Irrigation Pump for Small Farm - Inverter Problems

[kjmclark]

Soft start works great

OK, so I finally figured out the wiring diagram for the soft start. (I'm sure the rest of you would look at the diagram and say "duh", but I though it was a little unclear.)

So I wired it up as a test. I need to get a proper switch for it, but I also need to test it with relays, since I'm going to control the current to the pump with an Arduino. For the first test, though, I just plugged it into a socket at home.

Before the soft start, this smaller, 1/2hp pump was drawing somewhere around 8-9 amps of inrush current. I couldn't tell because the needle jumped quickly and dropped quickly back down to its run current of ~3amps. Could have been much higher and my meter couldn't move the needle fast enough. Actually, I'd have to assume it went up around 15-20amps for a very short period.

After the soft start, the same pump goes no higher than 6 amps. You can hear that the soft start is slowly ramping up the motor. It's still pretty fast - the pump settles down (no load right now) to its 3amps within 2 seconds. The soft start doesn't signal that it's reached full voltage for about 5 seconds though.

If I remember, I'll come back next year (or someday) when I've switched things to 240v to run the big pump, and report back on how well the soft start works with that.

Just wanted to close the loop in case someone else looks for this thread in the future and wanted to know if it worked out.

Thanks to everyone who replied for all the great advice!

[kjmclark]

Sorry, let me rephrase the question. The softstart is completely outside the electronics of the pump. From the pump's perspective, the softstart is the electrical cord coming into the pump. But the pump must already have an internal mechanism to switch between the start and run windings.

So the question is, what about the soft start, or the power it's supplying to the pump, could cause the pump's mechanism to fail to provide the switch from start to run windings? Is the problem that the pump's very simple electronics might fail to work with the lower voltage/current ramping up to normal? What can I break by trying it?

Also, the soft start manual says:
"The ATS01N1 soft start is compatible with most single-phase motor designs, including split phase induction, capacitor start, and shaded pole."

So it doesn't sound like I'm risking the soft start.

[inetdog]

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?

Unless you are generating the voltage electronically inside the soft start box, the only way you can get a phase shift is with a combinations of inductors and/or capacitors.
If you want a soft start box to provide a phase shift between start and run winding, you must connect both windings directly to the soft start box, and have the necessary capacitor(s) inside the box. A switch cannot provide a phase shift, it can just turn on and off.

[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?

[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.

[thastinger]

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

[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.

[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]

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]

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]

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?

[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.

[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.

[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 ...

[thastinger]

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