Shifting 110v AC mains by 180 degrees

Discussion in 'General Electronics Chat' started by MrSoftware, Jan 14, 2016.

  1. MrSoftware

    Thread Starter Active Member

    Oct 29, 2013
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    Is there a relatively inexpensive (couple hundred bucks) way to shift about 3kW 110v AC by 180 degrees? The load will be variable between 0 and about 3kW.

    This is so I can "upgrade" my home backup generator. Here in the USA our home power is delivered by two 110v lines that are 180 degrees out of phase, and a neutral. Most outlets are 110v outlets, with half of the outlets getting power from each 110v line. Outlets for larger appliances use both 110v lines to give you 220v at the outlet. My current backup generator has outputs just like the utility so it's a direct plug-in.

    My preferred replacement generator (inverter type) only has single phase 110v output, so the dilemma is how do I power both sides of my house using only a single phase source?

    I'm willing to give up the 220v outlets when on generator power, so I figured my two options are; either shift part of the generator output by 180 degrees and power the house as normal. Or, run the same phase to both sides of the house and be sure all 220v appliances are turned off at the breakers, though I'm a little nervous about unforeseen issues with this method. Does anyone have insight on this?
     
  2. crutschow

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    I don't see a problem with running 110V to both sides of house as long as any 220V appliances are disconnected.
    Even if a 220V appliance were left connected, the voltage across them would be 0V since both sides of the 220V would be at the same potential.
     
    Last edited: Jan 14, 2016
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  3. nsaspook

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  4. crutschow

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  5. MrSoftware

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  6. #12

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    nsaspook is more detail oriented than I am, so I vote for crutschow's method. If the power went down right now, I would have that generator single phase connected in 10 minutes. If you have the time and money, the phase shift method is, "proper", but 3KW isn't enough to start an air conditioner. You might run one stove burner on that much power. So, if you can't run squat worth of 240 volt appliances with 3Kw, is it worth the money?

    Hint: They are hooked up to 240 volts because they are relatively huge power loads.
     
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  7. nsaspook

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    True but if you have feeds with a shared 110 neutral normally connected to split phase it's possible to over-current the neutral return with a jump connection by pushing max loads on the hot wires as the neutral currents no longer cancel. For just 3 KW that might not be a problem but it's something the code might question.
     
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  8. nsaspook

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    Any 110 isolation transformer at the correct VA range will work.
     
  9. #12

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    Absolutely! It isn't a dumpster dive likelihood, but there are many ways much cheaper than a transformer with a box around it and blinking lights and switches.
     
  10. MrSoftware

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    I think I see what you mean. If I understand you correctly; assume the house has a pair of 20A 110v phases. There might be two 12ga hot wires in the house, and a single 12ga neutral. No matter how much load the total house has, the single neutral will never see more than 20A net. If all circuits are fully loaded, the neutral will net 0A. If I power both sides with the same phase, then the shared neutral could potentially net 40A max, which would be very bad.

    If houses in the USA are sometimes wired with a shared neutral this way, then there's no telling how my house is wired and it would be safest go to the transformer route and wire it like an autotransformer to get a phase shifted output. My house is old, and was re-wired to have air conditioning added, and behind the panel looks like a boy scout knot tying competition gone wrong, with all the same color wires, so it's not worth trying to figure it out.

    So the hunt begins for a reasonably priced isolation transformer! The generator is good for 3kw. If I can assume that the power factor probably won't be worse than .8, then would a 4kva transformer be enough to be safe in the case that all 3kw go through the transformer?

    Thanks for the info, I'm really glad I asked the question before trying it! :)
     
  11. #12

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    3KW at 120 volts is 25 amps. If the entire load tried to "neutral" on one wire, 12 gauge is a fire hazard. It isn't 40 amps, but it is dangerous to bet your life that you have at least 2 neutrals running current.
     
  12. MrSoftware

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    <edit> I re-read your post. I'm not sure what's inside the house, but yes assuming the neutrals aren't shared sounds like a bad idea, even with only 25A.

    But that brings up a good point. I'll bet the generator specs are 3kw with a power factor of 1, and if the power factor dips then I bet the generator output dips too. I wonder if a 3kva transformer would be sufficient?
     
  13. #12

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    I wish you wouldn't do that. This is a tech site. We can do math. Concealing the truth from us only gets you wrong answers.
     
  14. nsaspook

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    Look for something like this at the correct VA. You can wire it for 120 input with a 240 split-phase output.
    http://www.alliedelec.com/images/products/datasheets/bm/ACME_ELECTRIC/70266999.pdf
     
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  15. MrSoftware

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    I'm sorry about that, I thought that starting with "assume the house has" made it obvious that I was using made up values. The truth is I don't have any idea what size wire is inside the house. It was built back in the 1960's and partially rewired at some point before I owned it, so if at any point Florida code allowed for shared neutral of a size that cannot handle 25A then I would be in trouble. I'll go the transformer route to be safe.
     
  16. tcmtech

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    At 3 KW I wouldn't worry about the transformer or anything else. Just tie your generator feed into one of the 240 feed circuits with each side of that circuit tied to the live line from the generator then tie the generators common and ground lines to the fuse boxes common and ground connection points and be done with it.

    If both sides of the main feed are tied to together there is no need to even shut off the 240 volt equipment. It wont run anyway.

    No need to over complicate the issue worrying about what if's. Just make sure your main utility feed from the meter is disconnected before connecting the generator of course.
     
  17. MrSoftware

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  18. #12

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    Aha! Some facts to work with!

    My house is 1959, Florida. All the original wiring meets code for today. Only the crap installed by an amateur remodeler was improper. That is a reference point for you.
     
  19. MrSoftware

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    Great, thanks! I'll see if I can find an easy code reference, or better maybe an electrician who will give me 30 seconds of time to quiz him about this shared neutral business. :) Side note: I was horrified to find the in-wall 240v lines for my stove/oven are aluminum.

    @tcmtech - I understand what you're saying and I'm tempted to just wire it like that, but until I can rule out the shared neutral thing then wiring it that way would make me nervous. A pair of 15A 110V outlets that share a neutral and are normally on different phases would potentially put 25A (the generator max) on the shared neutral if I maxed the outlets, probably more than the wires typically used for a 15A oulet would handle safely.
     
  20. #12

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    So was I.:(
     
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