https://ludens.cl/Electron/trafos/trafos.html

                  Practical transformer winding   

[10kready]In the good old times it was a matter of fact that every
electronic hobbyist or technician would wind himself any power
transformers he needed, and rewind any that burned out.
Unfortunately, nowadays transformer winding is fast becoming a lost
art, and I have seen many people despair about where to find some
very specific transformer, or pull their hair out about the cost of
having one professionally wound to specifications.

Since I started in electronics, as a 12 year old boy, I have always
wound my own transformers. I started using the basic, but useful
instructions provided in The Radio Amateur's Handbook of the time,
and later I came to better understand how transformers work, which
enabled me to optimize a given transformer for the intended
application. 

Following a request by many readers of my web site, I've added this
page, which is complementary to the previously published Transformers
and coils. You should first read (and understand!) that page, before
trying to design any transformer. Then come to this more
practically-oriented page, to learn some tricks and hints about the
design process, and about hands-on winding.

This page addresses mainly single-phase power transformers in the
power range from about 1 watt to 10,000 watts, operating at line
frequencies, but much of what's described here can be applied to a
wide range of other transformers too.

[material]Let's start with the materials. To make a typical
transformer, you need the iron laminations for the core, enameled
copper wire of several different diameters for the windings, a bobbin
(or some material to make one), insulating material to apply between
wire layers, between windings, around the whole winding assembly,
and on exposed wires, and in most cases it's also a good idea to use
an impregnation varnish.

The photo here shows several stacks of iron E-I laminations, two
coils of wire (with cardboard protecting the wire from damage), one
roll of thick, stiff Pressspan, another roll of NMN laminate (we will
soon see what that is), two small bundles of spaghetti for wire
protection, and a can of transformer varnish. Add to this some glue,
cotton straps, ropes, adhesive tape, terminals, bolts, angle iron,
and other small material, and that's it.

All these materials are sold by companies specializing in
transformers and parts for transformers. Enameled wire is also sold
by many other distributors, but is usually cheapest at the places
that sell it together with the other materials. You will have to dig
into the phone book or some other directory to find these companies,
since they don't usually have a shiny nice store in the downtown
shopping mall!

[laminaEI]

Transformer iron is an alloy of iron with silicon and some other
minor components. It's characterized by a relatively high
permeability, very high saturation flux density, relatively low
hysteresis loss, and relatively high specific resistance. This latter
factor, along with the practice of using the material in thin,
insulated sheets, reduces the power losses produced by eddy currents.

The most common shape of these sheets is shown at right. It's the
classic "economy E-I" shape. Why it's called E-I should be pretty
obvious when looking at the photo! But the explanation for "economy"
might be a bit more elusive: It's because at the exact proportions
shown in the photo, the I's are nothing else than the cutouts to make
the windows in the E's, when two E's are cut facing each other! This
allows stamping E's and I's out of a large steel sheet, without any
wasting of material, except for the little round bits cut out of the
bolt holes. By the way, small laminations often don't have such bolt
holes, and such cores are held together by clamps instead of bolts,
or even welded.

The lamination in the photo is a large one, as the comparison with my
hand shows. It's an E80 (the center leg is 80mm wide), typically used
for transformers in the 3 to 10 kilowatt range!

In any E-I lamination you are likely to encounter, the center leg is
twice as wide as each of the other parts. This is because the entire
magnetic flux has to go through the center leg, but then splits up,
with one half of the flux returning through each of the side legs. If
you ever come across a lamination that has all three legs of the same
width, then you are looking at a lamination intended for three phase
transformers! 

Such an economy E-I lamination like shown here has completely fixed
proportions, beyond the rule above, that stem from the need to cut
the I out of the winding window of two E's facing each other: If the
center leg is 2 units wide, then the window is 1 x 3 units, the total
E is 6 x 4 units, the I is 1 x 6 units, and so on.

[arglam]

Not all laminations follow the "economy" proportions, though. Here is
an example of a lamination that comes in one piece, instead of being
divided into an E and an I, and that has the windows proportionally
much larger than the E-I lamination shown above. Such a lamination is
a bit more expensive to make, because the steel cut from the windows
is wasted, unless the manufacturer can find some other use for it.
But being able to accomodate a much large winding assembly, it has
some advantages in certain cases. 

These "non-economy" laminations were quite usual in Europe, many
years ago, but nowadays copper is so much more expensive than steel,
that transformers are usually designed to use more steel and less
copper. And for that goal, the economy lamination is very well
suited. So you won't very often come across a lamination like this,
unless you are restoring antique equipment.

[varnishing]

The laminations should be thin, and reasonably well insulated from
each other, to reduce eddy currents to an insignificant value.
Typical thicknesses vary from 0.2 to 0.5mm, but higher frequency
transformers (audio) use much thinner ones, while extremely large
transformers might use slightly thicker ones.

The insulation is often applied at the factory that makes the big
rolls of steel sheet, even before stamping the E's and I's. Different
kinds of insulation are used: A thin oxide layer, a thin layer of
enamel, or any of several chemical processes. Antique transformers
sometimes even used very thin paper!

When I was young, patient and overly eager to do things right, I
painted each and every E and I for my transformers, using diluted
transformer varnish, to make a thin, nice layer. The photo shows the
steel for a 200 watt transformer, drying. Later, getting old and
lazy, I noticed that the layer of rust on old, recycled laminations
is more than enough insulation, and that the very thin and imperfect
insulation that comes on new laminations is enough too, even if it
takes only a light scratch with the multimeter's test probe to
puncture it and get through to the steel. We don't need perfect
insulation between the sheets! We only need enough resistance to
reduce eddy currents to a low level.

 Transformer steel is not all born alike. Manufacturers will provide
data sheets about their products (often on their web sites), where
you can see what they offer. There are usually many grades, with
vastly different loss characteristics. At a given flux density and
frequency, a good material might have ten times less loss than a
cheap material! So it pays to look, investigate, and
decide intelligently what to buy. Thinner sheets normally have lower
loss, and the rest of the secret lies in the exact alloy. In any
case, you need to know what material you have, to be able to make a
meaningful transformer design!

Some transformer steel is grain-oriented. That means that when
rolling the steel sheets, a process is used to align the crystalline
grains in the direction of the rolling. This kind of material has
particularly good behavior when the magnetic flux is aligned with the
direction in which the sheet was rolled, but is worse than standard
material in the perpendicular direction. Such grain-oriented material
is ideal for toroidal cores, which are made by coiling up a long
strip of steel, but is not a large improvement for E-I laminations,
because in these a significant portion of the material has to work
with the flux perpendicular to the rolling direction.

[wires2]

Enamelled copper wire comes in many different diameters, and with
several different kinds of enamel. The diameters vary from less
than that of a hair, to about that of a child's finger. Different
standards exist for the wire diameter. A very common one is American
Wire Gauge, shortened to AWG, which is used in much of the world.
Britain has its own standard, and in many countries the wire is
specified simply by its diameter in millimeters.

Thick wires usually are coated with a sort of enamel that is very
tough, an excellent insulator, highly heat-resistant, highly
resistant to solvents, and that clings to copper even better than
dirt does to children! This enamel is usually yellowish clear, so
that the wire coated in it looks mostly copper-colored, but many
exceptions exist. To solder the ends of these wires, it's necessary
to scrape off the enamel, using a sharp knife or similar tool. This
procedure would be too difficult with a thin, fragile wire, so that
these thin wires are instead covered with an enamel that has most of
the same characteristics of the other one, except the heat
resistance: It will melt and turn into solder flux at a temperature a
common soldering iron easily achieves! This allows easily soldering
these wires, without previously stripping them. But transformers
using this latter kind of wire enamel cannot survive temperatures as
high as those using only the former kind of wire enamel. The red wire
on the right side in this photo has this kind of enamel. But be
careful with colors! The clear wire on the extreme left side also has
solderable enamel, while the dark violet one in the middle is of the
non-melting variety! 

The thickness of the enamel layer depends on the wire thickness, the
manufacturer, and can sometimes be chosen: Some manufacturers will
offer the wire with seeral different thicknesses of enamel. In any
case, the diameter specified by a certain AWG number refers to the
copper diameter, so that the complete wire, with enamel, will be
slightly thicker than what the AWG standard tells!

[wires]Here is a wire table for AWG wire. It shows the AWG number,
the diameter in millimeters excluding the enamel, the approximate
typical total diameter including the enamel (but this varies
somewhat), the cross sectional copper area in square millimeters, the
area of the square of window space occupied by that wire in a
transformer (including the enamel, of course), the current carrying
capacity at a typical, average value of current density, the
resistance in ohms per meter, and finally how many meters of that
wire come in one kilogram, because enamelled wire is usually bought
by weight, not length.

This table has wires from AWG #1 to #40, and for the thickest ones I
didn't calculate all data. But you should be aware that there are
wires exceeding this range! The thinnest I have ever used was #46. It
breaks when you blow at it! The photo here shows a #39 wire lying on
a #7 wire. The hairy thing below is my floor carpet. Note that even
this #39 wire is not much thicker than the hairs of this carpet!

It's interesting to note that every three AWG numbers, the cross
sectional area exactly doubles. Any deviation from this in my table
is due to approximation errors.

[bobbins]

Modern transformers of small to moderate size are usually wound on
plastic bobbins. Here you can see simple ones. Some bobbins have pins
or terminals molded into them, others have one or two divisions. Some
don't have the slits for terminals, which the ones shown here do
have.

Typically for a given size of E-I laminations, bobbins will be
available in two or three sizes, accomodating different numbers of
steel sheets. So you can vary the amount of steel in your transformer
not only by choosing the lamination size, but also the height of the
lamination stack!

[splitbobbin]

Here is a little transformer using a divided (or split) bobbin. This
is very practical, because it completely separates the primary from
the secondary winding, making it much easier to achieve the degree of
insulation required for safety. More about that later.

[bobbin1]

If you cannot find a plastic bobbin in the proper size, don't
despair! Bobbins can be easily made from materials such as strong
cardboard, or Pressspan, which is nothing else than a particularly
strong cardboard.

The bobbin shown here was made from 1.5mm thick Pressspan, which is
really too thick for this small bobbin, but I had nothing better on
hand. The pieces are cut to size using a sharp knife (X-acto or the
like), and glued together with cyanoacrylate adhesive (instant
bonder). The clever structural design of this super high tech bobbin
holds it together perfectly while the glue sets!

You must make the inner dimensions of the bobbin core a tad larger
than the transformer center leg, but JUST a tad, no more, unless you
want to waste valuable winding space! The sides can be made pretty
tight to the size of the laminations, because if they don't fit at
the end, they are easily enough cut or filed down, even after the
winding has been made. But the length of the bobbin must be smaller
than the window length of the core, by as much as 2 or 3%, plus any
tolerances of your manufacture! Because it is critically important
that the E's and I's can touch each other properly, without being
kept separated by a bobbin that deformed during winding, and grew!

[bobbin2] Be sure to at least break the corners as shown here, or
even better, round them off. Otherwise the wire is guaranteed to
tangle at the sharp corners during winding, and a wire loop sticking
out of the completed winding can ruin the whole thing!

Note that the junction of the bobbin's center piece is placed in the
middle of one side, and not in a corner. It's next to impossible to
produce a reasonably symmetrical and precise bobbin when placing the
junction in a corner.

If the material is thin compared to the bobbin size, the junction
should be made by overlapping the material. Of course, the overlapped
junction is always placed on one side that will end up outside the
core window, so that the added bulk has little detrimental effect.

To bend this thick material in reasonably clean right angles, my
technique is to use a sharp knife to cut out a 90 degree wedge from
the inside, along each bend line, leaving only the outer third of the
material intact.  After that admittedly cruel treatment, the
Pressspan eagerly bends to my will.

[argcoil]

Most antique transformers, and many of the larger modern ones, don't
use a real complete bobbin. Instead, they use only the center former,
and no sides at all! It takes some tricks and practice to wind a
transformer like this without having the whole thing come apart many
times over during winding, but for people who have acquired enough
practice, it's faster than making a real bobbin!

Further down, I will show you a trick to make this kind of
transformer, with high quality.

Note in this photo how the Argentinian maker of this transformer
(back in 1931!) used insulating material of several different
thicknesses for the bobbin center, the interlayer insulation, the
interwinding insulation, and the terminal support.

You might ask why any insulation material is required at all, if the
wire is insulated by its enamel layer! Well, the enamel is very thin,
and easily scratched. It might survive as much as a few thousand
volts, but it might also break down with a lot less! It depends on
type, condition, thickness, temperature, and other factors. So,
wherever the voltage can exceed a few tens of volts, some additional
insulation needs to be used. Specially between the primary and
secondary, safety regulations ask for an insulation good for at least
4000 volts, to avoid electrocuting somebody when there is a lightning
transient on the AC power network.

In antique transformers, the most usual insulating material
was paper, impregnated with something like beeswax, tar or the like.
This impregnation had several purposes: Mainly, it would seal the
pores of the paper, making it a really good insulator, while without
the impregnation it would only insulate as well as the same thickness
of air! But in addition, it kept moisture out, it helped stick the
thin wires in place during winding, and it improved the thermal
conductivity of the completed winding assembly.

It was also quite sticky, dirty, messy and gross.

Modern insulating materials are far superior. Plastic sheets such as
Mylar provide excellent dielectric strength and have no pores, so
they require no impregnation to realize their high degree
of insulation. Nomex instead, with its fibrous structure, behaves
like paper, but both Nomex and Mylar are much better than paper at
surviving high temperatures! This is a key characteristic of
insulating materials: The temperature class. It's coded with a
letter. Paper would have an A or B rating, telling that it is fine
for temperatures not much above that of boiling water. Different
plastic insulation materials instead are routinely available in
classes as high as F, G or even H! They can safely run much hotter
than paper can.

[nmn]The photo shows an NMN insulating sheet. This is a sandwich of a
Mylar sheet embedded between two layers of Nomex. The Nomex will
eagerly soak up and distribute the impregnation varnish (or the oil,
in an oil-inmersed transformer), while the Mylar will provide safe
insulation even in places that for any reason stayed dry! I love this
material. It's thermal class G, if I remember right.

Insulation materials come not only in many different variants, and
temperature classes, but of course also in many different
thicknesses. You choose the proper thickness so that it has enough
dielectric strength and mechanical strength, without taking up an
undue portion of your valuable window space!

Despite all modern materials, good old paper and cardboard is still
used sometimes. Mostly in its form known by the German word
Pressspan, which means "compressed chips", and is simply a very dense
paper or cardboard.

 

It's very good practice to soak a completed transformer in some
impregnation varnish. It will form fillets around wires, papers, and
anything else. It will improve the insulation, make the transformer
highly moisture-proof, glue everything together so that nothing can
rattle, come loose, or chafe through, it will improve thermal
transfer, and so on.

Varnish comes in several thermal classes, just like the insulation
material, and also it comes in variants that dry at high temperature,
or at room temperature. My experience is that no varnish ever fully
dries at room temperature, and when you start using the transformer
and it warms up, the varnish inside will start drying, and stink! So,
it's necessary to apply heat anyway, regardless of what sort of
varnish you use.

---------------------------------------------------------------------

 

Now that you have turned into a person very knowledgeable about
transformer materials, let's turn to those pesky questions such as
"how many turns do I have to wind?"  or "what wire size?" or "how
much power will I get?"

There are three typical situations:

1. You need to repair/rewind a transformer that burned out.

2. Your want to rewind an existing transformer, to produce the
voltages and currents you need, which are different from the original
ones. A variation of this case is when you want a certain voltage, at
the highest current that transformer can provide.

3. You have fixed specifications, want to design a transformer to
optimally meet them, and you will buy the core and all other
materials.

[unwinding]Let's start with the first case. You MUST  find out why
that transformer failed! A correctly designed, correctly built and
correctly used transformer is, for all practical ends, eternal. If it
failed, there is a reason. If you know that the transformer was
shorted, overloaded for a long while, exposed to intense lightning
transients, thrown into water, gnawed through by rats, exposed to
corrosive substances, or anything like that, then your best approach
is to unwind it, count the turns, measure the wire sizes, and rewind
it exactly as it was originally made.

The photo shows an antique speaker field coil, mounted in my winding
machine. I unwind coils by pulling off the wire while having the
thing spin in the machine, so that the turns counter in the machine
will do the pesky job of keeping count. The problem, as illustrated
here, is often that thin wires won't come off nicely! They are stuck
in place, and will break, then entire chunks of wire will come off
all together. This often makes it hard to accurately count the turns.

In such cases you might simply estimate how many turns you didn't
count. Or you can collect all the pieces of wire you removed, weigh
them, calculate the amount of wire from there, and calculate the
turns number from it. Or, instead of unwinding the coil, cut it with
a knife or better a Dremel tool, remove it in one block, measure the
cross sectional area of the entire winding, then remove a little
piece of wire to measure the diameter, and finally calculate the
number of turns from this. Any of these methods will usually be
precise enough for non-demanding applications, and none of it will be
precise enough when you need anything critical.

By the way, do you know how to precisely measure the diameter of a
thin wire, when you don't have a micrometer screw? Simple: You wind
10 or 20 or even more turns tightly on a former (such as a
screwdriver stem), measure the length of the coil with a common
ruler, then divide by the number of turns to get the wire diameter.
It's highly accurate. Sometimes it's even better than using a
micrometer screw, which can flatten the wire if you apply too much
torque! 

But there are cases when you have a burned transformer, and no good
reason why it burned. It might have been a manufacturing defect, a
huge transient, an overload that went undetected, but maybe - the
Gods of Electromagnetics forbid - that transformer might have been
misdesigned! In that case, painstakingly rewinding it with the same
wire gauges and turn numbers as original, will only produce a
transformer that will fail again. So, if you don't know why a
transformer failed, re-do the design, and compare your results with
what the manufacturer did! Many manufacturers are cheapskates, and
use substandard transformers, in the hope that most clients will
never use them intensively enough to blow them up! This sends you
straight to the section about the third case, further down this page!

In the second case, when you want to rewind an existing transformer
for new output values, very often the transformer will already have a
properly wound and healthy primary winding. In such a case, keep it!
There is no point in unwinding and rewinding the primary, if it is
fine. The calculation work for such  a transformer is quite simple:
Before taking it apart, measure the voltage delivered by the
secondary. Disassemble it, unwind the secondary, counting the turns,
and calculate the number of turns per volt from this. Calculate the
new number of turns you need for your desired voltage. Calculate
what's the largest wire size that will comfortably fit in the
available space. Get the wire, wind it, and assemble the transformer.
The power rating will be the same as before, and this allows you to
calculate the current you can safely draw, at your new voltage.

And if you also want a new primary, well, do the same as above, but
unwind and re-wind all windings, according to the value of turns per
volt you found out!

[wig]

Do you know what you can do with all that wire you remove from old
transformers? Well, a super trendy wig, like the one shown here,
modelled by my sister, is sure to catch everybody's attention!
Otherwise, there isn't really much use for such wire. It comes out
totally kinked, stretched, broken, scraped, with varnish, wax or tar
sticking to it. Don't even dream about ever re-using it in any other
transformer!

OK, now it's time to really start designing a transformer. Because
this is what you have to do in case 3, which is essentially creating
a transformer from scratch. I hope you are still fresh and aware of
everything you learned in Transformers and coils, because you will
need it here to understand what's going on.

The design process starts with guessing the size of core you need,
for the power you want. If you are experienced in electronics, you
will be able to make a reasonably close first guess. Otherwise, use
the data given by core manufacturers to get this first guess.

The core chosen will have a certain cross sectional area, and will
have a certain window area. The two multiplied are the "area
product", which can be related to the approximate power capability,
with a reasonably simple, but nonlinear curve. This curve also
depends on the quality of the core material, and several other
factors.

The next step is calculating how many turns per volt you need on this
core. To this end, you have to decide how much flux density you will
put through your core, and then you can apply the equations from
Transformers and coils. The optimal flux density might be anything
from 0.8 to 1.6 Tesla, and sometimes even outside this range! General
rules of thumb are these:

- Larger transformers use lower flux densities.

- Better core material uses higher flux densities.

- Transformers that are always energized, but rarely used at full
power, use lower flux densities.

- Likewise, transformers that work at full power whenever energized,
use very high flux density.

- Forced air cooled transformers use higher flux density.

- Oil-immersed transformers use even higher flux density!

- Higher flux density produces better voltage regulation.

- Lower flux density produces less base loss.

- Lower flux density is less likely to produce humming noise, and
magnetic stray fields.

- Lower flux density produces lower iron loss, but higher copper
loss.

 

I have seen many text books giving design equations that result in a
flux density of 1 Tesla in each and every transformer you calculate
by them, like if that were a sacred rule! If you come across any such
book, BURN IT! It's nonsense! While 1 Tesla indeed tends to produce a
workable transformer in most cases, in at least 70% of all situations
it's far enough from the optimal value to warrant some effort toward
optimization! Specially in small transformers, and in those using the
better core materials.

So, I suggest to start with a value chosen from the rules above, and
then calculate the transformer based on this value, analyzing the
losses, heating, voltage drop, efficiency, and so on. Then change the
flux density, by 10 or 20%, and re-do all the calculations. You will
see what I mean! There is a clear optimum value for flux density in
each particular case, and this value is very often sufficiently
removed from 1 Tesla to make you wonder why some book authors still
copy that "magic number" from other, long gone authors! Most likely
they have no idea about the matter they are copying.

That said, sometimes I do wind my transformers for 1 Tesla, because
there are cases when this is really a good value!

The loss calculation isn't very hard: The manufacturers of
transformer steel specify the loss of their products, as a certain
amount of watts per unit of volume or weight, under certain
conditions of frequency and flux density. And the better of these
data sheets also contain curves, or give equations, to calculate the
loss under different conditions. That's about the iron loss. The
total loss of the transformer also includes the copper loss, which is
caused simply by the current flowing through the resistance of the
wire. For a given flux density you get a certain number of turns per
volt, this allows you to calculate the total turns in each winding.
Dividing the available window cross section between these wires,
insulation, bobbin, and inevitably wasted space, you get the wire
cross sections. From the dimensions of the core, you can then
calculate the total wire length, from this and the cross sections you
calculate the resistances of the windings, and from that and the
currents you calculate the power loss. It's easy!

[trafoSheet]What? You are banging your head against the wall?  Come
on! Don't cry! I have prepared an Excel sheet for you, which takes
care of these calculations! It's really too time consuming to do all
this math step by step, every time. Click on the screenshot at right
to get the Excel file, and then play with it as you continue reading.

In this sheet, you enter your data in the green area, and watch what
happens in the orange one. The first three values you have to enter
are the ones that specify the core. First is the center leg width of
the lamination used, in millimeters. The 100mm width given in the
example is a pretty large lamination. You will typically use values
between 12 and 50mm. The sheet is based on the proportions of the
economy E-I core, so that you don't need to enter any other
dimensions of the lamination. If you happen to be using a lamination
that has different proportions than the economy E-I, you should still
enter the width of the center leg, but later you will need to
manually compensate for the larger available window, by increasing
wire size beyond that calculated by the sheet, reducing the copper
loss, and so on.

The second value you have to enter about the core is the stack
height, also expressed in millimeters. This is simply the height of
the stack of E's, well compressed. This sheet does not allow you to
enter the stacking factor, which tells how much of the stack is
actually steel. There's always some little space used by insulation,
and even wasted space due to imperfect compression. But the effect of
this is small enough to ignore, as long as you compress the stack
well enough!

Good stack heights to use are from the same as the center leg width,
to close to twice that. Often you have the choice to use a certain
lamination, stacked as high as the center leg is wide, or use the
next smaller lamination, stacked much higher, with both options
producing the same output. It pays to simulate both options, optimize
each, and compare the efficiency, voltage drop, weight, and cost! The
differences can be profound.

And the third value is the loss factor of the steel material. This
must be taken from the datasheet provided by the manufacturer of the
lamination. My sheet expects this loss factor to be expressed in
watts per kilogram of material, at a flux density of 1 tesla and a
frequency of 50 hertz. Many data sheets include the value in this
exact form, but those published by US companies might instead express
the loss factor in an eclectic mixture of metric, CGS and Imperial
units! If that's what you have, you will need to convert the value
into its fully metric equivalent. You might want to modify the
spreadsheet to do that.

The value of 2 W/kg @ 1T and 50Hz is pretty representative for modern
low cost laminations. A modern ultra low loss material might be a lot
better, while an antique or ultra-cheap material might
be significantly worse.

The next three values in that column are pretty obvious: You have to
indicate the primary and secondary voltages, and the frequency of
operation. The secondary voltage refers to the open circuit (no load)
voltage. And the frequency will usually be either 50 or 60 Hertz. If
you enter a frequency far away from this, it's quite possible that
the loss calculated for the material will be rather imprecise, so use
this sheet with caution if you need to design a transformer for a
very different frequency.

In the right hand column, you have four values which are design
decisions which you can vary somewhat. The first is the all-important
flux density. Just try varying that value, and watch how things
change in the orange output area! Specially, see what happens with
the iron loss. I have already given guidelines about what flux
density to use. Use them and see what happens in your case, when you
change it.

The next is the amount of copper cross section you will allow for
each ampere of current in the windings. Reasonable values are about
0.25 mm^2/A for very small transformers, increasing to 0.5 for large
ones. 0.35 is typical for medium sized transformers (50 to 300 watts
or so). When you adjust this value, the design of the transformer
doesn't really change, but the sheet will calculate a new set of
currents, power, voltage drop, efficiency and loss. With this
parameter, you basically are telling the sheet how much you will
stress a particular transformer.

The fill factor expresses how much of the lamination's window will
actually be filled with copper. It can never be very high, because a
lot of that area gets inevitably filled out with the bobbin, the
wire's lacquer, the air around the round wire, the insulation between
layers, between windings, and some space is always lost due to sloppy
winding, even if you are careful! The value of 0.4 used in this
example has proven in practice to be achievable without much trouble.
If you wind very carefully, and minimize the amount of space devoted
to insulation, you should be able to get up to 0.5. But don't push
this number too much, or you will end up with a transformer design
that you cannot actually wind! On the contrary, if you have never
before wound a transformer, and will do so by hand, in a ragged, ugly
winding, it might be a good idea to design the transformer with an
even lower fill factor, such as 0.3, to make sure you will be able to
fit all the turns! Of course, using a lower fill factor means simply
using thinner wire, and this means that at a given amount of loss and
heating, you get less current.

The fill factor can be pushed beyond 0.5 when you wind a transformer
with square wire (instead of round), or with copper tape separated by
thin layers of insulation. But square wire is hard to find and a
hassle to wind properly, and tape winding is acceptably easy only for
transformers that have rather few turns. This is often the case with
high frequency transformers used in switching power supplies, but not
at line frequencies.

The temperature rise defines how many Kelvins (same as degrees
Celsius in this case) hotter than the surrounding air and objects you
want your transformer to run. You need to carefully choose this
value, according to the highest ambient temperature (inside the
equipment!) at which your transformer will have to work, also taking
into account the highest temperature your wire, insulation material,
varnish, glue, etc, can survive. And what's most difficult, you will
also need to estimate the thermal gradient from the innermost wire
turns (the hottest ones) to the transformer's surface! Calculating
all this can be quite hard, and I can't give you simplified equations
for it. Maybe you can find them elsewhere. The value of 70 Kelvins
which I used in the example design is relatively high. This is so
because this transformer would work in open air, not inside a
housing, where the ambient temperature is never above 25 degrees
Celsius; also, I used class G or higher insulating material, wire and
varnish throughout; and finally, this transformer was carefully
impregnated with varnish, giving it a reasonably good thermal
conductivity between winding layers.

If your transformer will not be impregnated, or use class A or B
insulation material (paper), or run inside a cabinet that can be hot,
then you will need to use a lower value for allowed temperature rise
then my 70 Kelvins!

Finally, you can enter your local and current prices for enameled
copper wire and transformer steel laminations, to have the sheet
calculate the cost of these main materials for your transformer. All
the additional cost, for the bobbin, insulation material, terminals,
bolts and so on, is usually small compared to the copper and steel
cost. The most expensive item is usually the wire, by far.

The first two lines of the orange output area of my spreadsheet show
some basic results for that transformer: The cross sectional area of
the magnetic core and of the winding window, also the total copper
area (after applying the fill factor), the turns per volt constant
that will be valid for all windings on this transformer, and the
average length of one turn, which is calculated as the average
between the length of a wire that goes around the center leg touching
it, and one that goes around the entire winding package, touching the
outer legs.

Then you have a line for the primary winding and one for the
secondary. Each of these lines tells you the number of turns, the
length of the wire need to wind it, the copper cross sectional area
of the wire, and the nominal current that will flow at full rating.
The number of turns isn't rounded off, so you will have to do that,
because you can't wind a fraction of a turn. You can fiddle with your
voltage data to get the sheet to show round numbers of turns. The
wire length is based on the length of the average turn, so this will
be correct only if you wind the primary and secondary side-by-side,
on a split bobbin. If instead you wind the secondary on top of the
primary, you will need less wire than calculated for the primary, and
more than calculated for the secondary. Anyway, these lengths are not
very useful in practice, because wire is bought by weight, not
length. The main situation where they are useful is when you have to
wind a transformer with several thin wires in parallel, because these
are much easier to bend than one thick wire. In such a situation,
it's great to know how long the total winding is, so that you can cut
the strands and twist them together, before you start winding.

The remainder of the orange output area is divided into two columns.
The left one shows some important performance data of the
transformer: There is the input power, expressed in voltamperes,
which is really more correct than watts. The value calculated by the
sheet does not include the magnetizing current; Calculating it would
need additional information about the core. But in medium to larger
transformers, at least, the magnetizing current tends to be small
enough to be ignored.

Then comes the percentual power loss of the transformer, at full
load. This includes both the iron loss and copper loss. The output
power is of course the input power minus this loss.

Then we can see the voltage drop at full load. The value is
calculated only from the resistances of the windings. Any additional
loss caused by imperfect coupling between the windings is not
considered here. So, if you use a poor core or winding technique,
that results in bad coupling, you should expect a somewhat higher
voltage drop. Just for user convenience, the sheet also calculates
the output voltage under full load, which is based on the voltage
drop calculated above.

Below comes the weight of iron laminations and copper wire used in
the transformer. This has several purposes. One is knowing how heavy
the beast will be, of course. The other is knowing how much material
you have to buy! If you use side-by-side winding of the primary and
secondary, you need to buy one half the calculated copper wire of
each size (plus some extra, of course, to be on the safe side). If
instead you wind the secondary over the primary, you need a little
less than half of that weight of the primary wire, and a little more
than half that weight of the secondary.

And then, the sheet will calculate the total cost for laminations and
wire, and also divide this by the power, to give the cost per watt
for your transformer, which is a good figure of merit which you might
want to optimize. Even while this calculation doesn't include the
cost for insulating material and other odds and ends, it's still a
good reference.

The right side column of this area is about thermal matters.
These tell whether your transformer will survive, so don't take them
lightly!  The sheet calculates the power loss in the iron, in the
copper, and adds them to get the total power loss. Copper loss is
calculated at ambient temperature, though. When the wire heats up,
its resistance increases, and so its loss increases too! For this
reason, take the calculated value with a small grain of salt. The
same is true for the voltage drop end efficiency calculations!

You need to be aware of the fact that the iron loss is essentially
constant, regardless of the load placed on the transformer, except
for a little effect caused by voltage drop in the windings reducing
available magnetizing voltage, which causes a slight decrease in core
loss when the load gets higher !  The loss in the wire instead
increases with the square of the current taken from the transformer,
and the value calculated by the sheet is for the full rated current.

This gives you some big help in optimizing a transformer design. For
example, a transformer that will spend lots of time plugged in, but
idling or loafing along at low load, will see very little copper
loss, but the iron loss will be there all the time. So, you should
design that transformer with a relatively low flux density, resulting
in low core loss, accepting a higher copper loss instead, by setting
a smaller value of copper cross section per ampere. After all, most
of the time the rated design current won't be present, so that the
very high copper loss resulting in the calculation will be present
only very rarely, for short times! Transformers used in radio
communication equipment, in audio amplifiers, and many other uses,
are best designed in this way.

The opposite case happens with transformers that are energized only
briefly, but run at full output power whenever energized. Examples of
such use are microwave ovens and spot welders.  In such a
transformer, iron loss always happens at the same time as copper
loss, and you can optimize the transformer to get the lowest total
loss, regardless of how it distributes between the iron and the
copper. Even more, you might intentionally place more loss into the
core than the windings, based on the fact that the core is less prone
to be damaged by heat, has more thermal mass, and that the short
operation time won't allow the peak heat to distribute through the
transformer! And then, such a transformer that operates only for
short times can be designed to have a really huge loss, because it
will have time to cool off between uses! These things are what makes
microwave oven transformers that deliver 800 watts be as small as
a 200 watt transformer intended for continuous service at low rate,
and run at a flux density of 2 teslas or even more!

The heat produced by a transformer has to be dissipated to the
surrounding air. The spreadsheet calculates the approximate total
surface area of the transformer, and finally calculates a required
thermal transfer coefficient, which expresses how much power the
transformer needs to dissipate per unit of surface it has, and per
temperature rise allowed. This coefficient tells you how difficult it
will be to keep this transformer cool enough to survive! The violet
area below this coefficient includes referential values (not
calculated by the sheet) which you can use to try judging whether
your transformer will be OK, when you have it in a tight area, in a
more open area, cooled by a fan, or immersed in oil. I have my doubts
about these values, specially aout the value for the
oil-immersed transformer, so please take these with a big grain of
salt, and let me know if you have any further, better, or more
reliable data.

Anyway, my transformers designed for a coefficient of around 12 have
all survived so far, even while getting quite hot at full load, so
this value can't be too far from the truth.

---------------------------------------------------------------------

A typical design sequence using this spreadsheet would be to first
enter the tentative core size and loss, the required voltages and
frequency, then start with something like 1 tesla and 0.35mm^2/A,
leaving the fill factor at 0.4 and setting the temperature rise
according to your transformer's materials and environment. Then you
can observe the power and current it would operate at, and
the losses, efficiency, voltage drop, and also you would get the
thermal transfer coefficient which you can compare to the table to
gain an idea of whether the transformer will survive. You can then
tweak the flux density and current density, trying to get the
characteristics into the range you need, without exceeding the
thermal possibilities. If you just can't find a combination that
provides what you need, you will have to try with a larger (or lower
loss) core. Then you might want to explore several different core
sizes, optimizing each, and watch the cost, finally settling for the
design that best provides what you need, at the lowest possible cost,
while staying in the survivable thermal range.

This work with the spreadsheet is only the first step, though. When
you have arrived at a good design using the sheet, you need to tweak
it to make it buildable with real, available material! For example,
you cannot get wire in any desired diameter. The sheet doesn't know
that; It might ask you for a wire measuring 1.2345 square
millimeters, or anything else. It's your job to see what wire you can
actually buy, or maybe what wire you happen to have in stock, and
adapt the design. The AWG standard is quite finely stepped, so you
don't need to change the design very much to adapt it to standard AWG
sizes. But I have heard that in the US many stores only sell the even
numbered AWG sizes of wire! That's odd (pardon the pun), since even
down here in less developed Chile I can easily buy all AWG sizes,
even and odd.  If you are limited to even sizes only, you will have
to make bigger compromises.

A good approximation technique is to round the wire size to the
nearest AWG size, or if the values calculated fall just in the middle
between AWG sizes, you might want to use the next thicker wire for
the primary, and the next smaller wire for the secondary. That way
the final losses and the amount of space required will be almost
exactly the same as calculated by the sheet.

If your transformer uses lots of turns of thin wire, you are about
ready to start winding at this point. But if it uses a winding that
has few turns of a thick wire, you should check how well (or how
poorly) that wire fits in an integer number of layers, considering
the width of the bobbin, and about 5% of lost space due to the wire
not being perfectly straight. The problem is this: If the sheet
calculates you need 48 turns of a certain wire size, and it happens
that you can fit only 15 turns per layer, then you will end up with
three complete layers, plus one additional layer in which you have
just three turns! So the total height taken up in the window by
those 48 turns will be as much as 4 complete layers, that could have
60 turns! Consequently, the winding might end up too high, and won't
fit the window! Then you cannot assemble the core, and you have to
unwind that winding, throw the wire away, get new (thinner) wire, do
it again... you get the idea. Try to avoid such frustration!

When you are in the situation just described, it would be wise to try
the next smaller wire size. It's very likely that this smaller wire
size would accomodate 16 turns per layer, thus allowing you to wind
the 48 turns in three nice, clean, complete layers, and using up a
little bit less space than calculated by the sheet. This can in turn
allow you to use the next larger wire size for the other winding,
which will almost completely compensate for the higher loss and
voltage drop of the smaller wire you used for the 48 turns!

Putting it in short words, you have to pick the best wire sizes for
your transformer so that their cross sections are close enough to the
calculated values, but still allow a good, space-saving distribution
on the bobbin and thus inside the window area of the core.

Sometimes you might end up with a high current transformer requiring
a very thick wire. Such a wire is very stiff! If the bobbin is small,
you might not be able to bend that thick wire tightly enough around
the corners of the bobbin. The result would be a huge loss of space,
and the completed winding wouldn't fit in the window, making it
unusable. To work around this problem, you can replace one thick wire
by a bundle of seven wires, each of which is one third as thick as
the single wire. Such a seven wire bundle twists very nicely into a
round cable, and is more then 20 times as flexible as the single
thick wire! It costs only very slightly more money, and performs
great. So, this is the way to go when you have a need for such thick
high current conductors. It's good to know that an AWG number 10
units higher is roughly one third the diameter. So, if you would need
a #7 wire and this is too thick to wind comfortably, you can use 7
strands of #17, lightly twisted together.

Sometimes instead of using one thick wire you will also find it
convenient to wind with two or three thinner wires in parallel,
without twisting them. This technique can significantly ease the
distribution of a winding in entire layers.

Another hint: Most transformers use some thin and some thick wire.
For transformers that have the windings on top of each other (instead
of side-by-side), I suggest to always first wind the windings that
use the thin wire, then the ones using thicker wires, regardless of
which will be primary and secondary. This allows to wind the stiffer
wire on the outside, where the bending radius required is a lot
larger and thus less demanding. Whether the primary is under or over
the secondary has no significant effect on performance.

You might have noticed that my spreadsheet only considers
transformers that have a single primary and a single secondary. But
many transformers use several secondaries, and some use two or more
primaries! In such cases you will have to do some more work manually.
You should use the sheet to calculate the transformer, simulating
only the main secondary, tweaking it for the total power, and then
manually reduce the wire size of that secondary proportionally to the
percentage of the total transformer power this secondary will have to
deliver. Then you can add the other windings, calculating their turns
number from the turns per volt calculated by the sheet, and the wire
size from the current they have to carry, and the mm^2/A you
selected.

Or if you have two equal secondaries (or two equal primaries!), you
can let the sheet calculate a single secondary (or primary) of twice
the voltage. That will produce the correct number of total turns and
wire sizes. You only have to remember to cut the wire after having
wound half of the turns, bring it out, start again and wind the
second half!

You might have noticed that I didn't assign additional wire cross
section to the primary, to account for the magnetizing current. The
reason is that the magnetizing current is normally much smaller than
the main current, and on top of that, the magnetizing current is 90
degrees out of phase with the main current! The vectorial sum of the
main and magnetizing currents is so little higher than the main
current alone, that there is usually no need to consider the
difference.

You might also miss any discussion of core saturation. The problem is
that's quite hard to discuss saturation of silicon steel
cores, because they just don't saturate at a well defined level of
flux density! Instead, the saturation is quite gradual: It might
start at a level as low as 0.5 tesla, become more noticeable at 1
tesla, then the curve bends further, but even at 2 tesla there might
be a significant amount of permeability left! The effect of this is
that with increasing flux density, the magnetizing current increases
more sharply, but it would be really hard to reach a level where the
saturation makes the transformer stop working. So, the most important
consideration about flux density is the sharply increasing core loss.
Only when you are using very high flux density, would it be a good
idea to allow some additional cross section for the primary wire, to
accomodate the larger magnetizing current.

Enough calculations. Let's go to winding!

---------------------------------------------------------------------

[tc1trafo]Very often, winding a transformer for a hobbyist will start
with unwinding an old, burned one! The photo shows part of the guts
of a Heathkit tube tester from the 1950's, which was given to me
in damaged but restorable condition by my friend Renato Menare. Its
power transformer worked, but got extremely hot in just one minute of
operation, and started smelling burned after two minutes. 

After restoring the tube tester, without touching the transformer, I
used it for about a year, switching it on only for a minute at a time
to test one tube, and then letting it cool off. But eventually the
inevitable happened: The transformer burned out, filling my home with
smoke.

Such a tube tester transformer  is one of the worst nightmares any
transformer maker can get: It has two secondaries, each of which has
a huge number of taps to provide all the different filament and plate
voltages needed by any of the thousands of different tubes around in
the 1950s! It takes patience to rewind such a thing. Every few turns
you have to install a tap!

The first step is removing the transformer from the circuit,
unsoldering all wires and taking notes which wire goes where. The
fact that may wires are the same color doesn't help. Then, the core
has to be disassembled. To do that, you first remove the bolts, then
push a sharp knife between the first and second lamination, prying
them apart to break any glue or varnish or rust between them, then
grab the lamination with flat pliers and wiggle it out. Depending on
how much the core was compressed during manufacture, this can be
quite hard to do, and one or two laminations might be damaged in the
process. That's not the end of the world, the transformer should
later work even with one or two laminations less. Usually, after
removing the first few, the others come out easily. Sometimes each
lamination needs a little help with the knife to come loose, while in
other transformers the core falls apart on its own as soon as
compression is relieved.

[tc1roasted]Once the winding assembly has been freed of the core, it
is mounted on a suitable wooden core in the winding machine. Then the
windings are carefully unwound, letting the turns counter of the
winding machine do its job. You should take notes on the number of
turns of each winding, between each tap and the next, and anything
else you find worthwhile noting down. Shooting pictures with a
digital camera can also be very helpful, should you later suddenly
get doubts about where each tap connection should be located!

You should also save the wire, so you can measure its diameter and
find out what sizes of wire you need to buy. 

As the layers of wire and paper come off, things usually turn darker
and ever darker! This is because transformers work hottest at the
inside. This picture shows severely carbonized insulation, both the
paper and on the connection wires. Probably the paper slowly
carbonized, becoming slightly conductive, causing further loss,
further heating, until the beast failed for good. This is the problem
of paper insulation!


[debris]When I was done unwinding this transformer, I found that even
the cardboard winding core was totally carbonized, crumbly and
definitely unusable. I was left with the heap of burned,
charcoal-like insulation shown here, kilometers of thin, crumbly,
burned copper wire, burned pieces of hookup wire, these photos, and a
piece of paper where I had written down wire sizes and turn numbers.

Don't do such a job in your parent's bedroom, because as you can see,
it's messy! Even the kitchen might not be the best place to do it.
Neither mothers nor wifes tend to like this sort of transformer
autopsy!

When not even the bobbin is usable, you will need to make a new one,
or find one that fits. I was lucky this time, and found one in my
junk box (aka treasure chest) that just fit this core, and even had
side walls! It's a great thing that core sizes are standarized, even
if several different standards exist...


[tc1windingmachine]

It's probably high time to introduce my transformer winding machine.
Here you can see it in all its glory, somewhat obscured by the messy
background (my workbench). Amateur radio friend Enrique Villanueva,
CE5FSB, gave me this machine when I was still a schoolboy. That was
many years ago, Enrique is no longer in this world, but I remember
him every time I wind a transformer! He was a true gentleman, and
always helpful.

This machine is simply a motor and switchable two-speed gearbox with
central neutral position, that drives a three-pronged thorn, opposed
to which is an adjustable centering screw. It's quite handmade, but
does the job well. The fast speed is about 120 rpm, and the slow
one is about 15 rpm.

I added a turns counter to it. It didn't have one when I got it. This
turns counter came from a surplus store, out of a broken gas meter.
It advances 4 counts per revolution, so I coupled it to the machine's
drive shaft via a 4:1 speed reduction, getting exactly one count per
turn. To make this coupling, I made two gears! One was cut from the
lid of a coffee can, has 16 teeth, and is visible in the picture. The
other one was made from a piece of wire, has 4 wire loops acting as
teeth, and is hidden in the photo. It's crude, but was cheap and has
worked well for about 30 years now! I can hardly even try to
calculate how many transformers I have wound wit this machine!


[wiresupply]

When winding a transformer, you need to have the wire unwind straight
and cleanly from the spool on which it's sold. If you simply stand
the spool on the ground, the wire will twist while unwinding, curl
up, and kink. At that point, you can throw it away and start anew!
So, don't. Instead, you need to fashion some device that allows the
spool to rotate, letting the wire unwind without kinking. This photo
shows one of the methods I often use: A screwdriver, serving as axle,
held in a vise.

Sometimes I'm too lazy to set up the vise. In those cases I simply
hold that same screwdriver between my knees while winding the
transformer!

[tc1winding]

 Here you can see the junk box bobbin, made from Pertinax, mounted on
a wooden core in my winding machine, and with part of the primary
winding already wound.

There are several styles for winding transformers. The highest
quality one is to painstakingly lay the windings out in neat,
perfect, orderly layers, with every turn of wire placed precisely
next to its neighbor, in close contact, and never crossing over. Each
layer is separated from the next by a sheet of thin insulation
material, cut precisely to the width of the bobbin, and to such a
length that it will overlap a bit. This overlap is placed at either
of the outside sides of the bobbin, not on the sides that will end up
in the core's window. Such a winding is gorgeously beautiful to look
at, produces an excellent fill factor, but is very time consuming to
make. I tend to use that technique when winding few turns of thick
wire, but not when winding many turns of thin wire. One time only I
wound a 78,000 turn high voltage transformer in that way, because no
other technique would do, but I almost turned crazy, and never
repeated that feat.

Modern small split bobbin transformers are normally wound in a very
untidy way: The operator at the factory simply lets the machine run,
and fill the bobbin section with wire, letting it build up wherever
it wants to. There are no layers, no insulation between layers, it
looks poor, is less reliable, the fill factor is less good but still
acceptable, and it's fast and cheap to do.

The winding shown in this photo, instead, is a hybrid of both
techniques. The winding is split up into just a few layers, two or
three or so, but each layer is wound thick, with the wire turns
actually building up on each other. I let the machine run and guide
the wire to let it slowly fill the layer from one side to the other
(never going forth and back several times!), letting the windings
build up to the desired height, about two millimeters or six wire
diameters in this case. When that ugly layer is complete, I apply a
coating of self adhesive tape as insulation, and then wind the next
layer. It's almost as quick to do as a totally wild winding, but
significantly more reliable.

A word about tapes: Transformer parts stores sell self-adhesive mylar
tape, usually yellow, in several thicknesses and widths. This is a
good material, and you should use it. But I often don't have it on
hand, and use alternative materials. Vinyl insulating tape can be
pressed into service, but isn't great , because it gets very soft
when hot, and the wires can press through it. Much better, even if
you don't believe that, is painter's masking tape! It has just about
the optimal elasticity, is thin, cheap, comes in several widths,
holds up well when hot, and looks pretty good on transformers for
antique equipment, on which any plastic tape would look out of place!
Masking tape is what I used for this transformer.

[tc1terminal]

A question many newcomers have is how to handle the ends and taps of
windings. When the wire is rather thick and robust, you can simply
let the end stick out of the bobbin! In that case it's good practice
to protect that wire by a piece of spaghetti (I don't mean an Italian
noodle, but a piece of plastic or fiber hose that looks much like
it). It should ideally be only slightly thicker than the wire. You
slip it over the wire, and anchor it in the winding assembly, with
adhesive tape and the pressure of the windings. It will
protect the wire from chafing, and provide additional insulation at
places where the wire might pass very close to other wires or to the
core.

But thin wires should not be treated like that. They are too fragile,
and might very easily break later, forcing you to rip up and rewind
the whole transformer!  Instead, you should take some pigtails of
stranded, plastic-insulated wire, strip and tin the ends, solder the
thin enamelled wire to the end of a pigtail, and embed it in the
winding like is shown in this photo. You need to place enough layers
of insulating material both under and over the connection, to make
sure that no sharp tip or edge of the wires may puncture through the
insulation. This adds quite a lot of bulk, so of course these
connections must be done on the sides of the bobbin that will end up
outside the core's window!

When you solder these connection, make absolutely sure that the
enameled wire stripped properly in the solder bath. It's very
frustrating to end up with a nice new transformer, that has one
winding that doesn't conduct, because of a badly done connection! In
this photo, the first one and a half loops of the thin enamelled wire
around the pigtail didn't strip, but the other three loops did, so
the connection is secure.

One word about safety: Between the primary and secondary windings,
and at any place where there might be high voltage, you need really
good insulation. After connecting this pigtail, which is the end of
the primary winding, I had to apply such safe insulation. When doing
it with adhesive tape on a bobbin like this, it's done by winding
several layers of that tape, and winding it not only all the way to
the side walls, but even a bit up on these walls, forming a kind of
cradle bedding for the secondary winding. You need to get either a
perfect seal between the tape and the side walls, or having so much
tape, and the wire moved far enough toward the middle of the bobbin,
that the creepage distance from primary to secondary, around the
insulation, is at least 4 millimeters. This is even a legal safety
requirement!

[tc1ready]After winding the two secondaries with their lots of taps,
and re-assembling the core, the finished transformer looked like
this, in all the glory of its dozens of connecting wires! In this
Heathkit tube tester, the wires connect directly to the circuit. In
other cases, these wires would be neatly dressed and soldered to
terminal strips, which are either embedded in the outer layers of
insulation in the winding assembly, or bolted to the core.

---------------------------------------------------------------------

[10ksetup]A winding machine with turns counter is a great tool, but
it's not absolutely necessary. Transformers can be wound with much
simpler tools too. And despite having my machine, sometimes I have to
wind transformers that are larger than what the machine can handle!

Such was the case in 2008, when I had to build two transformers for
10kVA each, starting from scratch. These are the transformers I chose
as an example to put into the spreadsheet before uploading it to this
page! In the sheet, the primary voltage is 230V and the secondary is
2000V. In truth, one of the transformers works in this way, while the
other is opposite, the 2000V winding serving as primary. In the
following discussion, I will use "primary" to refer to the high
voltage winding, regardless of how the transformer will be used
later.

I built the simple but effective setup shown in the photo. A big
wooden bobbin was made, with dimensions such that the complete
winding package for the transformer would precisely fit inside. Note
that this bobbin is just the support for winding! It will be removed
before final assembly of the transformer. For this purpose, it's held
together by screws, and the wooden pieces on the inside are designed
so that they can be easily removed from the finished coil assembly.
This big wooden bobbin got fitted with a hand crank, and mounted on a
steel tube serving as axle, which was clamped to the workbench.

Each wooden piece of the bobbin was separately wrapped in kitchen
wrap (saran wrap, Sichtfolie) before assembling the bobbin. This
assures it can be disassembled after varnishing the winding assembly,
without the wood sticking to the coil assembly! 

The spools of wire for these big transformers weigh 30 kg for the
secondary, and 25kg for the primary. So I made a simple but sturdy
support structure for them and placed it at the work site.

[10kpressingbobbin]

Instead of a complete bobbin with side walls, I chose to employ only
a basic bobbin, made from a sheet of 1.5mm thick Pressspan (I
couldn't obtain a material strong enough with a better temperature
rating). The Pressspan was cut to size, half-depht cuts were made
with the knife at the bending lines, then it was bent around the
wooden bobbin. The overlapping side was smeared with epoxy glue, and
the whole thing was compressed with wooden boards and clamps like
shown here, to get it nice and straight.

[10kstraps]

Since the Pressspan might carbonize and become slightly conductive at
high temperature, I wound two layers of high temperature NMN laminate
over this core, to insure permanent safe insulation between the
winding and the core. It's temporarlily held in place with plain
office type adhesive tape, but this tape is later removed when
winding the wire, to keep it from carbonizing and possibly causing
trouble.

This transformer has sufficiently few turns and thick wire to wind it
in orderly layers, separated by insulating sheets. To keep the whole
thing from falling apart, I bound the layers together with cotton
straps, and to make sure the wire turns don't get closer to the edge
of the bobbin than 5mm or so, I wound cotton ropes at the edges, used
as spacers. These ropes will be removed later.

The photo shows the assembly, ready to start winding. A double rope
makes one turn on each side of the bobbin, enough spare rope for the
successive turns is coiled up on screws driven into the bobbin, and
the cotton straps are installed and coiled up too. The wire end,
protected by a piece of high temperature red fiber spaghetti, is
anchored in a hole drilled into the wooden bobbin. Time to start
winding!

[10kfirstlayer]The first layer has to be wound very carefully,
pushing each winding into tight contact with the previous one. When
that first layer is complete, be sure to count the turns to make sure
you actually got as many turns as you calculated! Otherwise, you need
to compress the winding a bit more, and then add the remaining turns.

When not using a turns counter, such as in this case, a good thing to
do is to wind a full layer, then count the turns, and write it down.
Don't forget! Otherwise it's just too easy to loose count of the
layers! That sounds stupid, but it's very real. A good way to count
the turns is to lightly draw a pointy, non-scratching object (plastic
or wood) over the wire turns, and count the clicks as you feel them.
Counting by sight tends to be confusing.

Since the wire isn't ever perfectly straight, it's impossible to
completely eliminate air between the turns. That's why you have to
consider a safety factor when calculating how many turns you can fit
in each layer!

The next layers instead are easier to wind, because the wire will
embed itself slightly in the depressions between two turns of the
previous layer, automatically assuming the correct spacing. Note that
this produces either winding layers which alternatingly have one more
or one less turn, or else the windings with the same number of turns
in each are alternately offset sideward by one half wire diameter,
respective to each other. You need to consider this when designing
the transformer, when you intend to do a neat, tight winding like
this.

[10kcompressingwire]

After each layer, the wire is pressed flat, using boards and clamps,
on the sides of the bobbin that will end up inside the core's window.
This further helps to actually make the winding fit. It doesn't
matter at all if the windings bulge out a bit on the other two sides,
because these will end up outside the core, where space is plentiful.

If you do this, be careful not to damage the wire's insulation. It's
quite tough, but not infinitely so. The boards should have a smooth
and not too hard surface. I used a thick paper as cushion between the
chipboard and the wire. 

After winding and compressing a layer, a single layer of insulating
material (NMN laminate in this transformer) is wound, overlapping it
a bit on one side that won't be inside the core window. Then the four
ends of the cotton straps are crossed over, pulled tight, and
reattached to the bobbin, and the cotton ropes are wound another
turn, taking care to avoid tangling the straps with the ropes! The
straps will remain in the transformer, while the ropes will be
removed. So the straps should hold just the wire and insulation
sheets, but not the ropes.

[10kbendingwire] If you simply let the wire unwind from the spool and
wind it on your transformer, it will get a bow shape instead of lying
flat, and have so much air below it in the center region of each
bobbin side, that later when compressing the layer you would get
excessive bulging on the other sides. To prevent this, the wire gets
a pre-bend in the opposite sense while winding, by guiding it with
the thumb like shown here. I used a woolen glove for this, because
the wire has to be kept pretty tight, is quite stiff, and it would
wear through my skin, despite being quite smooth!

 This photo was shot during the winding of an intermediate layer of
the primary.

The wire and the cotton ropes are placed between the ends of the
insulation sheet, for the transition from one layer to the next. Said
in a different way, the insulation sheets are simply wound into the
coil of wire and rope.

[10kstrapsattap]

Here you can see how a center tap is made. The winding was arranged
in an even number of layers, so that the center point ends up at the
transition from one layer to the next. At this point, the wire was
brought out and back in through a single hole, and protected with
spaghetti. The spacing ropes were also brought out and back in
through two holes, to keep them from interfering with the wire tap.
Finally, the cotton straps are threaded so that they will hold both
wire ends in place.

After this, the insulation layer can be wound, and then the ropes and
the wire for the next layer.

[10kprimaryready]

After winding the last layer of the primary, the spacing rope is
anchored in a hole in the bobbin. The end of the wire is treated just
like the beginning of it, and brought out through a hole too. I
drilled all these holes where needed, while winding. 

Then several layers of insulating material are wound, since the
voltage between primary and secondary can reach 10kV or more during a
nearby lightning strike! These transformers were built to send power
over a 600 meter long outdoor transmission line, in a mountainous and
lightning-prone area.

Since the secondary winding uses much thicker wire, the spacing rope
needs to be thicker too. I used one strand of thick rope for the
secondary, and two strands of thinner rope for the primary. As a
result, both windings are spaced about the same 5mm from the sides.

[10ksecondaryready]

The secondary is wound just like the primary. In this case it was a
little more complicate, because I actually wound two secondaries,
each of them having a wire half the cross section calculated in the
sheet. On top of that, each of the secondaries is center-tapped
too.This allows me to do several clever things with my transformers,
and as a bonus lets me wind with a thinner, more manageable wire.
This is #7, still stiff enough!

The photo shows the completed secondary, before applying the final
layers of insulation. You can see the spacing ropes, and how the
cotton strap will hold the last turn.

[10kfinishingstraps]

The spacing ropes are pulled through holes and secured, several
layers of insulation are wound, the final layer is secured with a
loop of masking tape, and then the cotton straps are pulled tight one
last time, and knotted together using a special pulling knot, the
same sort truckers like to tie down their cargo. If you don't know
how to tie such a knot, ask a trucker, a sailor or a boy scout.

At this point, the winding assembly is ready to remove part of the
wooden bobbin, and start the varnish impregnation process.

[10kcoveroff]

Removing one of the side walls of the wooden bobbin reveals not only
the structure of the inner part of the wooden bobbin, designed for
strength and easy disassembly, but also you can see the spacing ropes
partially falling out, leaving the insulation protruding nicely
between the turns of wire. This produces plenty creepage distance for
safe operation!

[10kstringsremoved]

After removing all those ropes, the winding assembly looks like this.
Note how the wires are nicely tucked in between the insulation
layers.

While the cotton straps do a great job holding the end turns of each
layer in place, preventing them from falling out and the whole thing
springing open, this assembly is still far too weak to be used like
this. Considerable forces act on wires of big transformers. They need
to be secured very well.

[10kstrapdetail]

And a further closeup showing how the straps hold the wires, and bend
the insulation, always maintaining a safe creepage distance.

In this photo you can also see the overlap of the Pressspan bobbin,
complete with some epoxy glue that seeped out before setting. Since
the bobbin was made 2mm shorter than the core's window length, such
seepage and similar imperfections cause no trouble.

Without removing the other remaining parts of the wooden bobbin, I
soaked the entire winding assembly in impregnation varnish. I
poured the varnish in from the top, until it flowed out profusely
from the bottom. Then I let the assembly soak, let the excess drip
off, and let it dry somewhat on the surface.

But this is an oven-drying varnish, so it needs to be heated, or it
will never dry. I do have a thermostatically controlled oven - but
not large enough to fit a winding assembly of this size! So I applied
enough DC current to the primary winding to slowly heat up the whole
thing to a temperature high enough to dry the varnish.

[10kvarnished]

Every transformer varnish has a recommended drying temperature. To
get it right, I occassionally removed the power supply, and measured
the resistance of the primary winding. Comparing to its resistance
when cold, and considering the thermal coefficient of the resistance
of copper, it's easy to calculate the internal temperature! Then I
adjusted the DC current to keep the temperature just right.

I removed the other wooden bobbin side, and the spacing ropes on that
side, when the exposed side of the assembly seemed to be dry. The
newly exposed side was still fresh, so drying continued with the
assembly supported only by the wooden parts in the center.

When the varnish stopped smelling, signalling that it was dry, I
removed all of the remaining wooden parts. This was quite easy,
thanks to the kitchen wrap, which stayed partially adhered to the
Pressspan core. In this photo you can see that. The brownish color is
that of the varnish, while the near black sections inside the
assembly are partially carbonized kitchen wrap. This stuff doesn't
endure the temperature the varnish needs to dry! But no harm was
done, except to the wrap, which is of course irrelevant.

At this point, the winding assembly has a monolithic feel, with
everything firmly glued together by the varnish. It could actually be
used without further work, but the edges of the thin insulation
material are quite fragile. Any object striking them could bend or
even break them, compromising the creepage distance and thus the
safety of the transformer! 2000 volts at 10 kilowatts is not
something to take chances with.

[10ksilicone]So I filled these areas with silicone caulk. This
material is available, inexpensive, easy to work with, tixotropic,
permanent, an excellent insulator, and endures very high temperature.
In short, it's a nearly ideal material for this task! There is only
one point to watch: Be sure to use the neutral curing type, not the
more common, acid curing one! The last thing you want to do is
releasing highly corrosive acetic acid into your new transformer!

The color of the silicone is your (or in this case, my!) choice.

I didn't do a particularly neat caulking job, but the point was to
get the edges sealed and protected, not to do a work of art.

The winding assembly is now ready!

 [10kcoreassembly]

The next step is assembling the core. I usually do this in the same
way for every transformer: I place the winding assembly on one side,
then I start inserting E laminations from alternating sides.
Distributed throughout the core, I occassionally insert two
consecutive E's from the same side. When almost all E's have been
inserted, things tend to get pretty tight, and forcing additional E's
in between the last one and the winding assembly, can lead to
damage to the latter!

[10klastlam]This is where the E pairs inserted from the same side
come into play: They form excellent guides for inserting a third E
between each two neighboring E's, even using a wooden block as a
hammer to drive them in, if necessary! This is the best trick I have
found to obtain a nice, tightly compressed lamination stack.

After all E's have been inserted, the I's are slid into the voids.
After that, two wooden blocks and a big hammer are used to knock all
the laminations into their correct positions, aligning them with each
other, and specially, align the screw holes of all the laminations!

[10kironassembled]

At this point, the transformer is functional, and you can hook it up
and try it, if you want. But it will hum like crazy, because all
those loose steel laminations will magnetically repel each other at
twice the line frequency rate, and vibrate. The core needs to be
tightly compressed to stop all that noise. The performance will also
improve when the core is correctly compressed.

In small transformers this is done with clamps. But big ones, like
this, use bolts. Often you won't find real bolts long enough and thin
enough, though! In those cases, buy some threaded stock, which is
sold in great lengths, cut it to size (a Dremel tool with a heavy
duty cutoff disk does this very nicely!), and use it with washers and
nuts on each end. 

[10kholeinsulation]

In large transformers, these bolts sometimes have enough shorting
action to cause significant additional loss and heating! For this
reason it's a good idea to insulate the bolts from the core. You can
use tubing for that purpose, or like I did here, slide in a sheet of
NMN laminate (or plain paper) rolled into a tube.

[10kassembled]

Installing the bolts just on the core leaves you with an unevenly
compressed lamination stack, and what's more important, with nothing
to mount the transformer! For that reason usually some angle stock is
used . It distributes the force over much of the core, and provides
convenient surfaces to drill mounting holes into.

Some transformers use formed caps instead, or complete steel frames.

Note the insulation sheets under the washers of the bolts! Depending
on the specific transformer, they might make any difference, from
barely noticeable to dramatic!

At this point, the transformer is truly ready for trying. If it still
hums, you can try tightening the bolts even further, and inserting
wooden or plastic wedge pairs between the winding assembly and the
core center leg, to compress the latter. If even this fails, which is
often the case, then you have no better option than once again
getting your varnish can, and soak the core in varnish! You can
loosen the bolts, let varnish flow into every space, then tighten the
bolts again and warm up the whole transformer by applying a suitable
amount of DC for at least a full day. I have yet to see a
transformer that still hummed after that treatment!

[smallferritetrafo]Depending on the application, different things can
be done with the connections. If they are made of flexible wire, they
might be directly wired into the circuit. In small transformers,
often plastic bobbins with pins are used, and the windings are
connected to these pins, like shown here, and then the whole
transformer is soldered to a printed circuit board. The example shown
here is a current sensing transformer, which also has a one-turn,
high current winding, which is brought out separately from the bobbin
pins.

[argtrafo]Often terminal strips are attached to the winding assembly,
and the windings are connected there. But in larger transformers, the
most common practice is bolting terminal blocks to the transformer,
and connecting the wires there. I did this with my large
transformers, leading to the final product shown in the very first
photo of this web page. That system is more solid and reliable then
most others, and allows repeated solderless disconnection and
reconnection, which I need a few times per year to reconfigure my
transformers for different conditions of usage.


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Some people wonder whether they should try winding their own
transformers, or if they should instead shell out big $$$ to have the
job done by a professional winding shop. I can only tell you one
thing: If you had the patience to read this long web page from start
to end, then you most certainly also have the patience required for
winding your own transformers!!!

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