OpenEnergyMonitor Community

RMS Input in US?

RMS voltage from the 9VAC transformer is reading in emonpi as 220V. Since this is on a US 120V outlet, do I need to calibrate that value to the value of L-N (122.5V) or L-L (245.8V)?

Do you have the US a.c. adapter? Have you set the switch in emonhub.conf - calibration = 110V. Don’t be fooled, that is a logical switch, not a calibration value, so you need it exactly as written; and if you need to finely adjust the calibration , you can still use the number in scales = ....

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Any idea why that value is set to 110V? That hasn’t been standard voltage in the US since the 1930s.

Yeah, it’s a switch value here, so the actual value is not really that meaningful, but still.

My theory is it was confused with the UK standard portable tool voltage of 110 V (actually 55-0-55 V). What I can’t explain is why it isn’t “US” vs “UK” (as it’s a binary value).

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Agree that “calibration” probably wasn’t the best choice of terms here… system type, maybe?

your mains power sure is weird over there… How’d the portable tool thing come about? That reeks of an approach only a government could love.

It adjusts the calibration constant in the front-end sketch, so that’s perfectly sensible. Why it changed it from one preset value to a different preset value is an historical mystery - hopefully to be removed sooner rather than later when the emonPiCM front end software appears.

So I take it that the AC transformer is expected to put out a nominal 9VAC for whatever the nominal input voltage is, whether UK 230V or US110V (it was reading 111V on an actual read of 122.5).

Based on what it was reading (under by 10%) at nominal input voltage, it may be expecting nominal to be 110V and calculating accordingly? I can adjust in the range settings, but the readings do suggest that somewhere in the code it may be incorrectly assuming US power is nominally 110V.

55 V to earth is relatively harmless - compared to the upper limit on the range of our normal domestic supply, which is 254 V. So that’s the reason for it. The tools are fed from portable transformers fed from the normal 240 V. Actually, our supply is a lot more sensible than yours (IMHO :wink: ): we have 240 V single phase with an earthed neutral for domestic and small commercial buildings, and 415 V 3-phase 4-wire for small & medium industrial (i.e. also 240 V L - N). Heavy industrial can be upto 11 kV or even more (I did a project for a mine winding engine connected at 11 kV (3-phase L - L) feeding a d.c. motor that pulled 8 kA to accelerate and 800 V on the armature at top speed).

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No it’s not. The constant in the front end is 130.0, which is the nominal unloaded voltage on the primary that gives 1.0 V at the ADC input, bearing in mind there’s also a voltage divider in there.

The discrepancy will be down to tolerances - 5% on the adapter, 1% (from memory) on the divider, a few more percent on the ADC reference and of course your meter. It looks as if they all stacked up against you. The “Shop” a.c. adapter specification is 12.0 V ±5% on no load, and that’s what the calibration is calculated from.

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I do a lot of (non-electrical) work in the maritime industry (cruise vessels, in particular)… The electrical systems and propulsion motors on those things are insane. Plus they have to provide both US and EU power outlets for the passengers…

So it was initially set to UK voltage and calculating my wattage based on that - even if I had set the scale value to 0.0055. when I set it to US and adjusted scale to 0.011, the values regained a much more normal appearance.

Well, it would be, wouldn’t it? The whole thing was designed in the UK and manufactured in the UK, and no doubt initially intended principally, if not exclusively, for the UK market.

To be honest, I get fed up with this sort of thing: a useless conversion in my calculator.
Screenshot 2021-07-12 22:30:13

1 litre isn’t 0.26 gallons, it’s 0.22. It’s only recently that “US” was added - until then the parochial person who wrote it didn’t know or care that an Imperial gallon existed.