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# Current Measurement, Part 2

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Rogowski Coil
11/15/2013 5:15:02 PM
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I must apologize for not mentioning the Rogowski coil for AC current measurement. I have never had the opportunity to use one, so there is not much that I can add.

For completeness, I guess I should mention the rotating disc that used to be on all power meters. For the life of me I can't remember that name of the effect that produces the rotation and I don't know of any modern application that uses this approach.

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Re: Rogowski Coil
11/15/2013 6:07:56 PM
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I think there is a Rogowski coil at NIST.

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Re: Rogowski Coil
11/21/2013 3:59:15 AM
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The rotating disc motor used in those kWh meters is the so called Ferraris motor (Galileo Ferraris, italian physicist, 1847-1897, was the inventor). Not to be confused with a Ferrari engine ;-)

A good description can be found on the dutch Wikipedia site:http://nl.wikipedia.org/wiki/Kilowattuurmeter

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Re: Rogowski Coil
11/21/2013 3:02:51 PM
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monle

The rotating disc motor used in those kWh meters is the so called Ferraris motor

Thank you. I thought I had known the name, but that doesn't ring any bells. I will try to remember it next time my trivia knowledge is challenged.

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Re: Rogowski Coil
11/27/2013 4:42:02 AM
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@antedeluvian - were you thinking of Eddy Currents?   Electricty meters have a voltage coil and a current coil acting on the disc, which gets eddy currents induced in it.  The interaction between the magnetic fields of the V/I coils and the eddy current in the disc causes it to rotate.

Here's the english Wiki article

http://en.wikipedia.org/wiki/Electricity_meter

I work for an electricity utility and any disc meters that go wrong are replaced with electronic ones.  Divider on the voltage, CT on the current and the rest is done by the processor, displayed on an LCD and downloaded to the meter reader via IR LEDs used as an isolated data link.  Usually a backup battery in case of power failure.  But they rarely fix them when they go wrong, just chuck 'em.

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Re: Rogowski Coil
11/27/2013 9:07:21 AM
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David

were you thinking of Eddy Currents?

yes I was- thanks. I must say though that "Ferraris motor" sounds much swankier.

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Re: Rogowski Coil
12/2/2013 11:25:07 PM
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"The interaction between the magnetic fields of the V/I coils and the eddy current in the disc causes it to rotate."

Which changes the speed of rotation, the fileds, the eddy currents, or both?

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Re: Rogowski Coil
11/22/2013 10:19:41 AM
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ooh, that is a cool piece of trivia right there! I'll have to file that in my "knowledge for no purpose" folder. Thanks!

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Re: Rogowski Coil
11/21/2013 2:59:18 PM
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Maybe that idea could be reduced to a mems device???

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Like the MAX4211, but digital
11/15/2013 5:23:55 PM
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Like the Maxim MAX4211 TI produces a (low side) current and voltage monitor with an SPI output, the LMP92064. Your processor will have to do the multiplication to get to power, but that's what processors are good for!

I believe TI has one or two others, but it will have to wait untll I get back to work on Monday.

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Current Sensor Magneto Resistive
11/15/2013 8:14:47 PM
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Need more bandwidht than a hall sensor or 1-5 Mhz consider magneto resistive.  Same techology used in many harddrive read heads.  Works like a hall sensor in that they need a bias current to read the restance change typicly aranged in a bridge.  Due to the fast response time you can pulse the current to save power.  Use two in oposition to cancel stray magnetic fields.  Note they can be magntized by large fields most have a reset winding.   Made by a number of outfits.  Have fun.

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Re: Current Sensor Magneto Resistive
11/21/2013 4:29:50 AM
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Used a MR sensor a decade ago in a compass chip ,  absolutely awful for anything analog, the scale factor, linearity and offset would change all the time, so needed to reset all the time .

One advantage of MR (for on/off applications), is that the magnetic field sensitivity is in the plane of the chip, where as hall effect is perpendicular to chip. More useful being oriented this way with SMD sensors for shaft encoders,  or current sensors when placed over a conductor. In both these applications a hall sensor would read nominally zero as its aligned with the null in the field.

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Current probes
11/23/2013 6:25:59 PM
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EMC engineers love to use current probes with oscilloscopes. take a look at Doug Smith's

You can also use current probles to inject pulses into wires and cables