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Kohler Electric Plant 15R61 low output

mrh0t

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Gentlemen, and/or Ladies,

This may be a good question for any old timers out there. This is my vintage (but I think not as old as me) Kohler Electric Plant, model 15R61, that I've owned about 15 years. The SN is 230934, and it's Spec 6701B5.7 The 15KW genset has internal windings to start the 4 cylinder Waukesha model 180GKB-5G gas engine on the front end. My local generator guy estimated it to be from the 1970's. If anyone can confirm that, or has a better idea as to it's age?

Except for the water pump, which got noisy and had to be rebuilt earlier this year, it starts right up, and it's been trouble free as long as I've owned it. It's single phase, and outputs 120V on each of it's two legs. Just recently it stopped putting out 120V, and now has only 4 or 5 volts on each leg. I checked the brushes and cleaned up the rotor, but no change. I was told these units used a coil to control voltage, instead of a voltage regulator? There is that big ol' coil inside the control box. Please see the photos.

I can't find anyone locally who's interested in working on this unit, so if anyone could point me in the right direction, I'd appreciate it.

Merry Christmas and Happy Holidays to all,
Harry
 

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I like oldstuff

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Last Subscription Date
11/09/2015
Lots of things to check.
Firstly, take the round cover off the end of the gen. You'll likely find a couple wire wound resistors that are adjustable via a sliding contact that touches the wires. Make note of the positions on the sliders. Take them off and clean the little pip that contacts the wires. Also clean the resistor wires with scotchbrite or fine sandpaper. Put it back together and see if it puts out power.

Those old selenium rectifiers (green finned thing in the control box) tend to fail with age. See if you have DC coming off of it. Should be the pinkish and yellow wire.
Also check the connections of your last pic for loose or corroded joints. The big wires coming from the generator into the terminal block.

Hang in there, the smart guys will chime in soon.
 

Vanman

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Last Subscription Date
07/10/2019
Definitely concur- the resistor under the brush cover is the first thing to check.

You'll want to replace that selenium rectifier with a silicon unit. In this application, when that rectifier fails open while the set is under load, the compounding transformer that supplies it will be destroyed and will then have to be rewound.

Failure of this circuit will not prevent the generator from building up and generating, however.

While the set is running and generating but with no load, there are only a few volts dc on the auxiliary field winding, supplied by the rectifier. This increases in proportion to load, up to perhaps 40 volts at full load.

Keith
 

I like oldstuff

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11/09/2015
Thanks Keith for bringing up the transformer failure should the rectifier croak. I forgot about that scenario to let the magic smoke out.
 

mrh0t

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Thanks Keith for your reply. I see the two wire-wound resistors, and the adjusting band has rusted through on the top resistor. The old band has a dimple in it for making contact with the resistor wires. Not a great design, but it is what it is. Should I fashion a replacement band, try to recreate the dimple, and solder it back on the lead, or try and replace the whole resistor? The old band left a good rust stain on the resistor, so I can get it very close to the original position if I make a replacement. Can you tell me the purpose of these resistors? Do they balance the output somehow?

I'll check the voltage at the rectifier when I get this problem fixed.

Best'
Harry
 

mrh0t

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I'm confused, ha, ha. "Oldstuff" was first to be thanked!

Now, thank you Keith, for the rectifier info. I'll try and see if I can find any specs on it.

Harry
 

Zephyr7

Registered
You can actually buy new bands for those wirewound resistors. I’d just make one out of a piece of brass strip. Easy to make the dimple, drill a hole in a piece of wood, ideally hardwood, then use a nail or awl to press in the dimple using the hole like a die underneath the strip.

You can clean up the wire on the resistor by polishing it a bit with paper towel. Make sure to wipe parallel to the wire, not across it, to prevent damage.

Bill
 

I like oldstuff

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There ya go, you found the problem and yup, either source new bands or make them. Take the resistors out to clean the windings. Take ALL the wire connections apart in that gen end and clean them up. It's an oldie so there's probably resistance in many of the connections and solder joints that could heat up and fail.
Once assembled you can slide the bands to fine tune your output voltage. *Shut it off when moving them so you don't get zapped.

Also carefully pull out on the end bearing seal and with a needle on your grease gun pump grease in it.

For the rectifier something like this will work just fine. I'm assuming yours is 3 phase.
https://www.ebay.com/itm/Bridge-Rectifier-50A-1000V-for-3-Three-Phase-Diode-PMA-Wind-Turbine-Generator-/271997001394

If your machine is single phase then something simple like this. You'll have a spare.
https://www.ebay.com/itm/2Pcs-1000V-50A-Metal-Case-Single-Phase-Diode-Bridge-Rectifier-KBPC5010-USA/382552292883?_trkparms=aid=555017&algo=PL.CASSINI&ao=1&asc=20160706104836&meid=03bca1c3f2c94e7da30ef3594ffd90a4&pid=100642&rk=1&rkt=1&&itm=382552292883&_trksid=p2045573.c100642.m3226
 

dkamp

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His Selenium rectifier is a single-phase. They probably only tapped off of two legs to get the feedback for field boost.

I would NOT waste any time in getting that Selenium out'a there... it's probably leaky as a cracked 5-gallon bucket.

Two white wires go to AC terminals... they're marked with a tilde ~, and it doesn't matter WHICH way you connect those.

I believe red goes to + and yellow goes to -.

IF you connect the DC backwards, as you add more load, voltage will drop...
...don't ask how I know... :bonk:


you can make a nice replacement strap by cutting a strip out'a a piece of copper pipe. If you don't have fancy tools, cut a piece of pipe about 4" long, hammer it flat on the back of your bench vise, then take a sharp cold chisel and cut a path along one of the folded edges. Once you've cut that clear, open up the pipe to a flat piece, then use the chisel to cut your strip... cut it a smidgen wider than you need, and clean it up with some 200 grit sandpaper.

Make the dimple by placing the strap on a piece of hardwood, and dull the tip of a fair sized nail...give it a light whack with a light hammer, it'll look professional. Wrap it, bend it, mark it, drill it and call it OEM.
 

mrh0t

Registered
Thanks everyone for your input. Sorry it took so long to get back to this, but with the holidays, and caring for my disabled wife, I'm really busy.

I'm sure I could fashion a band out of brass or copper, but the original band is pretty strong "springy" steel, and I don't know if a softer metal would work for long?

I thought I might just swap out one or both resistors, if they're not too costly. I got some Kohler info on them, and my model specifies either 4 ohm or 2.5 ohm resistors, with my variation specifying the 2.5 ohm unit. With my cheap VOM, which "zeros" out at 1.3 ohms, I measured 4.0 ohms across each resistor, so (4.0 - 1.3= 2.7 ohms), close enough to 2.5 ohms. The lower right resistor, with the band intact, measures (1.7 - 1.3 = 0.4 ohms) effective resistance, and the one with the broken band measures (2.8 - 1.3 = 1.5 ohm) resistance to the point where the band was situated.

I even got an original Kohler part number for the resistors, which is probably pretty useless at this point, but for reference, it's #265241.

I'll look at the rectifier issue once I get this squared away.

Thanks all,
Harry
 

Zephyr7

Registered
I think those resistor bands were usually made from something like tin plated brass. I doubt very much they were ever made from steel, and I’m sure not stainless steel. Myself, I’d probably look for some tin plated brass or copper, and if I couldn’t find that, I’d just use brass sheet. It’s not very critical.

I wouldn’t trust your meter for those very low ohm readings. Even the oxide on the surface of whatever you poke the probe at to make a measurement can result in big errors. You might want to try the poor-mans low-ohm measuring technique: use a light bulb and put the resistor in series with it. Measure the current drawn. Measure the voltage across the resistor. Resistance can be calculated as V/I=R, so divide the voltage you measured across the resistor by the current you measured (in amps) to get the resistance of the resistor. This will be more accurate in many cases than trying to measure the resistance directly with a normal ohmmeter.

Bill
 
B

BergmanJ

Guest
Lots of those "adjustable" bands were made from tin-plated steel as used in many different electrical applications (not just generators). Retired now; but, have seen quite a few rusted-through during my "working" life.

Regards, JLB
 

mrh0t

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Here's the latest everyone. I was able to buy a couple Ohmite steel replacement bands on eBay for $6 and change. They were 5 days late on delivery, but worth the wait. When the latest storms blew through we lost our power, so I started to fashion a band from plumbers' strapping. Setting the steel band on a 1/4-20 nut, and using a center punch, produced a nice dimple in the band. Then the lights came back on, and project home-made-band was abandoned.

When they arrived, I soldered one of the new bands in place, and positioned it where the old band sat. Cranked it over, and it's putting out 115V on each leg. Are these variable resistors used to balance the output of the two legs? How closely do the voltages have to be, when they're combined to get 230V? I'm not sure how that works in real life.

Yes, all the connections on this unit need a good cleaning.

I had to quit for the night, but this was great news. Thanks everyone!
 

I like oldstuff

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You can slide either or both bands to give you the presently accepted 120V. They affect the field so the output of both legs will be the same within a volt or two.

Glad you found the problem and fixed it. Ain't this place fun!
 

mrh0t

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Yes, isn't the internet great!

I'll pull the resistors out and clean all connections when I get time, and reset to 120V.

Thanks to all you Smokstak peeps!
Harry
 

Zephyr7

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Careful when you clean those resistors. Only clean GENTLY and PARALLEL to the resistance wire — don’t rub across it perpendicularly! Resistance wire is very brittle and breaks easily. Ask me how I know...

Bill
 

mrh0t

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Thanks Bill. I carefully cleaned the windings with an emory board, yes parallel. I appreciate learning from your mistakes, ha, ha. Harry
 
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