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Re: Why do primaries get hot?



Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Say you've got a 15/30 NST driving your TC.. That's in the area of 400W
power.  Say 10% of that is dissipated in the primary losses, that's 40W.
40W would warm it up noticeably.

The real question would be what is the RMS current (not average) through
the primary.  Charging up a cap, and then discharging it through the
primary, gives you fairly high currents.  That current gets squared,
jacking up the RMS current more than you would think..

Consider an example..

1 Amp continuous

10Amps, 10% duty cycle  

Same average current = 1Amp

RMS of the latter, though, is 3 Amps... Sqrt((10^2) /10 )

Consider Tesla coiling...

30 mA at 15 kV continuous vs

say, 0.01 uF,253 uH (100 kHz)  pk current is around 100 Amps..
Assume loaded Q is around 2, so current falls off to 37 Amps in 2 cycles,
or 20 microseconds.  Assume we do this every 8 milliseconds (each half
cycle, rounding to make math easier).. The duty cycle is roughly, then,
20/8000 or 1/400.   Lets assume that the current is effectively around 50
amps during that time (I know it's not.. but it's about that)...

RMS current = sqrt(50^2/400) = 2.5 Amps... 

compare this to the 30 mA RMS current coming out of the transformer...

Again.. if your primary's DC resistance were, say 0.05 ohms.. The 30 mA
would dissipate (.03)^2*.05 or 45 microwatts... 
The 2.5 Amps, though would dissipate (2.5)^2*.05 = 0.31 Watts... Given that
skin effect increases the AC resistance of the tubing a bit, dissipating a
few watts wouldn't be unexpected.. and even a few watts would make the
thing noticeably warm.


I realize the math is egregiously estimated and ball parked, but is just to
show that RMS currents in the primary can be pretty high, even though the
RMS current supplying it is low. And RMS currents are what make things hot.





Tesla list wrote:
> 
> Original poster: "Gregory Hunter by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <ghunter31014-at-yahoo-dot-com>
> 
> Dear List,
> 
> Why do primary coils get warm? I'm aware of the insane
> current pulses, but they are of only a few uSec
> duration. The Cu tube coil should integrate the
> various currents with respect to time, like a heater
> element, thereby arriving at the average current. The
> average power supply current of even a monster Tesla
> coil is under an amp. 1/4" or 3/8" Cu tube should be
> able to carry a few 100ma all day without warming up.
> Is it RF heating?
> 
> Cheers,
> 
> Greg
> http://hot-streamer-dot-com/greg