Re: AC current measurement with a current transformer

"The Phantom" <phantom@xxxxxxx> wrote in message
news:7fl3q1t13g2pinrkde16rbv9fnd61jndlu@xxxxxxxxxx
> Correct. "...pri AT minus sec
> AT..." is just exactly the mmf needed to support the flux in the core.
You
> seem to understand, but you forget this point later.

No, I just think either I haven't expressed my ideas clearly or you didn't
read them right.

> Now you're talking about inductors (no secondary), not transformers,
> right? I've been talking about current transformers.

Same thing: I'm talking about cancelling the primary and secondary
amp-turns. This leaves you with only the inductance of the primary and its
contribution to the core.

> >Peak current in a flyback converter,
>
> In a flyback converter, the primary current and secondary current

No, just an inductor. It works with a secondary too though, in which case
the secondary serves to remove the stored energy -- just the inverse of the
primary's purpose.

> >reactive current in a transformer, DC bias in a single-ended
> >output transformer, amps through the wire passing through the current
> >transformer, all the same thing.
>
> Not all the same thing.

All of these produce a certain field proportional to the primary amp-turns,
instantaneously and/or continuously, that magnetizes (or magnetizes and
demagnetizes) the core and can bring it near saturation. That's what I
mean.

> The voltage across the primary turn will
> be 100 mV/1000, or 100 microvolts. Integrate a half sine of a 100
> microvolt AC voltage for 1/120 of a second and you will have the flux in
> the core; you get the same result when you integrate a half sine of a 100
> millivolt AC voltage (found on the secondary) and divide by 1000 (the
> number of turns on the secondary).

Hm, in a flash I now understand the function of a current transformer. The
voltage drop is known because the *reflected impedance* is known. That
said, you can integrate the volt-seconds or whatever against the current
waveform (because it produces a V = IR) to figure the actual field the core
needs to withstand.

This also means inductance factors out and the transformer looks more like a
short resistor than a ferrite bead, while the added inductance (primarily
leakage inductance) is small, effectively a few additional inches of run.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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