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Grooved Bobbins For Segmented Windings

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Patrick

Sun Sept 25 2011, 01:47AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

jpsmith123 wrote ...

How many volts-per-layer will you have and at what approximate frequency? If your interlayer voltages are moderate and your insulating materials are not too lossy, you shouldn't have any problems.

But I've seen a few epoxies supposedly intended for electrical potting that are really lossy; with some having a DF as high as 0.08 or so in the frequency range of interest. At 800 to 1000 Vrms/layer, @ 30 to 50 kHz and using one of the lossy epoxies, I can easily see 10 to 20 watts of dielectric losses in your coil, far exceeding your copper loss.

It works out to 1kV per layer, at 100kHz. Maybe i should work out a device to measure dissipation factor for epoxies and food oils? Any idea anyone?

jpsmith123

Sun Sept 25 2011, 03:46AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

A while ago I calculated the approximate capacitance between two corresponding turns, in adjacent layers, using the dimensions and materials of one of my coils, and it was about 6 pF, IIRC. (In that calculation, I think the layer insulation was 0.014" thick).

So if you have for example 800 Vrms between two corresponding turns in adjacent layers, and an associated capacitance of 6 pF (although you may have more than that with thinner layer insulation), and a DF of, say, 0.015 at 100 kHz, you'll have about 36 mw of dielectric loss associated with those two turns, according to my calculations. This is a rough estimate, in part because there are usually multiple dielectrics involved with different loss factors and dielectric constants. With some materials there is also a significant temperature dependence. But it seems clear to me that dielectric losses are something that can get out of hand if you're not somewhat careful. (I seem to remember a discussion here a few years ago where someone built a transformer and was having some serious trouble with heat in the windings...and I think the culprit turned out be dielectric loss).

Patrick

Sun Sept 25 2011, 05:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Well then we'll have to figure out how to measure it then.

jpsmith123

Wed Sept 28 2011, 12:38AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I think for my second set of experimental coils I will use this (clear) epoxy:

I was going to use the following (black dyed) one, but it supposedly has more thermal expansion.

For some reason they don't specify a dissipation factor for the clear epoxy, but it shouldn't matter too much in this case.

jpsmith123

Mon Oct 10 2011, 02:56AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well I now have a few "second generation" coil assemblies ready for potting. I think tomorrow I'll order the epoxy from mgchemicals.

I reduced the width of dividing sections between the coils, making room for another coil section. The end grooves on each side are also offset towards the center a little bit, making more room for insulation (epoxy) at the ends. The plastic is also somewhat thicker (radially), which should lower the capacitance.

The feedthrough insulators are also redesigned and should have even less field enhancement (although there wasn't much to begin with).

Patrick

Mon Oct 10 2011, 03:34AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

You do exceptional work JP, that looks real good!

jpsmith123

Mon Oct 10 2011, 12:44PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well thanks for the compliment, but I have to give credit where it's due: that's the OCD talkin'.

(BTW, when are you going to get yourself one of these semi-crappy little lathes so you can suffer making round things too?)

Patrick

Mon Oct 10 2011, 05:05PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

What i really need is a CNC vertical mill, but im sure my apartment neibors wont like hearing it.

jpsmith123

Tue Oct 25 2011, 08:06PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I got the epoxy from mgchemicals today and I went to use it to encapsulate the coils, but when I opened it I found that the resin had undergone "crystallization"...supposedly the result of being stored at a relatively cold temperature.

So now according to the instructions I have to warm it up and stir it to restore it to normal. What a PITA epoxy is.

Edit:

It's taking forever to get the air out of the first coil assembly. It's been pumping for about 30 minnutes now and it's still bubbling, it's slowing, yes, but I'm amazed at how long it's taking.

And then right in the middle of everything, my vacuum TC gauge decides to die. I smelled that unmistakable burning electronics smell, and that was it.

Patrick

Tue Oct 25 2011, 10:50PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

It sounds like you would have the same or fewer problems with oil.

how much $ was the sensor?

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