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An electrical power supply system comprises an air-powered alternating current electrical generator in which pressurized air from a single source is used to drive an air turbine secured on the shaft of the alternating current electrical generator. One uncontrolled nozzle continuously directs air fro

An electrical power supply system comprises an air-powered alternating current electrical generator in which pressurized air from a single source is used to drive an air turbine secured on the shaft of the alternating current electrical generator. One uncontrolled nozzle continuously directs air from the single source to the air turbine. At least a second controlled nozzle directs air from the single source. A solenoid-controlled shut-off valve is placed in a conduit between the source of compressed air and the second nozzle.

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The invention claimed is: 1. An electrical power supply system comprising: an air-powered alternating current electrical generator in which pressurized air from a single source of variable pressure is used to drive an air turbine secured on the shaft of the alternating current electrical generator,

The invention claimed is: 1. An electrical power supply system comprising: an air-powered alternating current electrical generator in which pressurized air from a single source of variable pressure is used to drive an air turbine secured on the shaft of the alternating current electrical generator, there being one nozzle continuously directing air from the single source to the air turbine and at least a second nozzle directing air from the single source to the air turbine being in series with a solenoid-controlled shut-off valve; a rectifier and filter circuit for converting the alternating current output of the generator to a DC voltage; a DC-DC converter connected to the output of the rectifier and filter circuit for providing a DC output voltage appropriate to the intended load; and a comparator/controller receiving a voltage feedback signal taken from the output of the rectifier and filter circuit, whereby when the value of the voltage feedback signal falls below a reference signal, the comparator/controller produces a drive signal that actuates the controlled shut-off valve to direct air from the second nozzle to the air turbine. 2. An electrical power supply system according to claim 1, wherein the alternator is a three-phase permanent magnet alternator or DC generator. 3. An electrical power supply system according to claim 1, wherein the comparator/controller is provided with hysteresis such that the shut-off valve does not open and close at the same feedback signal value. 4. An electrical power supply system according to claim 1, wherein the single source is a brake pipe on a rail car. 5. An electrical power supply system according to claim 1, wherein when the value of the voltage feedback signal falls below any of a plurality of reference signals, the comparator/controller produces a drive signal that actuates one of a plurality of controlled shut-off valves to direct air from an additional nozzle to the air turbine. 6. An electrical power supply system according to claim 1, wherein there is a PWM amplifier in series with the solenoid-controlled shut-off valve for reducing the current to the shut-off valve after it has been actuated to a level sufficient to maintain its condition. 7. An electrical power supply system according to claim 1, wherein the control circuit receives a feedback signal indicative of the single source pressure and when the value of the pressure drops below a reference value, the comparator/controller produces a drive signal that actuates the controlled shut-off valve to direct air from the second nozzle to the air turbine. 8. An electrical power supply system comprising: an air-powered alternating current electrical generator in which pressurized air from the brake pipe of a rail car is used to drive an air turbine secured on the shaft of the alternating current electrical generator, there being one nozzle continuously directing air from the brake pipe to the air turbine and at least a second nozzle directing air from the brake pipe to the air turbine being in series with a solenoid-controlled shut-off valve; a rectifier and filter circuit for converting the alternating current output of the generator to a DC voltage; a DC-DC converter connected to the output of the rectifier and filter circuit for providing a DC output voltage appropriate to the intended load; and a comparator/controller receiving a feedback signal indicative of the brake pipe pressure and when the value of the brake pipe pressure drops below a reference value, the comparator/controller produces a drive signal that actuates the controlled shut-off valve to direct air from the second nozzle to the air turbine. 9. An electrical power supply system according to claim 8, wherein the alternator is a three-phase permanent magnet alternator or DC generator. 10. An electrical power supply system according to claim 8, wherein the comparator/controller is provided with hysteresis such that the shut-off valve does not open and close at the same feedback signal value. 11. An electrical power supply system according to claim 8, wherein when the value of the voltage feedback signal falls below any of a plurality of reference values, the comparator/controller produces a drive signal that actuates one of a plurality of controlled shut-off valves to direct air from an additional nozzle to the air turbine. 12. An electrical power supply system according to claim 8, wherein there is a PWM amplifier in series with the solenoid-controlled shut-off valve for reducing the current to the shut-off valve after it has been actuated to a level sufficient to maintain its condition.