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Re: Coiler Formers: a lossy mystery
Tesla List wrote:
>
> Original Poster: RWB355-at-aol-dot-com
>
> Hi All,
>
> I need some help:
>
> Iīm getting ready to build my 8x36" (winding length) secondary. I want to
wind
> it with AWG 20. Is this okay or should I rather go for AWG 22 ? Using AWG 22
> would get me above the 1000 turn number, tho. Also a coil wound with AWG 20
> should have a better Q factor
Either should work, but I'd go with the 20 AWG and fewer turns.
>
> Next problem:
> I have the choice between a 20cm (~<8") PVC pipe and a 21 cm (~>8") PPS
pipe.
> Which one should I go for?
> Not considering price or the fact that PVC reabsorbs water.
>
> I would go for the PPS pipe, but no paint in the world will adhere to PPS
> pipe.
> (i.e to keep the wire on the fomer, etc)
>
> Now, I know a lot of people say PVC is lossy. I had a look at some tables.
>
> Material Table:
>
> PVC: PPS:
>
------------------------------------------------------------------------------
> ---------
> R1: >10^15 >10^16
> R2: 10^13
> -----------
> K -at-1Mhz: 3.0 3.2
> LF-at-1Mhz 0.015 7*10^ -4
> kV/cm 350-500 595
> VLF: 600
> -----------
>
> R1: Resistance "thru" the material
> R2 Surface resistance
> K: Dielectric constant at 10^6 Hz
> LF: Loss factor at 10^6 Hz
> kV/cm: Puncture voltage in kV per cm thickness
> VLF: Leakage factor
>
> To VLF: This number tells you something about the ability of the material to
> prevent the voltage from "creeping" along the surface. Sorry, I donīt
know the
> english expression for this.
>
> What immediately springs into mind (looking at the loss factor) is go for
the
> PPS pipe and trash the PVC stuff. But after some discussion with a friend of
> mine Iīm not sure if it really matters.
>
> Our reasons why:
>
> Fact: the E-field goes from every turn of the winding to the base (or
ground)
> of the secondary.
>
> Given the second fact that the material is about 5mm thick and the E-Field
> lines are somewhere around 500mm "long", only about 1% of the E-field is
> contained within the plastic itself. Virtually all TC components are
going to
> have loss factors of way more than 1%.
>
> What would be of more concern to me is the surface resistance. Esp. as the
> material gets older. (i.e. the surface resistance will go down). The table
> doesnīt contain any number for weathered or aged material, so I couldnīt
even
> guess what the drop in numbers would look like.
>
> It is quite clear that building a cap with PVC (as dielectricum) would be
less
> than desirable because in this case the 95% of the E-field will be contained
> in the plastic sheet and only about 5% will be present in the oil and / or
> surrounding material.
>
> help wanted in germany to clear up the lossy mystery,
>
> Reinhard
I agree with your electrical analysis and assessment. The slight
decrease in Q due to dielectric losses in the coil form is of secondary
(NPI) importance. Of most concern should be the material's dielectric
strength and leakage resistance. Niether material will stand up very
well to arc-tracking, so control overcoupling/overvolting teh coil.
Either material has been used successfully by coilers around the world.
PVC may be a bit easier to work with since you can solvent weld it and
can use epoxy to glue other materials to it. And a surface coating will
tend to adhere better. If you do decide to coat the coilform and
winding, the poorer adherance on the PPS way create a cosmetic problem -
the coating may develop "fisheye" wherever the coating and PPS base
separate (with age, thermal/physical shock). Use either material with
confidence!
-- Bert --