Electrics

THE use of electrical generators to start engines from which they are driven is no longer novel. However, the duties required of such machines have become more arduous as a result of increases in the size of engines and in the complexity of electrical system specifications. At the same time, there is a continuing demand for reduction in weight on all equipment. With the starter‐generator designed for the SD3‐30 the Electrical Group of Lucas Aerospace has achieved a significant break‐through in that a hitherto straight dc machine has been redesigned to provide both dc and three phase ac output. The ac power will be used for windscreen heating, demisting and de‐icing duties, and supplants the normal method of using either a separate engine driven alternator or other conversion equipment. The new system has important advantages in simplicity and weight. The new generator is not significantly larger and weighs only 11 per cent more than a comparable dc only starter generator. It is designed to operate with the UACL PT6A‐45 engine and provides the power necessary to start the engine from internal batteries, or, an external power source at ambient temperatures down to −35°C. As a starter, the machine is thermally rated for a total of five starts with one minute rest periods between each start. From a 24 volt 30 milliohms source it will develop 34 hp at 1300 rpm, which corresponds to the engine ‘light up’ speed. In its generating roll the machine will provide 30 volts dc over a speed range of 5700–11 160 rpm throughout an ambient temperature range of −50°C to + 150°C. At ground level, it will produce 100 amps dc, self cooled by its internal fan. In flight at 7800 rpm, it will give an output of 250 amps dc plus 1·5 kW of ac, with cooling supplemented by blast air at 3 in. water gauge up to an altitude of 20 000 feet.