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TLDR: My shed AC unit (maximum draw 8A 120V AC) keeps tripping my 1100W inverter. It seems to have troubles occasionally when the fan turns on. It's pretty much guaranteed to have troubles when the compressor turns on. Looking at the power draw though, it never seems to draw more than 14A on the 12V DC line.

I've built a solar power system for my shed and so far it works well. In theory, I've got plenty of capacity for everything I need.

  • Solar panels producing 150W
  • Batteries capable of producing 50A at 12V for 5 hours
  • Inverter rated to produce 1100W at 120V AC

The system is more than capable of powering lights, chargers, and other small loads for as long as I've tried running it for. Thus, I doubt it's really a lack of capacity in the general sense.

I'm fairly sure the problem is the inductive load of the motors during startup. I've been reading a bit and I think one way to solve the problem is to get an inverter capable of handling roughly 8 times the load of the AC. My calculations would indicate that I need an inverter capable of handling (120V*8A*8) = 7680 Watts. Such a beast is quite expensive...especially if I can't be sure of it solving my problem.

Is my problem really just inverter size, or is there something I should look for when handling inductive loads.

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    Are you trying to power directly from your array or are you drawing from a battery. If your array is not producing the fla of the motor x 5-8 as you have read the inverter can not power the load, for high peak starting loads having a storage system really helps the battery sounds small for real motors. I have converted single phase to 3 phase with several different types of inverters and all were inductive loads. Do you have a charge controller limiting you?
    – Ed Beal
    Jun 1, 2020 at 23:31
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    Just to check. You wrote "Solar panels producing 150W". That seems small for the test of the system, as well as the application. Did you mean 1500W?
    – DoxyLover
    Jun 1, 2020 at 23:31
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    The solar panels go to a charge controller which charges the batteries. The inverter is powered directly from the batteries. I agree 150W solar is a bit low. However, practically speaking I will only be in the shed once a week for only a few hours at a time. Letting it take a week to recharge the batteries is ok for my use.
    – Elros
    Jun 1, 2020 at 23:38
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    8A@120V is 960 watts at normal running. That's nearly at the limit for the 1100W inverter. Is this thing a beast made by Schneider or Siemens that has wild overdesign built in? Or is it a Chinese cheapie that I wouldn't ever ask to run near spec, let alone continuously?
    – Harper - Reinstate Monica
    Jun 2, 2020 at 1:17
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    @Harper-ReinstateMonica My understanding is that 8A is supposed to be peak...though I guess if circuit breakers allow higher current for a short period of time, the startup current could be well beyond that. The inverter looks to be a German name. I’d expect some amount of over design, but I’m probably still overloading it.
    – Elros
    Jun 2, 2020 at 1:50

2 Answers 2

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It is not that the compressor motor is inductive, but that, at startup, it draws far more current than while running. This is a characteristic of most electric motors, but is exacerbated that until it is pinning, having a large moment of inertia, the compressor is pushing against the "spring" of uncompressed vapor. The locked rotor current draw might be five times that while running!

The short, high-current demand usually won't trip a circuit breaker (though GFCI-protected outlets and breakers may trip from the inductive voltage spike), because they have a thermal time delay for moderate overload. However, the inverter must handle that initial surge. A 40 Amp peak inverter would probably be sufficient (4,800 V-A peak), but the inverter could have a much lower continuous rating. For example, this US$140 2,000 W inverter has a peak rating of 4,000 W, which might be just a tad shy of your AC's needs. Considering your investment in 3 kW-Hours of batteries and the solar panel, this should be a relatively minor expense.

BTW, you might need far more than 150 W in solar panels to run an AC more than a few hours out of a week spent charging. Consider that sunlight is not available in the evenings, and that the panel will not produce full output on cloudy days nor when the sun strikes it at an angle, morning and afternoon. You'd be lucky to get 1 kW-H charge per day in summer, at most latitudes, and much less in winter. Of course, any other load, e.g. lighting or small refrigerator, would only make the situation worse.

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  • Thanks. Would be nice if I can find a way to handle short-term high-demand such as the peak at startup. Something like the way a cap across DC power lines handles current spikes. As for the panels, I’m lucky to get 2-3 hours a week. My plan is to add panels over time until I can recharge in a couple of days.
    – Elros
    Jun 2, 2020 at 1:45
  • Is there a good way to measure the start-up current? I’ve got an amp meter an adaptor with a 10x winding. I’m just not sure I’ll have the response time to see what the current is.
    – Elros
    Jun 2, 2020 at 1:53
  • As you say, both digital and analog meters have a finite response time... and the duration of inverter "peak" power may not be specified. Try to get the inverter from a store that has a good return policy, and test it a few times. BTW, a warm air conditioner (one that's been running and just shut) is much harder to restart because of the increased pressure of refrigerant. PV =nRT, as they say.
    – DrMoishe Pippik
    Jun 2, 2020 at 18:31
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It's easier to teach a violinist the bongoes than the other way 'round.

You should be shopping for an A/C unit with "soft start". This is one of the reasons for these units to exist - ability to start off inverters, generators etc. They use an inverter drive to run the compressor motor. They are also able to run the motor at different speeds to suit demand, so they don't do the slam-on/slam-off thing like simpler units do.

This platform doesn't allow product recommendations, but I would be asking on the off-grid/solar forums where people deal with that all day everyday.

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