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A permanent magnet fault tolerant generator 32 is driven by a variable speed shaft 34 to create AC power at terminals 36 , converted at 40 to DC power before being applied to a load 38 . The converter 40 is a bi-directional device which rectifies AC to DC and also acts as an inverter to synthesize A

A permanent magnet fault tolerant generator 32 is driven by a variable speed shaft 34 to create AC power at terminals 36 , converted at 40 to DC power before being applied to a load 38 . The converter 40 is a bi-directional device which rectifies AC to DC and also acts as an inverter to synthesize AC from the DC. This synthesized AC waveform is imposed on the terminals 36 to maintain a constant magnitude generator output voltage, regardless of the speed of the shaft 34 , and thus regardless of the EMF of the generator. This results in reactive currents flowing between the generator 32 and the converter 40 , but allows the generator design have a reduced current rating, resulting in improved generator size and weight.

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1. An aerospace generator arrangement operable to provide DC power from a rotating member, the arrangement having a permanent magnet generator driven, in use, by the rotating member to provide AC power at the output terminals of the generator, converter means operable to convert the AC output to pro

1. An aerospace generator arrangement operable to provide DC power from a rotating member, the arrangement having a permanent magnet generator driven, in use, by the rotating member to provide AC power at the output terminals of the generator, converter means operable to convert the AC output to provide DC power at substantially constant voltage, and control means which, in use, maintain constant magnitude AC voltage at the generator output terminals, regardless of the generator EMF. 2. An arrangement according to claim 1, wherein the generator is fault tolerant. 3. An arrangement according to claim 1, wherein the control means comprise a voltage synthesiser arrangement operable to impose a synthesised voltage waveform on the generator output terminals. 4. An arrangement according to claim 3, wherein the voltage synthesiser is a pulse width modulation-type synthesiser device. 5. An arrangement according to claim 3, wherein the voltage synthesiser creates the synthesised voltage waveform from the DC power. 6. An arrangement according to claim 1, wherein the rotating member is driven from an engine. 7. An arrangement according to claim 6, wherein the rotating member is driven from a compressor shaft of a gas turbine engine. 8. An arrangement according to claim 7, wherein the shaft is the low pressure shaft of a multi-shaft gas turbine engine. 9. An aerospace generator arrangement operable to provide power for use within an aircraft, the arrangement comprising an aero engine which drives a permanent magnet generator to provide AC power at the output terminals of the generator, converter means operable to convert the AC output to provide DC power at substantially constant voltage, and control means which, in use, maintain constant magnitude AC voltage at the generator output terminals, regardless of the generator EMF. 10. An arrangement according to claim 9, wherein the generator is fault tolerant. 11. An arrangement according to claim 9, wherein the aero engine has at least one rotary shaft connecting a turbine and a compressor, the generator being driven by or from the said shaft. 12. An arrangement according to claim 11, wherein the engine has more than one shaft, and the driving shaft is the low pressure shaft. 13. An arrangement according to claim 9, wherein the control means comprise a voltage synthesiser arrangement operable to impose a synthesised voltage wave form on the generator output terminals. 14. An arrangement according to claim 13, wherein the voltage synthesiser is a pulse width modulation-type synthesiser device. 15. An arrangement according to claim 13, wherein the voltage synthesiser is operable to create the synthesised voltage waveform from the DC power. 16. An aircraft incorporating a generator arrangement as set out in claim 1.