Turbines are conventionally started hydraulically, electrically or pneumatically. If hydraulic or pneumatic systems are used to start an aircraft auxiliary power unit (APU) electrical power is not normally available until the APU output shaft has exceeded some initial rotational speed to drive a generator. The consequences of not having initial electrical power include the inability to use electronic controller circuitry normally associated with APUs to perform such functions as opening and closing valves for the starting systems. Previously, when pneumatic or hydraulic starting systems were used, the operator had to manually close down the starting system after the APU began operation. This is not a reliable approach because if the starting system is turned off too early the APU will fail to complete the starting process; if, in the alternative, the starting system is turned off too late the starter motor may overspeed which makes failure of the starter system likely.
To address these and other problems, various previous methods have been devised. U.S. Pat. No. 3,812,378 to Coman describes the use of a permanent magnet generator for providing a first pulse train with each of the pulses having a duration that is a function of the rotational speed of the starter rotor. A second pulse generator is also provided, with the second pulse train having a constant pulse width; the second pulse train time width corresponds to a predetermined rotational speed of the starter rotor at which the starter is to be turned off. The first and second pulse trains are provided to a comparator circuit that produces an output signal when the duration of a pulse from the first pulse train is shorter than a pulse from the second pulse train, i.e. the starter rotational speed exceeds the predetermined rotational speed. This output signal from the comparator circuit is then used to turn the starter off. The clear disadvantage of this system is the requirement for electrical power from the very initiation of the starting process.
Another starter control system is described in, U.S. Pat. No. 4,747,270 to Klie et al, here a pneumatic starting device is disclosed. The pressure in a reservoir supplying gas to a pneumatic starting device is monitored, and the monitored pressure is exclusively used to end the starting process. As the gas pressure drops in the reservoir the maximum speed of the starter is limited. Since pressure drop in the reservoir is used to end the starting procedure, the reservoir must be constructed with only a capacity sufficient for carrying out one starting. This is an inescapable disadvantage, and is especially problematic for aircraft operating from austere airfields.