Overloading inverter?

[lukathonic]

I'm setting up a solar system for our shallow well pump. It will mostly be used during power outages (typically less than a day, but you never know), but if it works well I'll just disconnect the AC.

The shallow well pump is currently powered by a single 120v outlet. I have a single 12v dual purpose marine battery (group size 27) hooked up to a 2000w inverter. The battery is topped off by a trickle charger right now - I'll be replacing it with a solar panel later once I have the battery working properly.

My problem is that when the well pump runs, the inverter's alarm goes off. That either means a short or overload. I have a kill a watt electrical plug monitor that shows a draw of around 10amps while it's running.

Is it possible that the inverter is overloaded because the well pump is drawing more than 2000 watts when it first turns on? I've always thought that 1800w was the max on a 15a 120v circuit ...

Thanks for your help.

Luke

 

[sunshine_eggo]

Is it possible that the inverter is overloaded because the well pump is drawing more than 2000 watts when it first turns on? I've always thought that 1800w was the max on a 15a 120v circuit ...

Yes. Electric motors have huge surges, typically 4-10X. Pumps seem to have around 5X. Household electrical systems can handle these surges. It's short enough that the wiring and breakers have no issue with it, but the inverter actually needs to deal with all of it. Lightweight high frequency inverters don't really have a usable "surge," so even if your 2000W claims a 4000W surge, it's for such a short duration, it's not usable for starting a motor.

I have a 3/4 centrifugal pump moving water from tank to tank, and my 2000W inverter couldn't start it. It has a run power of about 1100W, which is probably very close to yours.

You likely need a 2000W low frequency inverter with a large surge capability. They tend to be 3X for 20-30 seconds.

 

[chrisski]

For the cheaper HF inverters, you seem to have to buy them to see if they work. I’ve seen recommendations inverters be 2x to 10x the size of the load.

I do know that kilawatt meters won’t measure the surge you’re looking at. I don’t know what piece of equipment measures surge, but it’s supposed to be expensive.

 

[time2roll]

You really need four+ of those group 27 batteries to properly drive a 2000 watt inverter. Voltage sag will be an issue along with run time.

Also need to connect with #2 wire minimum. 1/0 or 2/0 is far better. I assume the inverter alarm is for low voltage.

That 10 amps AC means 100 to 120 amps are coming from the battery and is a heavy load for what you have.

 

[lukathonic]

Thanks for the replies. Question about the number of batteries

You really need four+ of those group 27 batteries to properly drive a 2000 watt inverter. Voltage sag will be an issue along with run time.

Also need to connect with #2 wire minimum. 1/0 or 2/0 is far better. I assume the inverter alarm is for low voltage.

That 10 amps AC means 100 to 120 amps are coming from the battery and is a heavy load for what you have.

The inverter alarm is either for a short or overload according to the manual. Based on the comment from chrisski, it's likely the surge that's too much. I should also upsize the wires from the battery to inverter.

The pump typically runs for about 30 seconds at a time. Would I still need that many cells for short run-times like this? Thanks again!

 

[lukathonic]

Also, do you have any recommendations for low voltage inverters?

 

[time2roll]

You could put your voltmeter on the inverter 12vdc terminals at start up and run to get an idea of voltage drop.
Most inverters alarm around 11 volts and shut down close to 10.0-10.5 volts.

 

[acdoctor]

If the group 27 is in great condition and fully charged I wouldn’t blame it but, it is being pushed. I have ran a 1000 watt microwave on one with a 3000 watt low frequency inverter. Measure DC voltage at the inverter connections while under the load. I bet you the voltage is going low.

 

[MichaelK]

The proper way to do this if to first determine your pumps inrush current, then plan a system accordingly. The main problem here is you put together a rinky-dink toy solar system with no understanding what what it needs to do. What you are now finding out is that you need a "real" solar system. Here is what you need to do....

Step 1: Purchase a clamp meter with "inrush current" capability. I myself own a Uni-T 216C that is as accurate as my Fluke 274. Use that to determine the inrush draw of your pump. Also determine the running amperage, which is likely to be just 5-10A.

Step 2: Design a system with a battery that is at least 10X the size of your pump draw. Let's say you determine your pump draws 24A at startup, but only 6A while running. The startup watts would be 24A X 120VAC = 2880W, while running would be 6A X 120VAC = 720W. If you pump draws 720W while running, have at least 10X that battery capacity, 720W X 10X = 7200Wh of power. Now, 7200Wh/12V = 600Ah. That is a really BIG battery at 12V. More likely then not, your best option will be to up your system voltage to 24V, so you can reduce battery size.

Step 3: Wire enough solar to get 2X your running wattage. If your pump draws 720W, then install 1440W of panels. Right now that would be about six 240W grid-tie panels, that I am seeing on sale on Craigslist for about 75$ right now. That would run you about 450$.

Step 4: Purchase a low-frequency inverter that can handle your MEASURED inrush current. I have this one. I can tell you that it runs all my motor-driven workshop tools that operate in the same power band as you pump. It works.

Schneider Electric Conext SW4024-120/240 Inverter/Charger - RES Supply

Free Shipping! Schneider Electric Conext SW4024-120/240 Inverter/Charger, 4000W, 120/240VAC, 50/60Hz, 30A Transfer, 24VDC, 90A Charger, RNW8654024

ressupply.com

Keep in mind that these are MY numbers, which might not be the same as your numbers. But, without the clamp meter you have no numbers at all, so start with mine and work your way forward from there.