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An alternator for use in a brushless DC tachometer which produces are output voltage with a trapezoidal waveshape. The alternator includes a three phase toroidal winding on a plotless core. The windings are concentrated rather than being distributed. The rotor includes high intensity permanent magne

An alternator for use in a brushless DC tachometer which produces are output voltage with a trapezoidal waveshape. The alternator includes a three phase toroidal winding on a plotless core. The windings are concentrated rather than being distributed. The rotor includes high intensity permanent magnets configured to provide uniform flux density over a large segment at least equal to a commutation increment.

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1. In a DC tachometer, the combination of a magnetic core; a plurality of concentrated toroidal windings on said core separated from one another; a magnetic structure including at least one permanent magnet for establishing a magnetic field across an air gap between said structure and said core

1. In a DC tachometer, the combination of a magnetic core; a plurality of concentrated toroidal windings on said core separated from one another; a magnetic structure including at least one permanent magnet for establishing a magnetic field across an air gap between said structure and said core; commutation switching means for connecting said windings to provide an output for predetermined commutation intervals; and wherein said magnetic structure provides a substantially uniform flux density in said air gap during said commutation intervals. 2. A DC tachometer according to claim 1 wherein said magnetic core is stationary and said magnetic structure rotates. 3. A DC tachometer according to claim 1 wherein said magnetic structure provides a substantially uniform flux density over a segment of said air gap at least equal to the distance traveled by one of said windings during a commuation interval plus twice the width of said winding. 4. In a DC tachometer, the combination of a magnetic core; a plurality of concentrated toroidal windings on said core separated from one another; a magnetic structure including at least one permanent magnet for establishing a magnetic field across an air gap between said structure and said core; commutation switching means for connecting said windings to provide an output for predetermined commutation intervals; wherein said magnetic structure provides a substantially uniform flux density in said air gap during said commutation intervals; and said windings being interconnected in a grounded neutral, wye configuration, and wherein ##EQU##C≤M-2(W+A) where C is the commutation angle, M is the angular span over which said magnetic structure provides said substantially uniform flux density, W is the angular width of said windings and A is the equivalent angular span of the air gap. 5. In a DC tachometer, the combination of a magnetic core; a plurality of concentrated toroidal windings on said core separated from one another; a magnetic structure including at least one permanent magnet for establishing a magnetic field across an air gap between said structure and said core; commutation switching means for connecting said windings to provide an output for predetermined commutation intervals; wherein said magnetic structure provides a substantially uniform flux density in said air gap during said commutation intervals; and said windings being interconnected such that more than one winding at a time contributes to the tachometer output and wherein ##EQU##C≤M-2(W+A)-P where C is the commutation angle, M is the angular span over which said magnetic structure provides said substantially uniform flux density, W is the angular width of said windings, A is the equivalent angular span of the air gap and P is the angular displacement between the contributing windings. 6. A DC tachometer according to claim 1 wherein said core is made from a ferrite material. 7. A DC tachometer according to claim 1 wherein said core is made from an amorphous magnetic material. 8. A DC tachometer according to claim 1 wherein said permanent magnet is a rare earth samarium-cobalt magnet. 9. A brushless DC tachometer of the type including an alternator with windings for generating an alternating signal proportional to speed, a rotor position sensor, and a solid state switching circuit for connecting said windings, respectively, to provide an output during commutation intervals determined by the rotor position sensor, the improvement comprising: an alternator including; a magnetic stator core; a plurality of toroidal windings on said core separated from one another; and a rotating magnetic structure including at least one permanent magnet for establishing a magnetic field across an air gap between said magnetic structure and said core, and wherein said magnetic structure provides a substantially uniform flux density in said air gap during the commutation interval for each winding. 10. A brushless DC tachometer according to claim 9 said magnetic structure provides a substantially uniform flux density over a segment of said air gap at least equal to the distance traveled by one of said windings during a commutation interval plus twice the width of said winding. 11. A brushless DC tachometer according to claim 9 wherein said toroidal windings are interconnected into a three-phase configuration and wherein said commutation intervals are 60 electrical degrees. 12. A brushless DC tachometer according to claim 9 wherein said windings generate a signal having a generally trapezoidal waveform with the flat portions thereof being at least equal to said commutation intervals.