[TowerTalk] Inductor Design and Voltage Breakdown

[Jim Lux]

On 8/2/15 6:42 AM, Jim Thomson wrote:
> Date: Fri, 31 Jul 2015 14:28:11 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] Inductor Design and Voltage Breakdown
>
> On Fri,7/31/2015 2:11 PM, Howard Hoyt wrote:
>> I'd love to see the final product, Jim!
>
> So will I. :)  If it works well, we'll write it up. And maybe even if it
> doesn't, so others can learn from our mistakes. :)
>
> 73, Jim
>
> ##  As others have pointed out, your V per turn isnt an issue.   Another option is
> to use polyimide wire....which is just 15 kv rated  magnet wire.  Then you could
> butt the turns..tight up against each other..and uh would really increase a bunch,
> so less turns used  vs air spaced coil.  The 15 kv breakdown would mean
> 30 kv between adjacent turns.   Then just wind it on a low loss form..like
> fiberglass tube etc.   The polyimide in the larger gauges like 14-12-10-8 has
> the 15 kv rating.
>
> Plane jane airdux is plenty good enough.  You can get it in 10 ga.  A local
> buddy gave me a bunch of it a while back.... in 8 ga..and aprx 3.5 to 4 inch diam.
> You can also get airdux with different turns spacing. The slightly closer spacing
> stuff has more uh..which is ideal for low band applications.
>
> Even   10-12 ga  THNN wire would easily work in your application..with turns butted up tight.
>
> Dont confuse V breakdown with 160m  mobile coil setups.
>




I wouldn't rely on the voltage rating of the insulation to its limit. If 
you have two turns that are adjacent, the field is highly non-uniform, 
so the breakdown voltage is lower than you'd expect given the gap length.

Typical breakdown strength for Kapton (which is a polyimide) is about 
290 kV/mm (7400 V/mil) (compare air at 3kV/mm)

A heavy build insulation for AWG 10 would be 0.0015 mil.   So that would 
work out to about 10-12 kV.

There are triple and quadruple builds also available, but the 
relationship between breakdown strength and thickness is NOT linear for 
thin films (in the wrong way.. twice the thickness has less than twice 
the breakdown voltage).  This is why they use multiple layers in HV 
insulation (that, and because a void or defect in one layer is unlikely 
to line up exactly with a void in another layer)

However, there's a bunch of other factors that come into play: partial 
discharge and treeing are worse with RF than with DC or line 
frequencies, so you need to be more conservative in your ratings.

In any case, the turn/turn voltage in Jim's coil is so low, any 
insulation will work and it can have holes in it too, because it's below 
the minimum sparking voltage.

Polyimide is expensive (really, really durable, good for high 
temperatures, etc.).


http://www.mwswire.com/pdf_files/mws_tech_book/TechBook040114.pdf  is a 
nice source of information about magnet wire of all sorts.