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Wiring inverters in series
By Moogly, Section Homebrewed Electricity Posted on Thu Jul 22, 2004 at 05:56:28 PM MST
Is it possible (and safe) to wire 2 120V inverters in series to get 240V?

Is it possible (and safe) to wire 2 120V inverters in series to get 240V? And if it's possible, could two 500W inverters in series sustain a load of 1000W at 240V?
I know from my previous posting that the T240 transformer would transform 120V to 240V, but I was wondering if this method could be possible too.
Thanks,
Moogly

Wiring inverters in series | 11 comments (11 topical)

Re: Wiring inverters in series (3.00 / 0) (#1)
by mkseps on Thu Jul 22, 2004 at 06:09:56 PM MST

Don't even think about doing what you propose. Unless you have two inverters that have the capability of being syncronized, you will wind up with a variable output that you will not be able to control. You also have the potential problem of circuit isolation or lack of it between the two units. My suggestion -- don't do it. Get the correct inverter to do what you want to do.
Gene

Re: Wiring inverters in series (3.00 / 0) (#9)
by Ungrounded Lightning Rod on Fri Jul 23, 2004 at 03:50:39 AM MST

And if you're using a 240 volt inverter, you might want to look at the Goulds Pumps "balanced flow" system. That uses a digital signal processor to generate a variable frequency three-phase drive for the pump motor, running it continuously and regulating the pressure by adjusting the pump speed to maintain the pressure (with a TINY pressure tank) rather than cycling it to fill a big tank.

The important point for this service is that this device (like several others, by the way) does a "soft start", eliminating the enormous inrush current of switching on a stalled motor. That means you don't need the massively oversized inverter to power it, just to handle the couple seconds of enormous drain every time the pump starts.

(Caveat: While I have such a system I'm still line powered and haven't tried it with an inverter, or checked with Goulds to see if there might be some fly in this ointment.)

Note that if you have a topography where you can put a big water storage tank enough above your house to provide the pressure through gravity, you'd be ahead to fill it up when the sun shines or the wind blows, rather than run a pump off batteries whenever you turn on the tap. That way you store the energy in the water in the tank (which doesn't wear out after a couple hundred cycles) rather than the batteries (which do).

[ Parent ]

Re: Wiring inverters in series (3.00 / 0) (#2)
by RobD on Thu Jul 22, 2004 at 06:31:59 PM MST

I agree. The cheap inverters will work ok in some applications but here's where the better inverters like Sonny boy and Trace show their merrit.

Re: Wiring inverters in series (3.00 / 0) (#3)
by Demetri on Thu Jul 22, 2004 at 06:48:59 PM MST

Something similar can be done, it's called stacking inverters. Only certain inverters that can synchronise their wave forms will do this, trying it with ordinary inverters will get you a lot of magic smoke and a spendy bonfire. Other options, as mentioned, are a 120-1240v transformer or a dedicated 240 volt inverter. I'd suggest the latter option. I use two 240v inverters to run my septic system and well pump, both work well.
Demetri
Uncommon sense required.

Re: Wiring inverters in series (3.00 / 0) (#4)
by Moogly on Thu Jul 22, 2004 at 08:44:06 PM MST

Does Trace sell a 12V to 240V 60Hz inverter that could drive a 3/4hp well pump? How much would it cost? All the 240V models I have found are inverters/chargers and pretty expensive...

Thanks,
Moogly

[ Parent ]

Re: Wiring inverters in series (3.00 / 0) (#7)
by Demetri on Thu Jul 22, 2004 at 11:26:28 PM MST

http://www.donrowe.com/inverters/aims_3000_240V.html

I use two of these, one powers my 3/4 hp well pump and one powers the two 1/2 hp motors in my septic system. One of them also powered my 2 hp air compressor for a bit, though not at the same time as the septic system or well pump. One of these inverters would probably handle both the septic system and the well; these are some serious pieces of equipment.

Demetri
Uncommon sense required.
[ Parent ]

Re: Wiring inverters in series (3.00 / 0) (#10)
by RobD on Fri Jul 23, 2004 at 05:06:18 AM MST

You can get a T240 step up transformer from Trace that will run well pumps from your DR2412. I think it runs about $250.
RobD

[ Parent ]

Re: Wiring inverters in series (3.00 / 0) (#5)
by nack on Thu Jul 22, 2004 at 09:07:35 PM MST

Several of the Trace family can be stacked if you use the interconnect cables from Trace. I think you mentioned having a DR2412 in your previous post. That can be stacked with another DR2412 (US voltages only) and lucky for you, it seems that a lot of dealers are blowing these out for 800-900 bucks.

Re: Wiring inverters in series (3.00 / 0) (#6)
by Junkman on Thu Jul 22, 2004 at 10:40:07 PM MST

I agree with what everyone else said... This is a great way to smoke something. But, I wanted to add one little thing that seems to have been overlooked.
EVEN if you 'COULD' stack two inverters like you mention, you could NOT get BOTH twice the voltage AND twice the watts! Simply can't be done. That's getting more out than you are putting in. In capacitors, for example, you can wire them in series, and double the voltage they can handle... But the capacity is cut in half!
If you wired the same caps in parellel, you could double the capacity, but the voltage they could withstand would not change.
Transformers can be wired this way, and it MIGHT be possible to take the output of your inverters and run them through transformers to get the isolation you would need, and THEN put it all together, but you STILL won't be able to get twice the voltage AND twice the wattage! Simply no way. To double the voltage, discounting losses, you halve the amps, and vise versa. (Ohms law) Others on here will back me up. If you could 'stack' two inverters for higher voltage, the wattage would remain the same, just like the capacitors example I gave. If you could 'stack' inverters for double the wattage, the voltage would remain the same. To do what you wrote would take FOUR inverters 'stacked' parellel/series. Two to increase the voltage, and the other two to double the wattage. Sorry, but Robert Hienlien wrote it best many many years ago:
TANSTAFL: There Ain't No Such Thing As a Free Lunch!
Junkman

Re: Wiring inverters in series (3.00 / 0) (#8)
by Ungrounded Lightning Rod on Fri Jul 23, 2004 at 03:41:24 AM MST

EVEN if you 'COULD' stack two inverters like you mention, you could NOT get BOTH twice the voltage AND twice the watts! Simply can't be done.

Huh?

If you "stack" two inverters (hook the outputs in series) you add the voltage. But each puts out the same current it would if it were running standalone. So the current output of the stack is the same and the voltage is doubled, doubling the watts.

Each inverter also pulls its full current from your battery bank, so by stacking a second one on the first one you pull twice as much current from the batteries. So the energy works out correctly.

(Of course the output voltages only add if you can constrain the inverters to run in sync. Or better yet, run 180 degrees OUT of sync, so you can just tie the neutrals together and use the two hots as the two sides of a center-neutral 220 feed.)

It's the step-up transformer case where you use ONE inverter and the transformer treades doubling the voltage for halving the current, keeping the wattage the same.

[ Parent ]

Re: Wiring inverters in series (3.00 / 0) (#11)
by Junkman on Sat Jul 24, 2004 at 04:15:38 PM MST

<snip> If you could 'stack' two inverters for higher voltage, the wattage would remain the same <snip>
That's what I said.
There is NO way to get 'something for nothing', like increased voltage AND current together, which is what I said. True, using two inverters will indeed use more input power, whatever it's source, that's how he said he was thinking of getting the increased voltage... And the wattages of each inverter will still be whatever they would be alone.
To double the current, (or watts) in theory, anyway, one could do so with two inverters, OR double the voltage, but not BOTH AT THE SAME TIME. Not with only TWO inverters. That's what I said.
I stand by what I said, and don't see where I said anything not correct.
It is right, about transformers, one gets voltage at the expense of current, or the other way 'round.
In the case of two inverters, only one parameter will change... Either voltage or current, so energy is still conserved, just as in the transformer.
One can, and people do, accomplish the same thing with transformers too. Ever hear of transformers with multiple windings? That's one way... And people also 'stack' transformers too... And the same restrictions apply there as well. Not only must the outputs be phased correctly, but assuming two identical transformers, the combined input power will be double what it would be for one.
As you pointed out, the two inverters will each require input the same as if run singly, so again, energy is conserved. 2nd law of thermodynamics... Energy can neither be created nor distroyed, but only converted. Or, as I quoted, TANSTAFEL, There Ain't No Such Thing As a Free Lunch.
Hope I haven't confused anyone even more.
Junkman

[ Parent ]

Wiring inverters in series | 11 comments (11 topical)

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