Inverter Question For You Electrical Guru's

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Foto-n-T

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So....I've had a Cobra 1,000 watt inverter installed for some time now in my genset compartment.  Originally the intent was to power only my entertainment center while boondocking but as time goes by I've found that I might just like a little power in my garage (toy hauler) where I keep my beer fridge.  I don't intend to power the beer fridge off the inverter for any extended period of time, basically the thought occured to me that I could power the thing while we're in transit off the inverter and negate the need for icing beer down an hour or so prior to our arrival at what ever over-nite spot we choose.  I'd also like to have AC power back there once in a while when we're on batteries.  The other reason is that I'm bored and I just can't leave my mitts off things if I think they might be improved by .0001%

Yesterday while wandering through Lowes I picked up an outlet box, outlet, cover etc. and this morning proceeded to break into the 20amp wiring that I installed to bring power up through the garage floor a few years ago.  The outlet installation is a no-brainer and the fridge runs quite well off the batteries only drawing 110 watts, I am impressed by the low draw.  Here's the rub:

I took my multi-meter, set it to 200 volts AC (lowest AC setting) and checked the voltage and got 86-88 volts.  Hmmm, what's up with this??  The only thing that I can figure is that my multi-meter doesn't appreciate a square wave inverter so it doesn't give an accurate reading.  I ran into this issue when I originally installed the inverter but back then I hadn't discovered this forum so I had nobody to ask.  I plugged an incandescent light fixture into it and it lit the light at the same intensity as the shore power so I guessed it was the multi-meter not liking the sine-wave shape.

By the way, we are on shore power and the input voltage to the inverter is 13.7vdc and it's wired with 24" of if I recall correctly is #2 gauge stranded battery cable, suffice it to say the input voltage is fine and can handle quite a load, I tend to over-build stuff according to my wife.  She muttered something about the last outhouse I built was spec'd to commercial code.

Give me your thoughts on this if you will, I haven't had ANY problems with the entertainment center in the last 4+ years that I've been running this setup.  Also the voltage is the same at the inverter itself so it's not a drop in voltage due to distance which is approximately 40' with 10ga stranded copper.
 

Ned

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Most likely the meter just won't read correctly on a modified square wave inverter output.  Our Kill-A-Watt reads 0 on the inverter but everything works just fine.  A cheap plug in analog meter reads about 130V on the inverter, also wrong.
 

rhoag

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Humm very interesting.  I wonder if you looked at it with an Oscope if you would see a 120V peak to peak signal? 
 

Foto-n-T

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Ned said:
Most likely the meter just won't read correctly on a modified square wave inverter output.
That's kind of what I figured.  I'd assume that if there really was a problem it would have fried my satellite receiver, TV and sound system long ago.  I have found that some appliances with motors DO NOT like square wave AC.  I at one time took my espresso maker with me in my work truck that had an 1,800 watt inverter in the sleeper, I was looking forward to GOOD coffee the next morning only to find out that the pump on the thing would NOT work off the inverter.  I've never run it off of this one but I know that my FoodSaver works just fine on this one but that's about the only electric pump/motor I've tried it with.  I'd hate to cook my beer fridge but I guess this will be one of those hundred dollar experiments.

rhoag said:
I wonder if you looked at it with an Oscope if you would see a 120V peak to peak signal?
Yer showing yer age, I haven't seen one of those since my dad was building home-brew ham radios.  If I could get my hands on one it would be interesting to see the results though.
 

John From Detroit

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Davison Michigan
What is with this?  You have an MSW inverter, that's all.

There are two kind of volt meters, the low cost ones are peak reading, the more expensive ones can read TRUE RMS.

RMS is sometimes defined as "The area under the curve"  It is the area of the space between the sine wave and the base line. (Use the absolute value for negative numbers else you come out with a zero)

With MSW, the peak is lowered but it lasts longer, A peak reading meter, thus reads,,, Low.

A RMS meter, reads true.

Or you could upgrade to a better (True sine) inverter.. but if the one you have is workign for TV and radio.. No need.
 

Ned

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rhoag said:
Humm very interesting.  I wonder if you looked at it with an Oscope if you would see a 120V peak to peak signal? 

If you looked at the waveform for a normal 120V AC power source you would see a sine wave with  peak-to-peak voltage of 340V, 170V positive and 170V negative.  The RMS value of that sine wave is 120V which is what most meters are designed to read.  Any waveform other than a sine wave will read incorrectly unless the meter is a true RMS reading meter.  See this image for a comparison of sine, modified sine, and square waveforms, all of which are 120V RMS but have different p-p values.
 

Foto-n-T

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Jan 1, 2012
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Location
Cody, Wyoming - Sometimes
When I originally installed this inverter I did look at true sine wave inverters but I got this horrible pain in my wallet from just browsing them.  For now, until I manage to fry the inverter I'll stick with my cheapy Cobra as it does the job.

I do appreciate the information you "guru's" have posted.  I take comfort in the fact that there are folks out there who are much smarter than I and have no aversion to using their wisdom for my own benefit.  The difference between ignorance and stupidity is that ignorance is curable, I'm feeling less ignorant by the minute reading your responses, thank you.
 

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