Re: Current transformer compensation idea

From: Jim Thompson (thegreatone_at_example.com)
Date: 07/27/04 

Date: Tue, 27 Jul 2004 11:44:06 -0700

On Tue, 27 Jul 2004 18:25:49 GMT, analog <analog@ieee.org> 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 haven't done any active feedback to reduce the "voltage burden" of
the CT's winding resistance, but what I have done is use a DC loop
back to the "grounded" end of the CT to eliminate the terrible offset
that results from a direct connection to an OpAmp summing node.

(I used a DC-restorer to get the peak current value relative to
ground... current-limiting application.)

                                        ...Jim Thompson 

-- 
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

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