Author Topic: Rogowski current probe  (Read 1913 times)

Offline flyglas

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Re: Rogowski current probe
« Reply #20 on: September 28, 2020, 12:30:58 AM »
The fault is definitely in the coil.
If you wiggle at the output cable of the coil the output waveform sometimes becomes good. Therefore the integrator works fine.
So there have to be a faulty connection somewhere between the coil and the transition to the cable.
The next step is to open up the heat shrink at the coil and to dig deeper into the underlying plastic case (T-piece).

Offline flyglas

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Re: Rogowski current probe
« Reply #21 on: September 29, 2020, 07:45:22 PM »
The repair was a success!
I think the photos speak for themselves.
« Last Edit: September 29, 2020, 07:48:24 PM by flyglas »

Offline klugesmith

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Re: Rogowski current probe
« Reply #22 on: September 29, 2020, 08:08:22 PM »
Very nice work.
Can we see what the free end of coil looks like?  I guess it's presently clamped in place at the middle of T shape, perpendicular to the output cable and fixed end of flexible coil.
Is there a schematic that shows what those resistors are doing inside the T ?

Offline Hydron

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Re: Rogowski current probe
« Reply #23 on: September 29, 2020, 08:13:43 PM »
Good to hear it was a successful repair, and as expected, not a lot going on in the head.

I am also interested in any light you can shed on the coil construction and termination - it's not completely clear how the termination is done. Are the two termination resistors soldered together to one end of the sensing coil (with the other end being the red wire)? Or are they each going to a separate end (in which case, what does the red wire connect to)? Is the input to the integrator a normal piece of co-ax or is there something else going on?

Sorry for all the questions, the more I see about these the more it looks like a relatively simple DIY job to replicate!

Offline flyglas

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Re: Rogowski current probe
« Reply #24 on: September 29, 2020, 09:12:52 PM »
@klugesmith: I have attached some photos of the coil and the position of the free end inside the t-piece. The cap at the end of the coil is made out of metall.

The coil itself seems to be constructed around the inner part of a coax cable (plastic tube with center conductor).
At the free end the red wire is connected to the center conductor. After many turns the red wire reaches the t-piece.
The termination resistor in the t-piece is connected between the center conductor and the red wire.
The red wire and one end of the termination resistor is connected to the outer conductor of the coax to the integrator.
The second resistor is a series resistor and connected between the center conductor of the coil and the center conductor of the coax to the integrator.

As cable between coil and integrator a triax cable is used. The outer conductors (copper braids) are conntected inside the integrator but not in the coil.
In the coil the outer most conductor is unconnected.

Offline Hydron

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Re: Rogowski current probe
« Reply #25 on: September 29, 2020, 09:50:02 PM »
Thanks, makes a lot more sense with the diagram and explanation. I'm assuming the resistors you are seeing are the Rd and R0 as seen in this paper:
http://hotstreamer.deanostoybox.com/TeslaCoils/Misc/RogowskiCoils-816epe99.pdf
and that the rest of the circuit is similar to the non-inverting integrator they present there.
I suspect the major integration job is done by IC2, with IC10 as an output driver, though without knowing what's on the bottom side of the board it's impossible to tell exactly how it's laid out.

Some more relevant info:
https://gmw.com/wp-content/uploads/2019/02/ias_2000_pem-1.pdf
(I have a big stash of papers, patents etc that I've saved whenever I've gotten interested in these, just never found the time to do much more than some quick and dirty testing though)
« Last Edit: September 29, 2020, 10:02:15 PM by Hydron »

Offline Weston

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Re: Rogowski current probe
« Reply #26 on: September 30, 2020, 05:24:48 AM »
These are so simple inside, its crazy they sell for so much. If anyone is down to collaborate, I would be down to work on an open source design.

Offline klugesmith

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Re: Rogowski current probe
« Reply #27 on: September 30, 2020, 08:39:53 AM »
>>These are so simple inside, its crazy they sell for so much.

Let's talk in a few days after I have rolled my own flexible Rogowski coil, and materially supported your Bee probe.

Many simple products for niche applications, well designed and produced, command lots of money.
It helps if vendor is on Approved Vendor lists.  Selling accessories for six-figure instruments is like options on six-figure automobiles. 

For hands-free, solder-free probe positioning at home:

« Last Edit: September 30, 2020, 08:58:15 AM by klugesmith »

Offline klugesmith

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Re: Rogowski current probe
« Reply #28 on: October 02, 2020, 06:08:07 AM »
Flyglas, I hope this is not hijacking your thread.

Here is picture of a baby's first step.  Expected to grow into an open-able Rogowski coil for measuring currents at 60 Hz, without much concern for instrument risetime and waveform fidelity.
 
Is the commerical coil by PEM wound directly on exposed dielectric of a coaxial cable?  For my project that seems like needless trouble, and I have no coax with a suitable dielectric OD.  Finished coil must fit through a 7 mm gap. So I'm using the jacket from a multi-conductor cable, slipped over a straight steel mandrel.  Later it will slip over a coaxial cable dielectric.  The relatively thin, soft sleeve might help to prevent bunching of the small turns, on inside of the big loop.

This practice winding has 27 turns of 34 AWG magnet wire, made by twirling the mandrel by hand.

How do you all like to strip insulation from fine magnet wire?
« Last Edit: October 02, 2020, 06:12:52 AM by klugesmith »

Offline flyglas

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Re: Rogowski current probe
« Reply #29 on: October 02, 2020, 04:38:51 PM »
@klugesmith: No problem.

I do not exactly know the material of th dielectric inside the coil.
On some of my pictures you can see white material with approximately 4.5 mm diameter.

I like to strip fine magnet wire with a soldering iron using much solder melting the insulation layer away.

Offline Hydron

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Re: Rogowski current probe
« Reply #30 on: October 02, 2020, 05:06:36 PM »
I had an email from PEM stating the following regarding one of their much older probes:
"The Rogowski coil is manufactured as one continuous wire which is wound around a plastic tube, then the conductor returns down the tube centre."
If doing the same, I suspect it will be important to have a close fit of the wire in the tube to ensure it is as centred as possible.

Flyglas - did you get any pictures of the rear of the integrator PCB? Am trying to compare the implemented circuit vs what is written in papers/patents.

Offline flyglas

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Re: Rogowski current probe
« Reply #31 on: October 02, 2020, 08:59:48 PM »
Here is a photo of the backside of the PCB.


Offline davekni

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Re: Rogowski current probe
« Reply #32 on: October 03, 2020, 04:57:35 AM »
I had an email from PEM stating the following regarding one of their much older probes:
"The Rogowski coil is manufactured as one continuous wire which is wound around a plastic tube, then the conductor returns down the tube centre."
If doing the same, I suspect it will be important to have a close fit of the wire in the tube to ensure it is as centred as possible.

I'm wondering if it really matters about centering of the return wire.  If the bandwidth is high enough to need constant impedance of the "coax" of the return wire, then I can see.  But that high a bandwidth seems unlikely.  Otherwise, any field picked up by wandering of the center wire position would be tiny compared to all the turns of the outer coil.

For simple construction, how about some soft rubber high-voltage wire?  That has thick insulation with a center conductor.  Wind it with magnet wire and solder one end to the center HV wire.  My guess is that the biggest error will come from any non-uniformity in winding pitch along the wire.

Yes, as long as the magnet wire insulation isn't the high-temperature (typically crosslinked polyimide enamel of some sort) type, soldering is the best.  Red and green magnet wire is fine.  The gold/copper colored enamel is usually this high-temperature version.  For fine wire as 34AWG, I use careful sanding with fine-grit paper.  Other options include chemical stripping and welding instead of soldering.
David Knierim

Offline plasma

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Re: Rogowski current probe
« Reply #33 on: October 03, 2020, 06:54:25 AM »
Just wondering is the turns right hand rule or left, I'm guessing you would want the two inductor to add?

Offline Hydron

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Re: Rogowski current probe
« Reply #34 on: October 03, 2020, 11:20:37 AM »
In my attempts I've used kynar wire wrapping wire. Easy to deal with for testing unless you want the maximum number of turns on there.

Offline klugesmith

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Re: Rogowski current probe
« Reply #35 on: October 03, 2020, 06:22:34 PM »
Like Dave, I believe centering of the return conductor within the small helix is uncritical.
Its purpose is so the Rogowski coil has zero instead of one net turn encircling flux that's perpendicular to the big loop.  That cancels the largest stray field sensitivity.

Plasma, you don't need to choose between left and right hand twists.  Can put both in the same space, joined to each other at one place and brought out on the other side of the "break".  Never seen that on a flexible RC, but I made two rigid RC's that way.

Both have 60 turns and calculated sensitivity of around 37 nH.   One is wound on a thick flat nylon washer.  The printed circuit version went in waste area of a 6 mm thick board I designed at work about 10 years ago. I have about 10 of them somewhere; does any pulse power experimenter here want one to play with?

I like the kynar idea, to save work in a proof of concept coil.  But my 60 Hz sensitivity estimate (order of 10 mV from 200 A) is for small loop diameter of about 6 mm (constrained by gap between current conductors) and winding pitch of 6 turns per mm (150 per inch). As with close wound 34 AWG magnet wire, more than twice as dense as 30 AWG wire-wrap wire.  Maybe it's premature to contrive a geared lead screw to feed the wire at constant pitch, like I guess is done for some Tesla coil secondaries.  In this case we want the turns a bit looser than touching each other, to allow for bending without bunching.

Mod edit: removed partial duplicate post above this one.
kluge edit: Sorry 'bout that.  Pictures seemed to disappear so I put 'em back. 
Here's link to old 4hv post from which I scraped those thumbnails. It has more pictures, theory, and measurements. https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=82880#post-82880
« Last Edit: October 03, 2020, 08:27:09 PM by klugesmith »

Offline plasma

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Re: Rogowski current probe
« Reply #36 on: October 03, 2020, 06:27:26 PM »
Kludgesmith I've only got a DMM with inductance mH readings, it won't be sensitive enough, if I wrap two inductor on a common core and measure the inductance of one coil and pulse the second coil I should be able to workout the current,if the core is semi permanent.
I'm thinking it will be non linear, but you guys explained its only one turn, so wouldn't be that large.
« Last Edit: October 03, 2020, 06:46:15 PM by plasma »

Offline Hydron

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Re: Rogowski current probe
« Reply #37 on: October 03, 2020, 06:59:57 PM »
There are a bunch of papers out there regarding PCB rogowski coils, including stuff like integration into the board the power devices are mounted on. See also the following Ti reference design, with a differential PCB coil and digital integration (they include altium layout files btw):
https://www.ti.com/tool/TIDA-01063

Offline Weston

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Re: Rogowski current probe
« Reply #38 on: October 03, 2020, 08:46:41 PM »
What sensitivity do you need? It should not be difficult to design a high gain low noise amplifier given the limited bandwidth.

10 mV/ 200 A is probably more than enough sensitivity unless you were planning on feeding it directly into the scope input.


Offline plasma

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Re: Rogowski current probe
« Reply #39 on: October 03, 2020, 10:03:25 PM »
Weston 10mV / 200 then sqrt to give the nH, but the length of the centre wire has X inductance, so its a matrix of two (M1+M2)+(M1+M1)+(M2+M2)sqrt, its not just parrellel inductor, they are coupled.

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Re: Rogowski current probe
« Reply #39 on: October 03, 2020, 10:03:25 PM »

 


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