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Managing Inverter In-Rush

Managing Inverter In-Rush

Managing Inverter In-Rush

(OP)
I've built a cute little solar fountain- I love the sound of running water, and it complements my wife's award-winning front garden nicely I think- she was lukewarm at first but I think she's grown to like it too, but then again she's always been a tough audience for any of my projects- even the ones she "directs"...

Anyway, the original build started out with a nice simple option for the pump: a little 500 gph 12VDC boat bilge pump. I didn't expect it to last long, but of course they're cheap too. Regrettably, it failed after a couple months of 4-8 hr/d operation, which is obviously too quick.

DC pumps are expensive for some reason, and I'm chronically cheap. I bought a very small brushless DC pump as a test unit, and it too failed- even faster than the bilge pump.

So- the option seems to be 120VAC pond pumps, which are a dime a dozen, offered with warranty, and available at the local Home Despot. I bought one, and found it runs quite well on a little 400W inverter I had laying around. But regrettably, the inverter has a huge inrush current, likely to charge the input caps. While it works just fine when connected directly to my solar-charged deep cycle FLA battery, when connected to the load protection output of my charge controller, the inrush trips the overcurrent protection on the charge controller.

The charge controller I'm using is this one from Morningstar:

https://2n1s7w3qw84d2ysnx3ia2bct-wpengine.netdna-s...

The 10A 12V version.

My original strategy had a 12V timer turning the pump on and off at the right times of day, so it runs for enjoyment when people are around but shuts off at night and during a couple periods during weekdays to give the solar panel a chance to top up and balance the battery. The timer has contacts which are rated for AC, so I can use it on the output side of the inverter. But the problem is how to start up the inverter in the first place? Any time we get a string of dark days in a row and the load control feature of the charge controller kicks in to protect the battery from over-discharge, I'd end up with the inrush problem when it re-starts.

So far, the only idea I've had is putting a pre-charge resistor in series with the inverter inlet. The inverter draws about 2A at 12V when running the pump, and I have to keep the inrush below 10A to keep the charge controller happy. I'd likely have to put a time delay relay in parallel with this resistor to short it once the precharge is done, otherwise I'd be running the inverter at too low an input voltage for it to be happy. But of course the time delay relay is another component and more bother.

Any other suggestions on how to handle this would be welcome!



RE: Managing Inverter In-Rush

Why does the inverter need to get it's power through the charge controller? My cabin has a primitive single panel, charge controller, two 6V batteries and inverter. The inverter is connected directly to the batteries.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Managing Inverter In-Rush

The pre charge concept you describe is used in some of the large commercial inverters used in 48v systems to permit soft starting and would be a good solution.

When doing installations, one of the tools I made for charging the large input capacitors used in inverter and UPS systems, or for final checks before making connections in high energy storage 48VDC plants, was a small tool that used a 120VAC 100w halogen lamp and also had a 12VDC 55 watt halogen lamp. This device was used to first close the circuit using these "resistors", then if all is well, then make the circuit connection.

A time delay relay (a simple rc circuit driving the relay coil could do that) to start the inverter with the resistor in series, then relay shunt it out of the circuit would suffice.

Halogens may or may not work for you or, you if you use them, may need to experiment with the wattage rating, as their cold filament resistance is very low.

RE: Managing Inverter In-Rush

Feed the invertor through a relay controlled by the load control contacts of the charge controller. The charge controller will see the coil current of the relay and the batteries will buffer the inrush.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Managing Inverter In-Rush

(OP)
dgallup, the charge controller has a load interlock which I like to use for all the loads on the system. It protects the battery from over-discharge, shutting off all the loads whenever the battery voltage drops below a certain preset level. The battery was a spendy part of my system (remember how cheap I am!) so I don't want it to croak prematurely. I've only run 1 pair of wires to the front garden where the fountain is located, and it supplies all the loads. I have LED lighting which runs off the same pair of wires with a separate timer to control it.

DanEE- thanks for the suggestions- they're very helpful. I have a bunch of big wirewound resistors to choose from, which will do just fine I think, but only with the relay. If I use a normally open contact on the relay I guess I could use an RC circuit on the coil, probably with a time constant of a half second or so- will take some experimenting. When I've tried that strategy before I got an annoying buzz out of the relay as the RC circuit charged up, but this delay is so short that it may not be such a problem. Fortunately at least this inverter re-starts after its input is interrupted without having to go and push the "on" button again, and its on-state loss with no load is pretty minimal, so I'll likely put the timer on the AC side of the inverter rather than having to pre-charge it every time the timer for the pump turns on.

RE: Managing Inverter In-Rush

(OP)
waross: now that's a great suggestion! So simple, and obvious after the fact too! And I can still make it work with my single pair of wires running out to the garden! I'd lose short-circuit protection on my loads in the field, but the fuse I have in the circuit already will be just fine to handle that.

You get a virtual LPS for that one, and a virtual drink in the Pub too!

RE: Managing Inverter In-Rush

If the inverter is single-purpose and you're not drawing much power from it, I might be inclined to remove some of the input capacitance.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Managing Inverter In-Rush

That charge controller is supposed to have a 65 amp inrush capability, just how much current is the inverter drawing? I agree with MacGyver, maybe reduce the capacitance or just get a smaller inverter. My inverter has all the load and battery voltage protection necessary, no fears hooking it directly to the battery.

You could try just adding a hundred feet of extension cord between the charge controller and the inverter. That worked for a friend of mine trying to start wood working equipment from his generator. Just added enough resistance to slow the rush.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Managing Inverter In-Rush

(OP)
I was kind of counting on the ~ 60 ft of 14 ga speaker wire I have between the charge controller and the point of use to provide some of the necessary resistance. I got a whole spool of the stuff at a garage sale at random a few years ago. I did the testing last night right at the output terminals of the charge controller, so without the benefit of the long wires.

Did I miss that 65A inrush capacity on the spec sheet? Typical...

I haven't measured the inverter inrush except by observing the spark you get when you connect the input alligator clips to the terminals of the battery, which is surprising, though that's typical with DC circuits...It's a 400 W inverter driving a 25 watt motor, but the inverter I have is FREE (picked up at random at a different garage sale a few years ago), so I'll try to make it work. It's a little Chinese toy of a thing, but does have an internal fan which doesn't even bother to turn on to run this 25W motor load. I'm not sure whether or not the 75W toys that you can buy to plug into your car cigarette lighter would even start the little (presumably) shaded pole motor in the pump.

RE: Managing Inverter In-Rush

(OP)
Much ado about nothing, as the long lead wires running between my charge controller and the front porch where the inverter will be installed took care of the inrush. Once I (facepalm) realized that the inrush limit on the charge controller was 65A, not 10A, the problem wasn't as big a deal as I'd thought...Thanks again for your help, all!

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