build a TRANSFORMER

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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.

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.

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!
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.

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.
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.

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.

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.

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.

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.

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!

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.

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.

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.
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!
 
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!
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!

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.

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.

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!
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.
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!!!