I had a call from a guy whose AVR generator had suddenly stopped putting out power. I went through the usual fault-finding steps over the phone, including testing the stator resistances and connecting a battery to the brushes to flash the field. Flashing the field made voltage appear at the outputs but with the AVR reconnected the output still failed to appear. I concluded that the AVR was at fault and I supplied a replacement. However upon fitting the new AVR the problem persisted.
I was starting to suspect that the winding that drives the AVR had failed. I asked him to flash the field again but this time for 10 seconds. This time it did the trick and everything sparked into life. So I conclude that the rotor had been completely demagnetised . Some loss of residual magnetism is not uncommon in machines that have stood idle for a long time but I have not seen many instances where one day the machine is generating and next day the magnetism has gone altogether.
After some pondering, I came to the conclusion that the load (an electric drill) was not disconnected when the generator was stopped. The motor of the drill had completely demagnstised the generator rotor as it freewheeled to a stop. Electric drills are sometimes used to restore residual magnetism in generators but if turned in the wrong direction they will have the opposite effect.
Well that’s my theory. What do you think?
The moral of the story is : Disconnect the load before you switch off the generator.