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An electric power generator for generating a substantially constant frequency output from a varying mechanical speed input such as from a wind or water turbine, comprises a three-phase induction machine in which at least one stator winding is magnetically and electrically decoupled from at least one

An electric power generator for generating a substantially constant frequency output from a varying mechanical speed input such as from a wind or water turbine, comprises a three-phase induction machine in which at least one stator winding is magnetically and electrically decoupled from at least one other stator winding which is arranged to be connected to an external load, and a power source is arranged to excite said at least one stator winding with an AC current at a desired output frequency. The power generator can be directly connected to the mains grid without requiring a power electronics rectifier and inverter stage to convert the generator output to constant frequency AC.

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1. An electric power generator for generating a substantially constant frequency output from a varying mechanical speed input, comprising a three-phase induction machine with a rotor and a stator, the stator comprising three windings, in which at least a first stator winding is magnetically and elec

1. An electric power generator for generating a substantially constant frequency output from a varying mechanical speed input, comprising a three-phase induction machine with a rotor and a stator, the stator comprising three windings, in which at least a first stator winding is magnetically and electrically decoupled from at least one other stator winding which is arranged to be connected to an external load, and a power source is arranged to excite said first stator winding with an AC current at a desired output frequency. 2. An electric power generator according to claim 1 where the first stator winding is magnetically and electrically decoupled from a second and third stator windings, where the second and third stator windings are connected in series and arranged to be connected to an external load. 3. An electric power generator according to claim 1 where the first stator winding is connected in series with a second stator winding, the first and second stator windings magnetically and electrically decoupled from a third stator winding, where the third stator winding is arranged to be connected to an external load, and a power source is arranged to excite the first and second stator windings with an AC current at a desired output frequency. 4. An electric power generator according to claim 1 wherein the power source comprises an inverter arranged to convert DC power from a battery or capacitor bank to AC power at the output frequency. 5. An electric power generator according to claim 1 wherein the power source comprises a rectifier and battery or capacitor bank arranged to convert an alternating current from the generator to DC for recharging the battery or capacitor bank. 6. An electric power generator according to claim 1 wherein the power source comprises a rectifier-inverter arranged for bi-directional power flow between a battery or capacitor bank and the stator winding(s) for both exciting the stator winding(s) at the output frequency and recharging the battery or capacitor bank. 7. An electric power generator according to of claim 4 including a controller arranged to provide switching signals to the inverter for generating the AC current. 8. An electric power generator according to claim 1 wherein the rotor is a squirrel cage rotor. 9. An electric power generator for generating a substantially constant frequency three phase output from a varying mechanical speed input, comprising three stators or sets of stator windings, each comprising three windings, in which at least one winding of each stator or set of stator windings is magnetically and electrically decoupled from at least one other winding and in which in each stator at least one other winding is arranged to be connected to an external load, and a power source or sources is or are arranged to excite said at least one winding of a first of said stators or sets of stator windings with an AC current at a desired output frequency and with a first phase, said at least one winding of a second stator or sets of stator windings with an AC current at said desired output frequency and with a second phase, and said at least one winding of the third of said stators or sets of stator windings with an AC current at said desired output frequency and with a third phase. 10. An electric power generator according to claim 9 in which one winding of each stator is magnetically and electrically decoupled from two other windings of the stator or set of stator windings which are connected in series and arranged to be connected to an external load, and said power source or sources is or are arranged to excite said one winding of a first stator or set of stator windings with an AC current at a desired output frequency and with a first phase, said one winding of a second stator or set of stator windings with an AC current at said desired output frequency and with a second phase, and said one winding of the third of said stators or set of stator windings with an AC current at said desired output frequency and with a third phase. 11. An electric power generator according to claim 10 in which two windings of each stator or set of stator windings are magnetically and electrically decoupled from a third winding of the stator which is arranged to be connected to an external load, and said power source or sources is or are arranged to excite said two windings of a first stator or set of stator windings with an AC current at a desired output frequency and with a first phase, said two windings of a second stator or set of stator windings with an AC current at said desired output frequency and with a second phase, and said two windings of the third of said stators or set of stator windings with an AC current at said desired output frequency and with a third phase. 12. An electric power generator according to claim 9 wherein said power source(s) comprise one or more AC inverters arranged to convert DC from a battery or capacitor bank to AC at the output frequency and phases. 13. An electric power generator according to claim 9 wherein said power source(s) comprise a rectifier and battery or capacitor bank arranged to convert an alternating current from the generator to DC for recharging the battery or capacitor bank. 14. An electric power generator according to claim 9 wherein said power source(s) comprise one or more rectifier-inverters arranged for bi-directional power flow between a battery or capacitor bank and the stator winding(s) for both exciting the stator winding(s) at the output frequency and recharging the battery or capacitor bank. 15. An electric power generator according to claim 9 including a controller arranged to provide switching signals to the inverter(s) for generating the AC currents. 16. An electric power generator according to claim 9 wherein the rotor is a squirrel cage rotor. 17. A method of generating electric power of substantially constant frequency output from a varying mechanical speed input, comprising rotating the rotor of a three-phase induction machine, exciting at least a first stator winding, which is magnetically and electrically decoupled from at least one other stator winding, with an AC current at a desired output frequency, and taking power from said at least one other stator winding. 18. A method of generating electric power according to claim 17 where the first stator winding is magnetically and electrically decoupled from a second and third stator windings, and where the second and third stator windings are connected in series. 19. A method of generating electric power according to claim 17 where the first and a second stator winding are connected in series and are magnetically and electrically decoupled from a third stator winding.