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Re: Primary Coil Design


>
>It is often said that one should keep the tank circuit hookups as short 
>as possible. However, in a past TC design I noticed the metal parts 
>(capacitor housing & spark gap motor) getting hot. I attributed this 
>effect to inductive heating due to the close proximity of the parts to 
>the primary coil. 


Hi Jeff,

I,ve been working with induction heaters in the 200 kHz to 600 kHz
frequency range for about 5 years now.  One common material that is 
easy to heat inductively is graphite. It has  a resistivity about 800
times higher than copper.  This means it gets hot a lot faster than
copper does in the same RF field.

A slug of graphite placed at a spot where you suspect you have an RF 
heating problem would get hot long before any other object would.  Stick 
a cheap alcohol thermometer (the kind with the red fuild)in the graphite
slug and you have a primitve detector.  Don't use a mercury thermometer 
since mercury is an electrical conductor.  A half inch cube of graphite
would be about the right size. Drill a hole and epoxy the thermometer in
place.  If you leave the scale on make sure it is plastic and remove any
metal bits used in its construction.  

Note that graphite powder won't couple to the RF fields frequencies normally
used by TC buildes.  I use graphite powder as a thermal insulator for a
graphite crucuble that I heat inductively up to 1800 degrees centigrade
(3242 degrees F).  The graphite powder just doesn't see the 400 kHz RF.

I work at power levels ranging from a few hundred watts up to 25 kilowatts.
Below about 1000 watts you have to get within a couple of inches of the
coils before things start to get hot but then my objective is usually to melt
anything I'm heating.  Also, all my conductors are either 1/4 or 3/8 inch
water cooled copper tubing, so I never notice them getting hot unless I forget
to turn on the cooling water.  The smoke usually gets my attention.

Hope this helps.

Harry Adams