Diodes Blowing Under Load on my 550kw Diesel Generator

Cummins can help. Make the call.

Thanks PWSlack, but i had called them prior to posting this and They were suggesting i replace the alternator which is not an option for me at the moment as i want to try something else before comitting to the cost of a new alternator.

The alternator has been rewound both stator and rotor and all phases seem to be balanced. LIke i said before all phases at no load the voltage is a steady 400V, 400V and 399.0V

Generator rating is 400/230 V.

The diodes(70A) that are blowing are 3 of the +VE only. They blow at a load current of 700A and the rated full load current is 902A.

Any help as to what i must check would be great.

What is the voltage rating of the diodes?Check for loose connections on the load which could spike the voltage above the rating of the diodes.

If available, install a line monitoring recorder across the diodes to see what they are actually feeling when loaded.

What are the specs on your diodes?

How did you measure the load current? is it a new design or was it working until now? If it is a new design, did you check the "I²•t" of diodes?...Seems the given data is too short.-

If you have changed the diodes also with not same type .try to change them with fast diodes type (up to 500Hz ) high current more than (70 amp) and check the diodes heat after running not the heatsink plate ,should be little heat .check the rotor exciter(three phase coils connection ) and the main rotor exciter .

good luck.

thanks. i have used high frequency diodes and temp is ok. MY new problem that has arrised is when loading the generator now says low frequency on the computer and shuts down power to the loaded equipment.

Is this Alternator Brush-less? (Your Diodes are mounted on the Rotor Shaft?).

If Brush less: The AVR is driving the Exciter unit. This AVR has its own set of diodes to produce the required DC for the exciter. Are you referring to these diodes? if so, definitely the diodes should be rated more than 70A, more like 110 A for the 901A Generator. But then, the AVR is supposed to be already built for this rating!! Have you changed the AVR without checking the AVR's output?

This comment raises the relevant question, what is the current requirement of the exciter when the generator is at full load (900A) across the operating range of power factor. If more than 70A then this is likely to be your problem