MEP-804A Diode Ring Destroyed

[Evvy Fesler]

This post already has a confirmed diagnosis and root cause. I’m posting it to contribute to our knowledge base. Newly acquired genset had low voltage trouble light and lack of 120v on convenience outlet. Kurt Klopp refused to let me buy a voltage regulator until we did a little troubleshooting. Ultimately it lead to pulling the bell cap off to inspect and test the diodes. It was pretty obvious at that point. The diode ring was destroyed and unfortunately it gouged the stator windings on the way. I was also surprised to discover that this generator had the bolt on diodes and not the pressed-in ones that the TM says it’s supposed to have. It made me wonder if someone put in the wring part or if the generator was a different part than the original. Kurt suggested it may be a change in how they made them.

I’ve got pics to show. Click here!

Your advice is welcome. Some things you might comment on are:
- Rewind or not
- Where to find a rebuilt generator p/n 88-21005
- Sell the set and it’s trailer
- any other insights?

Evvy-

 

[Light in the Dark]

How about here? https://mibotparts.com/synchronous-...current-generator-88-21005-marathon-electric/

Its being sold for parts, presumably because the drive plate (which is steel, and can be brought flat) is bent. Price is right.

 

[Evvy Fesler]

How about here? https://mibotparts.com/synchronous-...current-generator-88-21005-marathon-electric/

Its being sold for parts, presumably because the drive plate (which is steel, and can be brought flat) is bent. Price is right.

Thanks! I think I also saw that one on eBay. It’s alao missing it’s rotating diode assembly and you can see that the stator wires are torn. They connect to the diode assembly. I suspect that the same thing happened to it as mine did. Still… it might have promise. I’ll write to them and see if they’ve tested the windings.

 

[peapvp]

This post already has a confirmed diagnosis and root cause. I’m posting it to contribute to our knowledge base. Newly acquired genset had low voltage trouble light and lack of 120v on convenience outlet. Kurt Klopp refused to let me buy a voltage regulator until we did a little troubleshooting. Ultimately it lead to pulling the bell cap off to inspect and test the diodes. It was pretty obvious at that point. The diode ring was destroyed and unfortunately it gouged the stator windings on the way. I was also surprised to discover that this generator had the bolt on diodes and not the pressed-in ones that the TM says it’s supposed to have. It made me wonder if someone put in the wring part or if the generator was a different part than the original. Kurt suggested it may be a change in how they made them.

I’ve got pics to show. Click here!

Your advice is welcome. Some things you might comment on are:
- Rewind or not
- Where to find a rebuilt generator p/n 88-21005
- Sell the set and it’s trailer
- any other insights?

Evvy-

Hi Evvy,

pretty sad story. This one seems to be a one of a kind, DoD Military Design most likely made under CAGE # of DoD and not Marathon.
That would explain the diodes and some of that wiring you had in there.
Maybe Guy can query the back office….
Peter

 

[peapvp]

Evvy,
The DoD designed many many parts themselves which where then made by a outside company but it would be listed under the NSN with one of the DoD’s CAGE Numbers.
Those items, with DoD CAGE # are almost all DMIL B or Q…….

 

[peapvp]

Evvy,
Sorry to be the bearer of bad news, but this board has been “occupied “ by civilians and when ever the insider say something then the civilians go

 

[peapvp]

NSN 6115013833124
Two versions with two CAGE #
the Made in Mexico has the screw type Diodes and is DMIL A1

 

[Light in the Dark]

Google Delk's Surplus Sales in NC. They may have a machine which they can separate for you.

 

[Evvy Fesler]

I talked to Marathon this morning. This may be helpful for others to know. The press fit diode assembly was the original configuration and later they discontinued it and the stud mount was the substitute part. There are equivalent commercial units that will fit, but not necessarily handle the surge current of heavy motor starting loads. I'll be researching these alternatives and will be able to provide additional information as this story unfolds.

Thanks to suggestions from all of you, I've been able to sort out three paths to explore:

1. Repair by rewinding. Since diode plates are still available, new, from the manufacturer this becomes a viable approach; albeit expensive. For some of you, this may be attractive because commercial units are available in single phase or 12-wire.

2. Stay military and buy an equal used unit or the 400 Hz unit and canalbalize it/them for interchangeable parts. This is viable since the diode assembly is available, which is the one part that I haven't seen on the so-called parts units. It would also mean removing and reinstalling the stator, which looks like a challenge; particularly a weight consideration. This would only be viable if the rotor windings are intact. I haven't checked them yet.

3. Go commercial and source a unit that'll fit.

4. Your ideas as they roll in.

Advice to self: Patience, research, capabilities and patience!

Evvy-

 

[Ray70]

Sounds like the next step is probably to get pricing on your different options.
Given your excellent ability shown thus far, I'm wondering if a commercial 12 wire generator conversion might fit your needs best?
It may be cheaper than a rewind and you could set it up for single and / or 3 phase output. ( as it sounded like you have a need for both )
My only warning with the 400Hz parts machine would be to first "research" as you mentioned. I would have to assume that nothing in a 400hz gen head would be of much use to you, but I have not looked into it at all.
Lastly, depending on price, another option could be to see if another 804 comes available where the current gen could be used as the parts donor / spare parts source.
All depends on PRICE!
Good work so far and excellent write-ups!

 

[FLCarguy]

There is a video on Youtube of a guy pulling the diode plate and the generator out and having the generator rebuilt and fixing the diode plate himself. I image it can be rebuilt fairly simply if you fabricate the mounting plate.

 

[peapvp]

This is interesting,
I looked at the two eBay listings and both seem to have suffered the same fate plus the one in the video FLCarguy posted as well.
That makes 4 Diode Ring failures.

This raises some questions.

The red material, where the diodes mount to, looks to be a fiberglass substrate. Somewhat difficult to see in the pictures / video I have looked at.
Fiberglass is usually very difficult to break unless the fiberglass has been exposed to high heat for a prolonged period of time.
The failure these disks experienced was pretty violent.

One of the government surplus auction sites had three heads with your part number listed and sold in the last 12 month.
I suspect that the two defective heads currently listed on eBay are two junkers which have been replaced with the NOS from the Government Auction Site.
The sales price on the Government Auctions was around $ 250.00 for each head without the fee's etc.

It would be interesting to know if the other units who failed had the same amount of run time on the genset as yours.

If that's the case, then, with a high degree of certainty, any other 804 A/B who has this head installed with the red Fiberglass Substrate / screw type diodes is probably going to fail in the same time frame as the other 4.

If the run time differs greatly and all 4 Genset's where picked up from Government auctions and produced no power when first started by new owner, then we can suspect that they were DEMIL'ed by DoD prior to release to Auctioneer.

DEMIL A can include a NON WORKING APPARATUS, but which can be rebuild / repaired after release from DoD / Auction. All the other DEMIL Codes require a complete scraping of the Apparatus with no possibility of rebuilding / repair.

Please keep this in mind when bidding on Government Auctions. Look and examine the picture / video carefully and read the listing text in its entirety before bidding on a Item.

Any Generator with the disclaimer "Starts/Runs but no Power / no excitation" in the description should raise a red flag.
Don't assume it's just a bad Voltage Regulator. There can be several issues for this "no power / no excitation".

Peter

 

[Evvy Fesler]

I picked up a little more information from Marathon Electric (the manufacturer). From the serial number they are able to determine that the unit has been modified. They also point out from my pictures (see above link) that the red paint on stator and rotor windings and lams is not their color, which is green.

It is evident that this generator has a "history." Marathon's replacement ring, as some are calling it, made of fiberglass as Peter points out, costs around $350 to purchase as new with diodes. It has holes for press in diodes and mounting brackets for stud mounted diodes. The diode ring is used in several models including commercial ones.

My opinion is that whoever did this work likely overlooked lock washers or perhaps used a thread lock compound, which may have softened with heat (think Iraq or Afghanistan). I lean toward the former because I'm not able to find any lock washers in the casing or on the floor of the sheet metal housing. I did find a couple of nuts for the diodes. It's also possible that a diode or two came loose and caused an imbalance condition.

Forensic analysis isn't a strength for me. Facts are 1) it's been modified and or repaired, and 2) the diode ring met a violent end and took out stator and or rotor windings with it.

I agree with Peter. It could be that the "new" fiberglass stud-mounted diode ring fatigues and doesn't stand up and my thought, assuming that's the case, is that MEP owners may want to inspect their diode rings yearly by pulling the bell plate and taking a look. It was relatively easy to do.

Evvy-

 

[Evvy Fesler]

There is a video on Youtube of a guy pulling the diode plate and the generator out and having the generator rebuilt and fixing the diode plate himself. I image it can be rebuilt fairly simply if you fabricate the mounting plate.

Great video @FLCarguy, thank you!

 

[Evvy Fesler]

There have been suggestions to look at a 400 Hz head for interchangeable parts. Here’s what I found out.

Marathon Electric, a very helpful company, said that 400 Hz head 88-21006, doesn’t have any (noteworthy) interchangeable parts.

Did you ever wonder why the RPMs have

RPM =120 x Freq / poles

60 Hz = 4 poles therefore
1,800 RPM = 120 x 60 / 4
1,500 RPM = 120 x 50 / 4


400 Hz = 24 poles

So what if we were to run the generator slower? Wouldn’t that be great? Less fuel, right? Well maybe, but we’d have to run it at 300 RPM. It’s doubtful it would hold an idle there let alone develop the Horsepower (HP) needed to overcome changing loads
Marathon Electric, a very helpful company, said that 400 Hz head 88-21006, doesn’t have any (noteworthy) interchangeable parts.

RPM =120 x Freq / poles

60 Hz = 4 poles
400 Hz = 24 poles

So what if we were to run the generator slower? Wouldn’t that be great? Less fuel, right? Well maybe, but we’d have to run it at 300 RPM. It’s doubtful it would idle there let alone develop the horsepower (HP) needed to overcome changing loads… like when the house HVAC energizes.

300 RPM = 120 x 60 Hz / 24 poles.

 

[peapvp]

Evvy,

These Diesel engines are optimized to put out the highest torque at app 1,800 rpm
At 300rpm you probably going to get 30% of the rated Torque which would reduce your maximum power output to 15KW x 0.3 => 4.5KW

Your better off with a 5KW Genset then

Peter

 

[Evvy Fesler]

Thanks, Peter... it was just an example for the knowledge base. Torque was a better word than horsepower that I used.

 

[peapvp]

Evvy,

here are the parts associated for the "Diode Ring"

50Hz / 60Hz:


400Hz:



Further,
by looking at the parts drawings in the TM I think it is clear of what happened here:




Peter

 

[Evvy Fesler]

Agreed, but let's not overlook that the manufacturer said that they discontinued the press-fit, as in the 60-Hz, first picture, and began using the stud-mounted similar to the one in the second picture. They went on to say that the replacement ring also accepts the press-fit diodes.
I suspect that mine may have been hand crafted, which would explain the presence of red paint on the back and absence of green paint, the manufacturer's color.

 

[peapvp]

So, this should be a warning to all 804 A/B owners:

There seems to be an x amount of 50Hz/60Hz 804's floating around out there which MAY {edit} have the 400 Hz Diode Ring Installed without lockwashers {edit}.

The Diodes are different:
The Diodes on the 50Hz / 60Hz Diode Rings have a Lower Current Rating and a lower maximum Voltage then 400 Hz Unit's

Theoretically, this should not be a problem but the impedance the Voltage Regulator "see's" is quite different in both setups.
That's why they have two different Voltage Regulators for the different frequencies.