lets throw a light how voltage is kept constant in an aeroplane,
SHAFT :- is a shaft which is inside the jet engine, it moves when the engine starts,
CSD : - stands for Constant Speed Drive
as u can see the diagram above which clear shows that in a jet engine there is a output shaft which moves when the fan of the engine moves, this shaft is connected with many gears, these gears, at the same time we have CSD which maintains the voltage of the aeroplane, as we have regulator in our local cars to control the voltage of the CAR so in aeroplanes we dont have any type of regulators we have CSD connected with the ENGINE output shaft, this CSD having various gears inside when engine starts the engine output shaft moves, when it moves, the various gears connected with this shaft also moves when the gear moves, CSD come in contact.
NOW THE QUESTION ARISES how come CSD come in contact, as we can see that CSD have a actuator, this actuator is controlled by hydraulic pump, the hydraulic pump controls the pressure to move this CSD accordingly, for example the rage of this is 5000 RPM to 50000 RPM, as we know the RPMS of the engine vary time to time for example when the plane is on the ground and engine starts this for example it attains 5600 produces 10 V but the when the throttle moves for more speed than its obvious that the RPM will also increase produces for example 130 V but the required is 110 V now the CSD is the unit which controls it.
so the actuator is device which is connected with hydraulic pump if rpm decreases so the CSD moves much closer to attain the required RPMS, if the voltage is in excess engine out put shaft produces 130 V which is in excess so the CSD moves OUT a little bit so that few gears are in contact, moving CSD with a little speed than the current speed (which is producing 130V)..
So the conclusion we get here is CSDs have many gears and the move right or left using a hydraulic pump supported by actuator.
How hydraulic pump knows when the rpm drops/increase?
there is a pressure sensor which tells the hydraulic pump when the pressure is decreasing/increasing.....