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diy solar

Need solar plan for small shop - needs to run 240v 3a compressor

CamaroMan

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Nov 26, 2022
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Hi all - I have a shop and looking into investing into a solar system.

99% of the time I charge power tool batteries and run maybe 60w of shop lights on 110v.

Once in a blue moon I need to run my compressor for sand blasting - its a 3HP 220v, last time i calculates the start draw is around 45 and running about 20-25.

I have PLENTY of sunlight, maybe 10 hours so dont need that much on tap solar power, its primarily for recharging the battery bank.

Now ive been tossing between 12v and 24v, but def leaning toward a 24v system if a 12v will not work AT ALL (i dont mind if it struggles a bit).. but 12v would equal a LOT of amp draw on the batteries.

Now inverter specs, where do I start. as far as ive read a LF would be better / thoughts? I will be going with a split inverter for 110 and 240v..

currently Im planning on a 280ah 24v battery lithium bank, maybe 400w solar and an inverter - everything will be wired within 20ft - ie REALLY close to where i work.

Let me know your thoughts - thanks!

also - ill be using the 3.2v cell system, not a complete battery, so yes I would need 8 for a 24v system..

Just not sure whether HF or LF, and if I can get by with 400w panels for recharging (mainly be using 200w max), mostly 100w max-

should I aim for more panels?

Thanks
 
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Once in a blue moon I need to run my compressor for sand blasting - its a 3a 240v, last time i calculates the start draw is around 45 and running about 20-25.

This makes no sense. You've given 3 current values:

3A
20-25A
45A

I'm going to assume "3a" is just a typo.

Was the 45A based on a measurement with a clamp ammeter with an INRUSH function or simply the "MAX" function on a clamp ammeter?

Surge is typically 4-10X the run current, so I would expect a minimum of 100A surge.

I have PLENTY of sunlight, maybe 10 hours so dont need that much on tap solar power, its primarily for recharging the battery bank.

The # of hours of solar exposure doesn't mean a whole lot. The intensity of the sun on the panel(s) matters.

You can simulate the performance of an array at your exact location, planned orientation and tilt at https://pvwatts.nrel.gov/
 
Correct, 3HP not 3a!

The reading was taken from the motor specs. 3HP 220v - i can test with a clamp. The LF inverters can handle 300% for 20s, so be more than fine - im leaning towards a 5000w split phase 24v 280ah system with maybe 800w panels, maybe 1200- i just need the batteries to have the juice to run the heavy load for 20 - 30 minutes.

Entered address and arrived at 1,682 kWh/Year for 1000w panel array.

low of 99 in Dec and high of 175 in Aug..
 
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Recommend this one:


Neighbor used it to start his 3hp grundfos deep well pump down at 700'.

It is relatively inefficient and has high idle loads, so it's a poor choice for light loads; however, this will be true for ANY inverter you choose capable of powering a 3hp motor.

Recommend two separate inverters - one for 3hp motor and one for everything else. The smaller one should be rated for 3X the typical load for maximum efficiency.
 
can that power 6k thru both the 110v and 220v simultaneously? ie 6+6? my compressor uses a 3hp single phase motor, that equates to 2200w, at say 75% efficiency maybe 2800w. So a low frequency pure sine wave from 4k up should quite easily be able to handle the 35amp start for 1-2s before settling down to around 3000w..

maybe a 5k or 6k would be more than enough - i run my compressor VERY infrequently and 99% of the time im using LED shop lights and charging power tool batteries and occasionally running a 110v bench grinder ...
 
No. No inverter can. A split phase inverter can output either 50% per leg or 100% across 240V. Many inverters can handle some imbalance between the legs, e.g., 3300W/2700W, but that varies.

I don't believe for a second that your 3hp motor will only need 35A start unless you've TESTED it with an INRUSH function on a clamp ammeter. Surge is typically 4-10X run.

What's the surge on the 110V bench grinder? Motors that have a long spin up time have much longer surges.

Recommend you thoroughly evaluate your equipment with MEASUREMENTS before you commit to hardware.
 
Another option to throw out there would be using 2 inverters. Get 1 that's a nice efficient simple single 120v inverter that runs most of the shop most of the time.

Then have a quality LF inverter that supplies a 240v distribution system for your heavy devices. Put a DC breaker on the feed to the LF and only turn it on when you're going to be using the 240v equipment. That way you're not burning the higher standby draw of the heavy inverter when you're not using it.
 
Assuming a "G" power factor for your 3HP 230V motor, a 45A startup surge is about what you should expect. Assuming a 24v system, you'll need an inverter and batteries that can provide ~500A of surge power. That's a heck of a lot of for a system that is normally only used to charge cordless tools and run LED lights. Why not just use a gas powered construction compressor for that "once in a blue moon" usage pattern?


 
Assuming a "G" power factor for your 3HP 230V motor, a 45A startup surge is about what you should expect. Assuming a 24v system, you'll need an inverter and batteries that can provide ~500A of surge power. That's a heck of a lot of for a system that is normally only used to charge cordless tools and run LED lights. Why not just use a gas powered construction compressor for that "once in a blue moon" usage pattern?



This is a single phase 220v motor 3hp, i cant see how a motor could draw 500 amps! Ill check the motor plate at the shop later- Im going solar for other lower current 220v machines like my engine block washer cabinet.

I think I need to come install the motor back on the compressor and run it with my clamp meter and see what it really draws- it does have a starting capacitor and I could always put 2 caps there or buy a $300 start load reducer -

https://www.micro-air.com/products_easystart_368_softstarter_microair.cfm

I have a 280ah 24v battery system ordered thats on its way-
 
ok so i found a guy testing a similar size motor, he got down to 85 amp draw and could maybe do more with a soft start.. isnt the amp draw on the battery bank size? I have 300ah 24v for now..

if a transformer LF pure sine inverter can handle 300% for a few seconds, why can a 8000w not handle 24000w while the compressor kicks on, what am I missing?
 
220v times 45 amps equals 10,350 watts. 10,350 watts divided by 24 volts equals 431 amps.

ok yes on the battery bank size, but not on the output side! That was my whole argument.. even a 507a draw for a millisecond would be fine on the battery bank - but at 220v thats way less, doesnt the inverter output x amount regardless of amps required?
 
Yes, people are concerned with that initial amperage across the battery cables. Once it's converted up to the 220v the amperage is MUCH less and that side of the equation is fine.


ok so i found a guy testing a similar size motor, he got down to 85 amp draw and could maybe do more with a soft start.. isnt the amp draw on the battery bank size? I have 300ah 24v for now..
So at 85a on the battery side that's only about 2Kw which isn't bad at all. The big concern is to get that startup amperage measurement on the battery side finalized to make sure the wires can handle it and that the inverter isn't going to cry itself to death trying to convert a HUGE amperage spike for a long time (couple of seconds is a long time in the electrical world).

One thing you can do is check out some of David Poz'z videos on Youtube where he does inverter reviews. He likes to use a chop saw as a test because it's a HUGE startup draw for a 120v device and a pretty good stress tester. You'll see quite a few times where the inverters, even when they SAY they should be able to handle it, fail and shut off when he tried to spin that thing up. Your compressors are going to be similar and you can see what happens when you overload the inverter with too much startup load.

So, if you can find a way to snag that startup amperage, or at least something very similar and plan to be able to handle that, then you won't have to worry about it in the future. A soft starter could very well help but they also tend to be expensive.
 
You want the opposite ends of the spectrum for your shop project, on one hand, lights and small power tools, (which is one type of system), on the other hand, compressors and motors, which is really another type of system, been there. The first small system is easy to set up using 12v, a small battery bank, a handful of panels. The inverter for this would have a minimal idle draw, (my 2500w inverter had 14 watts idling 24/7) and would run lights, fans, charging all my tools, even the occasional heat gun for short periods. For your compressor though, a bigger system is needed, and 24v is about the minimum in order to reduce cabling sizes to manageable. You will need more batteries (really more Ah), and bigger inverters, with related higher idling wattages, so more panels to maintain battery charge, and a bigger controller to supply the needed amps to the batteries. Really two different systems here, so some suggestions given make sense, like run the compressor from the grid or from gasoline if you want a small manageable system, or go for a much larger and more expensive system to run it all.
 
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