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Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Hi everyone. I've tried making a nicer secondary for my DRSSTC. its 6 inch outer diameter 24 inch tube. Winding length is 20 inches of 30 AWG. The winding was absolutely flawless. Just perfect, best secondary I've ever made. Things went south when I went to put the first coat of varnish on. What I did was kept my coil horizontal on the winding rig (just a steel rod through the coil to some pieces of wood, your typical homemade winding jig) and I slowly rotated the coil while holding the brush carefully against the windings going from left to right. The winding's immediately started scrunching together and overlapping. Within 5 seconds it turned into looking like a tangled fishing reel from hell (or just a tangled tesla coil from hell ) My theory is that the bristles on the brush I used, which was 100% natural bristle, must have gotten in between the winding's and pushed them together somehow, I really don't know. So thinking I figured it out, I went and bought a foam brush that is specific for polyurethane, ordered some new wire, and started over. This time the winding was excellent again. First thing I did was spray the winding's with a light coat of enamel so that the coil would be nice sturdy for the varnish, except the enamel sprayed on much heavier then I anticipated and weighed down the winding's, causing them to scrunch together again and end catastrophically. So here I am now, ordered some more wire, and going to try again in a few days. If anyone has any tips or pointers I would greatly appreciate some help here, Thanks. P.S. never got to take pictures of this, was too agitated, so no photos, sorry
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The symptoms you describe are clearly the result of inadequate 'attachment' of the wire to the former, which you would expect to get by tension and friction.
One mitigation would be to wind the former with a higher wire tension.
You don't say whether your winding rig is as automated as a lathe, with a leadscrew to apply the wire, or simply a method to rotate the coil and you run the wire on by hand. It could make a difference to whether the following is practical. I've had very good results with a lathe-type winding rig, using a piece of M12 studding as a leadscrew. This was rotated by some LegoTechnic gears, geared off the secondary. It took quite a few experiments to get the pitch/gearing right. This took care of the wire application, and I had a hand free to run some varnish onto the former a little ahead of the wire. It was tacky by the time the wire landed. The excess varnish was getting rotated, so didn't pool. But if you haven't got a leadscrew, you might need too many hands / not have enough concentration to use this method.
The thermal expansion of plastic is an order of magnitude higher than that of copper wire. If you wind at a high ambient temperature, then you could expect the wire to move under cooler conditions. If you can wind cold, and varnish warm, the wire would be under higher tension for the varnish. It would be an easy experiement to do, to see whether this effect was useful, with just a few turns of wire.
Some people have advocated putting a strip of double-sided tape along the former before winding. The discontinuity and possibility of trapped air in the dielectric could make it a weak spot for breakdown, so opinion is mixed on whether this is a good idea.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I've had problems with thermal expansion of plastic formers too. The wire looked great at the time, but the next morning it had slumped. I think I made matters worse by using pipe made of HDPE, a waxy, low friction material that doesn't hold onto the wire too well.
I've got sparks over 4x the winding length from a coil burnt in multiple places, repaired by soldering the wire ends together and wrapped in black electrical tape to hide the mess, so I don't think the dielectric integrity is super important. The winding itself acts as a quarter-wave resonant "metal insulator" and does a good job of grading the field and preventing breakdown.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You could give the former a coat of varnish/lacquer before winding .. should be dry (or slightly 'tacky') the wire will stick to/embed in the varnish whilst winding. One gentle light (dilute with thinners etc to allow the varnish to penetrate the winding) coat after winding allowed to dry (really dry, say 12 to 24 hours) will hold the windings in place whilst adding more layers/coats of lacquer/varnish.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I just did some very crude sums using the data from Kaye and Laby online.
Assume a former of PVC, polypropylene or polycarbonate (all of which have thermal expansion coefficients in the order of 75u/K). Copper has an expansion coefficent of about 15u/K, and a Youngs modulus of about 130GPa.
That's a differential expansion of 60u/K, so 600u in 10 degrees temperature change.
If the former is 100mm diameter, 50mm radius, if the wire is just snug at the winding temperature, then it will be 30um clear (50mm * 600u) all the way round if the temperature drops by 10 degrees. 30um doesn't sound much, but it's a lot if you actually want the wire touching. To put it another way, I estimate that the wire would have to tilt to 3 degrees to take up the slack.
If you strain copper by 600u, you will induce a stress of 78MPa (130G * 600u). If we assume that the tube wall is thick, so much stiffer than the copper, and wire that's 0.5 mm in diameter, so having an area of about 0.2u, that's a force of 15N (78M * 0.2u) or around 1.5kgf. If you wind a former with wire at this tension, the wire will still be in tension after a fall in temperature of 10 degrees.
Obviously the tube isn't infinitely stiff compared to the wire, so the tension figures will be lower than this ballpark, but they do give an idea of the size of the ballpark.
Hi everyone. I've tried making a nicer secondary for my DRSSTC. its 6 inch outer diameter 24 inch tube. Winding length is 20 inches of 30 AWG. The winding was absolutely flawless. Just perfect, best secondary I've ever made. Things went south when I went to put the first coat of varnish on. What I did was kept my coil horizontal on the winding rig (just a steel rod through the coil to some pieces of wood, your typical homemade winding jig) and I slowly rotated the coil while holding the brush carefully against the windings going from left to right. The winding's immediately started scrunching together and overlapping. Within 5 seconds it turned into looking like a tangled fishing reel from hell (or just a tangled tesla coil from hell ) My theory is that the bristles on the brush I used, which was 100% natural bristle, must have gotten in between the winding's and pushed them together somehow, I really don't know. So thinking I figured it out, I went and bought a foam brush that is specific for polyurethane, ordered some new wire, and started over. This time the winding was excellent again. First thing I did was spray the winding's with a light coat of enamel so that the coil would be nice sturdy for the varnish, except the enamel sprayed on much heavier then I anticipated and weighed down the winding's, causing them to scrunch together again and end catastrophically. So here I am now, ordered some more wire, and going to try again in a few days. If anyone has any tips or pointers I would greatly appreciate some help here, Thanks. P.S. never got to take pictures of this, was too agitated, so no photos, sorry
-Harrison
Sorry to hear your troubles; would be helpful if you could get a picture of what happened. I've wound many coils from 40AWG all the way up to 24AWG, almost all on PVC pipes , and never had any of the problems you described. What tube did you wind the wire on?
Here's how I usually wind my coils - I wind them by hand, trying to keep them as tidy and tight as possible, and I paste tape every few inches or so to keep the windings tight. At the end, I tape down the ends of the coil, remove the tapes in-between, and give it a very light coating of polyurethane using a brush (I've used all kinds of brushes, just make sure you try to get as many loose bristles out before).
* Many thin coats are better!
Recently, I've tried something else, using a polyurethane spray and very lightly coating the secondary the first time to hold the coils in place. Then I go using a brush to give many layers of thin coats (spray is expensive), and it always works out very well.
I suspect if the coils are too loose or if there is too much space in-between turns, capillary action takes over and the varnish fills up the gaps between the turns, spoiling the coils. I have never observed that in any of my coils before though.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I would have thought you'd want to warm up the reel of wire before you start winding, then hopefully it's thermal mass will keep it warm(ish) whilst you're winding. I've never tried this but I used to have similar problems with the windings collapsing the next day!
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Thanks guys! I didnt even think of the thermal expansion. That has to be it.
Dr. Slack wrote ...
The symptoms you describe are clearly the result of inadequate 'attachment' of the wire to the former, which you would expect to get by tension and friction.
One mitigation would be to wind the former with a higher wire tension.
You don't say whether your winding rig is as automated as a lathe..
The thermal expansion of plastic is an order of magnitude higher than that of copper wire. If you wind at a high ambient temperature, then you could expect the wire to move under cooler conditions. If you can wind cold, and varnish warm, the wire would be under higher tension for the varnish. It would be an easy experiement to do, to see whether this effect was useful, with just a few turns of wire.
Some people have advocated putting a strip of double-sided tape along the former before winding. The discontinuity and possibility of trapped air in the dielectric could make it a weak spot for breakdown, so opinion is mixed on whether this is a good idea.
Steve Conner wrote ...
I've had problems with thermal expansion of plastic formers too.
The steel rod for the winding jig is attached to the chuck of an old drill which is plugged into a variac to control power/speed. I must say the windings were under decent tension against the form. But as you mentioned, I made the coil inside at room temperature and took it into the garage to apply varnish, where it is freezing. So I guess that must play a huge role here. I will try the opposite this next time around. As for using double sided tape, I've done this once and it just slows the winding time down greatly as well as gives the coil an ugly indentation.
Sulaiman wrote ...
You could give the former a coat of varnish/lacquer before winding
I do this with enamel spray because when it dries, it's kind of 'tacky' and the wire goes on very smoothly but at the same time it doesn't slide around much as if it was just the bare plastic form.
loneoceans wrote ...
What tube did you wind the wire on?
PVC, I will take pictures when I wind next time.
Avalanche wrote ...
I would have thought you'd want to warm up the reel of wire before you start winding, then hopefully it's thermal mass will keep it warm(ish) whilst you're winding. I've never tried this but I used to have similar problems with the windings collapsing the next day!
Not sure how I would heat the wire while winding and at the same time keep the plastic form cold.
Thanks again everyone, I'll report back when I wind the new one. Hopefully with no problems
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
I use a process very similar to loneoceans. I wind by hand with a simple jig. Then for coating I switch in a small geared motor to turn the coil consistently to get even distribution and to stop pooling. Then spray on the poly from a can. Do about 6 coats and it turns out real nice and smooth.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Avalanche wrote ...
I would have thought you'd want to warm up the reel of wire before you start winding, then hopefully it's thermal mass will keep it warm(ish) whilst you're winding. I've never tried this but I used to have similar problems with the windings collapsing the next day!
Physics Junkie wrote ...
Not sure how I would heat the wire while winding and at the same time keep the plastic form cold.
There is such a large difference between the expansion coefficients that you don't have to. 75u for the PVC, 15u for the copper, 60u differential. You would have to heat the copper through 4 degrees to get the same effect as cooling both copper and plastic by only 1 degree. So it's not worth mucking about with the copper temperature. Put your ski hat, thick socks and body-warmer on, and wind the coil in the out house.
You only have to resort to difficult tricks like differential temperatures when you are dealing with two materials with the same coefficient. So you put your steel shaft in the freezer, steel ball-bearing inner race in hot oil, then get them in the right place quickly before the temperatures equalise.
If you do take them somewhere cold to wind, beware condensation. I'm thinking particularly about your breath. The out house and components might have reached a nice dry low humidity equilibrium, but if you go and spend an hour or two in there, you could raise the humidity such that it could condense on the former or wire. I don't need to spell out the problems that dampness could cause for varnish drying or subsequent kV operation.
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Varnish the secondary. Catastrophe
) My theory is that the bristles on the brush I used, which was 100% natural bristle, must have gotten in between the winding's and pushed them together somehow, I really don't know. So thinking I figured it out, I went and bought a foam brush that is specific for polyurethane, ordered some new wire, and started over. This time the winding was excellent again. First thing I did was spray the winding's with a light coat of enamel so that the coil would be nice sturdy for the varnish, except the enamel sprayed on much heavier then I anticipated and weighed down the winding's, causing them to scrunch together again and end catastrophically. So here I am now, ordered some more wire, and going to try again in a few days. If anyone has any tips or pointers I would greatly appreciate some help here, Thanks. P.S. never got to take pictures of this, was too agitated, so no photos, sorry
