Segmented Coil?

[Vietti]

I have wound a number of small, 1"x1", ignition coils for my magnetos with spotty success. I wonder if anyone has experience with winding segmented Coils in this application and would be willing to share their knowledge?

I realize this may not be of interest to very many members here, so if you like respond with a personal message.

Thanks

 

[petertha]

I'm not sure if this is relevant or even close to what you are after, but Ken has done some coil work. I emailed him on occasion & although English is not is first language, he did reply.

http://modelicengine.la.coocan.jp/kinner index.htm

 

[dsage]

Segmented coils are abound on Ali Express. Search for what you see below. At that price for sure it's not worth winding you own. I just bought several to experiment with a CDI design I'm working on.

Or if you want more info on winding your own:
Find a copy of "Model ignition coils and magnetos in miniature" by Bob Shores. A real wealth of information on winding your own coils. I wound ONE from the book (Only one). It's a real problem winding 20,000 turns of something like #38 wire on a bobbin. So I won't do it again. But good luck. I just use Ford COP coils now.
Green twin made mention of the book here. It may still be available from Village Press.
https://www.homemodelenginemachinist.com/threads/bob-shores-model-engine-ignition-book.35714/
A really good book.

 

[mrehmus]

Segmented coils are abound on Ali Express. Search for what you see below. At that price for sure it's not worth winding you own. I just bought several to experiment with a CDI design I'm working on.
View attachment 163607
Or if you want more info on winding your own:
Find a copy of "Model ignition coils and magnetos in miniature" by Bob Shores. A real wealth of information on winding your own coils. I wound ONE from the book (Only one). It's a real problem winding 20,000 turns of something like #38 wire on a bobbin. So I won't do it again. But good luck. I just use Ford COP coils now.
Green twin made mention of the book here. It may still be available from Village Press.
https://www.homemodelenginemachinist.com/threads/bob-shores-model-engine-ignition-book.35714/
A really good book.

Yes, the Ford coils are cheap and good.

 

[Ken I]

The logic behind segmented coils is to keep down the potential difference between windings.

Let's say we want 20kV off 20000 turns over ten layers - so that's 2kV per layer - if we wind one layer perfectly (good luck with that) over the other by the time we get back to the beginning of the first layer then we have a 4000V difference - wire lacquer isn't going to cut it.

In above example the anypoint difference between two layers will be 4000V. And that's if you wind it perfectly - if you hand wind on a lathe, you might end up with two wires crossing from as far as 5 layers apart - 10000V difference (or worse) ARRGGHHHH !

You can add inter-layer insulation sheets or use DCC (Double Coated Cotton) wire to increase the anypoint interlayer insulation resistance and / or by segmenting the coils into a series of short in series coils.

So if we only wind one tenth of the length per segment as a series of 10 segmented coils of 200V per layer giving an anypoint difference between layers of 400V vs 4000V = more segments less voltage needs insulating.

It doesn't use any more wire but does occupy a slightly greater length in the inter segment divider insulation.

Apologies if I'm teaching you to suck eggs - but others might like to know the logic behind it.

Double coated cotton provides more gap between windings and is usually vacuum impregnated with lacquer - such as GE Glyptal - and cured before use - the cotton (usually Dacron) merely provides the latticework for the lacquer rather than insulation per-se.

Example:

My drawing for a 120kVA 200kHz induction heater feedback choke.

Regards, Ken I

 

[SmithDoor]

I have wound a number of small, 1"x1", ignition coils for my magnetos with spotty success. I wonder if anyone has experience with winding segmented Coils in this application and would be willing to share their knowledge?

I realize this may not be of interest to very many members here, so if you like respond with a personal message.

Thanks

I have look at Microwave transformer s.
Then outside is about 9,000 volts . The takeout low voltage side and rewind it for new voltage.

Dave

 

[Vietti]

Thanks for the replys. Yes I agree Bob Shores book is excellent. I met him years ago and actually sent him an example of my magnetos for his evaluation. I built his EK magneto and his coil winder. Good guy.

David, those coils you mentioned look interesting and I will get one. I worry though, that they don't have enough primary windings for a magneto. Many of these small coils are fed relatively large voltages by a CD or battery. In miniature mags I'm lucky to get 1 1/2 volt from the primary. I also need a a laminated core that protrudes for the armature of the mag. These coils may have a thru hole so a core can be inserted.

Ken you are exactly right about the potential between layers being a problem thus segmented a coil.

Lots of suggestions, but remember I'm trying for a coil 1 inch in diameter and about an inch long with at least a 3/8 square laminated core I often use about 200 turns of #24 wire for the primary and as many turns of #44 for the secondary as will fit. Often I can get 12,000 turns or so on the secondary but the interleaving also takes up space and you cannot wind up to the bobbin for fear a winding will drop and short out the coil.

My worry about a segmented coil is how do you keep the incoming wire to a segment from shorting out against the windings that are stacking up against it as the segment fills??

Obviously there are numerous easy ways to make sparks in our small engines esp. with CDI units coming from China for under $30. I think, or at least hoped for the last 30 years or so, I could interest people in building magnetos. In fact I wrote a howto article in MEB and Don Grimm followed up with a few articles to improve my design. Magnetos are quite a simple device to build except for the coil. Some commercial small coils are available though still kind of large. I feel like our antique appearing engines deserve a period correct ignition system when possible. Reminds me of the heat I took for using O rings instead of cast iron rings.