Generator eats up rectifier assemblies????

gregd(6)August 22, 2005

Well, I'm a bit stumped here. I have a 3250 watt Craftsman (made by Generac) generator that came with my house (Nicely wired with a transfer panel, thank you!)

It works great--but seems like it loses its magnetism relatively quickly. I can jump start it by "flashing" it via the "plug in a corded drill and spin in reverse" method for a while, but it never seems to regain it's own ability to produce power without this step. Eventually it can't be revived.

If I replace the recitfier/brush assembly, it works perfectly for a while. No flashing necessary.

I do start it once a month or so and run it for 30 minutes with a space heater (1500 watts) to keep it excersized.

Could I be doing something to burn out this rectifier assembly somehow? It's a Briggs part (91825GS) that I get for about $19.95 plus shipping from Jack's. It's easy to swap, but the 20 bucks every year or so (with very little use) is getting a little annoying!!!

(I'll admit that for my 30 minute "excersize," I don't ground the generator. Could this be somehow ruining the rectifier?"

Any ideas would be appreciated!



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Hi Greg, if the rotor assy wont hold its residual magnetism it is bad and should be replaced. If you start the generator and get no power at the outlets check the voltage at the outlets. if it is about 3 VAC you still have residual magnetism and the rectfier circuit and its wiring are not returning DC to the rotor to boost the field strength. WIRES, rectifier, Brushes, slip ring, or maybe wierd flying open in the rotor could cause this. HOWEVER

I cant really understand your post 100% since you are talking Civilian-Human and not (understandably) Technician :-) . A short in the rotor will destroy the rectifier. If you mean that after a while flashing doesnt revive the unit and changing the rectifier does then either your FLASHING PROCEDURE or the rotor excitation circuit is killing the rectifier.

Off the top of my head the power winding resitances should be a fraction of an ohm, and the rotor resistance should be on the order of one or 2 dozen ohms or so. The rotor wire is very fine and would not take say, 10 amps for very long. if the output of the rectifier is say 120 volts anything around 12 ohms would be too small and even 24 would be questionable as 5 amps thru fine wire for long periods is??

Check the rotor ohms and power winding ohms after failure, and the voltage in the outlets after failure, if you need help to do do, please repost. HTH Dave.

    Bookmark   August 23, 2005 at 2:02AM
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If you mean that after a while flashing doesnt revive the unit and changing the rectifier does then either your FLASHING PROCEDURE or the rotor excitation circuit is killing the rectifier.

Dave, thanks for the reply.

Okay, I think we're getting somewhere here. You understood my "civilian speak" correctly. I keep a spare recitifier on hand, I changed it out, and the power came bounding out just like it should.

For the next several "test runs," the generator will produce power without any help. It's just after a while has passed that it needs a "kick in the butt" to get going again.

I remember taking the DMM to the outlets once when it failed to make power, and there was about 3 volts reading.

Could you explain what the "rotor exitation cuircuit" is? Is it generallly part of the rotor itself? The good thing about Craftsman stuff is the availability of parts, but this part sounds expensive to replace, and probably out of my limited technical abilities.

I truly appreciate your taking the time to answer my post.



    Bookmark   August 23, 2005 at 9:49AM
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Hi Greg, if this were mine I would first carefully check all the wires behind the panel where the outlets are to make sure there are no loose wires. next I would remove the end cover and disconn one brush wire and cover it with insulating tape, then start the unit and check the voltage at the outlets. Should be about 3 volts ac. If thats not 3ish there is no residual magnetism. if you have 3ish voltage shut off, remove brush assy NOTING CAREFULLY HOW IT AND ITS WIRES ARE INSTALLED. (should be pos brush closest to bearing at end of rotor). Check the resistance of the rotor(slip ring to slip ring) and post it here. For extra test have an assistant rotate starter rope while while you check resistance. Also check resistance of rotor via the brushes, its uncommon but they could possibly be glazed over or open. Post that here too, if you please.

Next take ohmeter, engine still off, and test the resistance between the two wires that come from the stator(the big stationary can0to the rectifier, please post here.

A couple of key points, I am not sure about what features this model has as I do not memorize that kind of info. i assume you have no idle down circuit or voltage regulator, just a plain rectifier. Other point is this description fits a somewhat unusual problem, Usually rotors just go open when they fail, or their resistance is so low from a short they eat rectifiers almost immediately.

I will explain how they work in sepaate post later. HTh Dave.

    Bookmark   August 23, 2005 at 11:30PM
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Wow, awesome tech article, techdave!

I will get out my manual to make sure I'm testing the right things you've described above.

This unit does have the brushes and rectifier combined as an assembly.

When I have some results for you, I will post here.

Thank you so much for taking the time to educate and help an electrical pre-schooler!


    Bookmark   August 24, 2005 at 6:43AM
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Hey guys, I'm back.

My son was born 1 day after my last post, and we must not have lost power since then as this project obviously got shoved to a back burner.....

I'm dedicated to resolving this generator mystery....more to come.

    Bookmark   July 20, 2006 at 11:34AM
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I have a Generator without full power. I get 3.7 volts tested @ outlets. what is the resistance between rotor slip rings and @ Stator wires removed from bridge rectifier? this generator was used once and was sitting for 15years (in-laws) I think I will change the brush assy that comes with rectifier attached. hope you can help.


    Bookmark   December 16, 2014 at 8:06PM
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