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Onan 7.5JB Separating T2 From Ground


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  #1  
Old 10-08-2018, 08:42:34 PM
scoops scoops is offline
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Default Onan 7.5JB Separating T2 From Ground

I have a spec P Onan jb with the magneciter. The manual from the 60’s states that T2 must be connected to ground.

The current NEC states that’s if you are going to use a transfer panel that only switches the “hots” and not the neutral then you must have only one bonding point.

The dillema here is that this particular generator states that T2 must be connected to generator frame. This is most likely needed to complete the auto flashing circuit.

Yesterday I decided that after hours of studying the manual that I would go against the printed words of the manual by separating the “neutral” (T2&T3) from frame ground at the generator, and will instead rely on the connection point at the transfer panel at the neutral to ground bonding point.

By doing this will allow me to use all 4 wires (2 hots, neutral, and ground) and be in compliance with the latest NEC codes.

I realize that I may effect the longevity of the magneciter exciter and regulator by doing this, and will use this post as a means to document the ongoing tests.
I will be purchasing some power diodes and voltage control diodes in the event of a failure.
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  #2  
Old 10-08-2018, 08:53:56 PM
len k len k is offline
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Default Re: Onan 7.5JB separating T2 from ground

If you think about it in concept there are really 2 grounds: Battery DC ground and AC safty ground.
With gens in practice they are the same metal gen frame.


I think it will work, was thinking same for my 7NHM after I open N/G bond at gen.

I suspect reason Onan ties T2 to gen frame (which is DC ground and AC safety ground) is to give battery flashing current a way to get back to battery ground. I believe the +12VDC flashing goes thru magnaciter , thru rotor, back to magnaciter and out the 120 neutral , to gen metal frame thru thru T2 connection to gen metal frame, and back to battery. I haven't looked at magnaciter circuit for a while.

Once gen is making AC I suspect magnaciter won't care if T2 is connected to gen frame as it's sensing, and is powered by, 120V from one hot to neutral.

Just a good idea to ground the neutral so that static doesn't make high voltage between windings and frame causing a spark and insulation damage. I've read in ungrounded utility systems transients can make voltage from hot to earth 7X normal hot to neutral voltage. That's one reason why they ground them, other is safty so breaker can kill power if short makes frame live (over current fault clearing).
.

Last edited by len k; 10-08-2018 at 09:54:20 PM. Reason: add summary at begining.... I think it will work
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Old 10-08-2018, 09:09:22 PM
YellowLister YellowLister is offline
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Default Re: Onan 7.5JB separating T2 from ground

how is your onan going to be installed? will your ground and neutral be always connected to the houses ground and neutral?if so.. you could... unbond the ground and neutral at the main breaker panel and use the onans ground and neutral bond as the houses.. as long as the wires are the same size as the homes.. in theory

but... i would seek a licensed and insured electricians advise before doing so.. un bonding t2 is a sure way to let the magic smoke out of the magnaciter..
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Old 10-08-2018, 09:12:27 PM
scoops scoops is offline
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Default Re: Onan 7.5JB separating T2 from ground

Yellow, yes it will be always connected to house panel.

I have thought about it, if I should ever choose to use it as a local system with local outlets, I’ll just reconnect the neutral to ground.

I ran it yesterday for a few hours varying loads from no load to full load, no magic smoke as of yet to report, I don’t suspect there will be any either.
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Old 10-08-2018, 09:14:03 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB separating T2 from ground

Onan says it “may” damage the magneciter. Ok it may work as you suggest the wiring but you must live with the thought that, maybe some day, for example during a severe lightning storm, it might fail when most needed. I defer to the Onan designers and followed their requirements and my JB continues to work reliably now going on 48 years.

The technical reason why Onan insists on grounding T2 to the chassis is hard to explain. What I do know is that the magneciter’s exciter board is powered by one of the stator windings T1-T2. So you are not going to ground T2 at the generator. So you are going carry the T2 lead all the way back to your transfer switch and then complete the grounding circuit by running another ground wire all the way back to the generator chassis?

Therefore, when the revolving field is flashed that 12 dc circuit has travel out, to the transfer switch and then all the way back to the low side of the 12 dc starting battery. Let’s hope a nearby lightning induced transient does not damage the exciter.

Something else just as important to think about is the following. That is the magneciter voltage regulation circuit monitors the voltage across T1-T2. By making the grounding loop much physically longer, you may find times pick up, electrical noise, and what ever coupling occurs inside your transfer switch jeopardizes the ability of the magneciter to regulate the generators output voltage to within 3%. Anyways, that is how I see it.
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Old 10-08-2018, 09:15:41 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Onan 7.5JB separating T2 from ground

Quote:
Originally Posted by scoops View Post
...The dillema here is that this particular generator states that T2 must be connected to generator frame.
Actually it says it must be grounded. I haven't seen anywhere where Onan ever said it had to be 'grounded to the generator frame', or as others have said 'solidly grounded to the chassis'.

Quote:
This is most likely needed to complete the auto flashing circuit.
Yep, agreed.
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Old 10-08-2018, 09:17:46 PM
len k len k is offline
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Default Re: Onan 7.5JB separating T2 from ground

I think in general it will work, haven't thought about lighting and capacitive coupling.
Absent that I suspect regulation will be Ok

To clarify.
Was not talking about connecting one of the other T lines to frame.
Was talking about whether or not to connect ONLY the T2 lead to metal gen frame, somehow (even with house N/G bond).

Without T2 connected to frame SOMEWHERE, I think flashing +12V current will have no way to get back to battery. So it won't flash, so if gen lost iit's residual magnatizem gen won't bootstrap it's way up and make AC.

IF you connect say T1 to gen frame I suspect you will basically be putting 120V in series with 12V flashing. And that likely will smoke things VERY badly. Haven't looked at schematic for a while.


Same issue comes up when I thought about opening N/G bond in my 7NHM. It has AC voltage reg that needs flashing also. Was thinking of interlock, leaving N/G bonded in my house panel.

Last edited by len k; 10-08-2018 at 10:03:43 PM.
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Old 10-08-2018, 09:54:14 PM
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Default Re: Onan 7.5JB separating T2 from ground

Any chance a moderator could add 'magneciter' to this thread's title? (It applies ONLY to magneciter 7.5JB sets)
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Old 10-08-2018, 09:59:03 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB separating T2 from ground

Jim for crying out loud. Onan reiterates the requirement that T2 be grounded to the chassis in several documents. The machines are even provided a ground lug for that purpose! This requirement is stated in the Owner Operator and parts manual, 967-320, in the JB/ JC Major Overhaul Manual, 967-500 and in the Magneciter repair and trouble shooting manual.

I rest my case? Good evening.
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Old 10-08-2018, 10:05:29 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan 7.5JB separating T2 from ground

Interesting to see this thread after Len mentioned T2 grounding needs to me in another thread (this thread answered my question that I asked you there BTW)

If you really want to open the ground/neutral bond at the set, you could put a low-voltage high-joule MOV (metal oxide varistor, a surge suppressor element) between neutral and ground at the set. This would help to keep transients from smoking the regulator. It could be a really small voltage rating, like 7 volts or something, since there shouldn’t normally be any voltage between neutral and ground in your case.

Ground doesn’t necessarily mean earth ground, it is the return path. There can be multiple types of “ground” which can get confusing.

I usually advocate for keeping the bond at the genset for mobile units that aren’t permanently installed, in my opinion it is safer that way (assuming there is a breaker at the genset). For permanently installed units it’s not as much of a safety issue to move the bond to the transfer switch.

From what some others have mentioned about how the magneciter voltage regulator works, I would think that moving the bond may result in worse regulation if the voltage drop around the ground/neutral loop to the switch under load effects voltage sensing. The neutral when under load will show more voltage drop than when unloaded.

Bill

---------- Post added at 10:05:29 PM ---------- Previous post was at 10:02:06 PM ----------

Quote:
Originally Posted by Leon N. View Post
Jim for crying out loud. Onan reiterates the requirement that T2 be grounded to the chassis in several documents. The machines are even provided a ground lug for that purpose! This requirement is stated in the Owner Operator and parts manual, 967-320, in the JB/ JC Major Overhaul Manual, 967-500 and in the Magneciter repair and trouble shooting manual.

I rest my case? Good evening.
I will add that from my understanding of how UL listings work (from what my electrical fabricator has told me and they have to deal with this all the time), if Onan provided the connection inside the equipment, and documents that it must be that way as well, then changing it voids the UL listing for the unit.

Probably best and safest to NOT remove the ground connection to T2

Bill
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