It is desirable to be able to control the output of a generator so as to maintain the voltage produced by the generator within acceptable limits. However generator output voltage may vary with generator speed and electrical load. A variable frequency electrical generator is shown in FIG. 1. The generator comprises a pilot exciter 1 in the form of a permanent magnetic generator (PMG) having a permanent magnetic rotor 2 co-operating with a three phase stator winding 3. Power from the stator winding 3 is provided to a stator field winding 5 of a main exciter 6 via a generator controller 4. The main exciter 6 has a three phase rotor winding 7 arranged to supply current to a main rotor winding 9 via a half wave bridge rectifier 8. The main rotor winding 9 co-operates with a three phase main stator winding 10 which is connected to the generator output 11. The permanent magnetic rotor 2, the exciter rotor winding 7 and the main rotor winding 9 are attached to a common shaft 12 so as to rotate together. The shaft 12 is connected via suitable coupling means (for example a gear box) to a prime mover, such as a gas turbine aero engine. The generator controller 4 is connected to the output of the generator stator windings so as to monitor the output voltage produced by the generator.
It is known to reduce the current supplied to the stator field winding 5 of the exciter if the generator voltage rises above an acceptable threshold. Similarly, it is known to increase the current supplied to the stator exciter winding 5 if the output voltage of the generator falls below an acceptable voltage. Measurements of generator output voltage may be performed on a time averaged basis and also on a peak basis. Whilst this control system works well, there is a risk that complete failure of or a poor or broken connection to the generator output voltage sensing system within the controller 4 could result in the generator voltage becoming unregulated and raising to a level where equipment supplied by the generator may become damaged.