brentz
Model 4370-2, 20kw, NG, 1.5L Mits, 120/240V 1 phase, 400 hours run time
Put In-service 2002

2 years ago problems began, motor starts fine and runs followed by starter engaging every 10-15 seconds even though running. Took the easy way out and threw new VR at it, no joy.
In the last few weeks, decided to get serious about figuring out what was wrong and followed some service manual diagnostics and got the following:

Polished/cleaned brushes and slip rings then did static rotor test:

Applied 12V to wire #4
Rotor current 1.3 A running and not running
DPE voltage (6,162): 75VAC
Mains voltage (S15,S16): 6VAC
Isolated two main windings, (22,11):80 VAC, (33,44):78 VAC
Both main windings had shorts to ground
Conclusion..bad Stator

I removed the stator and inspected and found this:

image5.jpeg 
 
Clearly melted copper and obvious damage. Called up local electric motor place and they said bring it by and they would tell me if they can do anything with it short of spending the equivalent of replacing the thing.

Then I noticed the rotor had about 30 degrees of play in it attached to the fan plate. Smelling more trouble, I removed the assembly and took it apart to find the square key sheared and some damage to both the end of the rotor and the fan plate. See below:
image4.jpeg 
image3.jpeg 

image2.jpeg 

Fan Plate Damage:
image1.jpeg 

Soooo...is this when I should say replace the rotor/stator/fan plate? Or, since the rotor seems operational, no shorts in windings and it ohmed out at about 9 ohms, should I try to make reuse any of this stuff? My engineering training says replace it all, but all my cars have over 200,000 miles on them...you get the picture. Appreciate any experienced opinions..
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78buckshot
New stator and rotor will set you back $1500.00 - $2000.00 IF they are available. If you call Ziller Electric with the serial number from the main nameplate they can give you more info on parts. I know other parts of that vintage are obsolete.
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Peddler
You can get a new 22KW air cooled for about what it would cost to get your generator repaired professionally.  The 1.5  engine has a few parts you can't get any more and I think the upgrade would be worth it as they are a good reliable unit and can handle anything the 20KW liquid cooled did. 
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Cobranut

I'm with Peddler.  If engine parts are getting harder to find, and you have this much damage to repair, I'd go with the new 22kW.

You used this one only 400 hours in 18 years, so the air cooled should last at least as long.

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Peddler
You can still use your existing transfer switch and the pad and gas should be simple to repipe.
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brentz
Thank you for your experienced advice! I know that genset technology has evolved from 20 years ago and is much more "appliance" like now. When I bought the thing new, ran about $5K and now to find an equivalent liquid-cooled unit, starting at $9K. So during the tornadoe outbreak of April 2011, we were without power for almost 2 weeks, would an air-cooled unit keep it together for that long?
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Peddler
Yes we have had many air cooled unit run 2 weeks plus without trouble.  Having said that you would still need to do maintenance during the outage.  Your old generator only held 3 quarts of oil and should have been checked on a daily basis during the outage.  An air cooled only holds two quarts of oil and it too should be checked every day after about the third day  but can run forever with checking the oil and changing the oil and filter on a weekly basis during an extended outage.
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brentz
Good News! I have nursed this old thing along for a little bit longer. Replaced rotor/stator with a used one from a 25kw model, same engine and it now is purring.
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