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Wiring 7.5A27 kohler

Vanman

Subscriber
Last Subscription Date
07/10/2019
Sounds like you'll want to mount a receptacle to the generator set that matches your existing plug. The generator full load output current will be more than 60 amperes, so may want to mount a small panel, maybe even with just two breakers in it, so you can properly protect the receptacle at it's rating, and maybe have an extra receptacle served off the other breaker.

L1 and L2 are the 120 volt output. You need to follow the wiring and determine with certainty which one is bonded to the generator chassis. That will be your neutral, and needs to connect to the neutral on the receptacle.

Keith
 

Kevin 7.5A27

Registered
So from what i can tell l2 runs to the neutral/ negative side of the battery. So i will only need 2 wire running out of the generator?
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Yes, the "2" in the model number means that this is a 120 volt, two wire generator. But you also need to run a separate ground wire as well. I am *assuming* that your boat is also 120 volt, since it has only a three prong plug? (Hot, Neutral, Ground)

So, yes, Neutral connects to the terminal that is common to the battery and bonded to the chassis, L2 as you have traced out.

Then your "Hot" comes from the other terminal, should be L1. For a ground, ideally, you'd connect it to the point where battery negative and generator neutral are both bolted to the chassis, normally in the bottom of the controller. Make sure this connection is clean and tight.

Keith
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Yes, that would be a concern. Check the speed of the set by checking the frequency. 60 cycles equals 1800 rpm. At no load it should be a bit higher, perhaps 61 ~ 62. Perhaps someone has monkeyed with the governor. Raising the speed would raise the voltage on this type of generator. Also you might check with a small load on it. Sometimes digital meters give erroneous readings due to the nature of the contact between brushes and collector rings. A small load cures this.
 

Zephyr7

Registered
Is this one of the gensets that can be configured for 120/240 output? 208v would be low in this case, and would show up as ~104v on the 120 legs.

If it’s only a 120v winding, then 208v is crazy high. Might be a regulator going full field.

Bill
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
The 120 volt Kohlers usually (always?) have only two collector rings and therefore cannot be reconnected for 120/240 three wire service. No regulators on these. Instead they are compounded via a transformer, rectifier, and auxiliary field winding. This works very well, and is very reliable.

But, if your rectifier is the original Selenium unit (finned device) you'll want to replace it with a silicon rectifier before applying a load to the set. The Selenium rectifiers fail with age and, in this circuit, when it fails open, the compounding transformer will be ruined by high voltage across it's secondary. Primary is in series with the load.

BTW, please post a good photo of the nameplate, and of the whole set, just so we can be more certain of what you have.

Keith
 

Kevin 7.5A27

Registered
I dont have a picture of the generator right now but after a little more messing with it, I got it pushing out 120v and running power. What had me for a loop what the auto start from the AC side. Hooked up a test part to it and fliping a switch on started it right up and producing recommended power
 

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Vanman

Subscriber
Last Subscription Date
07/10/2019
Yes, the Kohler Automatic controllers are ingenious, dating at least as far back the early '20's and probably earlier.

Your serial number looks to be from 1971. I'm reasonably certain that will still have a selenium rectifier. A look inside the controller will reveal the answer.

Keith
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Any of the electronics supply houses will have them. Digikey, Mouser Electronics, and a few others I'm not thinking of right off hand. Probably even fleabay.

What you're looking for is a Single Phase Full Wave Bridge Rectifier. They're about one inch square by maybe 1/4" thick. They have a single mounting hole in the center and four 1/4" tab type terminals, two AC input, two DC output. They cost around $3.

This circuit runs at around 40 volts and a few amps at most. All that matters is that the ratings of the rectifier are well above the voltage expected. I ordered a ten pack of 50 amp, 1,000 volt units to get the bulk discount. They don't mind excessive current for a short time, even ten times rated, but absolutely will not tolerate excessive voltage.

In the event of a short circuit on the generator output, the rectifier would be briefly exposed to considerably higher voltage, so I probably wouldn't get one that's less than 400 volts. A 20 amp one would be well more than adequate.

Keith
 

Zephyr7

Registered
I’m always a little nervous buying components through amazon since they tend to be China direct specials, so questionable QC. I wouldn’t risk those for anything important. When you buy from digikey, mouser, or any other reputable electronic component distributor you know you’re getting quality parts and not seconds.

I wouldn’t try to save money here since a failure takes out an expensive part just like if the original selenium rectifier were to fail.

Bill
 

LWB250

Registered
I would also suggest that you get a strong MOV or surge suppressor to place across the AC terminals of the bridge rectifier. Snag one from the same supplier as the bridge and combine your shipping costs....

When the fields collapse from the removal of a load, the resulting spike will come back down the line and hit the bridge. This is what typically trashes the selenium rectifiers after they get old. With an MOV across the AC side, it will absorb the spike and prevent the bridge from getting smoked.

Kohler used to sell a kit with a bridge and a huge MOV, but to be honest despite installing tons of these things I couldn't begin to tell you what the value of the MOV is....

Dan
 

Zephyr7

Registered
All you need is an MOV that will fire above whatever the normal
Operating voltage is but below the PIV rating of the rectifier. If you’re using a 1000v PIV rectifier, I’m sure a 270-300v MOV would be fine.

Note that MOVs are rated for both DC and AC firing and operating voltages. The AC voltage ratings will be lower than the DC ratings. Make sure you know what you’re ordering!

Bill
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
When the exciting current decreases and the field collapses, the bridge rectifier will merely function as a freewheeling diode. No more current can flow than was flowing in the circuit to begin with, and the voltage will merely be the forward bias of the rectifier.

The risk of high voltage on the rectifier would be from a short on the generator output. That would result in a voltage spike across the auxiliary field and it's rectifier. I'd bet it wouldn't peak out at more than a couple hundred volts though.

I installed a 50 amp, 1 kV rectifier. It was cheap. I bought a ten pack for less than $30. I'll have to check the brand though. To see if it is reputable.
 

Zephyr7

Registered
When the exciting current decreases and the field collapses, the bridge rectifier will merely function as a freewheeling diode. No more current can flow than was flowing in the circuit to begin with, and the voltage will merely be the forward bias of the rectifier.
A bridge rectifier can only act as a free wheeling (reverse biased) protection diode if the inductance is connected across the DC terminals. Is that the case here? I couldn’t find a schematic that clearly showed the rectifier connections.

There is no DC current path through the AC terminals of a full wave bridge rectifier due to the way the diodes are internally connected. No reverse transient protection is offered to anything connected to the AC terminals of a full wave bridge rectifier.

Bill
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Yes, the rectifier allows the DC auxiliary field winding to be in series with the AC load. The transformer makes it so that the rectifier and winding do not need to carry ~50 amps.
 
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