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Author Topic: Split-phase current sensing - best practice?  (Read 2816 times)
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Santa Rosa, CA
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Okay, this is something of a follow-up to my previous post, since it looks like Rogowski coils are a dead-end for measuring current accurately in household electrical service.

Considering most current transformers (at least the affordable ones) are sized such that you can only wrap them around a single conductor on the incoming mains, how do you deal with both legs of a U.S. split-phase service? I only have one place to wire a current sensor to an Analog Devices ADE7753.

I read in a blog[1] that you should have a CT around each leg of the split phase, then wire the CT's in series in order to sum the current measurements. Does this seem correct? I'd like opinions on this in order to reach a sensible course of action.

Thanks!

[1] http://draythomp.blogspot.com/p/test-html-code.html
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Unless you have a neutral or ground-fault the current in each phase is the same.
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Whoops, my error - it depends in that supply only supplies 2-phase appliances - in which case what I said is true.  If you assume that the phase voltages are equal and opposite you can wire current transformers in series - multiply the current you measure by 110 rather than 220V.  And get e polarity right (!)
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If you assume that the phase voltages are equal and opposite you can wire current transformers in series - multiply the current you measure by 110 rather than 220V.  And get e polarity right (!)

Thanks, and I gather that if you didn't get the polarity right, you'd see seriously out-of-whack results (i.e. it wouldn't damage anything in particular or cause a fire). I'm pretty sure a voltmeter/ammeter will be able to sort this out before you wire it into an actual circuit.

I intend to do most of the breadboarding and proof-of-concept using "light bulb" loads for 120V and perhaps rigging both a symmetric and an asymmetric 240V load (though I will have to put in a 240V circuit in order to do that, I have a sub panel in my workshop with which I can make that happen).
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I have found the Allegro Hall-effect current sensors far superior to current transformers. They produce an analog output that is perfectly linear with current, are 100% isolated, and you can get AC versions that have an output that is .5 the supply at 0 current.  You can't directly add the output of two devices, however.
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Hi I am new to Arduino and from the UK. Hope you can help. I want to monitor AC current with CT and at a fixed point switch a relay. For example when 1kw is sensed a relay will switch on a water heater via a relay.  I am monitoring the AC power generated by a PV system after the inverter (AC), I only want the water heater to work is the array is out puting 1kw or more. Can anyone help ??
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If you assume that the phase voltages are equal and opposite you can wire current transformers in series - multiply the current you measure by 110 rather than 220V.  And get e polarity right (!)

Thanks, and I gather that if you didn't get the polarity right, you'd see seriously out-of-whack results (i.e. it wouldn't damage anything in particular or cause a fire). I'm pretty sure a voltmeter/ammeter will be able to sort this out before you wire it into an actual circuit.

I intend to do most of the breadboarding and proof-of-concept using "light bulb" loads for 120V and perhaps rigging both a symmetric and an asymmetric 240V load (though I will have to put in a 240V circuit in order to do that, I have a sub panel in my workshop with which I can make that happen).

Hello nearby bay area arduino fan!

I think the two CTs with secondary wired in series is going to give some problems. Household power as you stated around here is split phase 220. There is a L1, L2, and a neutral wire. All the 110 circuit breakers  are usually wired alternately to the L1 and L2 buses and neutral and any 220 loads are wired just to L1 and L2 branches. So if you have a CT on each of L1 and L2 and series their outputs that would give you a total of what all the 110 branches are consuming, however won't the 220volt loads be 'double counted' in that scheme, as the 220 load current will be passing through both CT's?

Lefty
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You cannot double-count current that way.
However, series-wiring the CT's may give incorrect values since currents are out of phase by 1800.
Presumably that would place CT voltage outputs out of phase and canceling.
The way I'd do that is to send each CT to a separate adc and then let Arduino add up the conversions.
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Georgina Ontario
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Maybe digging around on this site will give you some ideas.
http://etherbee.com/products/ECM1240/default.htm

They sell the sensors as well.I am thinking of buying some sensors and doing the rest myself. Sensors and a full wave rectifier should give a voltage proportional to the current.  -- The sensors produce an AC signal.
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Hello nearby bay area arduino fan!

Greetings! I hear we are legion here in the bay area, but all I hear up here in Santa Rosa is the crickets!

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I think the two CTs with secondary wired in series is going to give some problems. Household power as you stated around here is split phase 220. There is a L1, L2, and a neutral wire. All the 110 circuit breakers  are usually wired alternately to the L1 and L2 buses and neutral and any 220 loads are wired just to L1 and L2 branches. So if you have a CT on each of L1 and L2 and series their outputs that would give you a total of what all the 110 branches are consuming, however won't the 220volt loads be 'double counted' in that scheme, as the 220 load current will be passing through both CT's?

I would figure that the 180º phase difference between the legs would work in favor of tying them together. Perhaps parallel instead of serial...?

I wish I had paid more attention in physics...

--R
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I would figure that the 180º phase difference between the legs would work in favor of tying them together. Perhaps parallel instead of serial...?

I wish I had paid more attention in physics...

--R

Well it's not 2 phase power for a 220v device wired directly across L1 and L2, it sees a single phase voltage. My clothes dryer has just three connections on it's power cord, L1, L2, and a safety ground that carries no current unless there is fault/short condition. So it's just a simple 2 wire 220vac circuit for 220v devices and if you have two CTs measuring the same 2 wire circuit, how can adding their outputs together not give twice the true current flow value? Forget the 120 loads for the moment and just think about simple 220v loads.

From: http://en.wikipedia.org/wiki/Split-phase_electric_power
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A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the original Edison 3-wire direct current system.
And to complicate it more, I bet you need to rectify and filter the CT's secondary voltage to actually be able to measure it with a standard micro based system, so all phase information if even useful is lost. Oh what evil webs we weave somtimes.  smiley-grin

Lefty
« Last Edit: March 30, 2011, 01:22:40 am by retrolefty » Logged

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*bump*

Any update on this?

I too am looking for an accurate method of measuring my energy consumption and most options I see are (at best) 85% accurate.  Not that I need absolute accuracy but I might as well find the most accurate method available smiley
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