Generators and air conditioners

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Generators and air conditioners

Postby 48Rob » Sat Jun 03, 2006 4:04 pm

Hi all,

I'm working on sizing a generator to run my air conditioner.

The air is 5050 BTU, rated at 4.8 Amps.

I'm having a bit of trouble understanding the readings I'm getting when checking the Amp draw.

Unit turned off, meter hooked up, turn unit on, reading is 19.7 Amps.
Unit won't start, fan comes on, stays on.

15-20 seconds later, the compressor kicks in, Amps go to 4.7

Quickly drops to running Amperage of 3.0

So, I let it run for a bit, then turn the thermostat until it drops out.
Amps go to 0.30, fan only.

Give it a few moments, then turn thermostat again.
Amps jump to 18, but won't kick in.

After 15-20 seconds, compressor kicks in, Amps are 3.5 at start of compressor, then drop to normal running Amperage of 3.0.
Unit runs fine.

If the startup draw is 19.7 Amps, then I'm going to need a 2500 Watt generator...and the whole idea goes to heck (too big and bulky).

If the starting Amperage is 4.7-5.0 then I can get by with an 800-1000 Watt generator (very compact and easy to store).

Any thoughts?

Rob
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Postby Sonetpro » Sat Jun 03, 2006 4:34 pm

Rob,
That's one of the reasons I went with a petcool. Running is only 2.6 amps but startup is 8.6. Startup of the compressor is usually 2-3 times the running amps. If it stalls like you were describing it is very hard on the compressor. Do you have a power saver on the genset? I have one on mine and was very dissapointed that my 1000k wouldn't run it. But then I realized that when the compressor kicked on it wanted amperage right then, the compressor wasn't fast enough to sense the load and kick up. When I turned the power saver off it has no problem. I ran it last week for 6.5 hours on a tank. I got it so cold in the cabin there was frost on the outside of the windows.
You might try to go to a hotter start capacitor. If you go to Graingers and ask them they can guide you in the right direction. I had one of the AC guys at work do that for me. It has no problems now.
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Postby 48Rob » Sat Jun 03, 2006 5:42 pm

Hi Steve,

The tests were performed with the unit plugged into the grid, I don't have a generator yet.
I suspect it stalled because of the clamp meter adaptor I put in line to check the Amp draw...

I did this test on both the air conditioner in the trailer, and a comparably sized unit in my shop, both stalled when hooked to the meter, but neither stall when plugged directly into an outlet.

The real question is; which of the readings I got is the actual startup draw?

Could the 19 Amp reading be because of the clamp meter adaptor, or is 19-20 Amps the real start current need?
If it is, then why does the meter only jump from .30 Amps, to 3.5 when the compressor starts?

I'm not questioning your knowledge, I just don't have a great understanding, yet.

Thank you!



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Postby GeorgeTelford » Sat Jun 03, 2006 6:59 pm

Compressor starts are usually in the region of 4 to 5 times running amps at least, hence the figures you are seeing. The surprising thing is a 20 Sec start (before compressor kicks in)

Some of this is Power factor Now this is a really doozey to try and explain, its power thats got to be available but isnt actually used, that is why Steve having a bigger capacitor fitted as tricked the compressor into starting with a smaller genny than it normally would.

The reason the power jumps around is that after the initial start the compressor from stall the power requirement to keep it rolling is lower also the PF is no longer drawn.

The figures I have handy relate to a fridge compressor (at 12v)

On start up it pulled 55 Amps, for normal running it pulled 11 Amps, but the average consumption per hour was only 2.9 Amps
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Postby 48Rob » Sat Jun 03, 2006 7:12 pm

George,

Thank you for the explanation, I can relate better now...

With Steve's setup, the capacitor stores more power, so, if I had my A/C buddy change it, how many hard starts would the compressor accept before I would have to replace the unit?

Would the unit even start with a 1000-1500 Watt generator?

At $70.00 each, and only being used a few days out of a year, the air conditioner may be cheaper to replace every couple years, than it would be to have/deal with, a large generator.

Rob
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Postby madjack » Sat Jun 03, 2006 7:17 pm

Rob, let me get this straight...is the unit plugged into some kinda meter adapter and that adapter plugged into the grid, or is plugged directly into the grid with a meter clamped AROUND the power cord....
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Postby Sonetpro » Sat Jun 03, 2006 7:24 pm

Rob Think of the compressor as a rotory engine. When stopped the rotor is on the magnet. It takes alot of power to get it spinning off the magnet but once spinning it doesn't take much at all.
I agree with George completly. I my honest opinion I don't think that small a genset is going to do it.
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Postby Sonetpro » Sat Jun 03, 2006 7:27 pm

madjack wrote:Rob, let me get this straight...is the unit plugged into some kinda meter adapter and that adapter plugged into the grid, or is plugged directly into the grid with a meter clamped AROUND the power cord....
madjack 8)

I wouldn't test it that way. I have a Fluke 87III with a clamp attatchment. It will record so you can see what the true start amps is. It only pull's it for a second so it would be hard to see on the meter.
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Postby madjack » Sat Jun 03, 2006 7:31 pm

Steve, you said you ran your PetCool onna 1000w gentset...is the PetCool, the 2500BTU model and what make/model genset are you using...........................
madjack 8)
p.s. did you do any other mods to the PetCool besides the hotter capacitor
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Postby 48Rob » Sat Jun 03, 2006 7:39 pm

Jack,

The a/c is plugged into an adaptor (that splits common and hot) the clamp meter is clipped over that adaptor on the hot side, the adaptor is plugged into the grid, ie. my shop outlet.

The meter won't measure if it is just wrapped around a cord, the wires have to be separated.

I looked a little harder, the adaptor is only rated for 15 Amps, perhaps part of the reason the a/c is dropping out after the big spike...?

Steve,

You're probably right...I just keep thinking that if I wish hard enough, some day it will work in my favor :worship:

The high reading I got was only visible on the display for a second, but it was clear.

Hmmm, perhaps a hand crank to give it a spin or two before flipping the switch ;)

Thank you all for your help, it is appreciated! :thumbsup:

Rob
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Postby Sonetpro » Sat Jun 03, 2006 7:39 pm

Yep, Put a hotter cap in it and it run's fine. it's a 2000 btu. 1K Kipor.
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Postby bdosborn » Sun Jun 04, 2006 12:39 am

Rob,
There's two things going on here I think. One is starting a single phase AC motor. They have notoriously bad starting characteristics. Not only do they draw a ton of current on start up (the poles of the motor tend to align when they stop like sonetpro said and then there's very little starting torque) but the power factor can drop below 0.5. A start capacitor doesn't store current to help the motor start, its there to try and correct the power factor to somewhere around 0.8, which is typically the lowest power factor a generator is rated for. The other issue is compressor "hot start". When the compressor stops, refrigerant pressure builds up in the system and its even harder to start the compressor because its pulling a bigger load. Most RV A/C manufacturers recommend waiting 2 minutes between compressor starts. Thats usually enough time for the pressure in the refrigeration system to drop to facilitate starts. You should take some meter readings after waiting a couple of minutes between compressor starts.

I have a 2000 btu Petcool and my Kipor 1000 will start it, even when I cycle the compressor on and off with no waiting period. Its a big load and the generator barely pulls through it but it will start. If I wait a couple of minutes between compressor starts it will do it with no problem, even when its in economy mode. But, I've tweaked my Kipor with a high altitude jet and adjusted the idle mixture. I'm not sure it would do it if I hadn't tweaked it. I've read on other forums that the Kipor 2000 will start some 5000 btu units but not all of them. The Honda eu2000i usually starts a 5000 btu unit. I think you'll need a minimum of 2000w of generator and perhaps a hard start capacitor kit, depending on your A/C unit.

Here's a link to 210 pages of chinese generator stuff:
linky
Cheap generators that seem to be of adequate quality and acceptable noise levels.

Here's a link to Kipor generators.
linky2
Chinese Honda eu clone. They're cheaper than the Honda but US service support sucks. You pay your money and you take your chance.

There's a bunch of articles in that forum about starting A/C on generators if you use the search function. Hope this helps...
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Postby john » Sun Jun 04, 2006 1:51 am

I have an older 220 window unit in my house. If it has been running and is turned off and then quickly turned back on I can hear the compressor trying to turn over but failing, and in doing so kicks the breaker with in 2 or 3 seconds. If I wait 5 or ten minutes before turning it back on it has no problems getting going.

In my tear I have a small 110. If I turn it off then quickly turn it on I hear the compressor also trying to turn over but failing except in this case there is a fail safe that kicks in and foces a wait before the compressor starts up.

I believe the readings you are getting suggest a need to wait for the freon lines to equalize pressure so as to ease compressor start up. The difference in power needed between a low pressure startup and a high pressure attempt is interesting.
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Postby 48Rob » Sun Jun 04, 2006 5:49 pm

Bruce and John,

Thank you for the links and tips! :thumbsup:

I spent a bit of time today studying and digesting...and have come to the conclusion (as all the other good posters already had) that unless I want to spend big money on a large generator, it isn't going to work.


Oh well, not the end of the world, a shade tree feels pretty good too!

This whole exercise (besides learning something new, which is always a good thing) was an effort to cool the trailer while dry camping at old tractor shows and swap meets.
These events are always held on the hottest days of summer, with no power, and little shade.
I attend these things with my vintage car, which is not large enough to haul the big generator needed.

Like many before, I fell for the low dollar, compact, and "perfect for my needs" mini generators that abound on E bay.

It was a fun ride while it lasted, and I've learned much :applause:

Thanks to you all!

Rob
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Postby Kurt (Indiana) » Sun Jun 04, 2006 8:04 pm

48Rob wrote:
Would the unit even start with a 1000-1500 Watt generator?

Rob

Rob, I'm running a 5050 "Goldstar" A/C. I have been able to run it on my Honda 1000 watt generator. . I needed to turn off everything and turn up the generator to max power. Then turn on the A/C (fan on low). It worked in the garage, but I've never tried it while camping. I always have been able to use the campsite electricity.
Keep trying, cause the few nights that you will need it, you'll be glad you did. Don't forget those rainy humid days when you are sweating to death and nothing to do. I turn on the air and put in a DVD. :thumbsup:

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