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Re: [TCML] PFC and Line Filter

Hi Carl,

Actually, low-end NST's do not use built-in power factor correction caps. These are sold as uncorrected or "No Power Factor" (NPF) NST's. Higher-priced "high power factor" (HPF or H) NST's contain an internal PFC cap. A HPF transformer typically draws half of the mains current of an uncorrected NST of the same VA output. Instead of connecting the PFC cap directly across the mains, a HPF NST has an additional PFC winding that typically develops several hundred volts. By operating at a higher voltage, a smaller valued internal PFC cap can be connected across the PFC winding to provide the same degree of power factor correction.

An uncorrected NST looks like a heavily inductive load to the line due to the current-limiting leakage reactance that's built into the magnetic circuit linking the primary and secondary windings. However, the presence of the Tesla coil tank cap across the secondary cancels some (or even most) of the NST's leakage reactance. A large tank cap will resonate with the leakage reactance at mains frequency - effectively cancelling most of the NST's leakage reactance. Residual leakage inductance seen on the primary side can be counteracted by adding some PFC caps on the primary side of the NST to improve effective power transfer in the system. I suspect that even a system using a Larger than Resonant (LTR) tank cap will still "look" inductive to the line due to primary-side leakage inductance.

The size of the optimal PFC cap size varies depending on the NST(s) and tank cap value (i.e., how close it is to mains resonance). In general, the optimal value will be a fraction (often 1/2 to 1/4) of the value predicted using the short-circuit inductive reactance of the NST(s) alone. PFC caps do not improve TC performance, but they may allow operation of an NST farm from a circuit that would otherwise trip the mains circuit breaker.

Bert Hickman
Stoneridge Engineering, LLC
World's source for "Captured Lightning" Lichtenberg Figure sculptures,
magnetically "shrunken" coins, and scarce/out of print technical books

Carl Noggle wrote:

NSTs have PF correcting caps in them already.  It sounds like they are
not right for correcting for a TC.  (No surprise there.)  Is there
information on this?  Is there an easy way to measure PF? Does it
sometimes require a lagging correction (inductive)?


On 4/24/2015 7:52 PM, Gary Lau wrote:

I think either cap series is acceptable - this is a very non-critical
application, given the relatively short run-times involved.  It's not
it on a rooftop in an air conditioning unit running 24/7.  But I think
25uF value is too low; for a 15/30 NST at 120VAC/60Hz you want something
like 120uF.  For 240/50 it's less, not sure how much.  But again, this
is a
fuzzy target - there's not a sharp sweet-spot.

For line filters, I think most folks just use whatever they find on the
surplus market.  I put up a web page about how to hook up EMI filters
- it
may not be obvious in which direction to hook them up!  See

Regards, Gary Lau

On Fri, Apr 24, 2015 at 3:08 PM, Ayush Sharma <ayushsharma3210@xxxxxxxxx>


Thanks to all the people helping out in my previous queries. Here's
question out to all you folks.
While searching for a PFC Capacitor for my SGTC (15kV, 30mA) of rating
around 25uF, I am confused about whether it is acceptable to use
motor run
capacitors made by companies like EPCOS listed here (


) or focus on the dedicated Power Factor Correction capacitors made
by them
listed here (



I am able to procure the AC film capacitors (for motor run) pretty
in local shops, but am having a hard time locating the other type.
Has anybody previously used the EPCOS B323 seris of Motor run capacitors

Also, on a related note, any suggestions as to what kind of a Line
should i be looking at (Power supply is 240Volts, 50Hz).

Ayush Sharma
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