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Registered Member #570
Joined: Wed Mar 07 2007, 03:41PM
Location: Winchester Uk
Posts: 42
Hi All,
What are peoples thoughts on selecting a capacitor value? High or low impedance? I know a lot will depend on size of coil, IGBT's, coupling etc. For instance, if I have a coil running at 40kHz:-
If I use a 0.2uF at 1200A peak it will develop 23.9kV across the capacitor = 28.6MVA
If I use a 0.3uF capacitor it will develop 15.9kV = 19.1MVA
The energy will transfer to the toroid and produce sparks, but which would be better? I think I may have to simulate it looking at the number of cycles to get to 1200A versus coupling and streamer load.
My thinking is that the smaller capacitor is better as it has a greater reactive power level, but perhaps it takes longer to ring up so ends up less efficient.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Paul
I design for a loaded Q of 10. In other words I choose the capacitor so that when the tank circuit has rung up to the current I want, the peak voltage across the capacitor is 10*4/pi times the inverter's DC bus voltage.
To find out what primary current I want, I start with the desired spark length, then use John Freau's formula to get the required average power. I then calculate peak power assuming something like a 250us burst at a 100PPS repetition rate. Primary current is just peak power divided by DC bus voltage, give or take some 4/pis and sqrt(2)s.
Registered Member #570
Joined: Wed Mar 07 2007, 03:41PM
Location: Winchester Uk
Posts: 42
Hi Steve,
Thanks for the insights.
After further calculations I can see that in practice I am limited by the cost of the capacitors. As a result I will run with 152nF (that is what I can build up at the moment) and limit the primary current to 500A. I doubt I'll ever get to 1200A unless I get bucket loads more capacitors.
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Thoughts on DRSSTC primary capacitor value
Im sure everyone will be interested in your approach. Thanks!
