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Old 07-12-2017, 12:31 PM   #21
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Be sure to use a heavy duty RV extension cord if you'll need one. Start the air conditioner on low cool to minimize the start amperage draw. Once the compressor is running you can switch to high cool. I have a voltage meter that plugs into an outlet (I leave it pluged in above the sink) to monitor the voltage. If it drops below the green range (about 108 volts) I either shed other draws or turnoff the AC to avoid damage. So far, so good!
Another good thing to do periodically is to use scotch brite or equivelant to shine up the brass plug prongs to minimize loss there.
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Old 07-12-2017, 02:44 PM   #22
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BTW, the appliance doesn't die because of low voltage, but it will draw it's rated watts. So an appliance that requires 1200 w. and the voltage drops for whatever reason to 90v., the motor will draw higher current (13a.) to make up the difference.
That's what kills it.
Power (w) = volts x amps.
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Old 07-12-2017, 04:13 PM   #23
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No need for doom and gloom. It's more useful to communicate the issues and potential pitfalls.

The AC's themselves are installed with a 20amp breaker in the RV. So 20amps is sufficient for the AC running alone.

Like I said earlier, the resistance issue is a separate problem that should be understood for long runs. I will be using my 30amp cord for my short run anyways.
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Old 07-12-2017, 06:12 PM   #24
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How do I go about ensuring all my peripherals are off such that there's no other draws other than the A/C?
The A/C is on its own circuit breaker. Shut all the other Airstream breakers off (except the Main)?
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Old 07-12-2017, 06:51 PM   #25
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Running A/C on 20amp Outlet?

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Originally Posted by pteck View Post

Just wanted to know in the meantime, whether the 20amp outlet from the garage would work.

Yes it will work fine. Use your 30 amp cord and the wall socket adapter supplied by Airstream (or equivalent).

A Dometic Penguin 13,000 rooftop A/C draws 15.3 amps. Dometic recommends a HACR breaker ( which all modern breakers are). So your Airstream and your house should have the proper 20 amp beakers for the AC.

If you find the garage breaker tripping, shut a few other things off or isolate the AC unit by turning off unused breakers on the Airstream.
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Old 07-13-2017, 12:04 PM   #26
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BTW, the appliance doesn't die because of low voltage, but it will draw it's rated watts. So an appliance that requires 1200 w. and the voltage drops for whatever reason to 90v., the motor will draw higher current (13a.) to make up the difference.
That's what kills it.
Power (w) = volts x amps.
Hi

It's actually a bit more complicated than that. Motors don't really *like* 60Hz power. They would be much happier with DC or with < 20Hz. The why and how get a bit strange. They do make for fun reading if you are having trouble falling asleep.

To get a motor running on 60 Hz, you need a "start winding" or something like it. (Yes you could use a VFD, but that's not common). The start winding is a bit of a kludge but it's what we have. It sort of broke Edison's heart and made Tesla's day, again a different story. The start winding pulls a lot of current to get a whopping big magnetic field. That gets the motor up to speed and into an operating range where it's "happy" with the 60 Hz. The winding is mainly cooled by the mass of the motor (conduction) rather than by air. Run it at low(er) voltage and it stays on a surprisingly lot longer. More time is higher temperature. Higher temperature is sorter life. Same thing applies to the start capacitor. It may be where you can replace it. Blow the compressor motor on a sealed unit ... it's gone.

Bottom line, motors aren't constant power devices. Put in lower voltage and they simply run with less output power. The have no way to compensate for low voltage. Stuff that is based on switching power supplies (like a computer or some inverters) will indeed act as a constant power load.

Lots of fun !!!

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Old 07-13-2017, 04:00 PM   #27
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Originally Posted by Mollysdad View Post
BTW, the appliance doesn't die because of low voltage, but it will draw it's rated watts. So an appliance that requires 1200 w. and the voltage drops for whatever reason to 90v., the motor will draw higher current (13a.) to make up the difference.
That's what kills it.
Power (w) = volts x amps.
Almost right. Less volts means resistance in the path. Which actually lessen the potential current that can be delivered.

So less volts and less amps means not enough power.

The danger is that without enough power, a motor may not start. A motor draws its maximum power in a stalled state, potentially overheating and burning out.
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Old 07-13-2017, 04:31 PM   #28
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pteck, I think you need to go back to school. Your answer is helpful or correct.
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Old 07-13-2017, 08:49 PM   #29
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Feel free to correct me. I don't pretend to always be correct and certainly am open to learning. That's why we're here after all isn't it?
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Old 07-13-2017, 09:15 PM   #30
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Feel free to correct me. I don't pretend to always be correct and certainly am open to learning. That's why we're here after all isn't it?
Hi

Looked right to me. Maybe you misplaced a coma

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Old 07-13-2017, 09:58 PM   #31
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Hi

Looked right to me. Maybe you misplaced a coma

Bob
Try an Easy Start device from Micro Air and your high end power draw will be 17-18 amps.......all the time!
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Old 07-13-2017, 10:11 PM   #32
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Don't forget that resistive heating increases as the square of the current drawn.
(i^2*R=W).

Low voltage will result in higher starting current. Resistive heating increases exponentialy.
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Old 07-14-2017, 05:26 AM   #33
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Originally Posted by pteck View Post
Almost right. Less volts means resistance in the path. Which actually lessen the potential current that can be delivered.

So less volts and less amps means not enough power.

The danger is that without enough power, a motor may not start. A motor draws its maximum power in a stalled state, potentially overheating and burning out.
Ptech, You can not change the resistance of a device by changing the voltage. The resistance is a physical property. But, the compressor motor is mostly an inductive device. None of that really matters. The other poster was spot on. The inductive motor will start and draw its rated power. However, if the voltage has dropped, the current draw will necessarily increase. It is this higher current draw and heat in the windings that can destroy the motor either quickly or shorten its life. For any owner, this simply means do not run your AC if the voltage is low. Others have posted 108-110 volts; I don't know and will freely admit that. Both the Easy Start protection and my surge suppressor have low voltage protection built in.

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Old 07-14-2017, 02:43 PM   #34
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Hi

"Resistance in the path" in context, is resistance in the power cord or in the house wires. If you increase that resistance (and you very easily can) the voltage (and thus power) available at startup on a conventional motor is going to go down.

Bob
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Old 07-16-2017, 03:04 PM   #35
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Running a 13.5 or 15,000 BTU AC on a 20 amp circuit will eventually damage the AC unit. It may very well run for awhile (how long is anyone's guess) but make no mistake you are damaging the compressor whiule doing this.
.
I'll have to disagree with this. It is too general of a statement. For one, the AC is already running on a 20 amp circuit from the RV panel. Sure the run is short, which means minimal voltage drop. What if the OP is using a 20 amp circuit that originates close to his main panel? The VD could be less than a 30 amp circuit with a longer run. Only way to know is a VD calc or a recording meter.

It all boils down to VD, which is determined by wire size, circuit length and demand. The circuit breaker has no effect on VD, which is what damages motors.
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