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Onan Generators

Onan Gen Shutdown


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  #1  
Old 02-11-2014, 12:25:14 AM
mech1369dlw mech1369dlw is offline
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Default Onan Gen Shutdown

I have read a few times about not letting the generator shut down while under load by running out of gas. Why? What happens that is so bad about this happening? just curious. Thanx
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Old 02-11-2014, 12:34:53 AM
Jim Marcozzi Jim Marcozzi is offline
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Default Re: Onan Gen Shutdown

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Originally Posted by mech1369dlw View Post
I have read a few times about not letting the generator shut down while under load by running out of gas. Why? What happens that is so bad about this happening? just curious. Thanx

It can:

Kill the residual magnetism that's required to start generation
Overload the regulator since it will go full field to try to compensate for low output
Damage the load due to spikes, low voltage or incorrect frequency

I'm sure others will add
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Old 02-11-2014, 08:36:25 AM
Wayne 440 Wayne 440 is offline
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Default Re: Onan Gen Shutdown

In addition to the possibilities Jim mentions, add "considerably lighten one's wallet".
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Old 02-11-2014, 10:06:19 AM
Billy J Shafer's Avatar
Billy J Shafer Billy J Shafer is offline
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Default Re: Onan Gen Shutdown

The older ones with out regulators are not effected as much as the ones with them. It doesn't always damage them. But you want to avoid a loaded shut down as much as you can. Don't need to see the magic smoke come out.
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Old 02-11-2014, 10:34:48 AM
Leon N. Leon N. is offline
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Default Re: Onan Gen Shutdown

Shutting down a Magneciter designed generator under load will not damage this style generator but it may do harm to the connected loads. Just not a good idea to abuse your refrigerator compressor or even the TV. As mentioned already, it may damage the later designed Onan voltage regulator controlled generators.

Nevertheless, back in 1970 I designed and successfully installed my own version of a generator shunt trip device for the very reasons mentioned above. There are all kinds of so called shunt trip devices. However, mine monitors the generators frequency. When the frequency is within spec limit, + or - 3 hertz, a contactor closes there by making the 120/240 output available. The circuit is also damped to prevent nuisance tripping.

Leon
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Old 02-11-2014, 10:42:15 AM
Billy J Shafer's Avatar
Billy J Shafer Billy J Shafer is offline
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Default Re: Onan Gen Shutdown

They call that roll off protection now of days Leon. Sure saves some money
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Old 02-11-2014, 04:27:10 PM
Isaac-1 Isaac-1 is offline
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Default Re: Onan Gen Shutdown

Thanks for bringing up this topic, it reminded me that I had a 200 amp shunt trip molded case breaker laying around, maybe I will use it to build a safety out of range (high / low voltage or frequency) trip for my MEP-003a since they tend to go high voltage on AVR failure.
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Old 02-11-2014, 05:04:51 PM
scoops scoops is offline
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Default Re: Onan Gen Shutdown

Quote:
Originally Posted by Leon N. View Post
Shutting down a Magneciter designed generator under load will not damage this style generator but it may do harm to the connected loads. Just not a good idea to abuse your refrigerator compressor or even the TV. As mentioned already, it may damage the later designed Onan voltage regulator controlled generators.

Nevertheless, back in 1970 I designed and successfully installed my own version of a generator shunt trip device for the very reasons mentioned above. There are all kinds of so called shunt trip devices. However, mine monitors the generators frequency. When the frequency is within spec limit, + or - 3 hertz, a contactor closes there by making the 120/240 output available. The circuit is also damped to prevent nuisance tripping.

Leon
I'd be more interested in what you built Leon.
Care to share how it was constructed?
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Old 02-11-2014, 05:13:57 PM
Leon N. Leon N. is offline
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Default Re: Onan Gen Shutdown

Scoops, yes think I can assemble some information. I am not a computer wiz, but I will see what I can do.
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Old 02-11-2014, 05:32:27 PM
grif grif is offline
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Default Re: Onan Gen Shutdown

Quote:
Originally Posted by Leon N. View Post
Scoops, yes think I can assemble some information. I am not a computer wiz, but I will see what I can do.
Is that schematic from the Radio Amature Handbook for a voltage controlled trip using a GFI still floating around here?
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Old 02-11-2014, 05:49:13 PM
Leon N. Leon N. is offline
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Default Re: Onan Gen Shutdown

No Grif, this is 100% my own design. I based it upon my knowledge of AC theory, ala, a series resonant RLC circuit, an understanding digital electronics, ala, a Schmitt Trigger circuit and some miscellaneous stuff not necessary to go into at this time.

OK, What I did was build a series RLC circuit consisting simply of an inductor, a capacitor and a resistor that would be resonant at 60 Hertz. At resonance, the impedance (opposition to the flow of current) reduces to that of the series resistor. So then I built a Schmitt trigger which monitors the voltage across the above mentioned resistor. At a predetermined set point, the Schmitt fires and activates a relay which in turn applies generator voltage to the two main contactors K1 as shown. Each contactor is of a 2 pole style necessary to handle the 120 volt current from the Magneciter.

The electronics and the resonant circuit are show in the little aluminum chassis mounted above the main switch box. I also have a trip indicator light and a manual trip if needed.

In a nut shell that is all there is to it.
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Old 02-11-2014, 05:53:48 PM
grif grif is offline
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Default Re: Onan Gen Shutdown

The current (last year?) handbook has a voltage sensitive trip using a GFI as the active relay/switch. Your frequency sense would be an easy add on. I've been looking for my scans of it,,, can't find them.
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Old 02-11-2014, 05:53:58 PM
Power Power is offline
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Default Re: Onan Gen Shutdown

Your picture shows a clothespin that is not shown in the schematic - what is it's purpose?
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Old 02-11-2014, 06:02:38 PM
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Default Re: Onan Gen Shutdown

Quote:
Originally Posted by Power View Post
Your picture shows a clothespin that is not shown in the schematic - what is it's purpose?
Power I believe that's a specialty tool.
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Old 02-11-2014, 06:26:57 PM
Leon N. Leon N. is offline
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Default Re: Onan Gen Shutdown

The cloths pin. That pin must have been there for many many years. I don't recall why. Guess I should have cleaned things up abit before taking that picture. Sorry. I do not recall, unless it held a schematic while I did some trouble shooting way back.
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Old 02-11-2014, 06:35:41 PM
Power Power is offline
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Default Re: Onan Gen Shutdown

Good answer! You know I was teasing you.
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Old 02-11-2014, 07:54:17 PM
farokh farokh is offline
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Default Re: Onan Gen Shutdown

Couldn't you avoid this by using a 240V contactor in the transfer switch? As soon as the voltage drops off, the contractor will disconnect the load. The only question is how fast the contactor drops. I suppose that's something one could experiment with a 240V contactor and a variable power supply or rheostat to see at what voltage the contactor drops…
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Old 02-11-2014, 08:59:38 PM
Leon N. Leon N. is offline
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Default Re: Onan Gen Shutdown

Couldn't I have avoided this by---? No because of two reasons. 1) I was more concerned with maintaining frequency than voltage. In my opinion, the Magneciter is the stronger part or shall I say the more robust part of the JB power plant. Who knows, the prime mover could run out of gas, have a failure, or something that would lower RPM. Yes that would ultimately bring down voltage if bad enough. 2) It was fun, I enjoyed doing this at the time and still do watching the plant start up and get on frequency and the contactors close. And conversely, on shut down watching the reverse procedure.

Not so much now, but I use to use this device when experimenting with various dummy loads I can temporarily hook up in the shed to sort of load test the JB before putting it on line. I recall I had to expend some effort, getting the KN and the carb adjusted to operate correctly over the full load range of the generator.

If I had enough ambition, I would like to modify my shunt trip or create a similar circuit to shut down the plant if the frequency falls a significant amount to prevent damage to the Magneciter that can result from operating at a reduced RPM for any length of time. I do have installed the Onan over speed trip, but for the sake of protecting the Magneciter it would be nice to have a LF engine shut down..

That's the story in a nutshell.
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Old 02-11-2014, 09:14:48 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Onan Gen Shutdown

Quote:
Originally Posted by grif View Post
Is that schematic from the Radio Amature Handbook for a voltage controlled trip using a GFI still floating around here?
No, but here's one from QST:


http://www.nxtbook.com/nxtbooks/arrl...startid=43#/46
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Old 02-12-2014, 11:34:55 AM
Leon N. Leon N. is offline
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Default Re: Onan Gen Shutdown

Jim, I think the Magneciter has a higher MTBF (mean time between failure) than the electronic voltage regulators that came after the Magneciter. For those with the VR board, I would feel more comfortable employing transient suppression devices, such as transorbs, to protect the electronics. The Magneciters were built like the proverbial brick SH. No need for additional transient protection under normal installation and use. Just my 2 cents.
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