Re: YATQ (Yet Another Transformer Question)

I used to design transformers and wind prototypes. With the quantity
of surplus transformers available I have not had to resort to
rewinding or doing heavy modifications to transformers. Taking a
commercial transformer apart that has been sealed with varnish and
baked is not a fun task. The amount of labor only makes sense if you
are (A) desperate; (B) financially challenged or (C) morbidly
curious.

As for the dual primaries. Since Watts out is equal to watts in plus
transformer losses, the more current you pull from the secondary the
more current is drawn by the primary. Since Watts is approximately
the voltage times the current, running the primary at 110VAC forces
the primary to have to supply twice the amount of current for a given
secondary load. Because transformer size ($) is a function of core
size and core size is a function of wire size, the designer will not
typically oversize the wires to handle double current. That's where
running the two primaries in parallel come in. Now you have two
primaries each carrying half of the load...bingo, everything's back
to normal.

Basic rule of thumb: You can impress whatever AC voltage on a
winding it puts out. Any output can become an input. Another way of
saying it is that any winding could become the "primary" and the
others the secondaries as long as you kept them at their rated
voltage and currents. If you have a 110VAC to 24VAC/3A transformer
that means you could put 24VAC on the second wwinding and get 110VAC
out of the other. Now, don't try to put MORE voltage on a winding
than it's rated for because the windings are designed using a formula
that says: core saturation (= magnetic short) is a function of the
magnetics properties of a given core, (saturation in Gauss), the
number of turns of that winding, and the applied voltage and
frequency. As frequency goes down the core saturates faster. As
applied voltage goes up the core saturates faster.

Most transformers are designed to handle at least 125VAC for
a "110VAC" winding. If you find a 50/60HZ rated transformer it means
the designer did his calulations at 50HZ as worst case so 60 gives
you a little more "headroom".

> Note: if the above is correct, it's an interesting way to use a

110VAC fan when

> plugged into 220VAC.
>
> Does it sound O.K. to put 110VAC across 1 and 3, or would there be

any advantage

> to using both windings in parallel (1 and 2 jumpered, 3 and 4

jumpered)?

>
> I seem to remember there is a former transformer winder on this

list, and I'd

> appreciate some practical insite on how to use transformers.
>
> Thanks for your time,
>
> Steven Ciciora