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Grooved Bobbins For Segmented Windings

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Proud Mary

Mon Jul 19 2010, 08:37AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

jpsmith123 wrote ...

One problem is that I don't have a data sheet for this 26 gauge magnet wire I'd like to use, so I'm not sure what its breakdown voltage is.

Of course I searched the internet to get an idea of the range of breakdown voltages available in commercial magnet wire, and IIRC, the lowest rated wire I found was specified as "> 1 KV". (I was surprised to see that some magnet wire was rated at 5 KV, and I think there was some rated even higher).

In any case, to be prudent, I'm assuming my magnet wire is rated at 1 KV. Using this number, I can see that I need to be somewhat careful, but I don't need to get carried away.

If your winding is neat, there's no reason why the PD between any adjacent, subjacent, or overjacent* turns should exceed the breakdown voltage is there?

A winding style sometimes used at high voltage has a triangular cross section, with each layer having less turns than the one beneath it. This maximizes the slant height distance between the highest voltage turns at the top, and the lower voltage turns at the bottom - it also prevents a higher voltage turn slipping down the side onto the next layer.

* Subjacent is a word usually only found in house conveyancing contracts to mean the flat/apartment below, and I invented 'overjacent' for want of an existing word!

Matt Edwards

Wed Jul 21 2010, 04:51AM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

That is awesome. I was winding a secondary for a TC the other day and a buddy of mine suggested doing something like that for the primary coil. I thought it would look nice.

jpsmith123

Fri Aug 13 2010, 11:08PM

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

I finally got a grooved bobbin made...except for the slits (or whatever) to feed the wire from section to section.

One thing I decided to do was to have the ID of the bobbin a little bigger than the core, so as to have room for a few layers of polystyrene or polyethylene film, and some copper foil (for a shield). This should prevent corona around the core and lower the dielectric loss in the cpvc a little bit.

As far as the "slits" are concerned, I don't have a milling machine, but I do have a drill press with a cross-slide vise attached, so I'm thinking I'll try to mill small angled slits between sections using this arrangement, with a small (1/32") end mill in the drill chuck.

If this method of making a HV secondary ultimately doesn't work out, e.g., if I ruin it trying to put the slits in it, or by way of any other operation, I think the fallback method is to use the teflon wire with the straight bobbin, but to divide it up into 4 or 5 sections in series, using large cpvc washers as seperators.

Andyman

Sat Aug 14 2010, 12:20AM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

Have you considered using a slotting saw on a dremel? Those are pretty darn thin. Or even a razor blade, and carve a slot

jpsmith123

Sat Aug 14 2010, 06:13PM

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

Hello Andyman,

I've tried cutting cpvc with a razor knife, and unfortunately, I found it to be very difficult if not impossible.

The slotting saw idea is a possibility, but I don't have a dremel tool.

For $20 or so, it's probably worth getting one, as I'm sure I'd have other uses for it, but for this application, I wonder: How easy are they are to control?

I may ultimately be forced to try something like this, but I hate to take something I spent a few hours working on, making it all precise and everything, and then go to work on it free-hand, with a cutting tool...I can see myself slipping with it or or misjudging something and ruining the bobbin.

Martin King

Sat Aug 14 2010, 07:27PM

Registered Member #3040 Joined: Tue Jul 27 2010, 03:15PM
Location: South of London. UK
Posts: 237

jpsmith123 wrote ...

I wonder: How easy are they are to control?

Clamp the workpiece in a vice, for a plastic bobbin I would get two scrap pieces of plastic or wood and drill holes the same diameter as each end of the bobbin and use them to hold the bobbin in the vice jaws otherwise you risk damage to the bobbin. Hold the Dremel VERY firmly even bracing your arms against your body. Particularly with a razor saw blade cutting plastic the blade can grab leading to disaster for the workpiece or scars on your fingers (BTDTGTS). Make the cut a little bit at a time, removing the blade and allowing the plastic/blade to cool between each go. If you can, then protect nearby vulnerable parts of the workpiece with some scrap material, cut up bits of a steel can be bent to form shields. Practice on some scrap first if it's a valuable workpiece. Don't let this scare you off using a Dremel, they are very useful tools and I use them a lot, they just need a lot of care and respect!

Martin.

IntraWinding

Sun Aug 15 2010, 11:23AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

A model makers photo etched saw blade would make a nice thin cut if it can cope with the plastic you're using. A quick search found this example

To get a slip free repeatable cut, build a little jig with a blade guide.

jpsmith123

Mon Aug 16 2010, 08:39PM

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

Well, so much for the end mill idea.

After much thought, I devised what I thought was an ingenious method to hold the bobbin and steer it into the rotating end mill, but it was all for naught, as despite great care and a light touch, I nevertheless managed to break both of the end mills that I had.

Now it looks like it's either a saw blade on a rotary tool, or back to the drawing board.

jpsmith123

Sat Sept 18 2010, 04:50PM

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

Here's the latest (and hopefully final) version:

I cut smaller grooves in the "lands" between winding spaces, and then I put it in a cross-slide vise mounted on a drill press and milled slots with a 1/16" end mill. The whole arrangement is cheap and sloppy, and the work came out kind of sloppy, accordingly, but I guess I have to live with it.

Next thing is to try and wind it.

One potential issue I see already is the possibility of losing some space on the bottom layer, due to the finite bend radius of the wire. IOW, when the wire emerges from a narrow groove (through the vertical slot) and into the adjacent winding space, it has to turn a corner. This will probably cause the loss of a turn or two on the bottom layer of each segment...but I should be able to put a few extra turns in the narrow grooves to make up for it.

Proud Mary

Sat Sept 18 2010, 06:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

jpsmith123 wrote ...

Here's the latest (and hopefully final) version:

That's great progress!

I've always found that just dividing the available space by the wire diameter never works out in practice with my casual way of going about things, so I always leave a contingency margin to allow for 'winding creep' or whatever we choose to call it.

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