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A brushless DC tachometer including an alternator which produces a substantially trapezoidal output signal coupled to an FET transistor switching bridge. The trapezoidal output signal is achieved using a block permanent magnet for one pole and a consequent pole contoured to achieve a substantially u

A brushless DC tachometer including an alternator which produces a substantially trapezoidal output signal coupled to an FET transistor switching bridge. The trapezoidal output signal is achieved using a block permanent magnet for one pole and a consequent pole contoured to achieve a substantially uniform flux density for the winding over a distance sufficient to produce a trapezoidal output signal.

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1. In a rotary electromagnetic machine, the combination of a magnetic structure including at least two poles, a block permanent magnet providing one of said poles, with a nonuniform gap spacing, and wherein the other one of said poles is a consequent pole structure; and at least one coil movi

1. In a rotary electromagnetic machine, the combination of a magnetic structure including at least two poles, a block permanent magnet providing one of said poles, with a nonuniform gap spacing, and wherein the other one of said poles is a consequent pole structure; and at least one coil moving relative to said poles; said consequent pole being contoured to provide a nonuniform gap spacing relative to said moving coil so that the sum of the flux densities encountered by said coil moving relative to said consequent pole and the other of said poles is substantially constant for portions of each revolution. 2. The rotary electromagnetic machine according to claim 1 wherein said coil is energized and said machine operates as a motor. 3. The rotary electromagnetic machine according to claim 1 wherein said coil generates a signal and operates as a tachometer. 4. A rotary electromagnetic machine according to claim 1 wherein said sum of the flux densities encountered by said coil are substantially constant between 60 and 120 degrees and between 240-300 degrees. 5. A brushless DC tachometer of the type including an alternator for generating an alternating signal proportional to speed, a rotor position sensor, and a solid state switching circuit for producing a DC output signal proportional to speed from said alternating signal, the improvement compromising: said alternator having a magnetic structure including at least two poles, one of which is a consequent pole, and the other of which is provided by a permanent magnet pole surface with a nonuniform gap spacing; said alternator further including at least one coil movable relative to said poles to generate the alternating signal; and said consequent pole being contoured to provide a nonuniform gap spacing relative to said movable coil so that the sum of the flux densities encountered by said coil moving relative to said poles is substantially constant for at least intervals during which said coil is connected by said switching circuit for contributing to said DC output signal. 6. A brushless DC tachometer according to claim 5 wherein said permanent magnet is a high energy block magnet forming one of said poles. 7. A brushless DC tachometer according to claim 5 wherein said sum of the flux densities encountered by said coil are substantially constant from 60-120 degrees and from 240-300 degrees. 8. A brushless DC tachometer according to claim 5 wherein said alternator includes a grounded neutral, three phase, wye winding which includes said coil.