PV Inverters

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George Stolz

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Can two 120V inverters be configured to supply 240V loads?

I'm working on a standalone 12VDC solar house, which has a 240VAC well pump. Fortunately, I am not dealing with the PV system, but the homeowner mentioned that something along those lines was going to transpire, and it aroused my curiousity. It sounds like it would be difficult to ensure that the inverters would be sync'ed to provide 240V.

This house has been a real treat: It's a post-and-beam structure with "SIP's" panels. Essentially styrofoam with channels in them to allow for wiring. Nothing like fishing every exterior wall on a new house! :)
 

hurk27

Senior Member
Re: PV Inverters

Can two 120V inverters be configured to supply 240V loads?
The answer to this question is no as there is no way to sync the two together connected in this fashion. The inverters themselves has to have an output of 240 volt or come with a output transformer that will provide 240/120 volts.

There are many sites that have infomation on these systems, here's one:
PV system inverters

And if you are doing the LV side hook ups keep in mind about the NEC and UL requirments.
 

petersonra

Senior Member
Re: PV Inverters

actually the answer is yes. there are inverters that can be synched together. you can't just buy two cheapie inverters and hook them together to do this though. they have to be designed for this purpose.

unless you wanted 240V loads, what difference would it make if you did not use multiwire circuits. and PV systems rarely use 240V loads.
 

physis

Senior Member
Re: PV Inverters

It's possible to drive two units with the same clock but I didn't think anyone made one that has an option for it.
 

hurk27

Senior Member
Re: PV Inverters

Bob how do you connect two 120 volt converters to get 240 volts? You can't sync the clocks they use the same power leads? yes you can sync them in a parallel connection but not in a series connection. Unless there are units with isolated clock sync connections? I have never seen one? :roll:

[ July 03, 2005, 07:13 PM: Message edited by: hurk27 ]
 

physis

Senior Member
Re: PV Inverters

It would actually be excrutiatingly easy for a manufacturer to configure the clocks to be synced. But I'm thinking, why not just sell the one someone needs too.
 

George Stolz

Moderator
Staff member
Re: PV Inverters

I think the idea was that the second inverter would only run when necessary, and otherwise only one inverter would be in use. How this is to be accomplished, I have no idea. I think (IMO) that perhaps the HO has just enough knowledge to be dangerous. All work is being done by professionals, he's sub'ed out a PV contractor, and we're taking care of everything back to the "Main Disconnect." The terminations will be made there by the PV dude when he shows up.

The only 240V load is the well pump.

I've intentionally avoided multiwire circuits, since I had no information regarding the inverter(s). I figured "better safe than sorry." :)
 

physis

Senior Member
Re: PV Inverters

By George:

It seems to me that inverters couldn't be sync'ed, just because electricity seeks a path to source, and if you have two sources
Imagine the red and the black legs on a single phase service. Now imagine they're not 180? out of phase with each other. Now imagine that the difference in phase keeps changing, like the blinkers on two different cars at a stop light, at one point they blink at roughly the same time, maybe a minute later they're blinking opposite each other.

Well if the red and black legs aren't 180? out of phase you won't have 240. If they're in phase you'll have 0. With two unsynchronized sources what you'll likely end up with is an AC source that varies from 0 to 240 over some period of (probably) several minutes.
 

George Stolz

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Re: PV Inverters

I understand your point.

For our discussion, focus on just the "a" phase of Ed's diagram:
:D
 

George Stolz

Moderator
Staff member
Re: PV Inverters

Originally posted by ron:
I figure that if the Honda EU series of generator/inverters can be paralleled (syncing the outputs in phase), there has got to be an inverter manufacturer that will sync the outputs out of phase.
Well, that's different. Hooking two 120V generators up to aquire double the amps at 120V is different than hooking two independent generators up together expecting the two to perform as one cohesive 240V generator.

Sam:
I'm working on some really cheesy sine waves for ya George.
Looking forward to it, I'm confused. :)

[ July 03, 2005, 10:44 PM: Message edited by: georgestolz ]
 

physis

Senior Member
Re: PV Inverters

I haven't put amplitude (voltage) values on these in the interest of time. But there are only two values and the waves are proportionately sized. The values would also be RMS but that's not important here so I won't mention it again.

The first one is 120v. and we'll say it represents the black leg of a single phase source.



The next one is 120v. and we'll say it represents the red leg.



There are two important thing to notice.

1] The two waves are aligned exactly opposite each other. They're 180? out of phase.

2] Both of the last two waves are measured (or referenced) against 0 volts. That would usually be neutral. If you're using a meter or an oscilloscope to measure these waves, one probe would be on nuetral (or 0 volts) and the other probe would be on one of the two legs. (Notice I'm not calling them phases).

When you put both waves on the same graph they look like this.



If your using a meter or a single trace (can only read one signal) oscilloscope you will only be able to measure one of these waves at a time. Or, if you , instead of measuring against neutral, put a probe on each leg, you'll measure 240 volts and it will look like this.



The two legs are either side of a single secondary of a transformer. The neutral is a center tap between the two ends of the secondary coil.
______________________________________________________________________________________

What you're talking about with the two inverters is essentially two unrelated 120v AC sources. If you assume that they are in phase and each has a 120v. terminal and a 0v. terminal, you can connect the 120v. terminal of one to the 0v. terminal of the other and that connection would behave exactly like the center tap of the transformer secondary and the other two terminals would behave exactly like either end of the secondary coil.

Or, it would act just like a single phase source with a neutral.

But the phase has got to be maintained!

[ July 03, 2005, 11:42 PM: Message edited by: physis ]
 

George Stolz

Moderator
Staff member
Re: PV Inverters

Originally posted by physis:
If you assume that they are in phase and each has a 120v. terminal and a 0v. terminal, you can connect the 120v. terminal of one to the 0v. terminal of the other and that connection would behave exactly like the center tap of the transformer secondary and the other two terminals would behave exactly like either end of the secondary coil.

Or, it would act just like a single phase source with a neutral.

But the phase has got to be maintained!
That doesn't make sense. Can you elaborate?
 

physis

Senior Member
Re: PV Inverters

I'm having some quality problems but does this make more sense?



Edit: You're right, it doesn't make sense. I'm making it sound like DC. What I meant to do was indicate that the two sources have to be connected out of phase rather than in phase so that the two terminals that are connected together are at opposite voltages.

Edit again: I put a better image in.

Edit again: And if the two inverters aren't synchronized you'll get stuff like this.



In this case you still have 120v. between the middle and either A and B outputs but you'll have very little voltage between the A and B outputs.

Edit again:

How can two inverters comprise a "phase" of 240V? Even if the two were perfectly sync'ed, how would the two ungrounded conductors interact with each other, if they aren't from the same source?
You can connect any sources in either series or paralell. I recommend against connecting out phase AC in parallel or DC in parallel with opposite polarity or mixing AC and DC. But if you connect two equal sources in parallel (in phase or same polarity) you'll get the same voltage and twice the current.

In series it's just like two flashlight batteries. Negative to positive (it doesn't have to be that way but I'll leave the rest out). You'll get the same current and twice the voltage. Put a wire in the middle of the two batteries and you have a sort of DC neutral. It's half the total voltage to either end and the ends are opposite polarity.

It works with AC too in the same way.

The source isn't two unconnected ends of a circuit. The source has impedance just like the load does. the current goes in a circle. When you put two sources in series and connect a load you still have a circuit where current goes in a circle. Only there are two sources of force pushing on the electrons instead of just one.

[ July 04, 2005, 06:04 AM: Message edited by: physis ]
 

George Stolz

Moderator
Staff member
Re: PV Inverters

Okay, that makes more sense. :)

Now, wouldn't the use of one inverter turn the other one on? Some neutral current would be tickling the unused inverter. Like sharing neutrals on a GFI. Now, a GFI kicks if the currents aren't balanced; and inverter might not kick, but wouldn't it get confused and activate?
 

George Stolz

Moderator
Staff member
Re: PV Inverters

Here's a rough layout of the outside:

Here's my idea of how it'll come together:

Anybody see any glaring issues with this?
 

physis

Senior Member
Re: PV Inverters

Some neutral current would be tickling the unused inverter
The Samtronix inverters that I use aren't ticklish. :D

If you have a motor and an open switch connected in series across outputs A and B what's happening that would involve the neutral?

Edit: If the B output is used to make the 240 for the pump, and the pump switch is open, there is no circuit through the B inverter. No neutral concerns. Only the A output inverter is working.

[ July 04, 2005, 04:34 PM: Message edited by: physis ]
 

petersonra

Senior Member
Re: PV Inverters

Originally posted by hurk27:
Bob how do you connect two 120 volt converters to get 240 volts? You can't sync the clocks they use the same power leads? yes you can sync them in a parallel connection but not in a series connection. Unless there are units with isolated clock sync connections? I have never seen one? :roll:
I actually managed to miss the part about the 240V pump in the original post. you would need a transformer or a unit with an external synch input to do this.
 
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