Irrigation Pump for Small Farm - Inverter Problems

[kjmclark]

Too much duty cycle for pony pumps.

You guys are funny.

No, it is not haste. I couldn't care less how long it takes to fill the 500 gallons. The smaller pumps will be wearing themselves out doing that much pumping. They're 15min on/hr duty cycle pumps. I already have the 12v model running in the hoophouse to reload the overhead 5gal bucket when the drip irrigation is on in there. Their max flow rate is 360gph, but you'll burn them out fast if you try to run them that much. So really they're 90 gph at most. I'm probably going to end up using one of them, but I'll be going through at least an impeller per month. They're not really designed for this much pumping.

The 1.5hp pump, on the other hand, is designed for significant volume. It's a much better pump for the job, except for that pesky problem of the inrush current. If it had worked, it would have used much less power overall, and I wouldn't have to take it apart each month to replace parts.

What went wrong is that I used the inverters' guides to sizing. I looked at both the Whistler and Cobra inverters for this load, and they both said that pumps often have significant start-up loads, so the inverter needs to have surge capacity of double the rated power of the pump. That turns out to be nonsense. The first I'd heard of LRA was when someone mentioned it here, after several months of designing this system and looking up the power requirements online and in the Realgoods Solar sourcebook.

And *this* is the first place I heard the recommendation of asking no more than 1/5 of the Ah for the battery for continuous running, much less that the cranking amps aren't a useful guide for starting a pump, since for cranking purposes a voltage drop isn't a huge problem. I just happen to have an inverter that the starting current is going through, and the inverter *does* care about low voltage.

See how much I've learned from you guys in a few days?

So, I'm neither interested in burning out little six-spline impeller pumps, nor running a pump so big I can't get the load started. The best option for a soft start I've seen so far is over $400. Anyone know if I can use the first phase of a three-phase thyristor soft starter for a single-phase pump? That would be much cheaper if I can get away with it.

The best option for a pump I've seen are the 1/2hp centrifugal pumps, though the 1/2hp is still out of my power range. However, if inetdog's suggested experiment works at all, I *can* use a 1/2hp centrifugal pump for this. In that case, I'd add a solenoid controlled valve coming off of the 1/2hp pump. The start cycle would be to turn on the inverter, turn on the pump (solenoid valve is closed until energized), wait a few seconds (or until the battery current has stabilized), then open the valve. I can control all of that from the arduino just fine. That's not too far off from what I'm doing in the hoophouse anyway. Then the motor starts under a no-load condition, and only has its impeller to spin, without moving any water. When I open the valve, it will fairly slowly have work to do, but that will be at operating speed for the impeller, not starting from a dead stop.

Doesn't help that I'm right at the edge of the various pump methods. Too much work for an impeller pump, but a bit small for a centrifugal pump. Right in the right range for single phase 115v, but few soft start options for single-phase. Three-phase pumps have soft starters, but I don't have three-phase power, don't want a big pump, and the one I have is single phase.

[inetdog]

[QUOTE=kjmclark;78473
Doesn't help that I'm right at the edge of the various pump methods. Too much work for an impeller pump, but a bit small for a centrifugal pump. Right in the right range for single phase 115v, but few soft start options for single-phase. Three-phase pumps have soft starters, but I don't have three-phase power, don't want a big pump, and the one I have is single phase.[/QUOTE]

If you are very concerned about pump replacement and soft start capability, take a look at the Grundfos AC/DC input pump line. The prices are, at first glance, outrageous. But on second look they are still outrageous. Nobody else makes pumps like them though. The SQFlex line is designed specifically for off-grid DC or generator use, and the power input range is

Power supply to pump 30-300 VDC, PE.
1 x 90-240 V 10%/+6%, 50/60 Hz, PE

. (PE= Ground)
So either DC down to a pretty low voltage or single phase AC. The integral inverter controller feeds three phase power to the pump windings for good torque and efficient operation.

[kjmclark]

Soft Start!

Here we go:

http://www.alliedelec.com/search/productdetail.aspx?sku=70007402

This will soft start single-phase motors, up to 1/2hp at 115vac, and 2hp at 230vac. Grainger and others have them.

Still won't solve my current problem, but it will give me a better starting option for a 1/2hp pump this year and a way to start that bigger pump next year (with some changes, of course). So this year I'll get one of these and a 1/2hp pump and run it all with the current inverter.

Next year when I have more budget, I might add a few more batteries, switch over to 24v, get a 24v->220v inverter, and switch the 1.5hp pump over to 220. Then I'll use this soft start for the 220v system. Or maybe I'll just stick with the 1/2 hp pump for the tank and leave it at that.

Yeah, I've seen those Grundfos pumps, and decided they're for people with more money than me. Seem to be a lot of people in that boat.

[thastinger]

Have you looked at the on-demand RV water pumps or the solar hot water pumps?

[kjmclark]

Kick me now.

Have you looked at the on-demand RV water pumps or the solar hot water pumps?

Solar hot water pumps... Wish someone had mentioned that before. Heck, I might get one of those anyway. No, I hadn't looked at those and the closest anyone else came to it was mentioning the Grundfos pump.

I don't need the high pressure or high temperature features of those, but otherwise they're just about perfect. Fairly cheap, efficient, low start-up load, constant-duty, long life, designed to be run with PV. The only problem I've seen so far is that most of them have *really* low head requirements. But our total head is only about 10', so I'm betting I can find one that will work well.

A quick update while I'm at it. The soft start has arrived, and I'm setting up a test of that with one of those dirt-cheap Chinese 1/2hp water transfer pumps. The good news about the pump is that it has a brass impeller, and when you open up the electronics box, there's a great big, cheap plastic capacitor wired in. So, if I take the capacitor out and have the soft start doing that job, I should be OK (I think - anyone want to correct me?) Yesterday I bought some cheap extension cord to take apart and wire things up at home to try it out. Test results coming soon.

... And if it doesn't work out, I know what to try next ...

[inetdog]

and when you open up the electronics box, there's a great big, cheap plastic capacitor wired in. So, if I take the capacitor out and have the soft start doing that job, I should be OK (I think - anyone want to correct me?)

]

Make sure you know how the motor is wired before you try to change anything. A single-phase soft starter will not necessarily provide the phase shift between the start winding and the run winding. And some motors require a capacitor while running too.

[thastinger]

RV guys who remote camp with smallish gensets use a "hard start" on their AC units to get the compressor motor going. RV AC units are 115 so one of those might also be something to look into.

[kjmclark]

Make sure you know how the motor is wired before you try to change anything. A single-phase soft starter will not necessarily provide the phase shift between the start winding and the run winding. And some motors require a capacitor while running too.

Why am I not surprised that it wouldn't be that easy? Thanks for keeping me from diving in head-first. Any suggestion for where I can look up a diagram for how to figure that out? I understand the concepts, but the details of how to check the windings for a single phase motor are still beyond me.

I'm guessing (since I have to start this with some hypothesis), that this motor is what is described as a "Capacitor Motor" here:

http://www.emt-india.net/equipment_tips/electric_motors/eqp_motor_design.htm

So the capacitor is connected to the starting winding, with a centrifugal switch thrown in. When the rotor is spinning fast enough (probably doesn't take much), the centrifugal switch changes things over to the run winding. How likely is that to be the case, and how would I verify it?

[Naptown]

Not all motors have the centrifugal switch some only use a start capacitor and some have a combiination start and run capacitor (Three or 4 terminals with wires on them is usually a combination capacitor. Only 2 wires on a round capacitor is start only if the capacitor is oval generally it is a run capacitor.

[kjmclark]

Ok, so the capacitor is a perfectly round cylindrical, 18µF capacitor, with two wires coming out of it, connected to hot and a yellow wire. I haven't able to figure out how to take things apart enough to look at the winding connections.

I have fired up the pump, however. I get a running current of ~3 amps (pump claims 370W, so that's about right - no load), with a startup current of ~9 amps. So, if nothing else, the capacitor is keeping the inrush current to only 3x the running current. Is there any way to verify it's a start-only capacitor by test running?

[Naptown]

Correct that is a start capacitor.
You don't want to remove it. It is there for a purpose. Check with the motor manufacturer and see if a soft start kit is compatible with your motor.

[kjmclark]

Check with the motor manufacturer and see if a soft start kit is compatible with your motor.

???

That won't be remotely possible, I think.

1) I don't speak a lick of Chinese. (OK, maybe "gung pow" and "low mein", but I doubt it will get me very far!)
2) There is nothing I can find that says who made the motor.
3) It's a generic 1/2hp electric motor - there could be hundreds of manufacturers of these things in China.
4) Frankly, I suspect they'd say the capacitor *is* a soft start kit.

[thastinger]

Check the A.O. Smith website for the motor, you might find one really close to what you have.

[inetdog]

4) Frankly, I suspect they'd say the capacitor *is* a soft start kit.

Nope, they will tell you that without the capacitor the motor will not start at all unless you spin it by hand. And you will not be able to predict which way it will spin.

Now proper tuning of the size of the capacitor may allow you to trade off between high surge current and low starting torque. But you probably do not want to go there. (Some posters actually have done the experiment and found a precise capacitor value which enabled the lowest inverter drain at startup. But you should understand what you are doing to try that.

A soft starter (paradoxically also called a hard start kit) usually reduces the surge current by reducing the applied voltage or limiting the applied current (same thing really) during the starting process.
A true VFD (Variable Frequency Drive) used for soft starting will generate separately controlled out-of-phase voltages for the start and run windings, and vary both the amplitude and the frequency of those voltages during the startup.

[kjmclark]

Nope, they will tell you that without the capacitor the motor will not start at all unless you spin it by hand. And you will not be able to predict which way it will spin.

Actually, that was a bit of a joke about the difficulties of discussing technical topics with someone who has little knowledge in the topic, using two entirely different languages.

You're talking about what this is talking about:

"Single-Phase Problem
A motor's coils driven by a single AC phase all alternate at the same time, reversing north and south poles in unison. This creates a problem called zero starting torque. While it can run a motor already spinning, it has no "kick" to get a motor to turn from a dead stop. You can start it by spinning it by hand, but who wants to hand-start a vacuum cleaner?

Starting Capacitor and Switch
A capacitor, connected to a separate coil on the motor, creates an alternating electric current ahead of the main phase by 90 degrees. This happens because the current through a capacitor leads the voltage by 90 degrees. During a motor's start-up, a switch connects a capacitor and a special starting coil to the motor. After the motor reaches its operating speed, the switch disconnects the capacitor. If the capacitor remains connected to the motor, it puts a drag on the motor's efficiency."

That helps. I think of capacitors as being used to store energy for rapid release (like a flash bulb capacitor) or for filtering. I've never heard of them being used for phase shifting like that.

So, why *wouldn't* a soft start "provide the phase shift between the start winding and the run winding." Shouldn't there be a switch in the motor that is doing that?