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Gnerd
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# Posted: 9 Feb 2021 02:29pm
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I have a Kobalt 8-gallon air compressor rated at 1.8 hp (1342 watts). I measured it with a clamp meter and it actually drew 9.5 amps (1140 watts). I don't have a way of measuring instantaneous inrush current on startup.
In setting up an off-grid power system, I'm considering using a Xantrex Freedom X 2000 24V inverter, rated at 2000 watts continuous, 4000 watts surge for 5 seconds.
Would this inverter probably handle the inrush current for the compressor?
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toyota_mdt_tech
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# Posted: 9 Feb 2021 03:14pm
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I would guess so, but start up when the tank pressure drops to turn on will be higher than start up with an empty tank. Maybe a meter with a peak mim/max hold feature to read it?
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ICC
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# Posted: 9 Feb 2021 03:31pm
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The only way to tell for sure is to try it. There is also no cheap way to accurately measure the inrush current. The specs of the two units make it appear you could be in luck. But there is no guarantee. I did a search on the inverter and the page I came up with stated a 2 second 4000 watt motor surge would be okay.
Those Kill-a-watt meters and other like them do not sample the current flow rapidly enough to get an accurate reading.
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gcrank1
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# Posted: 9 Feb 2021 05:27pm
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At one point years ago I had inadequate power for a normal low pressure start up. Functionally got around it by switching off and just running down the tank pressure until it wasnt useful; then the start up was lots easier and I could let it refill for another go.
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Fanman
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# Posted: 9 Feb 2021 06:14pm
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Some compressor pressure switches have an unloader valve that dumps the air when it switches off. That needs to be paired with a check valve downstream of the switch.
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Brettny
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# Posted: 10 Feb 2021 06:00am
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Your inverter "should" handle that load. Should meaning the freedom X inverters are prety robust but compressors are also in rush hogs. They could be 3x-5x the running wattage. If you have a generator I suggest running it on that.
The real question is will your batteries be able to supply enough voltage to the inverter to not trip the low voltage cutout.
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rpe
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# Posted: 10 Feb 2021 06:51am
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If your needs for compressed air are limited, you might be able to modify the compressor to reduce inrush current at startup. As fanman says, compressors often start with discharge line unloaded, but quickly pressurize that line during the startup event. Maybe a ball valve on the branch of a 'tee' could be installed in the discharge line? Open ball valve, start compressor, air is easily dumping out the ball valve. Close ball valve a couple seconds later, and air is now forced into tank.
And to start an even bigger compressor, install starter recoil off a derelict small engine. Open ball valve, pull recoil to get electric motor spinning, power up compressor, close ball valve. Give me time and I'll come up with further complications to your startup procedure 
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Steve_S
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# Posted: 10 Feb 2021 07:41am
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I run a slightly bigger 120VAC/15A/3HP/20 Gallon Compressor.
- If the air tank is empty, the start surge is 2100W.
- If the air tank is at 50%, kick on surge is 3000W +/-100W
- If Tank is at 85-PSI (I have it set to go on @ 85psi and off at 110psi) surge start is 4500W+/- 200W.
- I've had the inverter TRIP ONCE (capable of 12kw surge) when I was running heavy air tools when it cycled must faster ~but~ I was only using a 12guage extension cord at that time, once switched to 10G it wasn't a problem.
My Inverter & other equipment register the voltages, amps & watts drawn which I can view that in detail at my leisure.
When I need to run the Compressor or MIG and such for an extended time, I use the Generator. If it is for a short time I can use my Inverter without issue. I figure there is no point in abusing my solar system for the occasional longtime use.
I am running 24V LFP (910AH upgrading to 1190AH) and secondary bank of Heavy Lead at 856AH Gross/428AH net).
The inverter is a Samlex EVO-4024.
https://samlexamerica.com/products/ProductDetail.aspx?pid=575
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Gnerd
Member
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# Posted: 10 Feb 2021 10:54am
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Quoting: Brettny The real question is will your batteries be able to supply enough voltage to the inverter to not trip the low voltage cutout.
The general rule is that battery amp-hours should be at least 4x the max sustained load. Adding in inverter efficiency loss, I get 51 amps at 24V DC. Four Trojan T-105s should handle that no problem. And I expect to use the compressor during daylight, when I'm getting an extra boost from the solar panels. And I want to set up a 24V alternator that runs on a tractor PTO, for if I want to run heavy loads for an extended time.
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Gnerd
Member
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# Posted: 10 Feb 2021 10:59am
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Quoting: Fanman Some compressor pressure switches have an unloader valve that dumps the air when it switches off. That needs to be paired with a check valve downstream of the switch.
I bet I could rig up a manual valve fairly easily. The real trick would be getting it to work automatically.
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