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post #1 of 15 Old 06-29-2012 Thread Starter
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Powering AC with genreator, handling surge load

Hello,
I recently got a 2000 watt generator -- a little 2.7hp model. To operate it I simply plug a standard adapter which converts a standard 120v plug into a standard 30amp shore power plug into its 120v and hook my shore power cable into that adapter, and into the shore power socket. It works great, and powers everything BUT the AC. My AC is 16000btu, and draws a modest 4-5 amps under running condition (4-5amps*120v should be around 600 watts, P=IV), however, when the compressor kicks on it draws between 15-30amps. This is no problem on shore power but this surge is beyond the generator's capacity and it cuts out.
I realize I could go to a larger generator, but I do not want the additional weight nor size.
I was wondering if I could run power to the shore power in parallel -- one side of the circuit coming from the 2000 watt generator, and the other coming from a 2000-4000 watt inverter attached to the deep cycles. I theorize that when the AC kicks on, it will draw additional current from the battery to deliver the the necessary amps to kick on the compressor, and then when it drops down to running level amps (4-5amps) I can simply disconnect/switch off the inverter from the batteries, allowing the generator to carry the load on its own for the rest of the time.

Is this feasible? Has anyone experimented with a similar setup?

Thanks!
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post #2 of 15 Old 06-29-2012
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Re: Powering AC with genreator, handling surge load

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Originally Posted by xluke View Post
I was wondering if I could run power to the shore power in parallel -- one side of the circuit coming from the 2000 watt generator, and the other coming from a 2000-4000 watt inverter attached to the deep cycles. I theorize that when the AC kicks on, it will draw additional current from the battery to deliver the the necessary amps to kick on the compressor, and then when it drops down to running level amps (4-5amps) I can simply disconnect/switch off the inverter from the batteries, allowing the generator to carry the load on its own for the rest of the time.

Is this feasible?
No, no, NO - absolutely not! You can't practically parallel up to shore power without fitting large and expensive control gear.

The only way you could get your A/C unit to work off of the generator you have will be to install an Inverter big enough to handle the current in-rush of, as you said, 15-30 Amps ie. 30A*120V = 3600 watts, and run that off a battery which you charge with the generator. The A/C would be running off the Inverter full-time. I don't know what a 4kW Inverter would cost, but it wouldn't be cheap - or light - but, by the sounds of it, that's what you'll need.

IMHO you are better advised to get a bigger generator.

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Re: Powering AC with genreator, handling surge load

I've read a number of posts about a "soft starting" AC. I don't know the details but it is some kind of option you can add to the AC unit that makes it pull a lot less amps when starting up. A number of people have it just for your purpose - they have a fairly small generator that works fine except for compressor start up.

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Re: Powering AC with genreator, handling surge load

Hartley is absolutely right about not paralleling two AC sources. To do this you need synchronizing controls so the sine waves are in synch. Without that you will get excessive current and damage things.

And something is wrong with your number for AC capacity or normal current reading. Typical EER of an AC unit is 10. So a 16,000 Btu/hr unit would draw about 1600 watts. That's close to the rating of your generator and will never be able to start it. A 6000 Btu/hr AC unit would be about 600 watts and that would agree with your 4-5 amps when running. Check your numbers.
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Re: Powering AC with genreator, handling surge load

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Hartley is absolutely right about not paralleling two AC sources. To do this you need synchronizing controls so the sine waves are in synch. Without that you will get excessive current and damage things.

And something is wrong with your number for AC capacity or normal current reading. Typical EER of an AC unit is 10. So a 16,000 Btu/hr unit would draw about 1600 watts. That's close to the rating of your generator and will never be able to start it. A 6000 Btu/hr AC unit would be about 600 watts and that would agree with your 4-5 amps when running. Check your numbers.
I'm betting the 4-5 amps is just the fan running by itself - no compressor.

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Re: Powering AC with genreator, handling surge load

PO needs to get a real time amp reading on the unit when it starts. The fan most likely runs on less the 2 amps. Depending on which compressor (recip or rotary) will dictate starting amps. if the unit doesn't have a "hard start" accessory. it will need it to lower the starting amps. Do not use the name plate rating for ACTUAL running conditions. I'm assuming the unit is water cooled? That would also make the unit a lower amp draw then a air cooled condenser.

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Re: Powering AC with genreator, handling surge load

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I've read a number of posts about a "soft starting" AC. I don't know the details but it is some kind of option you can add to the AC unit that makes it pull a lot less amps when starting up. A number of people have it just for your purpose - they have a fairly small generator that works fine except for compressor start up.
That's the way to do it. Never, ever combine two different incoming AC electrical sources. Never.

Google "soft start for boat air conditioning" and you'll find the sources.

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Re: Powering AC with genreator, handling surge load

Depending on which small gen you bought, the Honda and Yamaha can be linked to produce a nominal 4kw from two 2kw gens. But this only works because the control boards can talk to each other and sync up by a cat5 cable.

Trying this with dissimilar sources, as noted above, is both dangerous and a fire hazard.

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Re: Powering AC with genreator, handling surge load

Back in the day cities in the east like Phila pa. Had and and may still have some "2 phase" service. There used to be 2 phase wired BIG semi hermetic compressors in some old ac units.

Just for conversation, and some interesting reading
http://www.3phasepower.org/2phasesystems.htm

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post #10 of 15 Old 06-29-2012
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Re: Powering AC with genreator, handling surge load

They still have the two phase and the big machines get special motors which is a PITA

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