Author Topic: Current transformer for narrow spaces  (Read 372 times)

Online klugesmith

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Current transformer for narrow spaces
« on: September 13, 2020, 06:22:07 AM »
I want to measure the current in electric soldering guns, as part of a bigger project.  (Show the serious mismatch between copper replacement tips for low-V designs, and ferrous tips for less-low-V designs).

Traditional clip-on current probes are too big to fit through most soldering tips.   Picture shows an exception. 
Current starts at 225 amps. It becomes less as the element heats up & wants to damage the insulation on probe core.


Let's make a current transformer with extra-narrow core, out of a little control transformer from some MWO.  Don't want to take apart the interleaved E's and I's, so new primary conductors will need to be fished through the unbroken core.

I chose to remove the higher voltage winding, which occupied more than half of the core window.

Said a bad word after sawing one stroke too far, and breaking the low voltage winding. 
That was before I made the measurement with commercial probe in first picture.
Later I found another MWO body part with a similar transformer, and cut it more carefully. First measured the normal secondary voltage: 12 V with a tap at the 3V / 9V point.

Here is first-round test, using a wire loop bigger than original soldering iron element.  Soldering gun is plugged into a variac, and kluge transformer winding is connected to a multimeter on AC 400 mA or AC 10 A range.

CT secondary current went up linearly with indicated loop current, with ratio close to 250 (50 A : 200 mA).

Easy so far, with the current loop under test going around the thick center leg of transformer core.  When it's moved to an outside leg, to fit the Subject of this post, we get about half as much secondary current, and I don't trust the flux division to be linear.

We could stop most of the leakage flux in third leg by cutting through the core, or putting the laminations in a different configuration.  First I tried encircling the third leg with a shorted turn.  First length of bare stranded wire ran out after 20 turns, and restored the secondary current to 93% of original. 10 more turns filled the core window but didn't appreciably change the CT ratio, now 1:262.  It's practically linear up to (briefly) 120 A, and not sensitive to where the test current wire passes through the core window.  A quick test with the "real" soldering tip, on a different gun than we see in top picture, indicated 240 A dropping to 205 A.
« Last Edit: September 13, 2020, 07:21:09 AM by klugesmith »

Offline Weston

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Re: Current transformer for narrow spaces
« Reply #1 on: September 14, 2020, 11:18:14 AM »
Interesting to see the impact of shorting out one leg of the E core with a bunch of turns! I wonder how that compares to cutting an air gap in the unused leg of the core.

60Hz is pretty low frequency, which might make designing an integrator without low enough noise (given the small signal), but this seems like an application where a rogowski coil might be useful.

Online klugesmith

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Re: Current transformer for narrow spaces
« Reply #2 on: September 15, 2020, 05:33:39 PM »
I'm planning to carry on the experiments, including some actual transformer design calculations.

One thing that leaped out is heating of CT secondary.  When measuring a current of 200 A, there will be nearly 200 ampere turns in the secondary (no matter how many turns of any gauge wire).  For a conservative loading of 4 A/mm^2, the secondary winding would need to fill 50 square mm. 

That won't happen with existing windings on little control transformers, though it tips us toward keeping the bigger winding.  We can go above 4 A/mm^2 because the transformer is small (good surface to volume ratio). And farther still if we limit the duty cycle; an exercise is to figure the adiabatic heating rate of copper at 10 or 20 A/mm^2.

Online klugesmith

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Re: Current transformer for narrow spaces
« Reply #3 on: September 20, 2020, 05:31:25 PM »
More progress toward DIY current transformer, for 200+ amps, 60 Hz, able to link soldering gun tips. Among the most challenging are the early Wen units, which work best but are hard to find parts for. Ferrous metal tip needs more volts ( still < 1 ) and fewer amps than all-copper ones.

First a digression.  How about a coreless sensor, inspired by Weston's "little bee" probe? https://highvoltageforum.net/index.php?topic=1154.msg8316#msg8316  When there's 200 A in a straight wire, the magnetic field 1 cm away is about 40 gausses (RMS), eh? OK for inexpensive B-field sensor chips. But what about the B field from other parts of the soldering gun tip?  How about a gapped core, linked with soldering gun loop, with a sensor IC placed in the gap? Might need to be an uncommonly high-range sensor.

Current transformers have the benefit of being passive, insensitive to temperature, and naturally linear. They are sort of "closed loop", magnetically comparing the primary and secondary ampere-turn values; only the small difference goes into core magnetization and phase-shifted voltage drop.

Anyway, previous report ended with a thick shorted turn linking one window of E-I core. How thick must it be, and how does it compare with an air gap in the third leg of core?   I set up an 80 ampere-turn source and measured the CT ratio while progressively removing conductance from the shorted turn. 
Baseline is about 320 mA in secondary (250:1 current ratio), according to this set of instruments, with short practically filling one window.  Down to 312 with window half full and 300 at 1/6 full (five wraps of bare copper stranded 14 or 16 AWG wire). Then dropped rapidly to 160 mA with zero or one wrap.  Sawing a small air gap brought secondary current back up to 313 mA.
Observed that disconnecting the secondary burden (mA meter) made primary current drop. That's because CT core goes into saturation, inducing back EMF that's significant in the very low voltage primary circuit.

Moved on to another transformer from some wallplug battery charger, opened with a rubber mallet resulting in damage to bobbin.
 
Winding R's about 650 and 15.
Voltage ratio 8:1 in normal direction and 1:7.5 driven backwards, indicating turns ratio of 7.75 with 97% coupling.
The E's and I's are not interleaved, so we can cut the core open and arrange to clamp it back with only one loop for flux.  Will monitor core reluctance via the magnetizing current in HV winding. Originally 3.17 mA at 1/2 voltage and 11.44 mA at nominal voltage.  This is important because it produces a nonlinear CT error term, and secondary circuit resistance can be no less than the winding R.
« Last Edit: September 20, 2020, 06:29:33 PM by klugesmith »

Online klugesmith

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Re: Current transformer for narrow spaces
« Reply #4 on: September 23, 2020, 11:20:53 PM »
Before finishing the work with red-coil transformer, I ordered some commercial CT's rated for 100 A.
Looks like the core will fit through any of my soldering guns, if enough blue plastic is removed.

Won't be hard to characterize its performance details.
If it gets nonlinear before we reach 200 A,  soldering irons under test can be operated at reduced voltage.
The induced EMF in primary circuit may be significant in this application, and unavoidable, but we will know its magnitude.

I also looked into commercial flexible Rogowski coils, many specified with 60 Hz accuracy good enough for "revenue" metering.
Found none that would thread through my tightest soldering gun. 

Might try winding flexible Rogowski coil for the purpose.  Sensitivity is proportional to small-loop area and winding density (turns per mm), regardless of coil length. I predict on the order of 10 mV output for 200 A input at 60 Hz.   No need for electronic integration if we are looking at sinusoids of known frequency.  Maybe electric field coupling to unshielded coil will be significant.  Can find out by mechanically reversing the link to current under test, while triggering scope on "AC line".

How do you wind coils to tolerate bending?  Suppose we wind 34 AWG magnet wire on a tubular former with OD of 1/4 inch (6 mm).
Then bend that into an incomplete toroid that would fit around a broom handle.  How will the wire bunch up on inside edge of the big loop?  Or will it resist compression, so the neutral axis of the bent former is close to inside edge?
« Last Edit: September 23, 2020, 11:48:48 PM by klugesmith »

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Re: Current transformer for narrow spaces
« Reply #4 on: September 23, 2020, 11:20:53 PM »

 


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