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Power factor correction capacitors?

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nzoomed

Mon Oct 26 2015, 02:14AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

I see these capacitors listed locally on an online auction, i was wondering if these are beneficial for tesla coil use?

I dont know much about power factor correction, but i believe you need some pretty sophisticated equipment to perform this.

I have a variac, is it as simple as putting these across the variac output, or do i need more complex switching circuitry?

Signification

Sun Dec 13 2015, 03:05PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

A nice medium-sized Tesla coil with a simple NST (Neon sign Transformer) does not have to concern itself with power factor correction. Example: If you have a 12kV @ 30mA NST lying around and use 60Hz power, then a good matching capacitor (based n matching impedance's) would be around: 0.0067uF (6.7nF) @ 18kV to 20kv. IIRC, I got this capacitance value with a dozen carefully built small root beer bottles (using salt water, I got right at 0.56nF/bottle). This was in 1999 and today, the bottle cap series connection still reads 6.7nF!...honest...

Now, with a variac, can use:V=I/2*pi*f*C = 11,877 Vrms or 11,877 * sqrt(2) = 16,800 Vp, settable via the variac.
...maybe a start...

nzoomed

Sun Dec 13 2015, 08:42PM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Im building a DRSSTC, not a NST tesla coil.
I believe these need a better power factor to run efficient

GrantX

Mon Dec 14 2015, 03:46AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

nzoomed wrote ...

Im building a DRSSTC, not a NST tesla coil.
I believe these need a better power factor to run efficient

An NST is a linear inductive load with a sinusoidal current draw. Power factor can be improved by placing a capacitor in parallel with the primary coil (displacement power factor, a difference in phase angle). But a DRSSTC is a big SMPS, which is a non-linear load that only draws large currents at certain points of the sine wave, leading to distortion. With a rectifier and smoothing capacitor current is only drawn when the mains voltage rises above the charge on the capacitor. A few capacitors wired across the mains input will help filter harmonics, but an active switching PFC circuit will be required if you're chasing high efficiency.

Mads Barnkob

Mon Dec 14 2015, 06:09AM

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

You need a line choke/inductor or active PFC in front of a DRSSTC.

Very short and incomplete chapter on it:

nzoomed

Mon Dec 14 2015, 10:31AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

OK, interesting read. Looks like alot of money unless i can find the components cheap. I will worry about that until after my coil is complete and i can test its performance.

Mads Barnkob

Mon Dec 14 2015, 04:50PM

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

If you have a variac inbetween to ramp up the voltage it will also double as a line choke.

nzoomed

Mon Dec 14 2015, 08:34PM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Mads Barnkob wrote ...

If you have a variac inbetween to ramp up the voltage it will also double as a line choke.

Yes i do have a Variac that i will be using.
Im looking out for a bigger one though, as I dont think its rated high enough for me to run the coil at full power.

Its only 8A, but i should be able to push 10A through it.

Im hoping i dont need much more than 10A at the wall to run my coil at 850Apk
Problem is i have no idea how much power i can push my coil to, as it appears my IGBTs will be able to handle 1500A.
So i could easily run my coil at 1000A if i set the OCD on the driver to allow this.

Mads Barnkob

Mon Dec 14 2015, 08:51PM

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

My DRSSTC1 had no problems eating 11A from the wall, OCD set at 500A, depends on your on-time/BPS, not so much your peak current.

nzoomed

Mon Dec 14 2015, 10:04PM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Mads Barnkob wrote ...

My DRSSTC1 had no problems eating 11A from the wall, OCD set at 500A, depends on your on-time/BPS, not so much your peak current.

Yes, thats true, i will have to play around with the MIDI interrupter.
OCD is only for protection, but i want to set it at the maximum i can for my IGBT's.

That way i can control its output by means of the variac and MIDI interrupter depending on the supply available.

Others here have said that theirs of similar power to what mine will be will draw about 20A peak at the wall, but the 10A breaker will handle it because its not sustained long enough to trip it.

Is your mains 230v or 120v?
Ive been estimating based on similar coils using 120v mains, the current at the wall should be half of that with 230v mains if the coil is running at the same output.

The other thing i want to know is if there is any rule of thumb for the size of the coil and maximum power i can get before i get too many strikes to the ring?

I might have to raise the toroid if i get too many ring strikes.

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