Tank Cap Voltage Clarification
Aragorn, Tue Apr 01 2014, 04:48PMI've been trying to get my head round the voltage rating for an MMC in a DRSSTC.
I think its related to the period of the resonant frequency and the number of cycles completed during the on time? I read it somewhere and i have it in my head, but i cant remember where.
So in a full bridge your switching between +Vbus and -Vbus, each cycle the voltage will increase by V?
So with a resonant frequency of say 150khz, one cycle takes 6.66 us. If we have an on time of say 100us, then you'll get 15 cycles of rise, and thus the cap bank will be at 15*Vbus?
How much of an overhead should i apply to this calculated value to get the MMC rating? EG if my bus voltage is 380v, in the above example i'd get 5.7kv. So should i build the MMC to 6? or some larger value?
Thanks
Kev
Re: Tank Cap Voltage Clarification
Mads Barnkob, Tue Apr 01 2014, 05:02PM
I have two equations in a spreadsheet. I use the most conservative for my MMC calculator.
MMC Xc, reactance: Xc = 1 / (2 * PI * F * C). F is frequency in Hertz. C is capacitance in Farad.
MMC Zc, impedance: Zc = SQRT(ESR^2 + Xc^2). ESR is the combined ESR for the MMC. Xc is the MMC reactanse from above.
Voltage over MMC: Voltage over MMC = Zc * primary peak current
Conner also once presented a less conservative, more hobbyist, I do not mind a little heating or blown capacitors rating. Vbus * 10.
MMC calculator:
Mads Barnkob, Tue Apr 01 2014, 05:02PM
I have two equations in a spreadsheet. I use the most conservative for my MMC calculator.
MMC Xc, reactance: Xc = 1 / (2 * PI * F * C). F is frequency in Hertz. C is capacitance in Farad.
MMC Zc, impedance: Zc = SQRT(ESR^2 + Xc^2). ESR is the combined ESR for the MMC. Xc is the MMC reactanse from above.
Voltage over MMC: Voltage over MMC = Zc * primary peak current
Conner also once presented a less conservative, more hobbyist, I do not mind a little heating or blown capacitors rating. Vbus * 10.
MMC calculator:
Re: Tank Cap Voltage Clarification
Aragorn, Tue Apr 01 2014, 09:02PM
Ahha, i'd forgotten about your site, been on there before! cheers!
Will jam some numbers in to it.
Where it says "find the ESR rating for your resonant frequency" is this in the capacitors data sheet?
Aragorn, Tue Apr 01 2014, 09:02PM
Ahha, i'd forgotten about your site, been on there before! cheers!
Will jam some numbers in to it.
Where it says "find the ESR rating for your resonant frequency" is this in the capacitors data sheet?
Re: Tank Cap Voltage Clarification
Steve Conner, Tue Apr 01 2014, 09:07PM
The ESR should normally be so small compared to the capacitive part of the impedance, that you can neglect it with no ill effects.
The rule of thumb I suggested was actually 10 * (4/pi) * DC bus voltage, IIRC. That's the peak voltage you would expect in a tank circuit with a loaded Q of 10.
Steve Conner, Tue Apr 01 2014, 09:07PM
The ESR should normally be so small compared to the capacitive part of the impedance, that you can neglect it with no ill effects.
The rule of thumb I suggested was actually 10 * (4/pi) * DC bus voltage, IIRC. That's the peak voltage you would expect in a tank circuit with a loaded Q of 10.
Re: Tank Cap Voltage Clarification
Aragorn, Tue Apr 01 2014, 09:36PM
yeh i see that now playing with the calculator, it basically makes no difference playing with the ESR.
is there a guide to how much capacitance to aim for?
Other similarly sized coils seem to be around 0.2uf, but i'd like to know how to come up with the optimal figure?
Aragorn, Tue Apr 01 2014, 09:36PM
yeh i see that now playing with the calculator, it basically makes no difference playing with the ESR.
is there a guide to how much capacitance to aim for?
Other similarly sized coils seem to be around 0.2uf, but i'd like to know how to come up with the optimal figure?
Re: Tank Cap Voltage Clarification
Mads Barnkob, Wed Apr 02 2014, 06:40AM
Thanks Steve, almost as I remembered it ;)
I added the ESR rating to the calculator, so that if someone plotted in a data for a unsuitable capacitor with a high ESR, alarm bells should ring when looking at the numbers. We can argue that people that do not know about a too high ESR value will not interpret the results correct, maybe some day I will add more automation and guide help to the calculator. In web 3.0 maybe.
Trhough talks with Kizmo and Dalus on the IRC chat, we concluded that tank capacitor size is only limited by your wealth.
The resonant frequency of some size Tesla coil is often in the same league as most others, a medium is around 50-100kHz, a large under 40kHz etc.
So with larger capacitance you have more energy to dump into the arc, thus getting fatter and brighter arcs with more energy. But the peak currents also get higher as the primary impedance have to be lower to be in the same league of resonant frequency.
Higher peak currents means higher voltage over the MMC, high voltage and large capacitance MMC is a costly affair.
I think Dalus used 1 uF in his large coil and Kizmo might have experimented with 1,5 uF.
Not the most scientific explanation, but more from experience and talks.
Mads Barnkob, Wed Apr 02 2014, 06:40AM
Thanks Steve, almost as I remembered it ;)
I added the ESR rating to the calculator, so that if someone plotted in a data for a unsuitable capacitor with a high ESR, alarm bells should ring when looking at the numbers. We can argue that people that do not know about a too high ESR value will not interpret the results correct, maybe some day I will add more automation and guide help to the calculator. In web 3.0 maybe.
Trhough talks with Kizmo and Dalus on the IRC chat, we concluded that tank capacitor size is only limited by your wealth.
The resonant frequency of some size Tesla coil is often in the same league as most others, a medium is around 50-100kHz, a large under 40kHz etc.
So with larger capacitance you have more energy to dump into the arc, thus getting fatter and brighter arcs with more energy. But the peak currents also get higher as the primary impedance have to be lower to be in the same league of resonant frequency.
Higher peak currents means higher voltage over the MMC, high voltage and large capacitance MMC is a costly affair.
I think Dalus used 1 uF in his large coil and Kizmo might have experimented with 1,5 uF.
Not the most scientific explanation, but more from experience and talks.
Re: Tank Cap Voltage Clarification
Uspring, Wed Apr 02 2014, 07:56AM
Steve Conner wrote:
Uspring, Wed Apr 02 2014, 07:56AM
Steve Conner wrote:
The rule of thumb I suggested was actually 10 * (4/pi) * DC bus voltage, IIRC. That's the peak voltage you would expect in a tank circuit with a loaded Q of 10.That's a gamble. Loaded Q can be much larger during a ground strike or if your coil is out of tune. It's better to use an OCD set to some value and go by Mads equations.
Re: Tank Cap Voltage Clarification
Steve Conner, Wed Apr 02 2014, 08:02AM
Steve Conner, Wed Apr 02 2014, 08:02AM
Uspring wrote ...
That's a gamble. Loaded Q can be much larger during a ground strike or if your coil is out of tune. It's better to use an OCD set to some value and go by Mads equations.
The rule of thumb is one possible way of choosing the tank capacitor voltage rating at the design stage. In operation, you should indeed set the OCD to limit the tank voltage to below whatever voltage rating you decided on.That's a gamble. Loaded Q can be much larger during a ground strike or if your coil is out of tune. It's better to use an OCD set to some value and go by Mads equations.
Re: Tank Cap Voltage Clarification
Uspring, Wed Apr 02 2014, 09:32AM
My coils MMC is rated at about 30*Vbus and I need all of that. This is probably due to my way of running it. In particular long bursts and I'm not using PLL magic to avoid the primary current peak.
Uspring, Wed Apr 02 2014, 09:32AM
My coils MMC is rated at about 30*Vbus and I need all of that. This is probably due to my way of running it. In particular long bursts and I'm not using PLL magic to avoid the primary current peak.
Re: Tank Cap Voltage Clarification
Steve Conner, Wed Apr 02 2014, 09:51AM
That's remarkably high. :O What coupling are you running?
Steve Conner, Wed Apr 02 2014, 09:51AM
That's remarkably high. :O What coupling are you running?
Re: Tank Cap Voltage Clarification
Uspring, Wed Apr 02 2014, 10:14AM
0.14, probably a bit too low. I couldn't help it due to flashovers (and a cylindrical primary
).
Uspring, Wed Apr 02 2014, 10:14AM
0.14, probably a bit too low. I couldn't help it due to flashovers (and a cylindrical primary
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