Hi Gene,
I would like to take the opportunity to clear up some misconceptions.
> Core selection appears to be fixed mainly by the voltage across the
> primary
Not so. There are several factors to play with. Firstly, core materials
are not all alike, and some can work at a much higher flux density than
others, at a given level of loss. How much core loss is acceptable will
depend on the application, kind of service, environment (temperature,
forced air cooling, oil cooling). Then, of course, the designer of a
transformer can trade core cross sectional area for turns per volt,
maintaining the flux density constant. And once the cross sectional area
has been decided on, a taller stack of smaller laminations, or vice
versa, also leads to different iron/copper ratios.
For these reasons two transformers of the same nominal power and
voltages could have very different core sizes.
> (so that there is sufficient inductance in the primary)
In most power transformer designs one doesn't even calculate the
inductance! It's usually high enough to be of no concern. Instead the
flux density in the core is calculated, and this defines the amount of
turns that has to be used on a given core, for a given frequency and
voltage. Only in special applications would the inductance become important.
> and the peak current that flows (so that the core doesn't saturate).
>
Here you express another, very common misconception! Transformer cores
DO NOT saturate from excessive current (at least if you mean load
current). Instead they saturate from excess voltage per number of turns!
Interestingly, a transformer that is idling (no load) will be closer to
saturation than when it is delivering full output current. This is
because the current causes a (usually small) voltage drop, which reduces
the effective voltage setting up a magnetic field.
Related to this is the misconception that transformers that saturate
will cause flat-topping of the output waveform. That won't happen! If a
transformer saturates, it will do so when magnetic flux is highest, and
that doesn't happen at the waveform voltage peaks, but at the zero
crossings!
> So designers don't have that much flexibility in iron core selection,
> implying that they can't vary it as much as they can wire diameter.
Not correct. Actually there is MORE flexibility in core selection than
in wire diameter! The wire diameter depends on the maximum current that
will flow over a time long enough to exceed the thermal time constant of
the wire, it will also depend on the allowable heat rise, which in turn
depends on the kind of insulating material used, and some other factors.
Also in many cases it's necessary to consider the total allowable
voltage drop due to wire resistance. At the end, it boils down to a
rather small range, from about 2A per square mm of coppper for a large
transformer made with class A insulation and working in air, to maybe 4A
per square mm for a small transformer using class F insulation.
Core selection instead is far wider ranged, since the designer can
choose between several different qualities of material, and then he can
choose different shape factors, at the same time as the size.
Transformer design is not like first selecting the core, then
calculating the windings. Instead it's an interrelated procedure, in
which both the core and the windings are altered and the results
calculated, until the best compromise is reached for the application on
hand.
> Then, I tried to do an "on paper" conservative design, and the thing
> would have weighed over a hundred pounds if I had made it.
That has happened to me too! That's why I learned to do optimized
designs, rather than conservative ones! ;-)
> It was also unclear how much money I would have saved after I bought
> the iron cores at a single-unit price.
I don't know how well your costs (in the US) would relate to mine (in
Chile), but down here I can save about half the money by buying the
core, wire, insulation material, and doing the work myself, over
ordering a custom-made transformer. Instead if a ready-made Chinese
transformer is available in the rating needed, that's far cheaper than
winding my own!
One can often find suitable cores in a well stocked junk box. But that's
not too much of an advantage, because new silicon steel laminations are
inexpensive, compared to the wire. And the wire has to be bought new
anyway. Trying to recycle magnet wire from old transformers is a recipe
for disaster.
> I did not have the facilities to bake it or do a decent job of
> laquering it
My trick is to pour the varnish into the windings, let them soak, and
warm up the whole winding assembly by passing DC through the wire. That
way it dries reasonably fast. Without such heating, it takes forever and
a day to dry.
> or making sure that there was sufficient insulation between the
> primary and secondary.
A simple voltage multiplier, built from a stack of diodes and
capacitors, powered from the line through a lightbulb as current
limiting device, easily generates enough voltage.
> It also probably would have looked like junk if I had made it.
Maybe the first time. Mine did too. But after making a few, you will get
the hang, and make good looking ones!
By the way, the very first transformer I wound, at age 13, is still
serving well! It does look like crap, with unprotected magnet wire
pigtails coming out from odd places of the assembly, but it works!
> The plate transformer he made for me-- 3KVA output, 99 degree F
> temperature rise for CCS, and capacitive input -- ended up weighing
> 47 pounds, and I estimate that about 2/3 of that is iron.
Clearly he used high quality steel laminations. By using standard
quality material, it would have ended up a bit heavier, with at least
3/4 iron, to reach that same temperature rise.
> I like home-brewing, but I'm glad I did not try it myself.
Hey, I challenge you to change that attitude! Homebrewing transformers
is fun!
> There is a LOT that goes into making one of them, make sure that you
> know what you are getting into first.
That's right. For a novice transformer homebrewer, I would advice
starting with medium size, low voltage ones, such as used in 13.8V power
supplies. Those are the easiest to make. Very small ones use large
numbers of turns, and very thin wire, and high voltage transformers also
have lots of turns.
Manfred, XQ6FOD
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Visit my hobby homepage!
http://ludens.cl
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