Fluke iFlex - Rogowski Coil?

[synapsis]

I have a Fluke 376 with the iFlex i2500 option. I'm looking at trying to measure the current coming out of a capacitor bank during discharge (200,000 uf @ 450V into a coil gun.) Obviously, since the Fluke has a 2500A upper limit with the iFlex connected, I won't be able to measure it with that.

I was snooping around Google and found the mention of using a Rogowsky coil and an integrator circuit to measure the current (and detect backcurrent with a scope.) My question is: Is the iFlex a Rogowski coil? If so, I could just use that instead of building one.

I looked at the cable and the loop just has a plastic lock at the end without a conductor. The meter end has two banana plugs and a third, smaller connector. The V+ banana plug is just blank molded plastic in the iFlex. If I connect a meter (Keithley 2000) to the smaller connector and the COM banana plug on the Fluke, the noise from the leads causes the Fluke meter to register amps. If I just touch the COM with a pin, it registers about 1 AMP. If I touch the smaller connector with a pin, it measures 25-40 amps.

I'd rather not tear apart my iFlex cable (brand new, never used yet), so I was hoping someone had some insight on it.

[ConKbot]

Fluke specs the range as being from 0.1 to 500 Hz, if that's because of the current measuring mechanism, I dont think its a Rogowsky coil, as it should extend to the MHz range, though that could just be an upper limit from the internal amplifier.  Either way, 500 Hz probably isnt enough bandwidth for a coil gun, as you'd be limited to a 700uS rise time.  Are there any patent numbers printed on the case of the device? I couldn't find any online with a quick search. That should tell you more about whats inside.  Maybe it goes  beyond the 500Hz range, but 500Hz is all they guarantee accuracy on. 

See if you can rig it up to a scope, discharge a capacitor through the loop and see what kind of rise time you get.   Also, how big is this going to be? SCR/thyrisytor switched or IGBT? With the former, your bank capacitance might be a bit high and you could run into suckback issues.

[CosPhi]

Fluke specs the range as being from 0.1 to 500 Hz, if that's because of the current measuring mechanism, I dont think its a Rogowsky coil, as it should extend to the MHz range, though that could just be an upper limit from the internal amplifier.

It is 5 to 500 Hz. When I checked the specs I was surprised about the very low crestfector. The say you can measure 1000A with the Fluke 376 clamp meter with a crestfactor from 1.42. That is very low and in "these day's" environment by far not enough.

Crestfactor (50/60Hz)  3  < 500 A
                                    2,5 @ 600 A
                                  1,42 @ 1000 A

Special for a current measure tool should be able today measure everything with a crest factor at least >= 3.

And about your question ... I don't know ... maybe you could try to sniff with the scope between the meter and the coil 

[synapsis]

I have some IGBTs, but they're too small. I might end up making something out of bar stock and a pneumatic actuator.

I looked up the patents at work, but when I got home I noticed the casing said "Patent pending."

I have a smaller capacitor bank I could try it on with the scope and see what I get. Just need more time.

[turbo!]

Use a shunt.

[ATGM]

Apologies for late reply but replying anyway since this discussion has had a lot of views.  The Fluke i2500 is based on a Rogowski coil, as discussed here: <https://www.fluke.com/en-ca/learn/best-practices/test-tools-basics/clamp-meters/inside-flexible-coil-clamp-meters>  .  It probably has its own amplifier/integrator built in, powered from the non-standard plug (warning on the product spec page "For use with any Fluke product that accepts the iFlex™ connector."  A similar FLIR product which is intended for use with general purpose multimeters requires 2 AA batteries; see <https://www.itm.com/pdfs/cache/www.itm.com/ta72/datasheet/ta72-datasheet.pdf> .  The bandwidth limitation probably comes from the powered amplifier/integrator, not the Rogowski coil itself.

[Nivr]

Hey!
I was also wondering about how the iFlex worked and started messing around with it.
Must say I'm stumped...

Can't measure any voltage or current coming out of the two terminals of the iFlex cable, so I figured there must be some circuitry in the coil itself that might need power to come from the clamp meter.
However the 376 isn't providing any power on it's jacks in iFlex mode. To make sure there was no kind of 'handshake' going on between meter and coil, I used wires to connect the two together and hooked up my oscilloscope. Again nothing to be seen but noise on the scope, while the meter was showing the amperage just fine. The only time I could distinguish something was when I turned the load (measured by the coil) on&off, this would bring a quick spike.

Magic? Anyone else has one of these and curious enough to figure out how they work? Any suggestions on how I should try to measure?

https://www.fluke.com/en/learn/blog/clamps/inside-flexible-coil-clamp-meters

[2N3055]

Hey!
I was also wondering about how the iFlex worked and started messing around with it.
Must say I'm stumped...

Can't measure any voltage or current coming out of the two terminals of the iFlex cable, so I figured there must be some circuitry in the coil itself that might need power to come from the clamp meter.
However the 376 isn't providing any power on it's jacks in iFlex mode. To make sure there was no kind of 'handshake' going on between meter and coil, I used wires to connect the two together and hooked up my oscilloscope. Again nothing to be seen but noise on the scope, while the meter was showing the amperage just fine. The only time I could distinguish something was when I turned the load (measured by the coil) on&off, this would bring a quick spike.

Magic? Anyone else has one of these and curious enough to figure out how they work? Any suggestions on how I should try to measure?

https://www.fluke.com/en/learn/blog/clamps/inside-flexible-coil-clamp-meters

Google Rogowski Coil...
That is how it works...

[bdunham7]

The only time I could distinguish something was when I turned the load (measured by the coil) on&off, this would bring a quick spike.

That's how they work, they respond to dI/dt.  The meter or control circuit then integrates that.

[Nivr]

Thanks for the replies, yea I read that too, but that doesn't mean it can only detect a current when you turn loads off & on or change them, does it?

It means it can only detect an AC current since then  dI/dt ~ cos(t)


The coil should produce a time-varying voltage proportional with that ever changing current?
I also used the integrate function on the scope. No discernable difference between no load/no load.

[bdunham7]

Thanks for the replies, yea I read that too, but that doesn't mean it can only detect a current when you turn loads off & on or change them, does it?

No, it just means that dI/dt is much higher when switching than during normal AC current flow.  I don't have numbers for the iFlex, but I'd assume that the signal levels with a small 60Hz current are just too low for you to pick up.  Or perhaps the integration circuitry is built in?