Refer to figure.
A starter-generator combines, as it names suggests, the functions of both the starter and the generator. In fact, it is a unit that besides functioning as an air starter to spool up the engine, it also has a second series of windings that will allow it to switch to a generator after the engine core has reached a self-sustaining speed. When speaking about the engine core, it is referred to the HP compressor since the starter generator is permanently engaged to the shaft through a set of drive gears at the HP compressor stage. This type of generator is a simple shunt wind generator but with more winding as it is needed to produce a strong field and provide a high torque for starting. The main reason for using a starter generator is to reduce weight and therefore money on fuel burn and maintenance.