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Old 02-19-2011, 03:09 PM   #1
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Voltmeter Readings for Inverters

After some discussion on another thread about the problem of getting accurate voltmeter readings with Modified Sine Wave inverters, I did a little testing. My results agree with the general comment that AC voltmeters will read low on MSW waveforms, UNLESS the meter is a "True RMS" type. These are generally expensive, but to my surprise the inexpensive "Kill A Watt" meter read "True RMS" even though it's not labeled as such in most places. It is mentioned in the operator's manual.

Results follow:



Digital Scope Waveform "Pure Sine Wave"




Kill a Watt Reading of Pure Sine Wave



Fluke 189 "True RMS" and Simpson 160 Analog Readings of "Pure Sine Wave" - Both read 122 volts also.


**************************************



Scope Waveform of "Modified Sine Wave"




Kill a Watt Reading of Modified Sine Wave



Fluke 189 Reading of Modified Sine Wave

NOW FOR THE READINGS OF NON "True RMS" Meters




Simpson 160 reading 97 Volts




Fluke 77 reading 100 volts.


I have no affiliation with Kill a Watt, but I am just reporting what I have found. Considering all the functions that it performs, I find it to be a very useful meter at a low price.


If anyone else has found an inexpensive DVM that reads "True RMS", please post.
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Old 02-19-2011, 07:42 PM   #2
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I keep telling people.... The less than 30 dollar Kill-A-Watt has over 1,000 worth of meters inside it's case.

There is one caution.. Some time ago another poster on these forums said that the Kill-a-watt would "Burn out" if plugged into a MSW inverter,,, Something about the way they tap the power to run the meter.
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Old 02-26-2011, 07:29 PM   #3
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It may be sensitive to the peak spikes that a MSW unit can produce. (See my MSW scope trace). It's posssible that they can't include MOV protection for some reason related to the measuring circuitry, or that the high harmonic distortion of a MSW causes some problems.

Probably wise to limit its use when the inverter is running.
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Old 03-04-2011, 08:53 AM   #4
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I just purchased the Kill-A-Watt meter shown in the OP. It works fine in my home and in the MH when plugged in, however with just the Xantrex inverter on and the MH unplugged all I get is 0.00 Volts reading. The outlet is hot and runs the coffee pot fine. What am I doing wrong?
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Old 03-04-2011, 11:35 AM   #5
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I have no idea... But then my Xantrex is Prosine (True Sine Wave)
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Old 03-05-2011, 08:18 PM   #6
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Quote:
Originally Posted by Harm View Post
I just purchased the Kill-A-Watt meter shown in the OP. It works fine in my home and in the MH when plugged in, however with just the Xantrex inverter on and the MH unplugged all I get is 0.00 Volts reading. The outlet is hot and runs the coffee pot fine. What am I doing wrong?
Are you plugging a "Load" into the kill-a-watt? If the inverter does not sense a load, it may stay in "standby" and not create any output.
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Old 03-06-2011, 09:19 AM   #7
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Yes, the coffee pot is plugged into the kill-a-watt and works fine running through it but the meter still shows 0.00 volts.
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