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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.

Elros
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5 Answers5

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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.

Harper - Reinstate Monica
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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 because until it is spinning, 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.

DrMoishe Pippik
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There are now 48V DC mini-split inverted HVAC systems available that have soft start and continuously variable drive. You could expect a 6KW system (~.5 tonne) to draw a maximum of about 7A @48V. These bypass the inverter and can be configured to run directly off the solar panels or the batteries and can also be configured to prioritize the solar and batteries, but run of AC from the inverter if solar and batteries are insufficient. Just google 48V DC mini-split HVAC.

Joe
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Generally for an inductive load, it is important to know an AC motor when turned off creates a spike that SCR's do not like. The capacitive component in a transformer based inverter can accommodate such a spike. Further due to a varying power factor especially during start up, a transformer based Inverter has a larger tolerance for the such a inductive and varying condition than a SCR that converts this in additional heat, and this might not be kind to an SCR based Inverter. My humble opinion, in the case of a high inductive load, a safer bet is just go with a transformer based type inverter.

Johan Hees
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You are largely right, and the answers I see in response to your question are largely wrong.

You are facing a hard compressor start load in addition, or exclusive of, an inductive load issue.

It's widely understood that a hard compressor start can be expected to exceed average load by %4,000 across several cycles. A fridge that draws 13OW continuous can be expected to draw 5200W worst case on startup.

How inverters and generators react to this momentary overload is seldom adequately addressed with hard figures.

To convert a "hard start" refrigerator compressor motor into a soft-start load is non-trivial.

Mysterylectricity
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