Author Topic: tales of the rogawski coil?  (Read 868 times)

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Offline CopperCone

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tales of the rogawski coil?
« on: March 19, 2018, 02:55:49 am »
Anyone ever build one of these before?

Here is a nice design:
http://www.ee.bgu.ac.il/~kushnero/belles-lettres/Rogowskoil.pdf
You can make it from an old coaxial cable.

Seems to be alot of options for these guys, seems that high bandwidth = lower current and vise versa.
Also alot of options on how to do the loops.
 
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Online coppice

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Re: tales of the rogawski coil?
« Reply #1 on: March 19, 2018, 03:06:09 am »
Rogowski coils are widely used for high current measurements in grid applications. They can be problematic for small currents, as they are hard to screen from surrounding fields. The key positives are that no core means no core to saturate and no core to limit bandwidth.

I've never seen anyone use coaxial cable to make a Rogowski coil. In the slides it looks like they are just using it as a flexible former.
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #2 on: March 19, 2018, 03:28:58 am »
some designs will use the inner wire, basically you wind the coil up, and return the current through the middle of the coaxial cable, and terminate the other end of the cable on the shield, so you can use a regular BNC connector for it without having to mold some kinda mechanical fixture for a coaxial connector.

The routing arrangement has some differences in the pickup behavior of it I think.
« Last Edit: March 19, 2018, 03:30:53 am by CopperCone »
 

Offline ahbushnell

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Re: tales of the rogawski coil?
« Reply #3 on: March 19, 2018, 11:59:41 am »
We use Rogawski coils in pulsed power all the time.  High currents high bandwidth.  We use coaxial cable as the coil form.  Peal back the braid and wind the coil on the insulation and connect the other end of the coil to the center conductor.  We also add a shield of copper foil on the out side and connect to the braid to block efields.  You need to leave a slot in the foil to allow the field to reach the coil.  use tap to insulate to you don't put a shorted turn around the coil.  If you don't use the coax method still bring the wire back along the coil so you don't add a big loop that can pick up external fields. 

Remember the Rogawski coil can not measure DC. 

IEEE paper
http://ieeexplore.ieee.org/document/4084317/
you can find at a library if you don't have an IEEE account
 

Offline Quarlo Klobrigney

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Re: tales of the rogawski coil?
« Reply #4 on: March 19, 2018, 02:06:36 pm »
Looks like new design fodder for the Free Energy or Audiophool people.
Voltage, does not flow, nor does it go.
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #5 on: March 20, 2018, 02:02:52 am »
They are more meant to measure things like capacitor dicharge currents and pulsed loads... Igbt, power mos... I dont think it really has much use in audio unless your are measuring some kinda super massive speaker
 

Online coppice

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Re: tales of the rogawski coil?
« Reply #6 on: March 20, 2018, 02:10:15 am »
Looks like new design fodder for the Free Energy or Audiophool people.
Yeah, the very latest thing from a guy who published his work on current sensing in 1912, and died in 1947.  ;)
 

Offline free_electron

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Re: tales of the rogawski coil?
« Reply #7 on: March 20, 2018, 04:03:40 am »
I made several of those as a flexible pcb . simply wrap them around the wire to sense.
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #8 on: March 27, 2018, 07:44:59 am »
My current problem is shielding this fucking thing.

I made a coil, used 11nF mica cap and series/parallel resistor like in the powerpoint I linked.

I made a fast capacitor bank, consisting of 1.2uF @ 2000V using foil capacitors.

When I discharge it with a reasonable voltage, like 200V, the coil basically triggers just being near the spark. In fact, it triggers when I touch one end of the capacitor with my pliers (without creating a spark!). I noticed the capacitor has alot of pickup, so I shielded it with some copper tape. The coil is not in the circuit.

I was told that coil symmetry is the #1 factor I should be concerned about here.

I will try again with a better wound coil I suppose. I don't know exactly how to shield it though. I saw in the powerpoint that you need to terminate the shield according to the coaxial properties. I thought I could slit open a vinyl tube, coat it with copper tape/conductive epoxy, then curve it around the coil, to get known stable shield dimensions.

What should I focus on here?
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #9 on: March 27, 2018, 11:30:50 am »
also, why does it spike when I touch it with a insulated pair of pliers on one terminal without a discharge? I got a big pulse waveform from doing that (with the capacitors disconnected from the ground referenced PSU).
 

Online dmills

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Re: tales of the rogawski coil?
« Reply #10 on: March 27, 2018, 09:09:33 pm »
Rogowski coils sense dI/dt so really fast events produce large amplitudes at the output, charging up your pliers from a film cap sounds likely to count as a fast event to me. To see the current waveform integrating the output is required (either in some preamp electronics or with your scopes math mode).

Note that you need to wrap the coil right around the target conductor to capture all the flux (and to close the loop to minimise sensitivity to nearby external fields, physically bigger then it needs to be is not a figure of merit.

I made mine on a thin plastic tube of the type used as a spout on a can of air duster, chucked it in the lathe (poked the end into a collet in the tailstock to keep it straight) wound the coil then fed the return down the centre of the tube, covered it in glue lined heatshrink to keep the coil in place then screened with copper tape and kapton, that bit was a faff mainly because of the need to slit the foil.

Regards, Dan.
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #11 on: March 28, 2018, 12:56:04 am »
How much rejection does it have though? The coil can be laying like several inches away from the capacitor bank, without any wires in the loop and the signal it picks up looks the same and has the same amplitude values as when it is over the conducting wire.

I will make one with the center conductor looped back as you describe. I believe it is said to aid in rejection.

I removed the capacitor from my circuit because i thought it might be a source of pickup but this is not the case.


I read from one manufacturer that a way they shield them from large dvdt transients is by using the sheild as a return for the coil (rather then routing it inside) but i have yet to find and construction details or formulas about this design.
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #12 on: March 28, 2018, 02:55:32 am »
Also dan, did you terminate the shield or connect it directly to ground? 

I see in the pdf i linked they recommend terminating the shield from both ends
 

Offline CopperCone

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Re: tales of the rogawski coil?
« Reply #13 on: March 30, 2018, 03:09:42 am »
Do you guys think the loop back through the center conductor will give better performance? I am short on coppper tape and idk if i should make a new rogawski coil that has the compensating turn or if i should shield the one i have.

The compensating turn does not do anything for immunity to high electric fields right?

I would like to use lme600 cable but its a bit thick. All the cables i have are super wimpy, and i think simply putting a loopback wire through a tube without a centering former wol lead to erratic performance
« Last Edit: March 30, 2018, 03:12:02 am by CopperCone »
 

Offline mikerj

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Re: tales of the rogawski coil?
« Reply #14 on: April 01, 2018, 10:19:17 pm »
The uniformity of the winding in a Rogowski coil is very important for the refection of external fields. 

The company I used to work for used Rogowski coils for measuring the traction current harmonics in electric locomotives.  These were powered from a DC third rail, but used electronic drives to power 3 phase motors, so switching noise could be considerable and could interfere with the track circuit block signalling.  They were so sensitive that temperature gradients causing uneven expansion of the former that the coils were wound on could degrade external field rejection.
 


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