Inverter question

[wildbill hickup]

I just purchased a voltage inverter it's a Vector 700 watt modified sine wave. In the instructions it repeatedly says the following," Do not connect the power inverter to household or RV AC distribution wiring."  It also says " Do not connect the power inverter to any AC load circuit in which the neutral conductor is connected to ground." I have one curcuit in my barn that I have been isolating from the rest of the AC in the panel by shuting off the breaker and then hooking up a jumper from my inverter to the hot and neutral to power the lights. After some use my old inverter gave out and I thought it was just because it was old. Two questions, First: Could the way I had hooked it up been responcable for the failure as I was just using the breaker to disconnect the cicuit from the panel?  Second: If I fisicaly disconnected the circuit from the breaker panel both hot and neutral then can I use the "distribution wiring" in the barn to run the lights in my barn? I don't want to burn up another inverter.

[TomW]

Wildbil;

I have my office rigged up so I can plug different walls either into my inverters or an AC outlet near the inverters. It works like a charm. The outlets on the wiring are wired with 3 wires [hot neutral and ground] so there is no problem doing it this way. I basically pulled the Romex out of the breaker panel and put plugs on them. The "normal" wiring is probably wired such that the neutral and ground are tied together at the breaker panel but they are isolated at the outlets.

I feel it is perfectly safe but I am sure there will be other opinions on how I have done it for years.

Cheers.

TomW

[LEXX]

That's a hell of a good idea Tom, I think I'll try that when i get set up.  As for Wildbill, that is almost guaranteed to be your problem, since your ground is bonded to your neutral in the service panel itself.  I would do exactly like tom and pull that loomex right out of the panel and give it a plug for when you have no power for your inverter to invert and otherwise have your inverter right there so you can plug it in to it.  If you don't want to do that because the batteries can't be by your panel, yes, you need to take the neutral off of the neutral bar in your panel as a minimum, and taking your hot out of the breaker isn't a bad idea either, you never know when you will inadvertantly hit the switch POOF and there goes another inverter.

LEXX

[ghurd]

Floating Ground? Simulated Ground? Almost-A-Ground?

They call it something like that. The inverter's outlet ground is faked somehow in the circuits.

The inverter ground is kind of not a real ground, and if the inverter ground is connected to a 'real' ground, and if the battery is grounded, then there are things connected that shouldn't be, and it makes something pop.

A short happens through the fake ground, to the real ground, to the grounded battery, to the negative in the inverter.

Sounds like a bad idea at first, but what ever is plugged in thinks there is a ground, so it is safer for the way 99% of the people will use it (car, RV, boat, etc. where nobody will be driving in a 8' grounding rod).

Honestly can't remember if it was Vector, Statpower/ Port-A-Watz, or possibily another brand.

I do remember it was a problem.

G-

P.S.- Maybe they have to say that anyway, unless it's UL Listed for that application?

[richhagen]

It seems like you could use a double pull, double throw switch or relay to switch both the hot and nuetral from the mains to the inverter.  

With only disconnecting the breaker on the panel, you would be connecting your hot to the inverter, and the nuetral would still be connected to the panel which should be bonded to ground, which would effectively ground the nuetral of the inverter.

Rich Hagen

[ourfarm]

This seems like a pretty serious question.Maybe more lurkers will chime in.

If it is a ground loop problem,Why not try to isolate the battery side ground(also the battery charger(alternator) and frame and leave the original house wiring grounds and neutrals intact for your protection?

I assume that the internal transformer provides internal isolation.

Mike

[Tom in NH]

Hi Bill,

I'm not smart enough to know why, but I can see where connecting your inverter neutral to the power company neutral would spell disaster if power were returning through it from the two different sources at the same time.

I just got my own inverter/battery/solar system working and I wired up separate outlets in the house just like TomW described. It's probably a much safer way should a ground fault or a short occur. If you pull the wire out of the box so there is no connection at all to the powercompany, that wire could be connected to your inverter. I think the reason for the warning in your manual is to keep you from connecting your inverter output to the mains where differences in phase and wave shape will result in things burning up and posing a danger to power company employees who may be working on a line they think is dead. Now I'm curious. I'm going to have to go back and see if there is a similar warning in the manual that I got with my xantrex.

Tom

[wildbill hickup]

Hey Tom,

There is no danger of feedback to the grid as the Main breaker has been removed from the panel. For some reason the folks that owned the house in the past had a separate meter for the barn and we never got it hooked back up. This breaker box is just for experimenting. It is now connected to a lights only circuit in the barn. I feed AE and generator power to the box completely isolated from the grid, however the box is connected to earth ground. I wonder if I disconnected that, then I might be able to used the breaker box for distribution? Right now I just have the wires connected to the inverter, but it would be neater if I could use the box.

How the solar panel production going? My busted ones are still charging away on my LED system, 24/7, they haven't gone out yet!

Wildbill

[chux0r]

My busted ones are still charging away on my LED system, 24/7, they haven't gone out yet!

24/7?  Your solar panels work at night?

[Tom in NH]

Hi Bill,

As long as there is no connection at all to the grid, you should be able to use it with your inverter. My gut tells me the DC ground is not the same thing as the AC ground so I wouldn't try connecting them. You should be ok if your ac neutral is tied to an ground, however. Just don't tie your inverter's chassis ground to it. Someone please correct me if you know better.

Haven't started the building the panels yet. Soon. Glad to hear about yours.

Tom

[nothing to lose]

I was planning a double breaker throw here at the house, this is news to me and I really need to know if that's gonna fry something! I will have to rework my entire off grid plan if so!

Basically I was going to have 2 breakers edge to edge, main 100amp from grid and 50amp or less for inverter. Both connected at the flip to each other and in order for one to turn on it turns off the other, no way for both to be on at the same time.

 We did this with an office trailer for genny power and grid power and it worked extremely well, I figured it would work the same for an inverter too.

The thing is the house breaker box will be wired normally as for grid use and grounded to the trailer frame which is also of course grounded to ground rod.

Also even when the house is on inverter power, the neutral is comming from the outside box at the well. The well will still be on grid with power, something I just thought of as I posted this, glad I read alot here!

So my thoughts here were I have 220v to the well always, to the house when needed, shut off the 2 110 lines at the house main and turn on the 110 with the inverter main. Entire house is only gonna be 120v, all 220V stuff will be at the outside box at the well, and most likely only the well will be 220v anyway.

But all this still leaves the power grids neutral in the house box, and neutral to ground.

[ghurd]

I think what I had to say was lost.

But here's a quicky.  Keep the inverter's ground and neutral seperate from the grid's, and each other.

The inverters chassis can be grounded to a real ground.

There just can't be a connection between the inverter output and battery via ground.

[RC in FL]

Modified sinewave are actually stepped squarewaves.

The battery runs a high frequency (25KHz to 100KHz)switching DC to DC upconverter, usually producing about 150 vdc at its output.

The 150 vdc is taken to a MOSFET output bridge and chopped to produce the "modified sinewave" which is really a short across output plug for about 1/4 cycle, +150 vdc for 1/4 cycle, short again for 1/4 cycle, then reversed, -150 vdc for 1/4 cycle.  The RMS of this is about 120 vac.

Problem is with this output bridge chopping there is no ground common to either side of output plug. On the above described sequence, the first short across the plug occurs by turning on the two top MOSFETS of the bridge tied to +150 vdc supply so both prongs of AC plug are at the 150 vdc potential.  The second short across plug cycle is done by two bottom MOSFETS of the bridge tied to zero volts (ground) of the 150 vdc supply.

Now we get is dependancy on particular inverter.  On the DC to DC upconverter there is a feedback from the 150 vdc output to the switching control on the battery side of the upconverter.  This feedback can be simple resistor feedback, optical link or transformer link like most computer swithing power supplies.  If you have the feed back isolation at least you have the secondary 150 vdc isolated from the battery side ground.  

If you wire the modified sinewave inverter into your breaker panel you run into a problem with the bridge arrangement of the inverter.  If you don't disconnect the neutral from the breaker box ground (white wire) then one side of the inverter plug will be connected to earth ground through your fuse box.  If your inverter and battery are floating, not touching anything, it will appear to work.  If you are grounded (standing on ground or touching fuse box) and touch the inverter case or possibly the battery terminal, you will likely get kicked on your ass.

Only really save way is to put an isolation transformer between inverter and fuse box.