[Nut-upsuser] Tripplite SMX1000LCD input Voltage reporting

[Gene Heskett]

On Thursday 25 February 2021 22:50:05 Yogesh Bhanu via Nut-upsuser wrote:

> Hello all,
>
> I have a Grid Overvoltage issue in my Area 'cause of solar inverter.
> Unfortunately the Tripplite ups underreports the input voltage -- It
> happens with the Display Panel and with SNMP. I 'm aware that with
> SNMP I'm polling the system. So I can never  measure instant input
> Voltage. I'm polling at 3s interval without an issue.
>
> I have tested this with a Wattmeter and a Fluke Digital meter.  The
> input voltage is off by alteast 3 - 5 V.  So when the UPS reports
> 250.5 V or 251 V the input voltage usually is between 253.5 and 255V
> and this is the case when the grid input Voltage is high for 10s at
> which point Solar inverter resets.
>
> My question is,
> Is it possible the UPS Display is not reporting Instant input Voltage 
> ?

Its entirely possible since the invertor may take liberties with the 
waveform delivered, combined with whatever meter you are using's 
faithfullness to true rms response. With the more squareish waves output 
by an invertor, the measurement errors can exceed 10%. Bear in mind that 
most so called rms metering, is only truly accurate at 60 hertz, or 50 
hertz depending on the environment its meant to be sold into.  The only 
true way to find out where the error actually lies, is to look at the 
invertors output with an oscilloscope, but even the eye can be confused 
if the distortion is seemingly well filtered, in which case a comparison 
with the sine wave output of a Good audio test oscillator can be 
informative. See a good explanation for the term rms, which stands for 
the square Root, of the Mean of the of Sum the squares, which is a fancy 
way of saying how much heat is generated by a suitable resistor hooked 
across the src. One massive enough that its temperature is not pulsing 
from the frequency of the applied voltage which will effect its 
resistance. This is very difficult to do in a modern digital multimeter 
because to do it right, they would have to do all that squaring and 
adding for a complete cycle, or about 6000 times a second, then on the 
zero crossing, instantly calculate the square root of all that to 
display it.

Doing that, given the power src for such a meter making those claims is a 
9 volt battery expected to last 6+ months, is patently impossible.

I just bought a new scope, a 4 trace, 350 MHz rated, 2GHz sampler by 
Siglent, that may be able to do that. It certainly has the horsepower to 
do it, but I've no clue if the code has been written to do it.

Your other choices are a bolometer, generally used for signals above 500 
MHz, or calorimetric involving measuring the temp rise of the water 
cooling the resistor, meaning 2 very accurate thermometers, and some 
means of measuring the water flow, all accurate to .1 degree and .1% 
flow.

If its being a PITB, I believe I'd see if the invertor output could be 
turned down 1 or 2%. But if the scope says it outputting a square wave, 
flat across the top and bottom, I'd yell at the invertor maker. Its a 
BBLB invertor, so expect some additional expense for a smarter one.

> Kind Regards,
>
> Yogesh

Good luck Yogesh.

Cheers, Gene Heskett
-- 
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