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Onan 7.5JB - Wiring


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  #41  
Old 01-31-2017, 12:22:45 PM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

Quote:
Originally Posted by kiteoregon View Post
.....]..... It seemed that when the 12v was applied the generator seemed to be under load, once removed it was like the load was removed.....
Engine should not have loaded down. IF you did not have a large load connected to gen then you have a shorted winding somewhere.

I strongly suspect it's in the 2nd stage stator. Problem is either an internally shorted winding or you have the T leads connected wrong and at least one of them is connected in a dead short (bolted short). This can result in cooking and destroying the $$$$$ winding if you let it run.

I would not run it again till you check the wiring. I'ld mark and disconnect all the T leads.
Check insulation resistance of each winding to frame, there are 6..
Make sure insulation is not chaffed and allowing leads to short to each other or frame.

With all T leads disconnected and taped so they are not connected to anything, run engine and reconnect 12VDC to field hopefully engine will not load down again. IF it does then there's likely an internally shorted coil. IF it loads down remove 12VDC to field immediately to minimize damage.

Each of the following lead pair should be a winding ( from reconnection diagram).
With engine off, ohm them out to confirm you have continuity between each pair, in this test all T leads should not be connected to anything else.

T1/T4
T2/T5
T3/T6
T7/T10
T8/T11
T9/T12

Last edited by len k; 01-31-2017 at 12:41:43 PM.
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  #42  
Old 01-31-2017, 12:54:08 PM
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Default Re: Onan 7.5JB - Wiring

Ok let us start with some basic information. Have you ever seen this set work properly or was it like this when you got it ? What was the story when you got it? It does sound like a bolted short or a mislabeled pair, especially if tie PO messed with the windings labeling. It is critical that get sorted out before anything gets damaged. If you can test each of the 6 winding for shorts to itself that would be good information for us, the resistance of each winding pair should be quite similar to the others . Once it has been established that there are no shorts then we can proceed back to the 12V test , each separated pair should produce 140V. Cheers Dan
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  #43  
Old 01-31-2017, 01:11:03 PM
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Default Re: Onan 7.5JB - Wiring

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Originally Posted by Kevin K View Post
You might want to try disconnecting all 12 output leads, let them float, and run the test again applying 12 volts to F1 & F2...
Yes - do this next. Be sure the 12 wires don't short out to anything or to each other during the test. Keep the voltage regulator disconnected during this test.

Check for 120-ish volts on the 6 windings, if the genset isn't bogging down. Work fast while the 12V is applied so as not to stress anything.
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  #44  
Old 01-31-2017, 01:13:16 PM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

You can try to read resistance of each coil, but it's low (~ 0.1 ohm) usually need special meter or technique. Here is a cheap way to measure fractional ohms.
https://www.smokstak.com/forum/showthread.php?t=149217

Quote:
Originally Posted by Jim McIntyre View Post
Check for 120-ish volts on the 6 windings, if the genset isn't bogging down. Work fast while the 12V is applied so as not to stress anything.
If you do this I'ld have voltmeter all connected , then apply 12V ONLY long enough to take a reading, then disconnect 12V immediately.
Alternate idea is to wire up a light bulb across each winding, then connect 12VDC only long enough to see which bulb doesn't light, this will minimize time for damage to occur. ( got an old sting of big bulb 120V christmas lights you can cut up)
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  #45  
Old 01-31-2017, 02:12:12 PM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

So a short back story on the generator. This is new to me, came with this new house and prior i had never seen it run or output any voltage. The PO told me that is was hooked up to the house and worked great. One longer power outage he ran out of propane, so he disconnected the T wires and wired up a plug for his portable that ran on gas. From that day forward this set was never touched again. He said something about a rebuild a while back, but there is a fair amount of rust, so i don't know what he was talking about. So far all of the wires are in great shape, and look very new, all or the print is very readable.

To get it started, i had to clean the points and if the level of corrosion is any indicator this has probably sat unused for several years. It does sit under cover, but can see some moisture as it could blow in from the side.

When i get home tonight, i will pull apart all of the T wires, clean connections and verify per post above.
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  #46  
Old 01-31-2017, 03:30:12 PM
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Default Re: Onan 7.5JB - Wiring

I like Len's idea of the 6 litebulbs , much quicker than taking individual readings and safer too.Cheers Dan
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Last edited by BigBlockChev; 01-31-2017 at 06:58:04 PM.
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  #47  
Old 01-31-2017, 05:25:26 PM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

Len's idea.
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  #48  
Old 01-31-2017, 06:33:00 PM
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Default Re: Onan 7.5JB - Wiring

Just don't cut up an OLD string of Christmas lights. Just like everything else, they don't make THOSE like they used to either. If they have Bakelite sockets keep them. If they're just molded plastic, by all means cut them up into six test sockets.
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  #49  
Old 01-31-2017, 08:48:12 PM
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Default Re: Onan 7.5JB - Wiring

If you use the 6 light method, make all bulbs same wattage and color. It's likely the bad winding will make partial voltage, so making all bulbs the same will make it easier to see which one is dim, in a very short time.

Last edited by len k; 01-31-2017 at 10:36:13 PM.
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  #50  
Old 02-01-2017, 12:15:07 AM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

So pulled all of the T wires and separated them.



T1/T4 - Yes - and T4 to T6
T2/T5 - Yes
T3/T6 - Tes - and t3 to t1
T7/T10 none
T8/T11 - Yes
T9/T12 - no but T12 to T5

Does that make any sense?
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  #51  
Old 02-01-2017, 12:24:08 AM
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Default Re: Onan 7.5JB - Wiring

Uh oh. Those are continuity readings? With all leads disconnected from one another? That does not look good.
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  #52  
Old 02-01-2017, 01:12:06 AM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

Correct, those are continuity readings, nothing connected.
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  #53  
Old 02-01-2017, 02:34:58 AM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

Check that reg or it's transformer is not connected to the T leads, hopefully additional continuity is coming from transformer.

I only have a single phase JB, it has 2 leads tapped off one winding going to reg. Two leads for my reg just appear out of genhead, they are not tapped off the T leads EXTERNAL to the genhead. I have not analyzed wiring diagram for your 3 phase gen but I remember seeing some 3 phase gens using a transformer to power the reg. I have not looked to see how 3 phase JB is wired regarding transformer for reg, too late tonight

Maybe check that leads to reg or it's transformer are disconnected and taped to isolate them.

Hopefully no continuity between T7/T10 and T9/T12 is due to lead numbers being wrong. Can test that later after resolve above.

Last edited by len k; 02-01-2017 at 12:53:21 PM.
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  #54  
Old 02-01-2017, 10:30:22 AM
stretch67 stretch67 is offline
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Default Re: Onan 7.5JB - Wiring

I may be incorrect here but does it not look like the spade terminal going into the bottom of the VR on wire 1 and 2 looks like it touching the frame of the VR?

Just a thought..........
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  #55  
Old 02-01-2017, 12:55:45 PM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

Yes any leads going from genhead to reg ( or possible transformer before the reg) need to be marked, disconnected and taped to isolate them from anything else.
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  #56  
Old 02-01-2017, 12:56:44 PM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

The voltage regulator assembly is removed, the 4 wires that connect to the VR are floating and taped, so no chance they were causing the problem.
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  #57  
Old 02-01-2017, 01:30:01 PM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

Follow the reg wires from reg to genhead, hopefully you'll find a transformer along the way, or a short between wires.
Also follow the 12 T leads to the genhead and check for shorts between them.
Otherwise short is likley somewhere inside genhead.

Take round gen cover off and look at the color of the 2nd stage stator windings ( the big ones further back). Should be red-ish or maroon-ish in color. Black is bad. Give it a sniff test too, smell for burnt.
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  #58  
Old 02-01-2017, 01:33:11 PM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

If i find that there is no transformer, are we thinking it would be a bad stater?
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  #59  
Old 02-01-2017, 01:39:49 PM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

If no shorts between the T leads outside the genhead.....yes

With any luck mice chewed the T leads inside the genhead, but external to stator, so you can insulate them. But I wouldn't hold my breath, coils might be cooked already. Take round gen head cover off and look at main stator winding color. Post pic if can.

Each winding should ONLY have continuity to two T leads. MAin problem I see is you have continuity between:
-- ALL of the following: T1, T3, T4, T6
-- and separately ALL of following T2, T5, T12

Secondary problem of not having continuity between below pairs can hopefully be they are mislabled :
T7 and T10
T9 and T12

Someone else please check my interpretation of the test data.
I don't have hands on experience with 3 phase gens, but I have an engineering background.
I looked at the general reconnection diagram, and it matches the way you originally listed the T lead connections for double delta

Last edited by len k; 02-01-2017 at 02:03:18 PM.
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Old 02-01-2017, 02:11:35 PM
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Default Re: Onan 7.5JB - Wiring

I agree with everything you say here, Len. Unless there is some serious mouse damage external to the generator, it looks like he has a good engine in search of another generator.
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