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Kohler Gens are the devil!

TennGen

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Here goes nothing. My company has a Kohler 300kW, model 300ROZD71 standby gen. It was here when I got hired, and I'm trying to fix it to use as a standby gen when a customers gen goes down. The field exciter assembly crapped the bed on it, pieces of magnets everywhere. I have the part number/s i need but they are discontinued. The gen is from the early 90's , but im hoping someone may be able to help me.
Model: 300DOZD71
Spec: 132708-71
the part i need is #257963 Field exciter assy.
I'm attaching the parts manual for it. The part is on page 40&41 number 43. Thanks in advance.
 

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Birken Vogt

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How was it confirmed that it is NLA? I have no specific insight into this situation but seems a little funny.

I wonder if a generator rebuild shop could source some new magnets and repair this. PMG are not unique to Kohler and this can't be the only one that has ever broken.
 

TennGen

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How was it confirmed that it is NLA? I have no specific insight into this situation but seems a little funny.

I wonder if a generator rebuild shop could source some new magnets and repair this. PMG are not unique to Kohler and this can't be the only one that has ever broken.
It's a discontinued part, that info came from kohler when I reached out to them directly. Not sure what PMG stands for other than primary main gen. If that's the case, I'm not talking about the main gen. If not then please enlighten me. The gen model is from '91 and from what I've seen it's not uncommon for those kohlers to have discontinued parts. Had a small 5kw kohler a few months back that needed a new exhaust manifold. The manifolds were no where to be found. I happened to luck out that it had a ford I4 engine and was able to get a hold of a refurbished part from northern power products.
 

Birken Vogt

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PMG stands for permanent magnet generator. The reason I thought it funny is because they used this style generator from several years before this one was built all the way up to 10 or so years ago. But surely 300 kw units are not that common so maybe the last time they sold this part was 10 years or so ago, who knows. For something of this size and value there has to be someone who can make one. It is at base a chunk of metal with some magnets in it.
 

TennGen

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I agree totally, and thank you for the clarification. I'll add some pics next week. i got the exciter stator off today and the rotar is pretty tore up. I work at an electric motor company so rebuilding the rotor isn't an issue, but our winding dept. doesn't know much about the magnetic stator.
 

Birken Vogt

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It's just permanent magnets instead of laminations and windings so the concept is not too hard. The details of what the magnets are, how strong they are and what holds them there is what somebody will have to figure out.

Somebody that rebuilds generators of this size might have some knowledge about this. The other "generic" generator manufacturers such as Stamford Newage, Marathon, etc. also have permanent magnet generators as well but Kohler puts their own twist on it with the Fast Response system.
 

pegasuspinto

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Kohler's solution is likely an entire new generator end. Won't be cheap.

I'd be worried about the entire gen end. Magnets all balling up and sticking to everything will cause a lot of damage.
 

Birken Vogt

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Kohler's solution is likely an entire new generator end. Won't be cheap.

I'd be worried about the entire gen end. Magnets all balling up and sticking to everything will cause a lot of damage.
I'm thinking there is probably a lot of space between the PMG section and the main section so hopefully the magnets did not make it that far. But it sounds like they are a motor shop so identifying and repairing damaged windings should be no problem for them.
 

TennGen

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The bits of magnets went all over, back into the main gen assembly as well. Our winding guys looked at it and said the gen wouldn't be an issue to rewind. The unknown is the magnet portion with the exciter stator. Not sure if the cost of repairing it would out weigh the benefit. Also, through my research I'm thinking what caused this to happen was an over-speed fault from the engine. These Detroit diesel's were prone to that from what I've found. So I'm thinking it would just be a money/time dump.
 

Zephyr7

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The PMG setup works like this:

1- Permanent magnets spin on the shaft to induce a voltage in a winding in the stator.
2- The winding in the stator generates power to operate the regulator. Since this power is independent of the main output of the generator, you avoid any issues with the regulator if there is a fault downstream of the main generator output. The output from the permanent magnet generator (PMG, the combination of the rotating permanent magnets and the coil in the stator) is a high frequency AC.
3- the PMG generator lowers the regulator, which in turn sends a DC voltage to another winding in the stator. This DC voltage is controlled based on the output voltage from the main generator. This is how the main generator regulates its output voltage. This winding in the stator induces a voltage in a winding in the rotor.
4- a winding in the rotor is connected through a rotating rectifier assembly (usually mounted on the end of the shaft) to the field winding in the main generator.

The link between 3 and 4 is the “magnetic coupling”. In kohlers “fast response” system, the magnetic coupling is replaced with an optical coupling located at the end of the generator shaft so that the LED/phototransistor rotate without getting interrupted. The phototransistor is on the end of the main shaft, in the center, and the LED is stationary and mounted within a cover pointing at the phototransistor.

I don’t think Kohler used the optical fast response system on generators above 150kw.

I would try calling marathon electric and ask them if they can be of any help. Marathon electric supplies the generator ends for many (most?) it kohlers larger gensets so they are likely to have more familiarity with how Kohler does things than most.

Bill
 

LWB250

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Kohler used the Fast-Response optically coupled design from 20kW up to 300kW. Anything above 300kW of this vintage would have a Marathon PMG unit (same concept, different design.)

There is no "shortcut" or end run around this. If Kohler can't build an exciter, you're SOL. They don't keep these laying around, and never did, even when they were in production. Understand that the frames are assembled by gluing the ceramic magnet "blanks" in place using a special jig, much like a giant tailpipe expanding tool. Once the blanks are in place and the jig is tightened, the exciter frame is placed in the same ovens used to bake insulation. The epoxy used is a high temperature material and this cures it over the better part of a day of heating and cooling.

Once the exciter frame is cooled to room temperature, the jig is removed from the center and the frame is struck a number of times with a large dead blow hammer. If any of the magnet blanks have failed to adhere to the frame this will make them fall off. There is also a visual inspection to assure that the magnet blanks have not cracked or the edges chipped.

So now you have an exciter frame with a series of ceramic magnet blanks glued in place. No magnetism at this time, it's a totally inert assembly.

These are stacked on a pallet after inspection and then sent to the alternator assembly area.

When it's time to assemble the alternator, the exciter frame has a large set of coils placed inside of it. Think of a giant exciter rotor. It's positioned correctly, another employee verifies the setup, and a power supply consisting of a number of very large, like barrel sized, capacitors discharge into the coil that's inside of the frame.

The exciter is now magnetized. It's a 24 pole exciter with 8 magnets. So how many magnetic poles does each magnet have? (Trivia question.)

Dan BTDT, pushed the button
 

TennGen

Registered
Kohler used the Fast-Response optically coupled design from 20kW up to 300kW. Anything above 300kW of this vintage would have a Marathon PMG unit (same concept, different design.)

There is no "shortcut" or end run around this. If Kohler can't build an exciter, you're SOL. They don't keep these laying around, and never did, even when they were in production. Understand that the frames are assembled by gluing the ceramic magnet "blanks" in place using a special jig, much like a giant tailpipe expanding tool. Once the blanks are in place and the jig is tightened, the exciter frame is placed in the same ovens used to bake insulation. The epoxy used is a high temperature material and this cures it over the better part of a day of heating and cooling.

Once the exciter frame is cooled to room temperature, the jig is removed from the center and the frame is struck a number of times with a large dead blow hammer. If any of the magnet blanks have failed to adhere to the frame this will make them fall off. There is also a visual inspection to assure that the magnet blanks have not cracked or the edges chipped.

So now you have an exciter frame with a series of ceramic magnet blanks glued in place. No magnetism at this time, it's a totally inert assembly.

These are stacked on a pallet after inspection and then sent to the alternator assembly area.

When it's time to assemble the alternator, the exciter frame has a large set of coils placed inside of it. Think of a giant exciter rotor. It's positioned correctly, another employee verifies the setup, and a power supply consisting of a number of very large, like barrel sized, capacitors discharge into the coil that's inside of the frame.

The exciter is now magnetized. It's a 24 pole exciter with 8 magnets. So how many magnetic poles does each magnet have? (Trivia question.)

Dan BTDT, pushed the button
Thanks for your input, I'm sol with the gen. gonna just strip it down for spare parts before taking it to the scrap yard.
 

LWB250

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BZZZT!

Wrong. 8 magnets, 24 poles. Three poles per magnet. You can magnetize any number of poles into the material based on spacing. Magnets were N-S-N or S-N-S depending on position.

Dan
 

Zephyr7

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Must have been interesting varying flux levels with those 3 pole magnets. Magnetized materials are dipoles, so you’d have some interesting field summation going on in this assembly.

Bill
 

LWB250

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Dan... can it not be excited with some other source... like an external belted gennie?
How would you create a magnetic field around the exciter?

My recommendation if they have to get this thing operational - either replace it with another generator set or simply buy a complete 300kW alternator, either from Marathon or Leroy-Somer, someone like that. Kohler used SAE standard flywheels and flex plates, so you should be able to bolt any standard alternator to it.

Dan
 
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