Resonated chokes and inrush current

[KA1ZGC]

Okay, this has been bugging the crap out of me for weeks.

A while back, Timtron and I payed a visit to the Harris FM-25K we single-phased and resurrected for a customer.

When it first went to high power, the high voltage with no RF drive rose steadily towards 10kV, where the caps are rated. We cut the power before it got there.

To correct this problem, the first filter reactor after the rectifier, a 5H oil-filled unit, was resonated with a pair of .5uF 8000V caps seriesed-up in parallel with it.

Problem solved, well under 7000V unloaded, but that's not the mysterious part.

Prior to this change, the inrush current from turning on the plate supply was tripping the breaker (125A breaker, 80A nominal draw). After the change, you couldn't make the breaker trip.

I'm not sure I understand why this would happen. All I can figure is the capacitors are keeping enough of a charge across the 5H reactor during the discharge cycle to have it exhibit a current-limiting resistance faster than it would by itself? Would that potential/resistance be established that quickly?

For some reason I get the feeling I'm overlooking something.

--Thom
Kilovolts Attack One Zorched Gates Collector

[Gito]

Hallo Thom,in a resonance Parallel circuit,there's an equivalent Parallel resistance(at resonance) where R2=X x X/R1,.

X is the reactance of either  the inductor or capacitor,R1 is the resistance of the coil/choke.so if the reactance of the coil/choke at 120 hz is 3770 0hm and the internal resistance of the coil is say 100 ohm ,than the parallel resistance is 3770 x 3770/100=1421 Ohm.Although R2 (the parallel resistor) is not an actual resistor ,to the source of voltage,the parallel-resonant circuit looks like A PURE RESISTANCE at that value.

So the parallel resistance (R2) limits the in rush current ,and thats why the breaker does not trip..

   Gito.N

 PS The source voltage sees  the choke paralleled by C as R2

[John K5PRO]

Sure hope those 0.5 uF caps were low loss plastic film and oil. Paper/oil caps won't hold up to the high AC voltage that will be across them in that circuit arrangement. Polypropylene is the best to use, you find them in microwave oven HV supplies.