A novel start system for an aircraft auxiliary power unit

This paper describes the design and the main technical characteristics of a new start system for an aircraft auxiliary power unit (APU). By using the latest improvements in power electronics and microelectronics, this system eliminates the conventional DC starter by driving the generator installed on the APU as a motor to achieve the start. The developed start system eliminates one battery, dedicated for APU starting, the DC starter and its clutch assembly, while providing a faster start, assisting the APU acceleration up to 8,400 rpm (vs. 6,000 rpm provided by the DC starter). This new system extends the APU life by maintaining lower engine temperatures during start. The battery voltage control feature, that maintains this voltage at 18/20 VDC during start and the option of using AC power for start, to save battery power is another advantage over the classical DC. This new start system is installed in over 650 commercial airplanes and the operation in revenue service has proved its high reliability. Presently its MTBF is 600% higher than the time between overhauls (TBO) for the DC starter, while assuring an increased performance and safety in operation.