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Old 07-01-2012, 09:33 PM   #1
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Generator Voltage Fluctuation Under Various Loads Normal?

I've done some testing on our RV's generator. The generator is an Onan Microquiet 4000 and was added about 5 years ago. Less than 200 hours on the clock. I start and run it under 50% load monthly; have owned this particular rig for 7 months.

What I am noticing under load is some voltage fluctuation, up to 3 volts in a cyclical pattern. It is most dramatic when under the heaviest load, but quite stable when the load is light.

Tests include:

1) Roof mount A/C running on high (compressor not cycling).
2) A/C + Microwave
3) A/C Fan only
4) Microwave only
5) Minimal load (bath fan, charging batteries)
6) Shore Power, no generator

There are no apparent fluctuations in generator engine noise/RPM during these various tests.

What I am wanting to know is if others have ever performed these tests and if I should consider this to be "normal". My biggest concern is that some devices can be more sensitive to such fluctuations (the microwave, for example).

What this reminds me of is back in the day my brother had a 55 chevy with a generator. There were pretty significant fluctuations in the voltage like what I see here; replacing the voltage regulator didn't solve the issue. When he swapped the generator out for an alternator, power was stabilized. (he had not attempted to get the generator rebuilt). I realize that we are talking AC vs DC here with this story; just sharing a similar symptom seen in the past.

Could it be that the windings have not been exercised enough in this generator? Should I care about the fluctuations? I wouldn't feel comfortable using a computer in the above scenerio.

See this youtube video I took this afternoon to show what I am talking about.


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Old 07-02-2012, 06:03 AM   #2
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I don't think you're seeing anything out of the ordinary. We have a 5,500 watt generator and have been seeing the same types of fluctuation for 12 years and several more run hours. When our microwave starts there's a much larger initial fluctuation. Then within a couple seconds it smoothes out to about what you're seeing. Ours has a digital gauge so you can see minor variations easily. In all the years and all the run hours all the appliances are original and everything still works fine. You can download many of the Onan manuals from the Cummins/Onan website.

Cummins Onan

As for no fluctuation on shore power, most transfer switches are wired so the generator has priority. In other words as long as the genertor is running it's the sole source of power. Even though you're plugged into shore power the transfer switch won't switch over to it until the generator is off.

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Old 07-02-2012, 06:35 PM   #3
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there are a few adjustments on the genny, including choke, rpm/Hz, and sensitivity. the latter adjustment may need a bit of tweaking.
1999 Winnebago Minnie 29', Ford V10, Close to stock.
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Old 07-02-2012, 09:01 PM   #4
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Playing with the speed of the generator will affect teh Hz of the power, it should be 60 for most of North America systems, either 60 or 50 for other places. (Same as your shore power) IF it is running too fast, or slow, bad things can happen.

The voltage is controled by other means (That said if the generator is runnign too slow the voltage will be low)

The catch is this.. There is a lot of wire, between the coils in the generator and the wires into your rig and so on so there will always be SOME LOSS when a big load kicks in.. This is normal, worry more about frequency than voltage.

Kill-a-watt.. Measures both.
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Old 11-10-2012, 08:52 PM   #5
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Update - - today while preparing for winterization, I started up the genny as part of a regular maintenance cycle. Admittedly I had not changed the oil (which was full) since buying it a year ago. It was clearly due to be changed. I decided to use synthetic. (I anticipate this will start a controversy, but many small engine manufacturers now are recommending synthetic). Anyway - the interesting thing after the change is that the fluctuations I noted above (see video) are no longer happening - the meter reads solid while under the same load conditions.

Of course I will still keep the same oil change intervals as regular oil. In my case, apart from the regular exercising of the generator, I've not yet had the need to run the generator while camping.
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Old 11-11-2012, 09:10 AM   #6
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The video is no longer available? Says it was "removed by user".
Gary Brinck
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL
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Old 11-11-2012, 05:56 PM   #7
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re-post of video

Interesting. Here is the video that shows the fluctuation before changing to synthetic oil. re-posted here:


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