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Onan 7.5JB - Wiring


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  #31  
Old 01-31-2017, 01:00:28 AM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

If low voltage is real , my next guess is rotating diodes are blown.
Disconnect reg and run engine, my JB spec AA makes ~ 15-20 VAC out the 120 leads under these conditions, it's field has a small permanent magnet built in.

Basically the reg powers a field stator, it's rotor makes 3 phase AC that is rectified to DC by 6 diodes to power 2nd stage rotor that makes AC into 2-nd stationary stator. I think the magnet is in the feild of the 1-st stator.

My Onan 7NHM makes 1-2 VAC when reg is disconnected, it doesn't have a small permanent magnet.
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  #32  
Old 01-31-2017, 01:32:39 AM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

With the voltage reg assembly removed and the volt meter connected to the wires that connect to TB21-1 and TB21-2, i read 2.6 volts ac while running.

Based on the service manual, it would seem my next logical step is to flash the field, but since i don't have a 6 volt battery, i will pick a 20 ohm resister and wire in series per the manual and flash the field to see if restoring magnetism will result in output voltage.
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  #33  
Old 01-31-2017, 01:49:40 AM
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BigBlockChev BigBlockChev is offline
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Default Re: Onan 7.5JB - Wiring

You can try flashing the field but it is not likely to do anything. The next logical step is to test the resistance of the field across F1 and F2. You should get 12 Ohms or so. The exciter coil ( field ) is the next most common failure point in a YD genend then the rotating diodes as Len has suggested. If the exciter coil measures around 12 ohms you can try putting 12V across it while running, this should give you over 100V output. Get back to us with the results. Cheers Dan
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  #34  
Old 01-31-2017, 01:59:40 AM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

With the set NOT running, MM connected to F1 and F2 i get 11.6 ohms. Once started i get 0 ohms.

]Just so i understand, now that i see roughly 12 ohms, i should start the generator and apply 12V to the F1 and F2. Is this with the voltage regulator disconnected? Do i measure output voltage on F1 & F2?
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Old 01-31-2017, 02:04:47 AM
len k len k is offline
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Default Re: Onan 7.5JB - Wiring

On my single phase JB , 12 VDC ( car bat) across field yielded ~140 VAC out the 120 lines. Suspect your 3 phase will do ~ same.

Kiteorgan....just saw your post

11.6 ohms sounds close enough to call it good, even if couple of the ~ 10 coils were shorted I would think the 1-st stage should make enough excitation so 2nd stage should make something like 70-90% of 120 VAC .

With gen rotating ohming field is meaning less, voltage from residual magnatizm makes ohmmeter give false readings.
Need gen stopped to obtain valid ohm readings.

You apply 12VDC across F1/F2 with reg disconnected, run gen and see what you get out the 120 leads ( L1 to L0) and (L2 to L0). Looks like positive of 12V bat should be connected to F1. Negative to F2

But first, With reg disconnected, engine off, measure resistance from either F1/F2 to frame. Should be >~ 1,000,000 ohm (insulation test)

Last edited by len k; 01-31-2017 at 12:10:57 PM.
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  #36  
Old 01-31-2017, 03:14:24 AM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

Check on the insulation test, north of 10.78 m ohm.

Will apply 12v and check leads as you suggested.

---------- Post added at 11:14:24 PM ---------- Previous post was at 10:54:22 PM ----------

Ok, so applied the 12V+ to the F1 wire and 12V- to the F2 wire. MM connected to L1 and L0. Started the set, output voltage 48.4 - 48.8 ish. It seemed that when the 12v was applied the generator seemed to be under load, once removed it was like the load was removed. Voltage Reg has been disconnected through all of this.
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Old 01-31-2017, 07:39:13 AM
b74eqcm b74eqcm is offline
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Default Re: Onan 7.5JB - Wiring

Quote:
Originally Posted by kiteoregon View Post
]Ok, so applied the 12V+ to the F1 wire and 12V- to the F2 wire. MM connected to L1 and L0. Started the set, output voltage 48.4 - 48.8 ish. It seemed that when the 12v was applied the generator seemed to be under load, once removed it was like the load was removed. Voltage Reg has been disconnected through all of this.
OK, so this sounds like a short. I suspect you don't have the output leads connected correctly, and have at least one phase in a bolted short. Double and triple check your output leads before doing anything else, as this could cause serious damage. If you are absolutely sure they are right, it's time to check the stator windings to find the short.
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  #38  
Old 01-31-2017, 08:00:05 AM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB - Wiring

Quote:
Originally Posted by kiteoregon View Post
Per Jim's post i removed the penthouse and tested with a 60w incandescent light bulb. After some pot adjustments the light bulb will come to full brightness. So at this point we can rule out the voltage regulator and transformers located in the penthouse.
Where was the bulb connected & what was the voltage present at those connections when the bulb came to "full brightness".
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  #39  
Old 01-31-2017, 09:16:48 AM
Kevin K Kevin K is offline
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Default Re: Onan 7.5JB - Wiring

You might want to try disconnecting all 12 output leads, let them float, and run the test again applying 12 volts to F1 & F2. If the engine loads up when 12 volts is applied, you probably have a short somewhere. If it passes this test, check each lead for short to ground. If that is OK, you probably had a bolted short as b74ecqm suggested.
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Old 01-31-2017, 12:00:46 PM
kiteoregon kiteoregon is offline
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Default Re: Onan 7.5JB - Wiring

The bulb was connected to F1 and F2, i did not check the voltage at that point, but will later today. In the pic, the red wires are for the light, the white is the 110 source. I will hook this back up after work and check the output voltage where the light connects.
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