Irrigation Pump for Small Farm - Inverter Problems

[kjmclark]

Hello all,

We put in a irrigation pump for our small farm. The pump works fine, we can run it with our 4000w generator without problems (no grid-tie at the farm).

However, today I finally got our inverter connected, and it kept coming up with an overload error and shutting down.

So, some details. The pump is a Sta-Rite DS3HF - 1.5 HP centrifugal above-ground pump. Our well is very shallow, there's water at about 6' down. So the pump has no problem pumping it, as noted above, with adequate pressure and terrific flow rates for our purposes. The pump manual says it has a maximum current of 20 amps at 120v, 10 amps at 240v. We've been running it off our generator using 120v.

Our PV system is a single 240w panel going into a Tracer MPPT charge controller (yes, I know it could be better). The controller is charging our 4 deep cycle batteries in parallel, so our system is 12v. The batteries are connected together with 2/0 AWG interconnects. The batteries themselves are Tractor Supply Stowaway marine batteries, rated at about 100 amp-hrs and 700 cranking amps each (yes, I know not the best brand).

The inverter is a Cobra 2500w inverter (probably the biggest problem). It's connected to the batteries using Cobra 4 AWG cables, two pairs at 5' long. I connected the cables at the two middle batteries, so not perfect, but should get close to optimal current draw from the batteries.

My test runs were using a not-good-enough extension cord, 50' 14 gauge rated for 15 amps (yes, I know that's a problem, but I doubt it's *the* problem). When we use the generator, we use a 12 gauge, 5' power cord.

The pump *started* to shoot out water but the inverter very quickly indicated a low voltage fault, then changed it to a current overload fault and shut down.

So, I suspect my problems are: 1) too much in-rush current to the pump, 2) trying to run the pump on 120v, 3) trying to run the pump on a too-small consumer-grade inverter, 4) too small cable from inverter to pump (though a safety issue, not part of this problem, really), and 5) maybe too big a current draw from the batteries.

Going from least to most, I'm a little surprised that the inverter first says low voltage. The inverter said the voltage was 13.6v coming from the batteries. Would putting a starting battery in the mix help with this problem?

I'm going to hard-wire the pump to the barn before I'm done, so the not-good-enough cord shouldn't be a long-term issue. Besides, shouldn't that have limited the current draw at the inverter a smidge?

Would some kind of soft-starter help with 1 and 3? I've looked into this, but I want to be able to software-control the starter, and haven't found (yet, only been looking an hour) a soft starter for 120v, 20amps full load, that's 24v controllable.

Which leaves the 120v issue. This pump is selectable to run at 240, and the generator can produce 240 too. I was hoping to stick with 120v for various reasons, including cheaper inverters. Should I give up on that idea?

Thanks for any suggestions!

[inetdog]

Thanks for any suggestions!

The 120 versus 240 is not the problem. It is the 12 volt part. 20 amps at 120 volts will be 200 amps at 12 volts. The starting current for the pump could be twice the nameplate current or more.
For 200A you should be using 2/0 or larger wire to the inverter and although 2 x #4 should be comparable, you will also have a lot of resistance in the terminations and connections. For the purpose of limiting voltage drop, you should be using 600kcmil wires. You may also just be drawing down the battery voltage.

Was the 13.6 measured at the inverter the load or no-load voltage?

For that amount of power, not only should you be using a larger inverter, you should be using a 48 volt (or at least 24 volt) battery bank and inverter.

[Sunking]

You have two big problems.

The rated running power is 2400 watts, not counting Start Up current referred to LRA which can be as much as 6 times FLA or running current. So your inverter is way undersized 6 x 2400 = 14,400 watts. Not say it wil take that much more like a 5 or 6 Kw inverter.

Second huge issue is you are running a TOY voltage of 12 volts, with way undersized batteries. Minimum Amp Hour Capacity @ 12 volts to run a 2500 watt inverter is 1700 AH at 12 volts or a 1200 pound battery. To maintain a 1200 pound battery takes a 1600 watt solar panel.

I would stick with the generator if I were you.

But if you insist on solar here is the minimum requirement to get the job done.

Inverter = 48 volt @ 5000 watts.
48 volt Battery Capacity = 400 Amp Hours
Panel Wattage = 1600 Watts
MPPT Charge Controller = 40 amps

Good luck.

[kjmclark]

Hmm. Not Going to Work.

Was the 13.6 measured at the inverter the load or no-load voltage?

That was no-load. Thanks for both of your replies.

That's one of the problems with solar, you can get your foot in the door at the 12V all the batteries, lower cost inverters etc. work at, only to get your toes slammed when you try to do anything significant.

The funny part is I had my father-in-law, a EE who worked in the electrical power industry (Centerior Energy, he maintained the NE Ohio power grid), who suggested I just stick with 12V for simplicity, since that resulted in far fewer components. I bet he was thinking solar power systems are toys, compared to the stuff he was used to dealing with.

Actually, I didn't mention the duty cycle either. During the week, this pump (or the one I'm going to buy to actually use with my 12V system) will half fill a 1000 gallon water tank once per day. Total head is about 10'. I don't need much of a pump to do that. We wanted to use the big pump since it could fill the tank in a few minutes per day. No problem sticking with the generator on the weekend (we only go out there on the weekend, the tank is to feed drip irrigation lines during the week.)

A soft start isn't an option, right? I haven't found one yet, so I'm assuming it's not.

OK, so there's no way I can afford to upgrade the inverter to 24 or 48V this year. Then I'd need a step-down transformer for the 12V stuff too.

So, how big of a pump could I use with the 2500W inverter and minor changes to the cabling? Should I assume 1/6 of the rated capacity of the inverter? 1/3? (Since the inverter manufacturers claim the inverters can briefly double their rating.) Could I run a 1/2hp pump?

[Mike90250]

.... So, how big of a pump could I use with the 2500W inverter and minor changes to the cabling? Should I assume 1/6 of the rated capacity of the inverter? 1/3? (Since the inverter manufacturers claim the inverters can briefly double their rating.) Could I run a 1/2hp pump?

I'd say 1/4 of the inverter capacity. And your cables have to handle the starting surge for 2 seconds, or the voltage loss will shut down the inverter (again) Calculate the 12V amps, and use cable rated at that amperage with low loss (link in my .sig for cable loss spreadsheet) And the batteries have to be rated for the amps too. At some point their internal resistance will create loss, and the inverter shuts down.

And if your inverter is "mod-sine" inverter, add 20% to all the power ratings, as the non-sine power gets wasted as heat in the motor, more losses.

[kjmclark]

It's only 500 gallons per day...

I'd say 1/4 of the inverter capacity. And your cables have to handle the starting surge for 2 seconds, or the voltage loss will shut down the inverter (again) Calculate the 12V amps, and use cable rated at that amperage with low loss (link in my .sig for cable loss spreadsheet) And the batteries have to be rated for the amps too. At some point their internal resistance will create loss, and the inverter shuts down.

And if your inverter is "mod-sine" inverter, add 20% to all the power ratings, as the non-sine power gets wasted as heat in the motor, more losses.

Let me repeat that I'm going to use this to pump around 500 gallons per day. Sunking's estimates, for example, are for *much* more pumping than we're going to do. The significant pumping, for sprinklers, we'll do with the generator. This is just to top off the 1000 gallon tank everyday, with about 10' of head.

OK, I get that as about 4amps on the pump. Is that "it won't work at all" if I go above that, or "You'll eventually damage some equipment"?

How do I know if the batteries are rated for those amps? I thought I could use the warm cranking amps for that estimate, but I'm *way* above what I need for the 1.5hp pump by that measure, unless 700 cranking amps per battery * four batteries is not 2800 cranking amps. The batteries say amp hour capacity, cold cranking amps, and > 32F cranking amps. The pump is the only thing running (besides an Arduino), so for amp hours capacity I'm fine.

What about a reduced voltage starter? I found this really nice write-up of the locked rotor amps (LRA) that sunking wrote about: http://franklinaid.com/tag/locked-rotor-amps/. That write-up suggests soft starts, particularly reduced voltage starters. I found this reduced voltage starter for single-phase to single-phase, though the best fit is a 230VAC line to the pump: http://www.anaconsystems.com/text/opti_e2.html. I do need to see what kind of control is in the pump, since it's probably incompatible.

One question that all these great suggestions raises is what duty cycle you're all talking about. At this point it's clear to me that I should at least look (next year) to change to a 24V system, add cranking batteries, and get a 24V to 230V inverter that's rated for either 4-6x the load for the pump (@230V) or figure out a soft-start option that will limit the current draw for startup. Frankly, I'd rather get the soft start if it's an option. This is the only thing at the barn that will use anywhere near that current. No compressors and don't want them; 12v power tools which are fine for me; 12v lighting for the bit of lighting we have. I'm not outfitting a cabin or something. It all seems like tremendous overkill to buy all of that equipment to run a 1.5hp pump a few minutes a day, though I get that I need to have capacity for that inrush current.

[kjmclark]

Watch that duty cycle

You have two big problems.

The rated running power is 2400 watts, not counting Start Up current referred to LRA which can be as much as 6 times FLA or running current. So your inverter is way undersized 6 x 2400 = 14,400 watts. Not say it wil take that much more like a 5 or 6 Kw inverter.

Second huge issue is you are running a TOY voltage of 12 volts, with way undersized batteries. Minimum Amp Hour Capacity @ 12 volts to run a 2500 watt inverter is 1700 AH at 12 volts or a 1200 pound battery. To maintain a 1200 pound battery takes a 1600 watt solar panel.
[...]
Good luck.

Let's do a bit of math here from the other side. I need to pump ~500 gallons per day. That 1.5hp pump is about 70 gallons per minute at the head we have. That will take about 7 minutes. So the inverter runs a load for 7 minutes per day. I'm planning to use an Arduino to turn the inverter on, give it 20 seconds or so to start up, turn on a pump till a float in the tank says full, then shut down the pump and shut down the inverter. So really, it's 2500 watts for 7 minutes a day. (The inverter uses a momentary switch remote, which should be pretty trivial to produce a signal for with the Arduino.)

2500 * 7 = 17500 watt-minutes, or 292 watt-hours. 292 watt-hours / 12v is 24 AH, not 1700 AH. So that's off by a scale of 70.

I completely agree that the inrush current is my problem, but I think you're ignoring my duty cycle entirely.

[inetdog]

I completely agree that the inrush current is my problem, but I think you're ignoring my duty cycle entirely.

You cannot ignore the duty cycle when talking about the energy stored in the battery. But at the same time, the duty cycle will not help you maintain the battery voltage for the short term of your heavy load.
Because your pump is the centrifugal type, there are two factors in play:
1. There will be a surge current as the motor starts and comes up to speed.
2. But the power drawn by the pump once it is at speed will be proportional to the flow rate of the water and not just the pressure. If the pump moves no water at all, it will consume less power. A smaller pump would draw less power at full speed, but at the same time the pump you have would draw less power if you choke back the outlet pipe so that it moves water slower. From your description the pump may already be up to speed at the time the inverter finally shuts down.

Anyway, the surge load that a battery bank can handle without dropping voltage too much or being damaged is proportional to the size of the battery bank.
For FLA batteries, a load of C/5 (current in amps equal to the 20 hour AH rating divided by five) is about as high as you want to go continuously if possible.
If you have 4 100AH batteries, you do not want to be drawing more than 80 amps if you can help it. At full load (with a wide open outlet) your pump will draw 200A from the inverter.

Since you cannot get a smaller pump at this time, there are two things you can do:

1. Double check all of your wires and connections to minimize the voltage drop. To do this, measure the voltage at the inverter input and at the battery terminals as the pump is starting. This will give you an idea of where the problem is. If the battery voltage on all 4 batteries drops identically, and is close to the voltage at the inverter, then you just do not have enough battery. (The 700 cranking amps that one battery can produce may be with a battery terminal voltage as low as 9 volts, maybe lower.)

2. Try starting the pump with the outlet valve CLOSED completely. This will not hurt the pump. If it continues to run without dropping the battery voltage too much, open the valve slowly until the inverter shuts down. That will tell you the pumping rate that you can handle with the inverter and batteries you have. Leave the valve closed to a working level and see how long it has to run to refill the tank. If you have enough battery power to run it that long and only run the batteries down by 20% of their capacity, you can live with that for awhile.

[kjmclark]

What pump *can* I run?

Inetdog,

Sorry, that wasn't directed to your comment. I agree with you guys that I can't use that pump for now, and I *am* planning to get a smaller pump for now. I'm trying to figure this problem out for the future, and:

I *really* appreciate all the points everyone is bringing up!

I'm learning a lot fast about batteries, motors, etc. Thanks!

For example, the load rule of thumb you pointed out for lead-acid batteries is an important thing that Mike was getting at. The solar panel will be feeding power into the system at the same time, but it's still not enough to run the whole thing. (I can monitor things from the Arduino and only allow the load when the panel is providing power.) That's a very helpful metric I haven't seen before.

But how *do* cranking amps apply? If the running load is the sum of the running loads of each battery, shouldn't the possible startup load be the sum of the cranking amps? Is there a rule of thumb for how much current I could draw without dropping the voltage below, say 11 volts? (The inverter is designed to conk out at 11.3.)

I'll try the closed-off pump test to see what happens, but I'm more interested right now in what size pump I *can* use. A 1/2hp transfer pump, according to the chart in the Franklin Electric link I posted, will have an LRA of ~65 amps; around 7kW. Still too much.

Mike's recommendation, 1/4 of the rating on the inverter, sounds about right, since 6* for the LRA would then be about 75% of the surge capacity on the inverter. That's ~625 watts = ~3750W @ LRA vs. 5kW. But that same chart shows a 1/2hp pump having a full-load wattage of 670W.

Maybe this pump: http://www.grainger.com/Grainger/DAY...gal-Pump-5PXX9 ? 1/4 hp, 4.5amps?

[Wy_White_Wolf]

...Actually, I didn't mention the duty cycle either. During the week, this pump (or the one I'm going to buy to actually use with my 12V system) will half fill a 1000 gallon water tank once per day. Total head is about 10'. I don't need much of a pump to do that. We wanted to use the big pump since it could fill the tank in a few minutes per day. No problem sticking with the generator on the weekend (we only go out there on the weekend, the tank is to feed drip irrigation lines during the week.)...

This is were your thinking went wrong. Had you taken the oppisite approach and willing to let the pump work slowly and take all day to fill the tank you could have gotten by with a 12V system and a lot less cost. Look into solar direct pumping. Get a larger tank and use it's storage as your battery bank.

http://www.builditsolar.com/Projects/WaterPumping/waterpumping.htm

WWW

[FloridaSun]

Maybe this pump: http://www.grainger.com/Grainger/DAY...gal-Pump-5PXX9 ? 1/4 hp, 4.5amps?

At your stated need of 500gal. @ day at 10 ft lift this pump may better suit your needs.
1/10 HP, 1.6 amp @ 115v, 300gph at 10 ft lift

http://www.grainger.com/Grainger/DAYTON-Utility-Pump-5UXL7?cm_sp=IO-_-IDP-_-RR_VTV70300505&cm_vc=IDPRRZ1

[Sunking]

This is were your thinking went wrong.

Exactly. You want to pull some 200 amps off a battery. That means the minimim battery capacity needs to be 8 hours x 200 amps = 1600 AH.

Use a small DC pump and take all day and you can run much smaller battery and panel wattage. You took the wrong route.

[SunEagle]

Exactly. You want to pull some 200 amps off a battery. That means the minimim battery capacity needs to be 8 hours x 200 amps = 1600 AH.

Use a small DC pump and take all day and you can run much smaller battery and panel wattage. You took the wrong route.

"Haste makes waste" or being in a hurry like a rabbit will cost you more when it comes to a solar pv system. "Slow and steady like the turtle" is better.

[inetdog]

At your stated need of 500gal. @ day at 10 ft lift this pump may better suit your needs.
1/10 HP, 1.6 amp @ 115v, 300gph at 10 ft lift

http://www.grainger.com/Grainger/DAY...&cm_vc=IDPRRZ1

The closer you can come to running the pump for a long time but only when the panels are actually producing enough power that the batteries are only there for starting and stabilizing the voltage, the smaller the batteries you can get away with.

[Mike90250]

The closer you can come to running the pump for a long time but only when the panels are actually producing enough power that the batteries are only there for starting and stabilizing the voltage, the smaller the batteries you can get away with.

This is EXACTLY why my pump is also controlled with a generic sprinkler timer (intermatic clockwork switch) so the pump has to see the float switch enabled, and the timer (11A - 3PM, daylight hours, after the batteries have had a good filling) so the pump essentially runs off the panels, because it being a big load, would flatten the batteries in just a couple hours.

And a big honking inverter to start it up.

Mike