Is my generator underperforming or is this normal?

[JimGnitecki]

I have an Onan 7.5KW generator in my Eagle. I THOUGHT that should be sufficient to run both Dometic basement air conditioners when on the road, since I don't have bus air, but maybe I am wrong.

When running only one of the basement air units, I run the driver side one since it has a better ducting system for distribution of the cold air. When I want more, I try to turn on the passenegr side AC as well.

When I turn the thermostat setting to on, the fan starts up, and of course there is a programmed delay until the compressor kicks in. When the compressor kicks in, there is a voltage stumble visible on the voltage monitor, and the 2nd copressor never gets on line - its circuitry evidently detects the drop in volatge and prevents its startup.

Today, I am cmaping overnight at a Camping World to get a Splendide washer/dryer installed tomorrow, and the service folks graciously let me hook up to a GOOD source of 50 amp power.

So, I tired the dual air experiment here. The voltage did NOT stumble on startup of the 2nd compressor. It also remianed at 118 indicated volts regardless of whether 1 AC or both were running. Plus the fridge. The 2nd AC unit appears to be running and cycling as oyu might expect it to.

So, my question is: Why won't the 7.5KW generator keep both AC units going? Is the startup load of the 2nd compressor, added onto the fan plus fan and compressor of the 1st AC unit, plus the fridge, too much for it?

Or, is there somehting amiss with my Onan generator?

Pardon me if I don't repsond to answers here tonight. I am pirating onto a wi-fi system here at Camping World, and I suspect it may go away after the store closes for the night.

[Sunchaser Art]

Jim-

I probably should leave this to the smart people on the board here, but I'll write something and hope I don't embarass myself! If it's not throwing breakers, my primary thoughts might be a capacitor or voltage drop in the wiring somewhere along the line (especially if you have one of those auto switches from shore to generator). If you don't find a fault there, maybe a generator load test is in order. I'm not sure if you have an ammeter you can monitor your AC load, but that helps me out quite a bit on some diagnoses.

As another thought, I'm not sure what you have for an inverter, but on initial input (from generator or shore power), the invertor draws quite a load when it's trying to load charge the house batteries. So, if you just plug in and try to fire the airs, there might be sufficient amp draw from the combination of inverter and AC units that it's creating a voltage drop. If you've had electricity plugged-in for awhile and the inverter slows to a float charge, that's a lot less amperage. I'm not sure how much voltage drop you're getting with the generator, but I would think they should fire all the way down to 105-110 volts.

Good luck!

Art

[Hal]

It all depends on your load and of course the condition of the generator.
You didn't state which A/C units you have. Coleman made a 27,000 btu basement unit that pulls upwards of 30 amps(they require 2 dedicated 20 amp circuits). If you have those units you will have a problem without too much extra dragging the genset down. Remember, you loose about 3% power generation for every 1000 feet elevation plus the loss from the elevated temperatures.
You should check your loads with an amp meter to have a real handle on your loads. After you have some real numbers you can evaluate your generators performance.
Hal

[JimGnitecki]

It all depends on your load and of course the condition of the generator.
You didn't state which A/C units you have. Coleman made a 27,000 btu basement unit that pulls upwards of 30 amps(they require 2 dedicated 20 amp circuits). If you have those units you will have a problem without too much extra dragging the genset down. Remember, you loose about 3% power generation for every 1000 feet elevation plus the loss from the elevated temperatures.
You should check your loads with an amp meter to have a real handle on your loads. After you have some real numbers you can evaluate your generators performance.
Hal

I was told the Dometics in my bus are only 15 amp units, and there are indeed only 2 separate 15 amp circuit breakers in the panel for the AC units - one breaker for each unit.

I have been keeping the bus plugged in when not in use, so we SHOULD be talking only a float charge for the house batteries.

I'll try a very controlled test sometime soon, where I will ensure that the AC units are the ONLY draw on the generator.

[JimGnitecki]

Interestingly, ONE ac unit is "almost" enough even on a very sunny mid 90s Texas day if the blinds are shut and the windshield is not facing into the sun. One WOULD be enough if the distribution ducting were better - our bus appears to ahve been well insulated.

The basement units are just forward of the bathroom and under the kitchen, and so the bathroom and kitchen get REALLY cool and REALLY strong air flow, while the bedroom and especially the driver area get weaker air flow. I think one ofmy future projects will be to replace the cheap flexible corrugated insulated ducting with rigid insulated piping and real elbows that maximize air flow. Then, I suspect one will be enough most of the time.

The roof of our bus is coated with a white ceramic paint coating that the previous owner told me is notably cooler to the touch on sunny days than the uncoated metal right near it.

If I could get to where one AC unit is enough all the time, the electricity savings and the automatic "spare AC" would bot be nice!