Author Topic: Should inverter outlets be tested the same way as standard outlets for voltage?  (Read 658 times)

Offline manwithaladder

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Should inverter outlets be tested the same way as standard outlets for voltage?

I recently purchased the Sportsman 1kW inverter generator. When I metered the outlet it appears to be making 120V with L-L.
The neutral and hot are each providing 60V ( Neutral to Hot = 119V, Neutral to Ground = 60V, Hot to Ground = 60V ). I'm using neutral loosely as the part of the plug designated by NEMA that should be neutral, actual neutral should provide 0V.

I metered an inverter I had in the back of my car and hadn't needed to use yet and got similar results; ground to each other part of the plug giving less than expected voltage and only expected voltage coming from Neutral to Hot.

The same test on utility power: Neutral to Ground = 0V, Neutral to Hot = 120V, Ground to Hot = 120V.
Does the inverter change the output once it notices load? If something requires a polarized plug, wouldn't that cause and issue if it was expecting 0V and got 60V?

Offline Alan Georges

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Should inverter outlets be tested the same way as standard outlets for voltage?
No, not generally.  As you meter indicates, inverter outputs are usually two 60 VAC lines driven 180 degrees out of phase, so that there is a net 120 VAC potential across them.

That's all fine, as long as what's plugged in is electrically "floating," that is, not connected to any external ground.  For example, a 2-prong laptop charger plugged into a car's inverter just cares that there's 120 VAC across the prongs.

The situation gets more complicated when there's a 3-prong 120 VAC plug.  The third prong is a safety ground which is assumed to be tied to a ground rod.  So say if you have a washing machine with a metal chassis, the safety ground goes between the machine's chassis and a metal rod in the dirt.  This way, if the hot leg somehow shorts to the chassis, the power is safely shunted away and you don't die.

Look at this drawing of a standard grid power setup:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html
You can see that the neutral and ground wires are tied together at the panel.  That works, because the hot is 120 VAC.  You can also see how that is very different than two "half-hot" 60 VAC lines, and how that kind of tie-in would short out one leg of an inverter.  A loooong time back, there was a thread here about someone doing just that and learning an expensive lesson.

In fact, the situation with an inverter is very similar to the 240 VAC circuit shown in that drawing, which has 2 120 VAC lines driven 180 degrees out of phase, i.e., "push-pull."

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Does the inverter change the output once it notices load?
No.  Or at least, if would be a very unusual inverter if it did.

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If something requires a polarized plug, wouldn't that cause and issue if it was expecting 0V and got 60V?
It could be, but not always.  Remember, the device is just looking for a difference of 120 volts, and doesn't care if it comes as 120 VAC to neutral or as 2 60 VAC lines driven out of phase.  Trouble could happen if the device's chassis ever shorted to the "neutral" leg.  This could happen through bad design, bad repair, or just bad luck.  In the usual grid power setup you'd usually notice, but with an inverter it'd be a 60 VAC short to safety ground.

ps: Stop by at the intro thread and say hi: http://thesurvivalpodcast.com/forum/index.php?board=77.0

Offline manwithaladder

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Thanks Alan ;)

Offline Alan Georges

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Always glad to be of help.   :)