What do you have to switch on, alternator or generator?
If it's the first one, an alternator has to be disconnected when not used because it discharges the battery. In cars it is made when you turn the key before starter receives energy from the battery, that's similar.
If you have to switch on generator, it could be to let energy reach it because it is an starter/generator, it moves the engine in the starting and then it is disconnected and used as a DC generator.
I don't know why it's not made automatically.