said armature windings are cooled by liquid in a vessel that contains said armature windings. 15. A brushless electrical machine as described in claim 11, wherein:said armature windings are cooled by air flow which is provided through holes in said rim. 16. A brushless electrical machine as described in claim 11, wherein:said rotor rotates inside a chamber enclosed within a container that is maintained with an internal vacuum, and said electrical machine functions as a flywheel energy storage system. 17. A brushless electrical machine as described in claim 11, wherein:said rotor is mounted in an electrically driven vehicle and used for at least one of the following functions: electric power generation by coupling to an engine, energy storage in a flywheel system, and motive power for said electrically driven vehicle. 18. A brushless electrical machine as described in claim 11, wherein:said rotor is mechanically driven by wind turbine blade such that said electrical machine converts wind energy to electrical power. 19. A brushless electrical machine as described in claim 11, wherein:said electrical machine is used in a continuous power system such that said rotor is attached to an engine through a clutch; during normal functioning of utility power, said rotor is rotated by applying synchronous AC electric power to said armature windings; during an interruption of utility power, said rotor continues to rotate because of its rotational inertia, and provides power through induced voltage armature windings by flux therethrough; during an extended interruption of utility power, said engine is started, said clutch engages and said engine rotates said rotor for producing power throughout the duration of the interruption. 20. A brushless electrical machine as described in claim 11, further comprising:a housing containing said rotor and in which said bearings are mounted; an electrically non-conducting support ring attached to said housing on which said armature windings are supported; said support ring having sufficient mechanical strength to transmit reaction torque from said armature windings to said housing. 21. A brushless electrical machine as described in claim 11, wherein:said slot opens axially in said rim. 22. A brushless electrical machine as described in claim 11, wherein:said slot opens radially in said rim. 23. A brushless electrical machine as described in claim 11, further comprising:permanent magnets on at least at one of said opposed surfaces of said rim in said single magnetic air gap between each of said circumferentially spaced protrusions. 24. A brushless electrical machine, comprising:an annular rim supported on a hub for rotation about a center shaft, said rim constituting an annular shell made of two connected ferromagnetic rings, each having an inside diameter spaced from said shaft by at least the radial thickness of said rim, at least one of said rings having a nonplanar cross section; said rim having an exterior surface and a hollow core communicating with said exterior surface through an annular slot defining a magnetic air gap between opposed, spaced apart surfaces of said two ferromagnetic rings, said ferromagnetic rings and said air gap together providing a poloidal flux path in said rim; multiple circumferentially spaced protrusions around at least at one of said opposed surfaces facing said magnetic air gap; a stationary annular field coil in said hollow core and having electrical conductors for connection to a source of electrical power to electrically energize said field coil for generating homopolar flux to circulate in said rim in said poloidal flux path; a stationary annular armature supported in said magnetic air gap in a position to intersect magnetic flux in said flux path as said rotor rotates about said shaft, and having electrical conductors for connection to a source of electrical power, or operatively to a load, for electrically energizing said armature to generate torque in said rim, or for generating electrical power to said load. 25. A brushless electrical machine for converting between mechanical and electrical energy, comprising:a rotor supported for rotation about an axis, said rotor having an annular rim spaced apart from said axis and formed from at least two annular ferromagnetic pieces such that said rim, when assembled from said pieces, defines a partially hollow interior core and forms a single magnetic air gap between opposed facing surfaces of said two pieces; said rotor including a central supporting structure and a hub for connecting said rim to said central supporting structure, wherein said rim has a moment of inertia that is greater than about 90% of the total moment of inertia of said rotor about said axis; a field coil located in said partially hollow interior core of said rim for generating homopolar flux in a poloidal flux path in said rim around said core when energized by electric current; circumferentially spaced protrusions projection from one of said facing surfaces around at least at one of said opposed surfaces of said single magnetic air gap for concentrating said flux in rays of flux across said air gap; and multiple stationary armature windings supported in said single magnetic air gap for inducing AC power when said rotor rotates, or for producing torque on said rotor when to said multiple stationary armature windings are energized by synchronous AC power.