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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843
Herr Zapp, can you point me to any reference material online anywhere that gives numbers for the DF of CPVC? If it really is 3 or 4 times higher than PVC (which is bad enough), then I can say without any further calculations that it's useless for my application. Thanks.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Steve McC -
Here's my process for applying epoxy coating to secondary coils. It's a rather long writeup, but this process has been refined over years of use and the details are critical to obtaining the best results.
EPOXY COATING PROCESS FOR TESLA SECONDARY COILS REVISED 10-5-09
BACKGROUND I have used a two-part transparent epoxy resin for coating secondary coils since 1998, and have developed an application process that works very well for me.
The epoxy I use is a two-part, medium-viscosity material called "Envirotex-Lite", which is advertised as a durable "pour-on" coating for counter tops and other high-wear areas. The manufacturer is Environmental Technology, Inc ().
There are a number of consumer and industrial epoxy products that are probably suitable for coating secondary coils, but this particular one has consistently given very good results. This material has the viscosity, pot life, and cure time that make it ideal for this use. The cured resin is slightly flexible, at least in thin sections.
One ABSOLUTELY necessary item is a motorized "rotisserie" device to support and rotate the secondary at a slow speed. I just use my motorized winding fixture, and coat the secondary as soon as winding is completed. For best results the fixture should have an adjustable-speed motor (I use a DC gearmotor with integral right-angle drive) and a foot-switch with a toggle switch wired in parallel. The foot-switch allows you to stop the coil rotation if required to remove lint or dust, and the toggle switch is used after the resin application is complete and coil needs to rotate unattended for several hours. The adjustable motor speed allows you to set the optimum surface speed regardless of secondary diameter. The foot switch allows you to keep both hands free to apply the coating, and also allows you to stop the rotation if required to deal with bubbles or lint.
Having coated at least a dozen secondaries with epoxy, I can assure you that NO solvent-based coating can remotely begin to compare with the appearance of a correctly-applied epoxy coating. First of all, a single "light" application of epoxy provides a cured coating thickness of at least .030". It would take many, many coats of varnish to generate a similar thickness. The total "wet-time" of these multiple coats of varnish is many times that of the single coat of epoxy, greatly increasing the time that the coating is susceptible to picking up dust, etc.
Because the epoxy stays "syrupy" until it starts to jell, you can flow on a heavy coat (relative to the coating thickness obtained from a solvent-base varnish) very quickly, then allow the coating to "self-level" while rotating. This results in a finished secondary that appears to have the windings encased in a glass sleeve. Any type of solvent-based coating begins to lose solvent, thicken, and skin-over the instant that the lid is removed from the can. It's impossible to "level" the entire coating on a coil at once when using a solvent-based coating.
TOOLS AND MATERIALS Motorized coating fixture Two-part epoxy resin Disposable plastic measuring container Clean mixing container Clean stirring rod Heat gun 3†or 4†wide foam paintbrush Waxed paper Pin or needle (for removing dust or lint)
APPLICATION PROCESS
1. Set up a completely dust free, draft free area to work in. I use a bathroom, damp-mop everything, close all windows, etc. A carpeted floor is useless; any movement stirs up a cloud of fibers. Place a large piece of wax paper (inherently lint free) under the secondary to catch the inevitable drips.
2. Very thoroughly wipe down the completed coil with a clean cloth moistened with 90% isopropyl alcohol to remove any trace of oil from the windings. Even fingerprints may cause "fisheyes" in the completed coating.
3. After all traces of solvent have fully evaporated, I do a careful wipedown with a cabinetmaker's "tack cloth" to pick any residual lint or dust. Wipe slowly; if the humidity is low, even the varnish on the magnet wire can acquire an electrostatic charge and attract dust and lint.
4. Calculate the approximate volume of material required to coat the OD of the secondary, based on a thickness of .040" to .050". Always err on the high side. I know the panic of having to mix up another ounce of epoxy to finish the last few inches of an almost perfect secondary. As a portion of the coating will "wick" down into the windings, the final coating thickness will be less than .040" or .050".
5. Measure the volume of resin and hardner carefully to achieve a 1:1 volumetric ratio. Incorrect ratio or poor mixing can result in a coating that never cures properly, and remains "tacky". The ETI-USA website provides additional instructions for measuring and mixing. Mix the resin and hardner thoroughly in a meticulously-cleaned, completely dry container; glass is preferred, DON'T use a waxed paper cup. Use a clean piece of wooden dowel or a metal rod to stir. I end up with a froth of bubbles, but they seem to all pop during the coating application. You can vacuum-degass the mixed resin, but I don’t find this necessary.
6. Turn on the fixture drive motor & adjust the speed, depending on the diameter of the secondary (20 RPM is about right for a 6†diameter secondary). LEAVE THE MOTOR RUNNING UNTIL THE COATING IS COMPLETELY CURED! This may be 6-8 hours, depending on ambient temperature.
IMPORTANT: From this point forward, try to reduce your movements (walking around, etc) to avoid stirring up dust & lint. Have all your tools and materials laid out in front of you at arm's reach.
7. Holding the brush vertically, position the tip of a 3-4" wide disposible FOAM paintbrush (NOT a bristle brush) against the top of the secondary, and start pouring a very thin stream of resin immediately in front of the brush. The first rotation will only spread the liquid slightly, getting better coverage with each turn. At this point, the brush is being used as a squeegee. Allow just the very tip of the brush to lightly contact the secondary; do not press the brush hard against the surface.
8. Get an initial spiral of liquid applied from end-to-end of the coil, then go back and spread it into a uniform coat.
CAUTION! DON'T TRY TO APPLY TOO THICK A COATING! If too thick a coat is applied, raised circumferential rings may develop, even with the fixture turning at a low speed. At first it may seem that there is not enough resin to completely cover the windings, but continue to spread it with the foam brush. Add more coating only if it is impossible to get coverage. A small amount of resin can be spread to cover a surprisingly large surface area.
9. As the coating spreads over the surface, reduce the pressure on the brush until it is just barely contacting the surface of the coil.
10. Keep the coil rotating, and inspect for tiny pieces of lint, etc. that may drift onto the surface. Stop coil rotation and use a pin to lift any particles from the surface. Stop the rotation only long enough to remove the lint.
11. By this time, 99% of the bubbles should have disappeared. Use a hot air gun to quickly sweep spots with any remaining bubbles. DO NOT APPLY EXCESSIVE HEAT TO ANY ONE SPOT ON THE COIL!!!
Invariably there is air trapped under the windings. Applying excessive heat will cause this air to expand and bubble out around the windings and into the coating. Swept rapidly over the surface, the hot air gun temporarily thins the very top layer of the coating, and at the same time causes the air in the bubbles to expand. Most of the remaining bubbles will pop by themselves. Finally, if there are any remaining bubbles that don't respond to the hot air gun, use a pin to pop them.
(It has been suggested that carbon dioxide can be used to cause bubbles in the coating to pop; this is a myth. There is no physical or chemical reason why CO2 should cause the bubbles to pop. I tried directing streams of pure CO2 and pure nitrogen on early coils I coated, and the gas had no effect on bubbles. However, mild heat from a heat gun or hair drier works great. The heat causes the viscosity of the surface of the epoxy to decrease instantly, and the heat also visibly expands the bubbles. Someone has also suggested the use of a propane torch; this is a recipe for disaster.)
12. Inspect the coil for bubbles, pinholes, lint, thin areas, etc. When satisfied that everything is perfect, slowly walk out and SLOWLY close the door. Make sure no one opens the door until the coating is fully cured.
13. Resist the temptation to touch the surface to “see if it’s curedâ€, as there's a chance you'll leave an unrepairable fingerprint as a permanent reminder of your impatience. Check the hardness of the drippings on the waxed paper, or the residual resin in the bottom of the mixing cup. Don’t touch the surface of the secondary for at least 6-8 hours.
14. After 8 hours have elapsed, and you've confirmed that the coating is fully cured, remove the secondary from the fixture and admire your work.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Thanks for the write-up, Herr Zapp.
I've tapped holes end-on into the walls of HDPE pipe just fine. This kind of pipe is used for water and gas mains that operate at high pressure, so the walls are usually pretty thick, and there's plenty of room.
The last time I did this, the wall was 10mm thick, so I drilled 5mm holes, tapped them M6, and used M6 nylon screws to hold the end caps on.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Herr Zapp wrote ...
jp -
Here's one reference giving the dissipation factors for a number of engineering polymers, including PVC and CPVC. See Table 6:
Here's a manufacturer's datasheet giving the dissipation factors for CPVC at 10KHz, 100KHz, and 1MHz:
Regards, Herr Zapp
Hello Herr Zapp,
I think the values (table 6) in your first reference may be incorrect. They seem to be at odds with every other source I've seen.
Most sources I've seen give a value of between 0.009 and 0.016 for PVC (over the range of 1 Khz to 1 Mhz), whereas your first reference says 0.007 to 0.020 (although other than to say "low" and "high" your source doesn't specify a frequency range).
This is at least in the ballpark; however, for CPVC, your first reference says 0.030 to 0.075, which seems way too high, IMO. (Note that your second reference gives values of 0.012 to 0.017 for CPVC (between 10 Khz and 1 Mhz), which is in basic agreement with values I see stated elsewhere.
So, based on everything I've seen so far, I would say that the dissipation factor for PVC and CPVC are roughly comparable.
In my calculations, I suppose I'll use a value of about 0.015 to get an idea of the losses.
Also, with regard to the epoxy coating you use on your secondary windings, do you have any electrical specs on the material? Depending on its characteristics, it may be absorbing some significant power.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
jp -
I have to agree with you, the one reference giving the elevated dissipation factor for CPVC looks "questionable".
Initially, I noticed this, but seeing the relatively high percentage of chlorine in CPVC vs PVC, I thought this might explain the difference in dissipation factors.
Since CPVC is infrequently used in electrical applications, most resin manufacturer's don't seem to list the DF.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Herr Zapp,
Do you have any info on the electrical properties of the epoxy you use? I'm wondering how much it might affect the performance of your secondary coil in terms of additional capacitance and power loss.
Given the high frequencies and high voltages involved, I think if it were me I would hesitate to expose anymore dielectric to these efields than I have to.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
jp -
I think you're fixating on the wrong things.
The Envirotex-Lite epoxy I use for secondary coating is marketed for use as a protective coating for bars and table-tops. The manufacturer has no data on electrical properties. Regardless, epoxy materials are very commonly used as coatings, encapsulents and potting compounds for high-voltage, high-frequency components and assemblies. Any "power loss" due to an epoxy coating on the secondary in a typical Tesla coil application would probably be unmeasurably low.
On the other hand, damage to the magnet wire insulation on a secondary from accidental bumps and scrapes, etc is a very real and common problem that is greatly reduced by a protective epoxy overcoat. Additionally, it is my experience that supplementary insulation on the secondary can greatly reduce the incidence of secondary flashovers, which allows a higher primary-secondary coupling factor to be used and can ultimately increase streamer length.
This is very much like worrying about the dissipation factor of the material used for the secondary coilform (acrylic. PVC, ABS, wood, etc). The effect of former dissipation factor is so slight as to be inconsequential.
If you are looking to build an efficient coil (input power:streamer length), there are MANY other areas to focus your attention on that will have a MUCH greater influence on coil performance than a fraction of a millimeter thick epoxy coating on the secondary.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Herr Zapp wrote ...
jp -
I think you're fixating on the wrong things.
Well I wouldn't call it "fixated"; rather, I'm just somewhat more thorough than most people who basically just want to slap something together ASAP and make sparks, that's all.
wrote ...
The Envirotex-Lite epoxy I use for secondary coating is marketed for use as a protective coating for bars and table-tops. The manufacturer has no data on electrical properties.
Sometimes the manufacturer may have information beyond what they publish; e.g., from another customer who did something similar and gave feedback, etc. Or maybe you even made some measurements yourself (e.g., measure fo and Q before and after coating); I have no idea, but it doesn't hurt to ask, right?
wrote ...
Regardless, epoxy materials are very commonly used as coatings, encapsulents and potting compounds for high-voltage, high-frequency components and assemblies.
Well of course, but the devil's in the details. Epoxies differ, and generally speaking, I would say that most commercial epoxy-potted coil/transformer assemblies, for example, are going to operate at nowhere near the volts/turn and frequency of a TC secondary, right?
wrote ...
Any "power loss" due to an epoxy coating on the secondary in a typical Tesla coil application would probably be unmeasurably low.
I'd bet it would not be "unmeasurably low". Depending on the particulars, it may be small as a percentage of power input, and therefore tolerable in terms of efficiency, yes, but I think it's reasonable to consider what the temperature rise will be in the various dielectrics involved.
wrote ...
On the other hand, damage to the magnet wire insulation on a secondary from accidental bumps and scrapes, etc is a very real and common problem that is greatly reduced by a protective epoxy overcoat. Additionally, it is my experience that supplementary insulation on the secondary can greatly reduce the incidence of secondary flashovers, which allows a higher primary-secondary coupling factor to be used and can ultimately increase streamer length.
This is very much like worrying about the dissipation factor of the material used for the secondary coilform (acrylic. PVC, ABS, wood, etc). The effect of former dissipation factor is so slight as to be inconsequential.
Being an Engineer by formal training and experience, considering the factors involved in efficiency and associated thermal management issues is simply part of the way I think about things.
wrote ...
If you are looking to build an efficient coil (input power:streamer length), there are MANY other areas to focus your attention on that will have a MUCH greater influence on coil performance than a fraction of a millimeter thick epoxy coating on the secondary.
Actually I'm building a ferrite HVHF transformer, so the operating conditions are somewhat different than for an air core TC.
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What's the best way to secure the ends of single layer windings on a plastic form?
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