Re: [TCML] Why don't big Tesla coils use helical primaries?

["jhowson4@xxxxxxxxxxx" <jhowson4@xxxxxxxxxxx>]

James, 
One of the main reasons is that it is much harder to tune a coil when all the wire is insulated as in a close wound helictical coil. This has been done before but a seperate series coil is required for tuning purposes.   But digging into the issue a little deeper the other factors are coupling and skin effect.  

A flat coil is coupled to the secondary much less than a helictical coil.   Which reduces the probability of racing sparks destroying your secondary coil. This applies more to the bigger cooks that are pushing thousands of amps in the primary.    Having said that, electrum uses a helictical primary that is mounted much lower than the bottom of the secondary. 

At the frequencies a Tesla coil generally operates the skin effect applies. The skin effect means that the majority of the current will be in the outer shell of a wire.  So much so that it is more cost effective to just use a tube vs. A solid wire of an equal diameter, why pay for copper that your not using. 

If you are really set on keeping space down use a hybrid primary coil.  I had a coil in which I wound 7 turns of #9 wire with the thickest insulation I could find into a rounded multilayer bundle, that bundle feeds off into a standard flat copper tubing primary coil of another 7 turns.  Saves me a lot of space, while still allowing for tuning.   I just had to be careful that I lowered the coil below the secondary enough to keep the coupling low.     This coil used a 12/30 nst. 

-Jay howson-

Sent from my HTC Inspire™ 4G on AT&T

----- Reply message -----
From: "James Hutton" <b-u-r-t-o-n-boy@xxxxxxxxxxx>
To: "tesla help" <tesla@xxxxxxxxxx>
Subject: [TCML] Why don't big Tesla coils use helical primaries?
Date: Wed, Aug 31, 2011 1:39 am



Hello,
so I was just wondering why no one uses helical coils with insulated wire like this: http://www.johndyer.com/MakerFaireJD06.jpgwhen building big static gap tesla coils.
I have seen tons of small sgtc's with a primary like that, but it seems that with bigger sgtc's people always use copper tubing and carefully space them in a flat or conical spiral.
I don't understand this. Helical coils with insulated wire are so much easier to make, they cost less, their more durable, look better, and take up much less space.
Is it because helical coils are less efficient?
I need to build someone a nice looking tesla coil that is 900w (15/60nst) and makes ~4ft lightning. I dont want the base looking huge, and I can't spend a whole bunch of money on 50ft of copper tubing.
Should I still go with a pancake type primary using the usual copper tubing? or can I just get a pvc pipe wider then my secondary and wind an insulated wire around it/
and theres no way I'm air winding a copper tubing helical coil :P I dont know how people have the patience for that!
thanks for the help! 		 	   		  _______________________________________________
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