Old Generator power low

I am still betting 10:1 on one of those diodes breaking down. The diodes you have in there if I remember correctly are 200 PIV 12 amp diodes. If I had a few lying around here I could find and I thought they would work I would mail them to you.

Something I mentioned earlier was measuring the voltage drop across the diodes one by one with it running good then measure again across the diodes when it fails.

You lose 50% of your field and each diode controls 50% of the field on alternating half cycles from your excitation windings. Now I figure either you have a diode breaking down or one of the field excitation windings breaking down. You could measure the AC voltage of the excitation windings with both diodes disconnected, Just measure across the Yellow wires measuring AC Volts. If that AC Voltage remains constant you have a diode breaking down. Not going open like a disconnect but breaking down. Now if the AC from the excitation windings fails and drops to half you could measure from each yellow wire to ground and isolate which excitation coil is breaking down and failing.

That's my story and I am sticking to it.

Ron

Ron, the diodes are MR 1124R. I do keep going back to the fact that the field goes to half power when the genny fails. That number coinciding with the voltage when a diode opens/fails is just too much to be a coincidence. Yet I can't prove it using any test that I've tried.

So I put the meter across the two yellow field wires with the diodes out of circuit and note the AC voltage. Simple enough. I'm looking for it to drop indicating an issue with the windings.
It's the next step that confounds me. "Now if the AC from the excitation windings fails and drops to half you could measure from each yellow wire to ground and isolate which excitation coil is breaking down and failing." You asked me to measure across those two wires with the diodes out of circuit. Now you want me to measure from each yellow wire to ground? Aren't we looking at both ends of the same wire? And if the diodes are removed, doesn't one end of that winding measure the same as the other end? Or does the cap in the middle make the field exciter coil actually two coils?
A First I'll remove the diodes from circuit and measure the field voltage AC. Hopefully the genny will 'fail' so I can measure in failed mode.
Next
B) Am I supposed to reconnect the diodes and measure from the wire to ground on each diode?
C) Am I supposed to connect one diode at a time and measure across the diode to ground.

Random thoughts; If the problem is in the windings and it's thermal, the genny isn't making power so it's not making heat. So it may not fail. If the field winding is breaking down, does that work with the knowledge that I can restore the field dc voltage by removing the cap from the circuit? IF the cap is breaking down is it putting more current through the diodes and forcing those to break down?

Thank you for your insight. I'll continue to test. I was running out of things to look at and I hadn't looked at the field AC voltages. I know it will fail if I let it run.

I'm up for this and I'll report back with the results. I just wanted to be clear about the test setup so I get meaningful results.
Thanks - Mark

Hi Mark

Part of what I wrote was my bad so lets look at a cartoon similiar to what you have. Hopefully I can get that attached right.

OK, one thing I suggest is removing the two yellow wires from the diodes. What you now have is just the lines off the excitation windings. The blue wire is a center tap of those windings. So first measure across the yellow wires looking at AC Voltage. Just watch that AC voltage with the unit running. Obviously there won't be any generator output. You want to note that voltage and see if it drops after some run time.

Now the blue wire is the center tap of your excitation windings. What I am saying at this point is that while monitoring the AC on the yellow wires if it drops out then measure from one yellow to the blue and then the other yellow to the blue. One of those two yellow to blue should be about nothing as in zero AC volts.

Remember when measuring the voltage across the yellow wires with the diodes disconnected you are measuring the AC voltage off your excitation windings. That AC voltage once rectified by the diodes becomes your field voltage. If that AC voltage (yellow to yellow) remains a constant then it is time to look at your diodes.

As to the diodes what I suggest is placing your one meter lead to ground (this with diodes reconnected) which will be the Anode of both diodes in your case. Now measure the forward voltage drop by placing your other meter lead on the post or diode Cathode. That forward voltage drop should remain a constant for both diodes when the system is running.

Excitation windings don't generally fail or break down but measuring that excitation AC output will show if one of the coils is failing.

As to the capacitor. That caps job in life is to smooth the full wave rectified DC created by the diodes. I have seen plenty of caps fail in a shorted state and plenty fail in an open state but never saw a cap fail in a half shorted state and act like a zener diode. That is why I am reluctant to see a problem with the cap.

Ron

I agree is sounds as if one side of the field is the problem, but how do you explain why without the cap it puts out 90%?
And why did the new cap provide full output longer?
It is a strange one.

Umm, Ron? I'm thinking I'm the wrong guy to be putting up a post to help guys troubleshoot their little generators. Your clarification and improved schematic answers a LOT of questions. I really hadn't thought of the exciter coil as being two separate coils with a center tap. Understanding feels good. It certainly does sound as if I'm losing one of the diodes. If I could find the darned things without paying $10 shipping, I'd just replace them and be done with it.

Neal, isn't that just ODD? If one side of the field is going away I'm fine with that. But why does it come back when I remove the cap? And if the cap is bad, why does it work for 8 minutes? And if it's failing it really isn't shorting or opening. For now that cap just bothers me. Like the diodes, if I could find a real replacement without the big shipping surcharge, I'd just do it. But to just throw money at the problem doesn't teach me anything. This is a fascinating thread. I'm (re)learning a lot. Most of it is stuff I used to know and haven't thought about in decades.

Mark-I am quite sure I have the 1124R diodes-they are in my garage attic. In the 70's I did aircraft electronics and they were used in power generation equip.
Still is strange you get 95% without the cap, if a diode is failing and it is strange to have the cap fail as Ron mentioned.
LMK if you want some diodes and I will look for them
Neal

Without the cap, the output waveform is probably not a good looking sinewave, more skinny shaped peaks.

What has not been brought up is the field power is dependent on a good output being established. The whole thing is an inter-dependent loop. You probably have some small magnets mounted in the face of the rotor to get the process started. The magnets get the process started which creates some small output that creates some field current, which then creates more output, which creates more field current, and so on, until it reaches steady state. If you disconnect something it has a compounding effect.

When cap is in-circuit, the peak conduction current through the diodes goes up. Diodes would be next thing to replace.

Man - you guys are a GOLD MINE! As far as the the Inter-dependent loop; I figured that was the case but I never heard anyone mention it. Little magnets mounted in the rotor to initiate the building of interacting fields. Now THAT makes sense. RC, Your short post is full of little tidbits that just make all of this make sense. And removing the cap which drops the current through the caps explains a lot.

There has been a bunch of really good stuff posted in the past few days. There is a lot more going on than I was thinking about. Info about how to troubleshoot this thing has been invaluable.

I'm on board with the diode replacement. I've done a good bit of internet looking for 1124R diodes. This morning I tried a cross reference for 1124R to get me to something that I could find at a local outlet. No luck. If somebody has a bucket of "Probably good parts", I'm interested. I'll buy the parts and pay the shipping. This stuff is getting hard to find. We had a wonderful salvage store that was full of this stuff. Most anything for under a buck - Pembleton Lab. Great place and we miss it!.
What I need is a cap - probably and a pair of diodes. The CAP is an aluminum case, Mallory Type CG, 100MFD 200VDC, Max Surge 250 VDC, 85°C - 01 3481 235-7746A. Diodes are MR1124R

OK another cartoon attached with some added stuff. However in the pre cartoon mode here is a brief description that covers what RCinFLA brings up:

FIELD EXCITATION

When a dc voltage is applied to the field windings of a dc generator, current flows through the windings and sets up a steady magnetic field. This is called FIELD EXCITATION.

This excitation voltage can be produced by the generator itself or it can be supplied by an outside source, such as a battery. A generator that supplies its own field excitation is called a SELF-EXCITED GENERATOR. Self-excitation is possible only if the field pole pieces have retained a slight amount of permanent magnetism, called RESIDUAL MAGNETISM. When the generator starts rotating, the weak residual magnetism causes a small voltage to be generated in the armature. This small voltage applied to the field coils causes a small field current. Although small, this field current strengthens the magnetic field and allows the armature to generate a higher voltage. The higher voltage increases the field strength, and so on. This process continues until the output voltage reaches the rated output of the generator.

Click to expand...

What you have and what many of its brothers and sisters are is a self excited generator. The start is based on residual magnetism and that sort of starts a snowball effect. Looking out the window maybe "snowball" was a bad choice of words?

Anyway the attached drawing contains a few examples of rectification just pretend the transformer is actually the excitation windings. The images are not my work unless we count adding the CR designations.

Someone mentioned building aircraft generators? That was funny as about maybe 25 years ago I worked for Lear Siegler Power Equipment Division and we designed and built aircraft generators and GCUs (Generator Control Units) and I also have a collection of diodes used in them. Problem is they don't have usable part numbers on them. I have no clue what they are rated at. Beefy little stud mount guys they are. If I knew they could handle 200 PIV @ 12 Amps I would send a few.

Also, somewhere around here I have a bunch of full wave rectifier blocks. I got to thinking (very rare happening). If you had a full wave block you could likely use three of the four legs in a configuration like what you have. Long as the diodes in the block meet the requirements it should work as a test anyway. That would be another option to at least test things. The best I can come up with is they should be between attic and basement. I found the stud mount aircraft diodes in a bag with assorted gun parts (owned a gun shop years ago). My organization really sucks so good thing my wife is patient as I have "stuff" squirreled away all over.

Ron

I need a smart guy. Nope, I need a well informed guy. I've spent hours on-line today looking for diodes. No luck at all. I've found what seems to be equivalent diodes with the following identifications; BYW 88/440R, BYX 78R, and BYX 99/60R
I find all sorts of diodes with all sorts of specs listed. I thought someone in this forum told me what I'm looking for if I were to go looking 'by the numbers'.
Any help or direction is appreciated.

With the information posted lately, I'm in agreement that a diode is failing. I didn't get the connection between the cap and the diode, but I understand the waveform and loading. Wish I'd of thought of that. Seemed so obvious when I read it.

An NTE5879 would be a suitable replacement. The minimums you want is PIV of 200 volts and forward current of 12 Amps. Any NTE distributor should either have it or be able to get it. They will run you about <10.00 EA. which is a bit of a rip off. Mouser Electronics shows them for just over $10 EA.


Ron

Thank you Ron.
While I was munching on supper and watching some worthless drivel on TV a thought came to me. My Ag-tronics is powered by a Briggs engine coded 1975. So the genny is probably about that age. In 1987, Ag-Tronics was bought by Coleman and became the PowerMate line of generators. The generator head on those older units looks just like my Aggie. The face plate is newer looking but the shape of the castings is the same. Dollars to donuts, the guts of the genny are the same. I'm going looking at repair centers locally for diodes for older PowerMate generators. A look-alike part is $7.50 but I can't read the part number. Gotta go shopping!

If that doesn't work, I'll go looking for a generic using the information from these posts. And I really thank all who are staying with me on this.
I'm having fun. The learning continues.

Oh yeah, before I forget a basic generic 1N number would be 1N1202R or 1N1203R (200 & 300 PIV Respectively).

Yes, they became part of the Coleman Power Mate line until Coleman dropped the line of Generators. Long as any rectifier diode meets the numbers I posted it should work fine and don't forget the "R" is for reverse as to what the stud is. Make sure you get a pair to replace both.

Ron

If you found a forward set of diodes just flip your capacitor and rotor connections over. As far as replacement diodes just look for a stud diode with the same threads with a 400+ volt rating and 10+ amps capacity. A higher amp and voltage rating doesn't hurt anything either.

I found something that may be of use to you in this. I knew I had a bunch of full wave bridge rectifiers around here. I have several of KBPC3504 Rectifiers. These are rated at 35 Amp (overkill) and 400 PRV. One of these could likely be configured like the attached image and though awkward would tell you once and for all if a few diodes will fix the problem. I hate to see you pay for diodes plus S&H and still have the problem. If you want one drop me a PM with where to send it and the wife can mail it tomorrow morning. I don't want anything for the bridge or postage so don't worry about that just let me know.

Ron

To all of you following this thread.
The project seems to have moved forward another step. Oddly, after Googling my diodes for several successive days, and then working with the replacement diode numbers I came away empty handed. Or nearly so. I really do want the chassis mount and they seemed to be no where to be found. Until today. I googled 1N1202 and up popped Newark.com. So I tried the inverted diode, 1N1202R and that one lit up at Newark.com. So I have a pair of diodes coming for $3.64 each and a bit of shipping. I should have an answer by the weekend. Or if not an answer, I'll have one more piece to the puzzle.

G'Nite

Hope they get here and solve the problem as I am curious.

Ron

Ron, Posted for the disillusionment of all are the latest results.
The Cap is replaced. The original was a polarized cap, the new one is not. IF a polarized cap is required, I'll put the old one back in. But the 2 1/2 minute failure is still present with the new cap.
The diodes are now a NEW pair of stud mount 1202R. Same polarity and same amp/volt range as the original.

Test Temperature = 40°F.
Generator Load = NO LOAD.

At 2 1/2 minutes the genny AC Output dropped from 125vac to 93vac
Brush DC voltage dropped from 67vdc to 37vdc.
Brush AC voltage dropped from (untested) to 25vac.
Diode voltage failed mode = 50vac and 41vdc for BOTH DIODES.

If the generator resumes correct output after it cools, I'll collect the pre-failed AC voltages at the Diodes and across the brushes. At this point, the genny is cool to the touch and is not returning to 'normal' output for the first 2 1/2 minutes.

It must be the weekend. Here I go again. Good thing I'm not in this for the money!

Sounds like you are down to a partial short in the exciter coils or rotor coils. Good chance its going to be a rewind or toss it.

One method to possibly confirm it would be to put a full wave bridge rectifier on one half of the exciter windings and power the rotor off of that and see if it does it. If so then switch it to the other side and repeat.

If it still does it you may have a partial short in the rotor windings. Then the only other option would be to externally power the rotor and see what happens. If it drops out just the same with an external power source its a rotor problem.

tcmtech said:

Sounds like you are down to a partial short in the exciter coils or rotor coils. Good chance its going to be a rewind or toss it.

One method to possibly confirm it would be to put a full wave bridge rectifier on one half of the exciter windings and power the rotor off of that and see if it does it. If so then switch it to the other side and repeat.

If it still does it you may have a partial short in the rotor windings. Then the only other option would be to externally power the rotor and see what happens. If it drops out just the same with an external power source its a rotor problem.

Click to expand...

That's not at all what I was hoping for but I'm thinking you're right. To prove exactly where the problem is might further my education, but it won't fix the rascal. This has been very interesting and certainly educational but to be honest, I've pursued this in anticipation of a repair. I'm not sure that the cost of the rewind is supported by the value of the repaired generator. A cost estimate would answer that question. I may unwind it just to continue my education.
Another thought - If I pull the rotor and put an ohmeter across the coil and center tap on each side - then gently add heat, Might I find a change? on one side? And I'm not discounting the external 70vdc power supply clipped to the brushes. Interesting.