Re: Current transformer compensation idea

From: John Popelish (jpopelish_at_rica.net)
Date: 07/27/04 

Date: Tue, 27 Jul 2004 18:12:52 -0400

analog wrote:
>
> A typical current transformer may have a one turn primary and
> a hundred or more secondary turns. The secondary is normally
> terminated into a small resistor (possibly through diodes)
> such that the core must support a small ac flux excursion.
> With secondary signals in the volt range, the primary voltage
> burden is minimal, usually a few millivolts.
>
> Although dc drift may be a problem for some configurations,
> a typical current transformer rarely comes anywhere close to
> saturation during normal operation. In spite of this,
> inductive signal droop may be a problem in high fidelity
> applications (magnetizing current is typically very non
> linear).
>
> I have been toying with the idea of using active circuitry to
> minimize magnetizing current. My first idea was to arrange
> the current transformer to drive the summing junction of an
> opamp rather than terminating it into a small resistor. This
> would tend to keep the voltage across the CT's secondary at
> zero, which would be a noticeable improvement over the
> standard arrangement.
>
> However, this would still leave the voltage burden from the
> sense current flowing through the CT's winding resistance.
> Even this could be largely nulled out by actively driving the
> "grounded" end of the current transformer with a feed forward
> signal proportional to current appropriately scaled just to
> equal the drop developed on the internal winding resistance.
>
> Okay, I have never built this circuit and don't have a real
> application for it, but the simulator says all works great.
> What I am wondering is whether anyone has used or seen such
> a technique before or could imagine a situation where such a
> circuit might prove useful. Note that this technique does not
> eliminate the dc saturation problem (although it does make the
> CT's core "look" much bigger). Comments or further ideas?
>
> analog

I think the best you may be able to do is to have a pair of bifilar
secondaries. Drive the nulling current into one but measure the
voltage with the other (which can be made of very fine wire. Of
course, this only provides for AC feedback, so you need an external DC
path to close the feedback clear to DC. 

-- 
John Popelish

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