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Tank Cap Voltage Clarification

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Aragorn

Tue Apr 01 2014, 04:48PM

Registered Member #18516 Joined: Sat May 18 2013, 09:09AM
Location: Lancashire, UK
Posts: 38

I'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

Mads Barnkob

Tue Apr 01 2014, 05:02PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

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:

Aragorn

Tue Apr 01 2014, 09:02PM

Registered Member #18516 Joined: Sat May 18 2013, 09:09AM
Location: Lancashire, UK
Posts: 38

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?

Steve Conner

Tue Apr 01 2014, 09:07PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

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.

Aragorn

Tue Apr 01 2014, 09:36PM

Registered Member #18516 Joined: Sat May 18 2013, 09:09AM
Location: Lancashire, UK
Posts: 38

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?

Mads Barnkob

Wed Apr 02 2014, 06:40AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

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.

Uspring

Wed Apr 02 2014, 07:56AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

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.

Steve Conner

Wed Apr 02 2014, 08:02AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

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.

Uspring

Wed Apr 02 2014, 09:32AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

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.

Steve Conner

Wed Apr 02 2014, 09:51AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That's remarkably high. :O What coupling are you running?

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