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More thoughts on protection chokes
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From: Gary Lau 26-Feb-1998 0837 [SMTP:lau-at-hdecad.ENET.dec-dot-com]
Sent: Thursday, February 26, 1998 7:52 AM
To: tesla-at-pupman-dot-com
Cc: lau-at-hdecad.ENET.dec-dot-com
Subject: More thoughts on protection chokes
From: Greg Leyh [SMTP:lod-at-pacbell-dot-net]
>>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.
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?
>>[snip]
>> What I do see in the PSpice simulations however is my choke and bypass
>> cap resonating and ringing down as soon as the gap conducts. The choke
>> ringdown current here peaks at tens of AMPS and is due to the bypass caps
>> discharging through the chokes, gaps, and damping R's.
>Yes, indeed!
>For all of you 'empiricists' out there, this result of
>Gary's simulation is quite REAL, and is not at all obvious
>from looking at the schematic. It was this same choke
>ringdown current that _melted_ a 4" x 20" RFC in my old
>coil! PSPICE revealed that the choke current was not the
>expected 5ADC, but rather 20A RMS AC worth of ringdown!
>Changing from an L-C to an L-Diode arrangement fixed this.
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.
I believe there is some other undiagnosed and unmodeled mechanism at work here.
Gary Lau
Waltham, MA USA