Century "five Star" 295ac/250dc Welder

amuller said:
Just my own line of thought! The fan is attached to the rectifier assembly and I won't be able to inspect the coil until I take it out, but as far as can tell the coil does not seem burnt, and I don't know why it would be--the bearings are free and it was running fine, and it's *cold* here in Minnesota. So hoping it is a lead connection problem. In any case, the secondaries of the transformer and the choke are aluminum, and the terminations rather rudimentary, so taking them loose for a cleanup and treatment with joint compound is likely enough a good idea.

Thanks again.

Alan
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Well, if it is that cold. It could have a thermal fan switch. And it’s just off because it’s so cold?
 
Well, I see I started this thread SIX years ago and just got back to working on the machine. And here is the thread just waiting....

Found fan with a 115 volt motor wired across the line, so no too surprising it burned out. So I cleaned up connections, checked the rectifier diodes, hung a fan on the outside, and it worked nicely for about five minutes before one of the rectifier diodes shorted. Disconnected the DC side and it works ok as an AC machine. So it uses 150 A 300 volt stud mounted diodes and these are a bit expensive. Cheapest I've found so far is $19 and most vendors are around $40. I'm not sure it makes sense to replace just the one. On the other hand, a complete 200 amp bridge rectifier in one piece is about $20 and a 300 amp is $30-40. Does anybody know if the rating of the one-piece bridge is equivalent to one individual diode, or do I need to apply a factor? And, what sort of spike protection should I put on the new diodes or new bridge. I guess this is more of an electronics question but people here usually know stuff.
 
Hi Alan, I would replace just the one diode. Motorola and International Rectifier are good brands. I'm not sure I trust the import bridge rectifiers I have seen. Something USA made I would trust. Besides it's more work, doing a conversion.
As far as spike protection, you could add a snubbing capacitor across each diode, something like 0.22 uF (microfarad) at 400 volts mylar type certainly wouldn't hurt.
-Mark
BTW, the airflow is very important to make those diodes live. Make sure any replacement fan has good strong flow, especially if the diodes are mounted on a smallish heatsink and there are cables and such that interfere with the airflow path, and you want to do high duty cycle dc operation
 
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Just zip-tie a Big box fan to the side of it and wire it in.......lol

Just like my friends refrigerator.........lol
 
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