On Mon, 11 Mar 2013 07:41:41 +0000, Jef wrote:


The second one is a current transformer. They both consist of coils
around a magnetic core driving some kind of load. The difference is
the source of power and that causes them to behave very differently as
well as being constructed differently.


What you are describing is a difference in the source NOT a difference
in the transformer!

The transformer behaves exactly the same in both cases, the inversion is
merely down to ohms law.

A transformer is a transformer. The only time it gets tricky is when the
core and saturation etc etc comes into play.

Jeff

Very true. A transformer is a transformer and all obey the same
equations. But that doesn't prevent real transformers from being called
voltage or current transformers. And it doesn't prevent them from being
constructed differently or their outputs behaving differently.

Part of the problem is that real-world AC current sources are rather
rare. So current transformers are seldom connected as shown in the
example that was given. They are usually connected in series between a
voltage source and some other load, frequently for the purpose of
measuring the current. This is what gives rise to the different
constructions.

Voltage transformers have to withstand the source voltage across their
primaries so they have to have enough turns to prevent the core from
saturating. The actual number of turns depends on the voltage,
frequency, and core size and material.

Current transformers are generally constructed to have minimum primary
voltage drop so they have few turns on the primary, frequently only one.
Considerations of frequency, core size and material still apply, of
course. The differences in the name and construction are determined by
the intended use. Many current transformers consist of a secondary
winding on a toroidal core. The primary is supplied by the user by
passing a wire through the hole in the core; thus, one turn.

For a loop antenna with a secondary, there is voltage across the antenna
winding and current through it. Which is the secondary responding to?
The current, since this is what produces the magnetic field. But, since
this current is flowing through some number of turns and developing a
voltage, it could also be considered a voltage transformer, which is why
I said the distinction is blurred.

--
Jim Mueller

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