Generator problem

[gaelen1]

Our generator has stopped generating... Power went down slowly ( in about an hour) from the normal 240 V to about 60 volt..
Generator is a Motori Morelli , about 12 KW .. Current no load voltage is 60v , Have disconneted the AVR , still producing 58 volt..
Am I right in thinking the AVR is the most likely problem?? Generator has no brushes..

 

[ScottyUK]

Sounds like it. Check for obvious faults in the windings: megger between phases and phase to earth, and check the phase winding resistances at least match each other if you don't know the exact values. If you don't find any problems relating to winding damage then you should be able to substitute a lab-type variable power supply (maybe 30V / 2A or something of that order) in lieu of the AVR and connect it to the field windings. ***Carefully*** raise the supply voltage from zero and you should see the output voltage increase. If the voltage comes up to a normal value then this pretty much confirms that the AVR is damaged or dead. If you don't see the voltage increase then it may point to a problem in the field or the on-shaft rectifier. Try the test and report back.

 

[waross]

I have used 12 Volt automotive batteries in the field where no power was available for the variable power supply. A reverse polarized discharge diode across the field is a good idea. It is left in place across the field to provide a discharge path when the battery is disconnected. Every model set is different but I have found that a lot of sets will produce about 50% of rated voltage with 12 Volts across the field.
You don't have a Permanent Magnet Generator feeding the AVR do you? A progressive winding failure in a PMG may have those symptoms.
Generally the AVR feeds a brushless exciter on the shaft. When the set is stopped, an AC voltage (variable,- ramp the voltage up slowly) on the field leads should produce an AC voltage at the rotating rectifiers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gaelen1]

Had to go away so haven't yet tested. Many thanks all for the useful replies. Will get back when I've done the tests.