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Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Hi !
Iwold say that for me the polyester (Mylar) film insulation is the more suitable insulation. 0.05 and 0.1 mm thichness and proper calculation of the corona starting voltage, creepage voltage, a good mechanical jig to keep the film and wire layers in proper place is fundamental. Dry and impregnation with alkid clear vernish is warranty for long years of continuous service. A have paid US $ 2,00 or less for 1m x1m square sheet 0.1 milar. from a local motor rewind shop. Exept for working temperature for teflon has similar dielectric characteristics of polyester. Having corona inside the winding, poly, teflon, werever the winding will fry... Regards Newton
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I ended up using other methods (so far), so at this point, I can't say from experience how well it works.
As I see it, the three main electrical issues with winding a coil are: (1) to minimize the capacitance; (2) to minimize the dielectric loss; and (3) for unencapsulated coils operating in air, to keep the electric field below the corona threshold (say 30 kv/cm) as much as possible.
The silicone tape supposedly has a dielectric constant of about 3, so it's comparable to mylar and other materials in that regard. It has a low dissipation factor of only 0.0005, or something like that, which is orders of magnitude lower than mylar, so it would seem to be very good in terms of minimizing dielectric losses. And it's also relatively thick, which would tend to lower the capacitance and electric field between layers.
As far as minimizing corona is concerned, if you assume a corona starting voltage of 30kv/cm, and the tape is 0.02" (0.05 cm) thick, then I suppose you would want to limit the peak voltage difference between layers to (0.05 cm * 30 kv/cm) 1.5 kv. So depending on the details, you might have to limit the number of turns per layer to a value siginificantly less than what you have room for.
Anyway, in my experience, the other issues with layer insulation are mechanical properties. In addition to the availability of the material in convenient lengths, widths, and thicknesses in the first place, there are also things like the stiffness and "slipperiness" - things that relate to the ease of actually working with the stuff.
In the case of silicone tape, one possible disadvantage may be the fact that it is only commonly available in a 1" width.
Overall I would say it's worth experimenting with...I would say get some and give it a try.
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
oh good. 3M makes the stuff now. I'd trust buying from them more than the infomercial companies I've seen advertising it. I'll pick some up when I start getting paid in a few weeks. I wonder how well their scotch tapes hold up to hv. Is the aforementioned mylar insulation adhesive or fusing in any way? That's a nice perk to have when you're winding lots of layers. on a side, but related note: is superglue a good insulator for hv applications? I use it quite extensively to hold everything in place but I'm not sure if it's helping or hindering the insulation of the windings. It's certainly not good for high temperature applications, so I wouldn't use it there.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Here's another idea I had regarding the use of this silicone rubber tape.
Suppose you get the 1.5" wide tape, and you wind layers of 28 gauge wire, for example, that are about 0.75" wide, giving you a 0.375" margin (approximately) on each side.
Then, when you're done winding the coil, you put it into a specially designed vacuum chamber and pull a vacuum.
And I'm just thinking out loud here, and the key is this: the wall of the vacuum chamber is cylindrical and just a little bigger in ID than the OD of the coil, and it's mechanically weak enough such that after you pump down the chamber, you tighten a "hose clamp" on the outside of the chamber and compress the coil radially inward...the idea being that after the air is pumped out, you compress the layers of silicone rubber together so that the rubber cures with little or no air trapped in the coil.
Instead of actually using a mechanical clamp of some kind, what if the vacuum chamber had a double wall, i.e., a container within a container, and you pump both containers down at the same time; and then, while maintaining vacuum in the inner chamber (with the semi-flexible wall), you open a valve and allow the outer chamber to come up-to-air, thus squeezing the coil radially inward.
I haven't worked with this silicone tape yet so I don't know if this idea is feasible or not, but if it is, it would be a way of "vacuum encapsulating" the coil without the hassle and the expense and the mess of using epoxy. And even if there was a little air left in the coil, it probably wouldn't matter, as the stuff is supposedly extremely corone resistant in the first place.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
jpsmith123 wrote ...
Here's another idea I had regarding the use of this silicone rubber tape.
Suppose you get the 1.5" wide tape, and you wind layers of 28 gauge wire, for example, that are about 0.75" wide, giving you a 0.375" margin (approximately) on each side.
Then, when you're done winding the coil, you put it into a specially designed vacuum chamber and pull a vacuum.
And I'm just thinking out loud here, and the key is this: the wall of the vacuum chamber is cylindrical and just a little bigger in ID than the OD of the coil, and it's mechanically weak enough such that after you pump down the chamber, you tighten a "hose clamp" on the outside of the chamber and compress the coil radially inward...the idea being that after the air is pumped out, you compress the layers of silicone rubber together so that the rubber cures with little or no air trapped in the coil.
Instead of actually using a mechanical clamp of some kind, what if the vacuum chamber had a double wall, i.e., a container within a container, and you pump both containers down at the same time; and then, while maintaining vacuum in the inner chamber (with the semi-flexible wall), you open a valve and allow the outer chamber to come up-to-air, thus squeezing the coil radially inward.
I haven't worked with this silicone tape yet so I don't know if this idea is feasible or not, but if it is, it would be a way of "vacuum encapsulating" the coil without the hassle and the expense and the mess of using epoxy. And even if there was a little air left in the coil, it probably wouldn't matter, as the stuff is supposedly extremely corone resistant in the first place.
I think the question is 'how would this compare to vacuum-degassed oil insulation?', for example.
Maybe it's possible to answer this using maths rather than practical experimentation, I don't know.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
In my view, oil is just so messy and generally annoying, that I see it as a last resort...an extreme solution.
I think it boils down to how easy it will be in practice to work with the silicone tape. Maybe the layers of rubber will stick together immediately, as the coil is being wound, thereby permanently trapping some air inside. In which case pumping it down and then squeezing it probably won't accomplish very much.
Or maybe there's some other negative physical/mechanical aspect of the tape that'll reveal itself when someone tries it.
I think it would make for an interesting experiment. I may try it myself someday but not right now as I presently have too many other irons in the fire.
Edit:
I see that McMaster-Carr has some 3" ID fairly heavy wall plastic tubing. You could put the coil inside this tubing, and then put the tubing inside another container and then pull a rough vacuum on the outer container...just enought to keep the tubing from collapsing inward as you pull a high vacuum on it.
Then when you have it pumped down, you release the vacuum on the outer container, causing the semi-flexible tubing to squeeze the coil radially inward. Would it work in practice? I don't know. Unfortunately the 3" tygon tubing is expensive.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Ash Small wrote ...
I think the question is 'how would this compare to vacuum-degassed oil insulation?', for example.
Another factor that a person needs to consider when using any new or unfamilar insulating product that will be immersed in oil for long periods of time, is whether that product is compatible with oil. Since this particular tape being discussed here is made of Silicone, it most likely will be Okay for use in oil. But some rubber and plastic insulation material will turn into a mushy, sticky mess after only being immersed in oil for a short time period.
A simple test that I use is, I put a small amount of insulating oil (such as Shell Diala AX, which is what I use for X-ray tube head repairs) into a glass bowl, and immerse a small piece of the material being tested. I pull it out of the oil after 24 hours, and simply feel it with my fingers. If it doesn't feel mushy, sticky, or crumply, then I'll immerse it again and check it after about a week. If the material will last that long without physically changing, then it will likely last forever in oil. Of course you won't need to test the material if the specification sheet mentions whether or not it is compatible with insulating oils.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
@JP, I think if you could build a mechanism that actually winds the tape under vacuum you may achieve some worthwhile results.
Winding the tape, then placing under vacuum will surely just result in the air expanding within the windings, leading to deformation, and, maybe, breakdown of the insulation. While 'some' air 'may' escape, some will surely be trapped within bubbles within the coil.
While I admire your attempts at epoxy and silicon encapsulation, I can't help thinking that, maybe if you had applied your time and energy to using oil, you may have made more progress by now.
I agree that oil does have some drawbacks, not least expansion/contraction with regards to heat, and the possibility of leaks, your other attempts to date, especially regarding vacuum epoxy encapsulation, do, however, seem to be just as messy, if not more so, than using oil.
While I'm not suggesting that you give up on epoxy/silicon encapsulation, it does seem that all the methods you've tried so far have some unforseen drawbacks, and that maybe oil has some advantages after all.
@Xray, I had meant using oil 'instead' of silicon tape, not as well as. I do agree with your points, though.
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Silicone Self-Fusing Tape As Layer Insulation
