Re: Current transformer compensation idea

From: analog (analog_at_ieee.org)
Date: 07/27/04 

Date: Tue, 27 Jul 2004 23:10:55 GMT

"Martin Schönegg" wrote:
 
> Hi analog,
>
> whats about LEM? They compensate their cores to zero. DC and low
> frequencies with a hall sensor and higher frequencies inductive.
> See their literature for details.
>
> MArtin

Hallo Martin,

Ich hab' grad mal nachgeschaut... hast recht, sehr interessant.
Viel thanks for das Tip. I noticed that they do their Hall based
sensors just like Tektronics does their clamp-on current probe.
Looks like they have same drift and dc bias problems too.

Their non-Hall, high performance triple coil construction feedback
based sensors are very intriguing, but they look expensive. What
has been your experience with their products?

Gruss -- analog

> "analog" <analog@ieee.org> schrieb im Newsbeitrag...
>
> | 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

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