AC Ammeter Calibration - Page 1
Forum section: electronics / general

author: kenglaze
author_rank: Newbie
timestamp: January 20, 2021, 07:55:48 am
content: Hi All,I have recently put together an off-grid solar system. I have a number of solar panels feeding a charge controller which manages a 12 VDC bank of batteries. The batteries feed an AC inverter which is connected to an APC Smart Switch AP7752. The smart switch is setup to prefer source A (my inverter) but when that is not available it allows utility power (source B) to supply electricity to part of my house. With this setup any time the inverter is turned on I am using stored power from the batteries and when the inverter is turned off the switch automatically changes to source B (utility power). To automate the system I have the inverter being controlled by a microprocessor so that the inverter automatically turns on at 4 PM when the (time of use) cost of my electricity from the grid doubles! The microprocessor does a number of other functions as well but more pertinent to this post it reads and logs the voltage coming from a AT1-10-000-FT AC Current Transducer indicating how much AC current is being passed through the system. Logging this is important to me for understanding how much power the system is producing daily and over time to calculate essentially the value of the power produced over time. What I have noticed is that when my load is relatively constant down stream, the amount of current being reported by the microcontroller seems to vary with the system state. Most notably running off the inverter I would read 1.6 amps while switching the system over to utility power it would report about 2.1 amps. Clearly there is something affecting the readings being taken by the microcontroller since I know the system load didn't change and this is very repeatable. I have done a number of things to look at my design to figure out what might be the root cause of the issue however every time I check for something new I have to bring down the whole system and it is quite a pain. I will continue to troubleshoot the hardware aspect of this but it still leaves the same lingering question even if I fix it. What is super easy to do is to update the software to add an offset or scaling factor to the current measurement to correct the readings based on the system state. This leads me to my main question.So here it is: What is the best way for an engineer at home without business resources to perform a calibration on an AC ammeter?e.g. I have a Fluke DMM and a KillaWatts meter but it is unclear to me what the best source of truth might be for me to calibrate my systems sensor to. Is there an easy way to produce a known load using a resistor for use with 120 VAC, is there some way to use a VARIAC, or a device like a current shunt that might be used for DC?I have considered buying a clamp meter since it is really convenient and relying on the vendors calibration but the range is typically huge like 1 to 200 amps with a spec accuracy of 2 or 3 % + 0.5 amps which wouldn't help since the delta in current reading I am trying to detect is way inside the error of that type of meter.TLDR:Any friendly advice on a good way to calibrate an AC ammeter would be very much appreciated!Cheers,Ken

author: richard.cs
author_rank: Super Contributor
timestamp: January 20, 2021, 11:20:43 am
content: So the same load measures 1.6 A from the inverter and 2.1 A supplied externally. A small difference would be expected as the voltages won't fully match, but something this large suggests perhaps that your current measurement is being confused by the noisy or distorted output waveform from the inverter. What is the inverter model number? Is it modified sine wave by any chance?

author: bob91343
author_rank: Super Contributor
timestamp: January 20, 2021, 08:15:52 pm
content: Calibration is more involved than simply taking a reading and adjusting or compensating.The waveform is very important. You can calibrate with true rms or just the fundamental or the average, etc.When measuring power line values, the readings will vary if the signals aren't sinusoidal. Random noise has to be accounted for.So the first thing is to decide just what you are calibrating. If you are using a shunt with a voltmeter across it you will need to make connections with contact resistances in mind as well as the burden of the shunt.It's never simple.

author: kenglaze
author_rank: Newbie
timestamp: January 21, 2021, 08:08:48 am
content: thank you!The inverter I am using is a AIMS Power PWRIX120012S 1200 Watt Pure Sine Inverter so it shouldn't be a noisy signal, maybe the utility power is noisy and maybe I should put a scope on both sources to see. All I have currently available is a DSO NANO pocket o-scope which I now see has a max input voltage of 80 Vpp, Sigh.I have run the final output of the system on the lines leading into the house though AT1-10-000-FT Current Sensor (https://www.tequipment.net/NK/AT1-010-000-FT/) so regardless of what power source is being used it is sensing the current being sent into the house. This sensor has some signal conditioning built into it such that it outputs 0 to 10 VDC signal that is linearly proportional to the current range selected by a jumper which in this case is 0 to 10 amps, so 2 volts = 2 ampsWhen I ran into this issue the first time I assumed that somehow the AC source was affecting the DC reference voltage of my microprocessor's ADC. To try to eliminate this I incorporated the use of an external ADC (ADS1115) with the channels configured for taking differential readings so that there wouldn't be any shared grounds like you would have with single ended signals. Alas, the issue persistsUsing a shunt to measure an AC voltage across the shunt and somehow take those readings in a way that can be ultimately funneled into a microprocessor sounds complicated. I think that is one of the reasons I purchased a rather nice inductive current sensor with a signal conditioned DC output. But if there is an easy path there I am not seeing I would love to hear more about it!

author: richard.cs
author_rank: Super Contributor
timestamp: January 21, 2021, 09:38:43 am
content: Assuming you are totally certain there's no change in load under the two conditions then step zero then has to be to compare the DC output of the current sensor, just measured with a DMM, on the inverter and on the external supply. If it's different then your problem relates in some way to what's happening inside the sensor which is not your control and that would suggest it's new-sensor time rather than trying to fix it with calibration.

author: kenglaze
author_rank: Newbie
timestamp: January 30, 2021, 05:44:57 am
content: The sensor should be good. I validated it by doing the following: I setup a Killawatts meter on a socket in the house. From that I ran an "extension cord" where the "hot" wire runs through the induction sensor. At the end of the extension cord I plugged in a number of different devices that would draw a range of currents from about 1 amp to 4 amps. When each device was powered on, and with a fluke DMM reading the output of the sensor I was able to confirm that the amps indicated by the Killawatts meter matched the correct proportional output being read by the DMM. So, I have concluded the sensor is good. This in my mind only leaves the possibility that something bout my microprocessor and ADS1115 is being influenced when the system changes power sources. e.g. the microprocessor's ground reference may be getting biased one way or another when the source is switched.

author: bdunham7
author_rank: Super Contributor
timestamp: January 30, 2021, 06:33:28 am
content: Could you draw and post a quick diagram of the system? Are you saying that the current goes through the same AC transducer/converter/measurer whether the utility or the inverter is supplying power, but the reading changes depending on the source?And to answer your question, you can't calibrate your sensor properly unless you have a low-distortion AC current calibrator or current source + load resistor. Your sensor is 'average-responding, RMS-translated', not True RMS. And even if you do, this is actually a very inaccurate way to measure power in an AC mains systems and you will have built-in errors of 20-30%, perhaps more, because of waveform distortion and load power factor.

author: bob91343
author_rank: Super Contributor
timestamp: January 30, 2021, 07:04:20 am
content: I think your reasoning would be okay if not for the waveform issue. Harmonics are very prevalent and important in situations such as this. You definitely need to put a scope on the two voltages and you probably will get your answer immediately.If your scope isn't up to the task, get one that is. And don't try to use a transformer to bring the voltage down unless it's of instrument quality, not cheap.

author: richard.cs
author_rank: Super Contributor
timestamp: January 30, 2021, 12:07:07 pm
content: Do the same test on both sources of power. My expectation is that in both cases you'll get an output proportional to current, but that the constant of proportionality will be different depending on the power source due to the different waveforms. If that is the case then the only way to make things work with this sensor (a bit hacky though) would be to have the microcontroller know which power source is in use and apply a different multiplier in each case.