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Re: A few quick questions....

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Alan,

At 07:17 AM 2/4/2001 -0800, you wrote:
>Hi everyone -
>I hope you can help with a few quick questions....
>1. Variacs. My NST is a 10kV 100mA model run from the 230V UK mains.
>Consequently I'm expecting that when the TC is operating the current drawn
>from the mains will be 1000W/230V = 4.3 A and so am saving up to get a
>variac rated at 8A (the nearest rating is 5A but I thought it might be wise
>to get the next one up just in case....!). Now have I forgotten something or
>is this estimate ok ? I'm sure I've seen a web-site in which someone was
>using a similar size NST and yet was pulling currents from the wall much
>larger than a simple application of P=IV would suggest....?

The larger the variac the better.  They can take substantial overloads but
I try to get bigger ones too for those "special" occasions.  Simply placing
a fan on a variac will really help its power rating too but you have to
watch the brushes if you really get carried away.  All you calculations
seem right on to me...

>2. Power factor correction capacitors for the NST. It's a Tunewell
>transformer and the makers suggest using a pfc cap of 40uF. Is it actually
>worthwhile using a PFC in TC work.....if only to reduce the electricity
>bills ?! 

Your coil will not show up on your electric bill unless you live in
California where you will need a licence to use electricity for a Tesla
coil :-))  The PFC cap does lower the current drawn (sometimes in half)
which can increase performance due to lower feed line droop (you just crank
up the variac more).  Where a PFC cap may really pay off is if your coil
(with sync motor) draws 20 amps like mine but you only have 15 amp circuit
breakers.  Now it draws about 12 amps...

>3. Positioning of the NST. I'm thinking of putting the MMC etc under the
>primary and secondary coils as in the most popular TC designs but was
>wondering if there are any undesireable effects of having all this equipment
>sitting in the magnetic field of the primary ? Particularly a huge lump of
>soft iron in the form of the NST !  If the NST is under the coil so that the
>mains leads run across the floor has anyone had a secondary strike the mains
>leads ? At the moment I'm leaning towards having the NST and NST protection
>filter at a distance from the coil and running the EHT leads across the

Having a bunch of "stuff" under the coil does tend to affect the magnetics.
 However, it has less affect than trying to locate the parts at a distance.
 Try to keep things reasonably far from the primary but you apperaently
don't have to worry too mauch about it.  I think others have studied this
and they know more than me.

>4. A question for Terry, I think: I'm thinking of using your multiple static
>gap design (the one in which you have lots of short copper tubes fixed to a
>wooden baseboard). Is that going to be ok for a 1kW coil run for short
>periods or do I have to start thinking about cooling fans and perhaps the
>TCBOR design ? How hot do the tubes get....I've visions of 3mm transparent
>polystyrene sheet melting so I'm wondering what (cheap!) material for the
>baseboard might do....un-copper clad matrix board...?

The tubes will get hot but the wood or epoxy never failed.  I would be
careful of using plastic due to the heat but a fan would easily cool it



>With thanks,
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