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Pearson current monitor model 1330 teardown (100kA)

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Mads Barnkob

Sun Feb 28 2016, 08:45PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Finn Hammer wrote ...

The core was a strip wound, very thin lamination strip, wound from something looking rather funky, I guess high nickel or cobalt content, from the shinyness of it.
Calibration of a Pearson is a joke, as calibration is same price as a new one, and guesss why that is so.
My guess is, they toss the old one, and ship a new unit.

Cheers, Finn Hammer

Yep, there is nothing to calibrate, you can only check its calibration. That is what I meant, we hope that the 2nd hand commercial CTs are still up to specifications and then we, in some way, have a calibrated monitor :)

The 1330 is properly made of the same strip material, just with sharper corners.

Wolfram

Sun Feb 28 2016, 11:28PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Excellent thread! I've always wondered what kind of magic they put under the green paint to get their famously good performance.

On a related note, I've found a good description of a DIY wideband CT with very usable specifications. The description comes from the book "Inductors and Transformers for Power Electronics" by Valchev and Van den Bossche. The page in question is openly available on Google books

, and I've reproduced it here in good faith.

1456702097 33 FT175696 Wideband Current Probe

Note that the resistor loading half of the secondary should be 330 ohms, not 33, this is stated in the errata for the book.

Mads Barnkob

Mon Feb 29 2016, 09:19AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I have a few pages from some journal, forgot where I have it from. I thought some of the ideas described inhere with multiply smaller cores in parallel and opposite direction coils etc was some I would see in the 1330. Turned out not to, but here is the 4 pages PDF for your entertainment.

]high-frequency__current__transformer.pdf[/file]

DerAlbi

Mon Feb 29 2016, 10:45AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Note that the resistor loading half of the secondary should be 330 ohms, not 33, this is stated in the errata for the book.

May i ask, just for curiosity, what this half load does in the first place??

johnf

Tue Mar 01 2016, 08:54AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

It is used as a Q killer
and it will remove any resonances from stray "C" in the windings

klugesmith

Tue Mar 01 2016, 03:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

This thread stands out from the chaff. Thanks, Mads.
And thanks to John for sharing wisdom about the damping art.

Makes me want to test my matched pair of Pearson CT's. They have about the same size and color as the one autopsied above. A lucky flea market find a few years ago, and never yet used by me.

[edit] As requested by a moderator, I moved my off-track comment about broadband Current Viewing Resistors to a new thread. Now it's Albi's turn.

Bert

Sat Apr 30 2016, 05:18AM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Thank you for your patience in tearing apart the large CT and for sharing the photos with us!

Dr. Paul Pearson's original 1964 patent appears to shed a fair amount of light on the theory and early construction of a similar double-shielded large wideband CT.

A more recent Pearson Electronics patent (from 1998) appears to cover some design approaches used in their smaller toroidal CT's.

A relatively recent paper discusses differences between core materials (strip wound Si steel, Ni-Fe alloys, and nanocrystalline iron allloys), remenence effects and mitigation, and frequency effects of core materials in wideband CT's.

Finally, there's a very interesting paper from CERN that compares performance between a Pearson 7546, a Stangenes 3-0.002, and a PEM CWT 150XB Rogowski coil versus a precision current shunt resistor. All three CT's show excellent results.

Mads Barnkob

Mon May 02 2016, 06:34AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Thank you Bert, the first link to the 1964 patent really sheds some light over it, and also confirms the schematic I drew from disassembling.

The patent describes the segmented construction pretty well in a short chapter.

In making a transformer which will monitor high currents, it is necessary to make the value of the load resistance relatively low. If the resistance wire has to traverse the same length for a very high current model as for a low current model, the ratio of resistance to inductance becomes appreciably worse. To combat this problem, the load resistance is broken up into short lengths as shown in FIGURE 11 in which the resistance portions 14a are separated by relatively wide, high conductance portions 15. The individual resistors are short and possibly thick and wide, thus lowering the inductance of the individual resistance well below the inductance of the resistance if it had a length equal to the length per section. The connecting wires are secured to the ends of the resistance portions 14a. Wide copper strips may be used for the high conductance portions. The resistance portions are connected in shunt with corresponding .coil sections 13a.

Hydron

Wed Aug 10 2016, 05:26PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

More info that may be of use to understand the construction of these:

My measurements of the exposed parts of a 3525 split CT (datasheet: http://www.pearsonelectronics.com/pdf/3525.pdf):

1470850015 30656 FT175696 Pearson 3525 Measurements

Measurements support the theory of distributed parallel termination throughout the coil, either with no series resistor (coil DC resistance is ~10R), distributed series resistance, or a lumped series resistance (minimum of 2 lumped parts, half on each side of split).
Also could see the location of the anti-shorted-turn split line in the case - it's between the centre bore and the hole in one of the top plates.

Have done a quick check against a pearson 4100, see 10k-10M freq sweep and test setup below (50R termination makes little difference, sweep plot is without it):

1470850015 30656 FT175696 Pearson 3525 Vs 4100 Freq Sweep

1470850015 30656 FT175696 Pearsons Under Test

The 4100 measures a dead short across the BNC - obviously there is no series resistance being used with this 1V/A unit, only the winding (dead short at DC) and a parallel termination.

Edit: it appears that the phase shift at high frequencies is affected significantly by the position of the wire within the core. Best is with it centred - worth taking care to do this if operating above 0.1x of rated 3dB frequency.

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