Feeding an inverter it's own AC

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n1st
n1st Solar Expert Posts: 77 ✭✭✭✭
in Solar Beginners Corner #1
Folks,

My manual transfer switch/breaker panel has as inputs, Inverter output and Generator output. One of the AC circuits from this panel feeds the AC input of my Outback inverter to charge batteries. Not that I would do this on purpose, but it could happen that I forget to shut off the charge circuit when I switch to inverter input. Would feeding the AC output of the inverter back into the AC input of the same inverter cause any problems?
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #2
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    Re: Feeding an inverter it's own AC

    Likely blow it up unless its a very robust inverter ... On the Xantrex XW, it can sense this condition, will fault and shutdown
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  • arkieoscar
    arkieoscar Solar Expert Posts: 101 ✭✭
    #3
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    Re: Feeding an inverter it's own AC

    On the SW's, it won't accept the input on AC1. No drama.
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    #4
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    Re: Feeding an inverter it's own AC

    I doubt this would hurt anything. The inverter would synch to the AC before trying to turn on a few seconds later. Then, the AC in would be connected, via pass through, to the inverter's AC output. When it starts charging, it would drag down the AC output and probably just shut off.


    boB
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: Feeding an inverter it's own AC

    When an Outback sense AC at its input it switches all loads to that input and changes inverter operation to charger. Since there is then no AC output from the inverter, there would be no AC input and the unit will switch back - exactly as though the generator or other AC source had been shut down. There is a programmable delay for switching to AC in, a matter of cycles to allow for generator warm up et cetera.

    So what would happen if you feed an FX's AC Out directly to its AC In? It will sense an 'external' AC source, count to switchover, move the loads (in this case its own input) directly to the input and go into charge mode. Since this will stop AC output (and input) it will think the 'external' source has been shut down and immediately switch back, whereupon the whole process will start up again.

    Probably dangerous to the inverter if allowed to continue for some time, as they are not meant to be turned on and off in this manner over and over again.
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  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    #6
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    Re: Feeding an inverter it's own AC
    Cariboocoot wrote: »

    Since this will stop AC output (and input) it will think the 'external' source has been shut down and immediately switch back, whereupon the whole process will start up again.

    Right. DUh !
    Probably dangerous to the inverter if allowed to continue for some time, as they are not meant to be turned on and off in this manner over and over again.
    There is a time delay so it would just "exersize" the relay. The FX/VFX is very tough.

    Now, backfeeding an inverter output with grid is very hard on it though. The FX/VFX is designed to withstand this but is still hard on them.

    boB
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  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    #7
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    Re: Feeding an inverter it's own AC

    Thanks very much for the information folks.

    I'm thinking for another $15 and some time, it'd be a good idea to draw the charging circuit directly off of the generator output before the transfer switch so there's no confusion.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Feeding an inverter it's own AC
    boB wrote: »
    Right. DUh !

    There is a time delay so it would just "exersize" the relay. The FX/VFX is very tough.

    Now, backfeeding an inverter output with grid is very hard on it though. The FX/VFX is designed to withstand this but is still hard on them.

    boB

    boB you're right; Outbacks are tanks in the world of Inverters. But I still wouldn't do this to it!
    n1st wrote:
    I'm thinking for another $15 and some time, it'd be a good idea to draw the charging circuit directly off of the generator output before the transfer switch so there's no confusion.

    I think that would be confusing and more likely to cause a problem with wiring going astray. The Outback has a built-in automatic transfer switch. Unless you have loads it can't/doesn't handle there's no reason not to let it do its job. GEN -> AC IN, AC OUT -> LOADS. Put a load on the GEN separately and you could get confused and 'loop it back' to the inverter's AC OUT.

    The Outback also has a built-in charging circuit, so what charging circuit are you wanting to tie directly to the generator? Separate battery charger? Just trying to understand what your proposed set up is.
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  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    #9
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    Re: Feeding an inverter it's own AC
    Cariboocoot wrote: »
    The Outback also has a built-in charging circuit, so what charging circuit are you wanting to tie directly to the generator? Separate battery charger? Just trying to understand what your proposed set up is.

    The application is for emergency backup with nothing using the inverter output under normal circumstances. My inital thought was to use a transfer switch with it's inputs being:
    A) generator output
    B) inverter output
    ...and the switches output going to a small circuit panel. Grid AC would be fed to the AC input of the Inverter.

    With the input I've gotten here, I'm thinking that the better configuration is to feed Grid and Gen power into the transfer switch. The switches' output going to the AC input of the inverter. Inverter output going to the small circuit panel. With that configuration, I can make use of grid input most of the time, and when the grid is down, flip the transfer switch from Grid to generator and use the gen when loads are too much for the batteries, or to charge the batteries. Does this sounds like a valid approach?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #10
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    Re: Feeding an inverter it's own AC

    Sounds perfect. Grid+Generator + backup battery/inverter.

    You do have the choice of getting a combination unit (internal transfer switch, inverter + internal charger) or doing it with separate components.

    Before I choose which way to go... I would look at your power requirements and look for a modern PSW/TSW (Pure/True Sine Wave) inverter + a power factor corrected battery charger.

    The TSW inverter is a better choice for clean power and the PF corrected charger can allow you to get more charging current from a smaller genset.

    I am a minimalist in terms of power for an emergency. Smaller battery bank, smaller genset, less fuel burn/store, fewer batteries to purchase/maintain/replace, cheaper components.

    Unless you are planning on running AC or a Well Pump--your largest load will probably be a refrigerator starting load. Typically, that will need a larger inverter--1,000 watts or a bit more. Otherwise, a small inverter will power a few lights, TV, Radio and laptop.

    The AC and Well Pump may be better to run from a separate genset (just run them when needed to preserve fuel).

    If you have or will have a large solar array and Grid Tied power eventually--then looking at the total system--A hybrid GT/Off-Grid setup like the Xantrex XW line would be interesting to look at.

    Once you decide on your exact needs (peak power, average power, length of outage, what you will be powering)--your choice of hardware will probably be a lot easier.

    A 300 watt 12 volt system is much less expensive and complex vs a Hybrid 6kW 48 VDC system with auto-start genset and solar array. But--they both have their place.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
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    Re: Feeding an inverter it's own AC

    Yes, as Bill says you need to determine your loads first. What do you really need to run when the power goes down? This would be things like refrigerator/freezer and electric supply for heating controls on furnace. Then there's the things you might need to run occasionally when power is off like a water pump, which could be powered from the generator on an as-needed basis.

    Knowing this will help you determine the best arrangement for wiring, where primary critical loads could be directly connected through the Outback or have a separate transfer switch if you don't want to keep the inverter active all the time. The generator would have its own transfer switch to power up secondary loads and recharge batteries - which could be either through the inverter or with a stand-alone battery charger which could also be powered from the grid to keep batteries up without having the inverter active. If you use a separate battery charger, you'll need a good one like the Iota with the complete charging stages, not just an 'automatic' type as used for car batteries.
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