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Rogowski coil construction

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aonomus

Mon Jan 26 2009, 05:30PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

So I've been doing some reading on various members rogowski coils that they've made and found a few PDFs which left me with a few questions... I'm trying to build a rogowski coil to measure current in my CG, and my future cap bank.

1. How many windings and of what gauge? They need to be evenly distributed and from what I've seen most people have used fairly fine stuff, but higher windings generally means larger output voltage range? Target
2. Active vs passive integrators, how hard is it to build one over the other since I'd like to have multiple-uses (mostly CG/pulse usage).
3.

describes target inductance at about 200-300nH, is this a solid number, or can it vary?

Overall I've just seen formulae and pictures of construction, however I don't understand [multivariate] calculus so I get confused when trying to figure out the math behind it all. What I'm looking for is something, anything with some solid values to work from since I can't figure out the calculations :S. Sorry if it seems like I'm digging for answers but I've seen Rogowski coils pop up all over the place with no listed values...

Related links:

Proud Mary

Mon Jan 26 2009, 05:38PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Let us suppose that the output from the coil is a very fast, steep, transient - I wondered how you intended to record that.

aonomus

Mon Jan 26 2009, 05:46PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Harry wrote ...

Let us suppose that the output from the coil is a very fast, steep, transient - I wondered how you intended to record that.

Either use a sound card oscilloscope (first guess), or worse case scenario, borrow a DSO from an acquaintance.

My discharges should be in the range of milliseconds. Sound cards can handle well into 15-20kHz which means a resolution down to 0.0x ms. I'd likely use extra isolation just in case* but I think the sound card scope should work for this purpose.

Dalus

Mon Jan 26 2009, 05:51PM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

Might be handy and easy -->

aonomus

Mon Jan 26 2009, 11:46PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I could never really find the schematic for what TDU made... thanks Dalus.

As for the coil itself, its simply a function of winding it, and calibrating correct?

Steve Conner

Tue Jan 27 2009, 12:03PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If I remember right, the mutual inductance between a Rogowski coil and the conductor it encircles, is just the loop area of a turn, times the number of turns, times the permeability of free space.

You don't need to be Stephen Hawking to work out the calibration coefficient from that and the time constant of your integrator.

aonomus

Tue Jan 27 2009, 12:12PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Stephen Hawking can't move his hands so I suppose he'd have trouble building a rogowski coil -.-

Also I see the term (n x A x Uo) coming up in alot of the forumula (

). Now things are starting to come together (I didn't even know what Uo was (google returned crap about ultima online).

Steve Conner

Tue Jan 27 2009, 03:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, it's too bad that keyboards don't come with Greek letters and subscripts. Try googling "mu nought" instead.

If I remember right, the sensitivity of a Rogowski coil with an integrator is:

(Vout/I) =(1/RC)*(n*A*u0)

and if using a passive "integrator" you should also choose RC such that the time constant is at least 10 times longer than the phenomenon you're trying to measure.

I used them to measure Tesla coil primary currents (over 5kA peak in my OLTC experiments) and I used a passive integrator with a break frequency (1/(2*pi*RC)) one-tenth of the coil's resonant frequency. There are details in the old forum archives somewhere.

aonomus

Wed Jan 28 2009, 01:54AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

So today I bought 20 ft of RG6 coax and spent a little while stripping a little under 12" of sheath, braid, and foil to get to the dielectric (also put the BNC connector on the other end). Tomorrow when I get any free time, I'll wind the entire length with 30AWG magnet wire (leftover wire).

I also bought a metal case, small transformer, and some other bits, I'll have to find 1% resistors and pick decent opamps for the integrator (I'm going to go with active). Any recommended 'budget' opamps? I've looked at a few and they are all $10-20/per

One other question is whether or not I should ground the metal case or not, on one hand I have a AC transformer inside, but on the other hand I don't want to form any freaky ground loops with any equipment. Should the shield of the rogowski coil be grounded as well?

rp181

Thu Jan 29 2009, 02:01AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

cool! now you don't need to double post =p

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