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AC Current Measurement with Current Transducers

AC Current Current Transducer U3

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#1 Paul.L.

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Posted 17 March 2015 - 04:04 PM

This is in part an email conversation with LabJack support. LJ support comments are quoted. 

I’m interested in a low cost system for measuring the current of 120V/60Hz AC lighting circuits using current transducers and a DAQ. The lighting is fluorescent with electronic dimmers, resulting in a non-sine wave AC pattern. The CT output is 1V/2A of current for up to 20 Amperes, suitable for the U3-HV DAQ. 

An electric utility tool lending library recommended using a Dent power meter/data logger that measures both current and voltage to use to correlate to the output of the CT as converted by the U3-HV ADC. This way I can run the lights from 0 to 100%, find the correlation between the Dent and the U3, and apply the adjustment in post processing of the ongoing monitoring of lighting current. (opt 1)

A second option is to buy current transducers with lower outputs to be able to use the lower ranges of the U3. I think this would improve resolution? LabJack support, is that right? (opt 2)

A third option is to buy Hall Effect transformers. This is a more expensive option. For more on that see this post: https://forums.labjack.com/index.php?showtopic=7068 . (opt 3)

 

Here is the LabJack support comments.

“1.  I am guessing that the sensor produces 10V peak output for 20 Amps, so we are expecting a sine wave that goes from -10 to +10V peak-to-peak.  Sounds fine for AIN0-AIN3 on the U3-HV.  You will use stream mode to collect waveform data:

http://labjack.com/s...3/users-guide/3

2.  Here is a forum topic with a link to the sensor on Amazon, but the cheapest one is now $94:

https://forums.labja...?showtopic=6840

For  more options you want to search for AC current sensors with DC signal output.  Ideally ones that can be powered from 5V, as that is what you can get from a LabJack. 

3.  The Hall Effect sensor still gives you a waveform output like #1. The difference is that I believe they have good high frequency response and thus can accurately transfer distorted waveforms, not just sine waves.”

Related Forum posts I’ve found on this topic:

https://forums.labja...?showtopic=4856

https://forums.labja...?showtopic=4261 

 

Part of the reason for this post was to freshen this topic. I saw posts going back to 2006. Electricity hasn’t changed, but products/prices/options have.



#2 Paul.L.

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Posted 04 April 2015 - 04:13 PM

I have a U3-HV and am looking to match it with a current transformer. This company's http://www.sentranco...ransformers.php Model-LCF is a suitable size and configuration but the AC voltage output options, 100mV, 250mV, 333mV, 500mV, or 1V, are not a good fit with the 0-2.4V, 0-3.6, or +-10V input capabilities of the U3-HV. Should I keep looking or is there a way to fully use the U3-HV input range?



#3 LabJack Support

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Posted 06 April 2015 - 08:26 AM

You have a couple options for bipolar inputs on the U3:

 

    1.  With a low-voltage input set to differential, the input range is +/-2.4V with 1.2mV resolution, but no input can go below -0.3V.  That means in a situation like this where the negative channel is tied to GND you could measure a small bipolar voltage up to +/-250mV or so.

 

    2.  The high-voltage inputs have a +/-10V range with a 5mV resolution.

 

 

Keep in mind that those output ratings might be RMS.  For example, if the output is 250mV RMS, that means if it is a perfect sine the signal swings from -354mV to +354mV.

 

With the sensor you linked, can you use the mA output option and add a resistor such that it gives you +/-10V peak-to-peak?



#4 Paul.L.

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Posted 09 April 2015 - 11:03 AM

From 1. above:

"With a low-voltage input set to differential, the input range is +/-2.4V with 1.2mV resolution, but no input can go below -0.3V". That looks like a contradiction with +/-2.4V ?

 

And, if the "negative channel is tied to GND" isn't this a singled-ended input and the range becomes -250mV to + 2.4V ?

I note that if the Sentran LCF is +- 250mV peak-to-peak then this would be a good fit with the U3. 

 

From 2. above:

This is a nice option. I'm still researching a split-core (clamp-on) CT for 120V/60Hz, with an appropriate output and an accuracy of 1% from 0%-100% of rated current. The load I'll be measuring will vary from 0-100 percent. What would be ideal, and I'm looking, is a DC output in relation to the current. I'll post what I find. 

 

Understood about RMS and peak-to-peak, thank you.

 

I've sent a query to Sentran asking about mA output. 



#5 LabJack Support

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Posted 10 April 2015 - 09:29 AM

 

"With a low-voltage input set to differential, the input range is +/-2.4V with 1.2mV resolution, but no input can go below -0.3V". That looks like a contradiction with +/-2.4V ?

 

It is confusing, but not a contradiction.  A valid example would be if the positive input is at 0V and the negative input is at 2.4V.  Thus both inputs are in the valid range versus GND of -0.3 to +3.6 (note 5 in Appendix A of the U3 User's Guide), and the difference (positive - negative) is -2.4V.

 

This is called pseudobipolar, rather than bipolar (2nd paragraph of Section 2.6).

 

Also see the Differential app note:

 

http://labjack.com/s...l-analog-inputs

 

 

 

 

And, if the "negative channel is tied to GND" isn't this a singled-ended input and the range becomes -250mV to + 2.4V ?

 

Yes, pretty much.  The signal is connected as single-ended, but you are acquiring the reading in differential mode.

 

When the ADC does a single-ended conversion, the conversion result range is 0-2.4 volts, so if you want to get a little bit of negative with a single-ended signal you have to do a differential conversion where the conversion result range is +/-2.4V.

 

So why don't we just always use the +/-2.4V range?  Because if 0-2.4V works, it has better resolution.  The resolution with 0-2.4 is 2.4/4096 = 600uV, whereas the resolution with +/-2.4 is 4.8/4096 = 1.2mV.



#6 Paul.L.

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Posted 16 April 2015 - 10:18 AM

I've done some research on CTs for the U3-HV DAQ device.I was looking for something that required minimal installation cost.  I explored several brands and types; analog out, current out, and DC out, all split core and self-powered. 

The ideal would be DC out, positive only, in a range matching input options of the U3; 2.44V, 3.6V, or 10.3V. 

Analog out is most readily suitable. The SCT model can be built to a specific output. A con is that the output is AC. A wave must be captured at sufficient frequency, then perform calculations to obtain  peak or RMS. The DC output I found have impedances that are too high. The current output CT requires incorporating a board with resistor to create a voltage. This does allow creation of a custom output range. 

I did not explore Hall effect CTs because they usually require power to operate, requiring additional electrician's work, therefore additional cost. Please post if you've seen other options and with specifications. 

 
 

Attached Files



#7 LabJack Support

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Posted 21 April 2015 - 10:10 AM

Good information.  Thanks.

 

Forum topic 7068 linked above has more info about hall effect sensors.

 

With the T7, you can use the FlexRMS extended feature to measure true-rms of a waveform signal.  An easy and convenient way to work with low cost current transformers.





Also tagged with one or more of these keywords: AC Current, Current Transducer, U3

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