Kohler Generator Voltage Problem

[grehard1]

I have a 1977 Kohler 10R0P23 generator that when it starts comes up to about 25VAC then goes right down to 2.5VAC and stays there. I have verified all the components I can see. The only component I don't know how to test is the transformer. Also where is the voltage regulator? The transformer has a1 and a3 legs as the input and the output through the selenium rectifier to the auxiliary field winding. Any help would be appreciated. Thanks.

 

[STB]

Is this the same generator we we're talking about earlier? I think I was supposed to get back to you and forgot? Sorry! Sorry!

The schematic is approximately the one attached here?

Did you see my post below about how the transformer and diode work as the regulator?

-- https://www.trawlerforum.com/forums/s3/10kw-kohler-generator-10r0p23-schematics-69486.html

Can you remind me about exactly which components you checked and exactly how?

Thanks!

 

[grehard1]

Thanks for the reply. It is the same generator. The engine runs fine. Oil pressure and temp sensors work. Motor trips after about 20 seconds because voltage has not come up to 110. I had to replace resistor R7 and the diode RE2 plus IR2. I cleaned all the connector bands. The resistance of R1 is only 4 ohms. All the other resistors check out ok. 1CR, CC and CR are all working properly. I pulled the leads and cleaned them. I verified 1CR worked by disconnecting the A1 lead and put 110VAC to simulate power coming off the generator lead. I could hear 1CR close and the power available light illuminated on the generator panel. When I put 110VAC to A1, I got 30V at the leads to the selenium rectified and 30V DC output. The brushes are ok and continuous. I removed them and ohmed the rotor. It checked out ok. Could not megger it. I ordered a bridge rectifier to replace the selenium rectifier since it is almost 50 years old. I don't know how to test the large transformer. however the winding are continuous on both the A1 and A3 leads. The surge protector looks ok and the leads are solid. Start solenoid works properly and all leads are good. Also cleaned all the leads on the terminal block. Thanks for the help.

 

[STB]

Sorry. I've been swamped at work. Give me another day to where I can pull 5hat schematic and trace through this.sounda likenyouve done some good checking so far.

 

[grehard1]

Thanks for all your help. It is greatly appreciated.

 

[STB]

The transformer is what is called a conversion transformer. It is basically an autotransformer that bleeds off a little bit of the AC current, proportional to the load. This current gets rectified to DC by that selenium bridge rectifier and the resulting DC is fed into that AUX field winding. The result is that the greater the load, the more currentnis fed into the DC winding to mitigate the droop that would otherwise occur. In this way the transformer and diode worl together as the voltage regulator
At least that is my take.

A problem here could result in low voltage. But I wouldn't assume voltage as low as you are seeing. That is well beyond droop and there is no load.

I'm trying to think if this part of the circuit could be bleeding off current instead of supplying it, pulling the voltage down.

Did you ever replace the rectifier?

Don't do something just because I think I would. But my next test would be to disconnect the regulator circuit and see what the voltage does.

 

[grehard1]

Hope you had a good Thanksgiving. Thank for the response. I did replace the selenium rectifier. No help. I'll bypass the regulator circuit and see what happens.

 

[grehard1]

Here is the latest. I removed the A1 leads from the transformer and started the generator. It did the same thing. Voltage now peaks at around 20VAC then settled back to 2.5VAC. To re-verify the brushes and rotor, with the transformer out of the circuit, I backfed the generator with 110VAC for a second. It motorized the rotor so I know the lead, brushes and rotor are all good. Unless there is something I am missing, I can only think that there is something wrong with the stator windings. The wiring diagram says the 4 elements are wired in series so there could be a break in one of the wires or one of the stator pads is bad. At any rate, I guess the only way to be sure is to pull the electrical end and have a rewind facility take a look at it.

 

[STB]

Here is the latest. I removed the A1 leads from the transformer and started the generator. It did the same thing. Voltage now peaks at around 20VAC then settled back to 2.5VAC. To re-verify the brushes and rotor, with the transformer out of the circuit, I backfed the generator with 110VAC for a second. It motorized the rotor so I know the lead, brushes and rotor are all good. Unless there is something I am missing, I can only think that there is something wrong with the stator windings. The wiring diagram says the 4 elements are wired in series so there could be a break in one of the wires or one of the stator pads is bad. At any rate, I guess the only way to be sure is to pull the electrical end and have a rewind facility take a look at it.

Unfortunately, that is now my thinking as well. Good news is...there is a next step.

 

[grehard1]

Well, at least I have learned quite a bit about how the electrical end of a generator works. Would the next step be to get 4 big magnets and put them around the case to set up a permanent magnetic field? Thanks. R/Greg

 

[grehard1]

One thing I didn't do was measure the resistance across F1 and F2. It was 1 million ohms (1020 on the 2000k scale). Does that mean there is a break in the line somewhere?

 

[Bmarler]

One thing I didn't do was measure the resistance across F1 and F2. It was 1 million ohms (1020 on the 2000k scale). Does that mean there is a break in the line somewhere?

are those leads isolated from everything when tested? if so, then yes, you most likely have a bad field winding.

 

[STB]

F1-F2 shouldn't be open. I don't know what it should measure, but I'd guess a few ohms to a few dozen ohms? Too much more than that and I don't see how there is enough current flow to get much of a field. And, any less than that and I'll bet it is shorted to ground or something like that and you are measuring the same short from different distances. Since it is wide open, you might want to check for a broken wire at either of the connections.

My problem in understanding your situation, though, is I think that is just the auxiliary field. Basically, my understanding is that the steady-state excitation is coming entirely from the exciter armature, the part labelled "EXC ARM". My understanding is that the auxillary AUX field is just supplementing the steady state field to better support non-linear loads.

My basic understanding is that a key limitation of the exciter armature (EXC ARM), operating alone, is that if it gets hit with a spike in the load, it can't provide enough current to maintain the field, and the field collapses, and then there is no field to build back up from to recover. My understanding is that the concerning situation is mostly a modern load which might present the generator with a lot of transient, non-linear loads, e.g. switching power supplies, ballasts, etc.

My understanding is that the idea behind the auxiliary field (AUX FIELD) is that it is a separate field, generated by the spinning of the rotor, rather than directly from the load current. So, even if the primary field collapses, this is enough to bootstrap the field so there is some output so it can all grow and recover back normal.

I don't think it is a perfect solution, because I think that there is still some magnetic coupling with the load, but other than a field from huge permanent magnets, which are truly independent, I think it is as good as it gets.

But, I'm not seeing how this could be causing your problem, because in my thinking, the aux field is just to provide a push in the direction of voltage recovery, not the primary field. I can't see that it is the difference between 2v and 120v. And, also, since you don't have a load, you don't have a short on the load, or any harmonics from a non-linear load. I don't see any reason simple output-dependent excitation shouldn't work.

Maybe I'm missing something and, with absolutely no load at all, it needs the aux field to build up. You could try putting a simple resistive load, e.g. an incandescent light or resistive heater, on the output of the generator. I don't see how that will change anything. But, I don't see that it can hurt to give it a /quick/ try? Maybe, without the AUX field, it needs some output current to maintain the EXC ARM field. But, I don't see why.

When you first start the generator, you are getting 25v, right? Then it drops down to 2v, right? And, it didn't start to go to 25v until after you replaced R7 and RE2, right?

So, I think R7 and RE2 are providing the initial field current to the field via the #248846 start solenoid labelled as "C", which is enough to get the voltage up to 25V. Then, when the generator starts and "C" turns off, the generator is losing its excitation via this path and the voltage is dropping.

If this is the case, to me, this points to a problem with the exciter armature, the part labelled "EXC ARM" in the diagram. This is the part that provides the steady state exciter current.

You might try checking that path from where it meets RE2 at the main "shunt" field (SH FIELD), all the way back into the generator back end and see if you see a problem. The main field is called the "shunt" field because it is in parallel or across the armature (shunting it) versus in-line or in series with it as is the case with a SERies field.

I don't at all remember what the backend of this generator looks like. It has been too many years since I had my possibly similar model. But, some generators have a bunch of rectifier diodes around the circumference of the armature that rectify the AC to DC for excitation. If yours has these, then, in theory, you can disconnect these diodes, check each on of them, and then check the windings they are attached to. If yours has this arrangement, I'd expect the diodes to test like any other rectifiers and the same as each other, and the windings in between them to not be shorted and have the same amount of resistance each.

I really think you've debugged this to the point where you have really good reason to believe the problem is in the electrical backend and that the next step is take it to an electrical shop. Unlike me, who has "some sense" as to how this "might" all work, they'll be able to see it, know exactly what you have, and have fixed problems like it a million times before.

But, be really careful playing with sparky. Especially if it has power or is connected to anything that can make power. You can get shocked, start a fire, burn yourself, get caught in or otherwise mangled by moving machinery, etc, etc, etc, etc.

I'm happy to enjoy the thought experiment at my end -- but I /really/ /truly/ don't want anyone getting hurt in a real world experiment. At a certain point, in the real world, it is time for the repair person or repair shop.

 

[grehard1]

Thanks for all the really good info. Even if I were to get the generator to produce 120V, with the auxiliary field windings open, there is no real way to control the voltage under changing loads. All that to say, it is time to pull the electrical end, get everything cleaned up and see if there are any breaks in the lines. It's really tight in there and I can't see anything obvious.

 

[STB]

Thanks for all the really good info. Even if I were to get the generator to produce 120V, with the auxiliary field windings open, there is no real way to control the voltage under changing loads. All that to say, it is time to pull the electrical end, get everything cleaned up and see if there are any breaks in the lines. It's really tight in there and I can't see anything obvious.

Correct. I thinknthe AUX winding is both providing some regulation against drooping as well as providing resilience against a collapse of the main shunt field under load.

What is puzzling me is that a bad aux field would make the output voltage so very low and makes me believe that there is another failure, also. It seems this unit may have/have had a few things going on with it at the same time.

Seems unlikely...but something(s) are the problem...and a seemingly.more likelymsimple failure doesn't seem to be the ticket.