fooljoe
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 10:51 am

nsayer wrote:I don't think that will work. You're still pulling current from the hot line that you're not returning to the neutral. That will always trip a GFI.
As far as I know a GFCI can only sense whether there's an imbalance of current between hot and neutral - it has no way of knowing whether the currents on the two wires are actually from different circuits. The idea certainly seems like it'd work in theory, but I have no idea how one could exactly match the current in practice.
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mrrjm
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 11:30 am

I'm driving a Tesla. Sorry to jump on the Leaf forum. This would be a last ditch effort if I was stranded (unlikely) but possible. Yes we are only talking 12 amps. 15 amps if the outlets are 20 amp. I wouldn't even consider bypassing a GFI unless my only other option was to tow the car. Even then I'm sure I could find Non GFI outlets.

nsayer
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 11:31 am

fooljoe wrote:
nsayer wrote:I don't think that will work. You're still pulling current from the hot line that you're not returning to the neutral. That will always trip a GFI.
As far as I know a GFCI can only sense whether there's an imbalance of current between hot and neutral - it has no way of knowing whether the currents on the two wires are actually from different circuits. The idea certainly seems like it'd work in theory, but I have no idea how one could exactly match the current in practice.
But that begs the question. How would you pull current from somewhere else that you could feed back into the GFI neutral without unbalancing *that* source's GFI? And if that source doesn't have a GFI, then why not just use *it* for the quick-220 instead of the GFI source?

fooljoe
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 11:47 am

nsayer wrote:But that begs the question. How would you pull current from somewhere else that you could feed back into the GFI neutral without unbalancing *that* source's GFI? And if that source doesn't have a GFI, then why not just use *it* for the quick-220 instead of the GFI source?
You're not "pulling current from somewhere else", you're siphoning off some power from the existing 2 hots (assuming they're on different legs), stepping the voltage way down, and (somehow) dynamically adjusting the current in this low-voltage "neutral to neutral" circuit so that it matches the current in the high-voltage "hot to hot" circuit that you're using for charging. Of course whether and how you can perform this dynamic adjustment precisely enough and quickly enough not to trip the GFCIs is the big question.
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nsayer
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 11:50 am

Oh, I see. You're counting on the GFI not noticing a voltage difference in the returns.

I don't see why that wouldn't work right offhand, but the whole thing is already a Rube Goldberg machine even before you start down that road... :)

Oh, and the current between the two hots is not static. Your "compensator" would need to vary the return current to keep the two GFIs happy.

EDIT: Oh you said that already. Yeah.

I have my doubts on the 'quick enough' part.

A typical GFI is just a CT coil with the hot and neutral running through it. Any time the current on the CT exceeds the trip current divided by the Te of the coil, the GFI will trip. I can't envision a reaction time fast enough to overcome that arrangement.

johnrhansen
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 12:42 pm

how could the current between the two hots be different... the neutral is not connected. where are the electrons involved going to/coming from?
2013 S model with quick charge package.
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nsayer
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 12:46 pm

[oops - erase erase erase]
Last edited by nsayer on Tue Jul 22, 2014 12:48 pm, edited 1 time in total.

nsayer
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Re: Do It Yourself: 240v from two 120v sources

Tue Jul 22, 2014 12:47 pm

johnrhansen wrote:how could the current between the two hots be different... the neutral is not connected. where are the electrons involved going to/coming from?
No, no, it's that the current from one hot to the other will vary over time, and the current going into each neutral must *exactly* balance it at any given instant.

That's going to be tough to arrange.

eHelmholtz
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Re: Do It Yourself: 240v from two 120v sources

Tue Nov 18, 2014 8:06 am

This past April I posted about EMW's creation which sold for $49 I think. Here's the link: http://www.mynissanleaf.com/viewtopic.p ... 04#p361604" onclick="window.open(this.href);return false; Thank goodness the forum response to this device prompted EMW to remove it from it's for sale list. However, there are still these devices in the wild. A friend from the Orange County LEAF gatherings had this device for use with his EMW juice box. Again this is not for the faint of heart...

Image

nsayer
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Re: Do It Yourself: 240v from two 120v sources

Tue Nov 18, 2014 8:16 am

Yeah, that is just a stunningly bad idea.

I'm building another of my version of this device for some friends who have a (non EV) need for one. I personally would never consider using anything less than the "3 relay" circuit. And that circuit, all built, actually winds up being well north of $100 once you buy all of the parts.

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