Machine selection and initial design of an aerospace starter/generator

A starter/generator combines the electrical starting and generating functions, necessary for combustion engines, into one machine. This requires both low-speed high-torque and highspeed constant-power operation, which are conflicting requirements. Aerospace gas-turbines require starting times in the order of 10-30 s and to avoid overheating, the starting performance of the electrical machine becomes as important as the generator performance. In addition, safety requirements dictate a failsafe machine. In the case of PM machine this requires a high inductance to limit fault current, further constraining the design. This paper qualitatively compares various machines types for use as a S/G and then selects and optimizes a number of PM machines, highlighting the trade-offs to be made between the different operating modes and how this affects the weight of the machine.