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15RM82 rectifier burnt out


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  #1  
Old 11-02-2009, 10:52 AM
bsutherland bsutherland is offline
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Default 15RM82 rectifier burnt out

I have a 15RM82, spec #: 86287C, serial #: 392679 Kohler generator that has burnt out the rectifier. I use this unit to power some machine tools and to supply backup power in case of power outages.
Last week I went to use my lathe and it wouldn't run. Discovered that the incoming voltage was just dropping off to almost nothing, but my mill would run. the mill has a smaller motor. I went to the generator and found that a mouse had set up home inside the control box and looks like the debris in there caused the rectifier to overheat.
Does the rectifier control the output of the generator?
An why on starting the unit would the rectifier be arcing inside. I don't see a connection to the starting circuit unless it's connected to the fields that turn the generator over as a starter motor. Right now I can't start the motor unless I short out the cranking solenoid and turn the switch on to energize the gas solenoid. If I don't do that the cranking solenoid just chatters. I've managed to locate the successor to the original OEM of the rectifier and they will make me a new one just waiting on price. I would like to know if there is something else I should check over because I don't want to burn up a new one.
The wiring diagram sheet # is D-268594.

Brian
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  #2  
Old 11-02-2009, 03:18 PM
BergmanJ BergmanJ is offline
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Default Re: 15RM82 rectifier burnt out

BSutherland,

I have a 15RH82 - 67120C which may be similar to yours. If you could forward a copy of the D-268594 drawing, I may be able to help a bit during my break & lunch times.

Is your rectifierof the older finned selenium variety? Silicon diodes may possibly used with a series-resistor as replacements; but, I need to see the drawing to try helping.

Regards, JLB
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Old 11-02-2009, 04:13 PM
K D Redd K D Redd is offline
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Default Re: 15RM82 rectifier burnt out

The older selenium rectifier have a lower forward voltage drop (.3 volts) than silicon (.7 volts) do. I would think one would NOT need any series resistors. The selenium rectifier will go bad setting on the shelf. The mouse may or may not have been the cause of failure. Old age may have been the problem.

Kent
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  #4  
Old 11-02-2009, 05:49 PM
BergmanJ BergmanJ is offline
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Default Re: 15RM82 rectifier burnt out

K D Redd,

I find myself having to respectfully disagree that series resistance should not be required - maybe, maybe not: Ive done this before several times and can state that: "It depends on the surrounding driving and driven circuitry". (And, yes, I'm hedging my reply! - just because "it depends".)

Unfortunately, in some applications, replacing selenium with silicon does require some low value of series resistance because, even though the "switching voltage" is lower, the internal resistance is much higher for selenium than for silicon, which can affect how the load circuit is driven. In my opinion, the substitution may, indeed, require some series resistance.

If 'twer me, I would prefer to err on the side of caution by using some series resistance ( 0 -- 10 ohms ??? -- wattage will depend on current and voltage drop) and adjust to obtain comparable performance.

My Kohler, for instance, has series/shunt wound main fields, with the selenium rectifier supplying a seperately wound "auxilliary/control" field for fine regulation; i.e., most of the operational field current is supplied by the series/shunt fields, with a (relatively small) proportion from the rectifier to accomplish the "fine" control. This "small" control circuit is of relatively high impedance compared to the "main" fields; so, would most probably operate best with a "soft" supply circuit: Hence the suggestion for the series resistance when substituting silicon rectifiers for selenium.

Again, either way is "iffy".

Thanks, JLB
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Old 11-02-2009, 06:24 PM
pops pops is offline
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Default Re: 15RM82 rectifier burnt out

Let's see a picture of the rectifier. The selenium rectifiers are such a POS, you want to change them. Oh, yes nasty smell they put, is toxic. I wouldn't worry about adding a series resistance. How many wires connect to rectifier? This answer will NOT relieve the picture requirment however. James
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Old 11-02-2009, 09:58 PM
bsutherland bsutherland is offline
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Default Re: 15RM82 rectifier burnt out

Here's a picture of the rectifier. It is a finned selenium type made originally by Syntron now CKE.

Brian
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rectifier.jpg  
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  #7  
Old 11-02-2009, 10:54 PM
bsutherland bsutherland is offline
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Default Re: 15RM82 rectifier burnt out

Wanted add to my previous post that the Kohler part # for the rectifier is: 265319. CKE can supply a new rectifier but it will be several weeks as they haven't made this model in about 6 years.
Also the transformer that attaches to the burnt part on the rectifier appeared to have some of the varnish melted out of it but it seemed to be working ok. Haven't disconnected it yet and rung it out to verify.
The generator is a 208/120 3ph unit. The output of the rectifier goes to F1 an F2 which in the schematic is listed as Aux Field
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Old 11-03-2009, 06:27 PM
pops pops is offline
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Default Re: 15RM82 rectifier burnt out

I suppose the control transformer (current tranformer) is 3 phase unit with 3 wires coming from transformer to rectifier. If so you can either buy a 3 phase rectifier (like from Digi-key, Jamico, Mousier) or buy 6 diodes at 25A 100piv and "roll your own". A google on "3 phase rectifier" will land a few examples. My Kohler 7.5c61 uses the current xmfr method of voltage control (aux field), it has about 30 volts on the aux field at NL and 75 volts FL. My Kohler is single phase. It also came with a selenum rectifier. I promptly replaced it with a silicon unit. James
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Old 11-03-2009, 06:54 PM
BergmanJ BergmanJ is offline
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Default Re: 15RM82 rectifier burnt out

Ther're 3 seperate transformers going seperately to 3- two-diode full-wave selenium rectifiers, then on to the Auxiliarry Field.

JLB
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Old 11-11-2009, 02:51 PM
pops pops is offline
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Default Re: 15RM82 rectifier burnt out

All you need are the 6 diodes and you're good to go. If you buy stud mount diodes (which I would), be sure to buy insulator kits. Wire them up on a piece of al and bolt that into controller.
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Old 11-11-2009, 02:53 PM
bsutherland bsutherland is offline
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Default Re: 15RM82 rectifier burnt out

I received some additional info about the rectifier. It's electrical parameters are:
AC input max is 78V RMS, Max leakage is 40 milliamps at 78Vrms an max output is 2.25A DC.
Does anyone know what diode would match this rating?
also how do you check a rectifier out as far as continuity?

Also will it harm anything to use this generator with the burnt out rectifier until I can get a replacement? Most of the tools except for my one lathe start and run OK currently. The lathe just kills it, the voltage just drops off to nothing and then the thermal switch trips.
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Old 11-11-2009, 03:12 PM
Jim Rankin Jim Rankin is offline
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Default 15RM82 rectifier burnt out

You most likely can find a direct replacement from somewhere like Mouser or Digikey. Mouser has no minimum order and has an online catalog. I haven't dealt with Digikey. They will have plenty of diodes if you want to brew up your own.

I would not keep using the generator without fixing it. You are pulling excess amps through the good components of the exciter/regulator when part are not working.

To check a rectifier with an ohmeter, disconnect a couple of leads from it and check that the ohm meter shows low resistance (usually around 600-700 ohms) in one direction and one direction only through each diode. Connecting to it with the opposite polarity should show very high resistance

Failure modes are
Open-only very high resistance, either way you touch your test leads to the terminals
Shorted-low resistance both ways through the diode.

With a package full wave rectifier, you have to understand how the diodes are arranged inside to test it.
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Old 11-11-2009, 04:14 PM
Fred M. Fred M. is offline
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Default Re: 15RM82 rectifier burnt out

Brian-

You need a circuit diagram to be sure how the rectifier is wired. If it is a simple bridge rectifier, you can either buy one or make one from 4 silicon rectifier diodes. (If it is a 3-phase bridge, it might be made from 6 silicon diodes, 2 for each phase??)

Diode reverse voltage requirements depend on what it is hooked up to. With a Capacitor load, actual peak reverse voltage is twice the peak of the RMS value. For 78 VRMS, I'd go for at least a 300 Volt diode.

I would expect a 3 Amp 400 Volt rectifier diode would be a good choice. Radio Shack carries a 1N5404, catalog 276-1144. Radio Shack also lists two 400 Volt bridge rectifiers, one rated for 4 Amps and one rated for 8 Amps. Either should work.

I don't know if a series resistor is needed. If you try it without, make sure it doesn't overdrive the field.

Fred
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Old 11-12-2009, 09:45 AM
BergmanJ BergmanJ is offline
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Default Re: 15RM82 rectifier burnt out

bsutherland,

That's very helpful information for me, as I have a very similar unit, with, what I believe to be, the same selenium rectifier unit.

As for silicon rectifier diodes, see my Onan reply to another gentleman on post 433802 with regards to numbers and prices for some "cheap" & "bullet-proof" stud-mount diodes which I have used in another application. They come in both "normal" & "reverse" polarity; which makes this application much easier to physically build.

'Twer me, I would still be cautious in trying these as a direct replacement for the selenium without some "cushion" (small series resistance).

Should you try the silicon diodes "straight", another "issue" could be that your generator output voltage actually goes UP with increasing load, as the purpose of the output transformer regulation is to compensate for increased loading; and, if that regulation is too "stiff", it could "overcompensate".

Overdriving of the Auxilliary Field winding would still be [I]my[I] main concern, however; with possible consequent damage to that winding. This is one of those "we don't know until it's tried" scenarios.

I'm very much looking forward to your results, as I may need to do the same thing in the future.

Regards, JLB
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Old 12-23-2009, 04:52 PM
BergmanJ BergmanJ is offline
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Default Re: 15RM82 rectifier burnt out

BSutherland:

So... ...what's the scoop on any updates? I (and probably others) am (are) curious to know of your progress.

Thanks, JLB
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Old 12-25-2009, 02:55 PM
bsutherland bsutherland is offline
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Thumbs up Re: 15RM82 rectifier burnt out

I ended up buying the direct replacement rectifier from CKE. I just recently installed it as I wanted to check the rest of the wiring out before installing it and potentially having other issues. Installed it and ran it and my lathe works like a charm. I think it may have been going for a while because the lathe has always been a little sluggish to start but now it's not. The lathe is 550V 10KW 3ph, I run the 208 through a transformer to go from 208 to 550 and then to a splitter box that feeds the lathe, mill, saw and drill press. Everything other than the lathe are DOL starting, the lathe is Star-Delta.
I know some of you suggested that I replace the rectifier with a silicon version and I thought about doing that. But the one from CKE was priced where I could live with, although I imagine doing one myself would've been cheaper, and was just install and wire in. I also figured if this one lasts for approx 35 yrs like the old one I'll be in my 80's and by then someone else can do what they want with it.
I would like to thank all who offered me help
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