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More thoughts on protection chokes
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From: Greg Leyh [SMTP:lod-at-pacbell-dot-net]
Sent: Thursday, February 26, 1998 5:07 AM
To: Tesla List
Subject: Re: More thoughts on protection chokes
> >> Gary Lau wrote:
> >> VSIN is 7.5KV RMS = 10.6KV 60 Hz, half of 15KV/60mA NST secondary.
> >> L3/R3 are lumped secondary components, 352 Hernys and 2.75K.
> >> C3 is my bypass cap, 600 pF.
> >> L2/R2/C2 is my ferrite core choke, 14mH, 0.5Ohm, 14.8pF
> >> R1 is my series damping resistor, 500 Ohms.
> >> SW1 & SW2 are switches representing spark gap - on-at-8.333ms, off-at-8.7ms
> >> R4 is spark gap resistance, arbitrarilly set at 4 Ohms.
> >> L1/C1 are primary tank components, 30.1 uH, .01 uF (290 KHz).
> >>
> >> +------+--R1--+--L2--R2--+------L3--R3-+
> >> | | | | |
> >> | SW1 ----C2----+ VSIN
> >> L1 | | |
> >> | | C3 |
> >> | | | |
> >> | R4 +-------------+--GND
> >> | | | |
> >> | | C3 |
> >> C1 | | |
> >> | SW2 ----C2----+ VSIN
> >> | | | | |
> >> +------+--R1--+--L2--R2--+------L3--R3-+
> >>
> >
> >Your addition of an external C3 improves upon this, but
> >the task of de-Qing the filter still remains. My guess is
> >that the required de-Qing resistances will dissipate
> >quite a bit of heat.
> >-GL
>
> Are you suggesting that the high heat is due to requiring a high R value
> to de-Q the chokes, heating due to 60 Hz charging current I*I*R, or the
> R's dissipating the ringing of the chokes?
Given that the characteristic impedance of your filter
is about 4800 ohms, I would have expected that R1 would
have had to be somewhat higher to significantly dampen
the filter. 9600 ohms would critically damp it, in
which case the 60Hz currents would dump about 50 W into R1.
For the 500 ohm case, the heating might be still due to
the ringdown current for the most part.
> While these peak ringdown currents can be suprisingly high, the duty
> cycle is quite low and consequently, average power dissipation isn't that
> high. The maximum power dissipated in the R's due to ringdown,
> excluding 60 Hz charging I*I*R, is .5*C*V*V*BPS, where C is the bypass
> cap and BPS is gap breaks per second. I doubt this could account for
> melting your choke, or my 500 Ohm R's dissipating >100W (finger test, not
> simulation).
> Actually, assuming your choke's resistance was reasonably low, I can't
> imagine why they would get hot at all, being a mainly reactive component,
> unless there was internal arcing.
Here's how the choke melted in my ill-fated original filter:
R2 = ~4 ohm (at DC)
L2 = 4 mH
C3 = 0.047uF
VDC = 26kV
The peak ringdown current when the gap fired was about 90A,
and the RMS function in PSPICE averaged it to about 20 RMS.
I*I*Rchoke = 1600W
With 15J sitting in the filter cap and a BPS of 300, approx
4500 VARs were available for filter cooking. I believe that
at _least_ 1/4 of the available VARs were doing just that.
With diodes taking the place of the filter cap, the filter choke
now has scarcely a noticable temperature rise.
-GL