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intro to MOT resonance

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lokeycmos

Fri Dec 18 2009, 02:04PM

Registered Member #2553 Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97

Hello everyone! this is my first post. i tried doing some searching on the site before i posted. A quick search didnt find what i was looking for. i have quite a bit of experience with flybacks and NST with just a little experience with arcing MOTs. im quite interested in setting up my MOTs with a resonance cap for an awesome jacobs ladder.i am aware of the dangers of MOTs (my dad is an electrician and gave me a pair of linemans gloves) I have several questions about how to go about it.

correct me if im wrong, but in a nut shell; i have to find the inductance of the secondaries, then 1/(2pi*sqrLC) to find the cap?

1. why do i have to short the primaries to measure the inductance on the secondaries?
2 is there a way to measure inductance with out a L meter?
3. if i have the secondaries in series would the cap i need be based on the series inductance value? and vice versa for parallel
4. would it be a benifit to put a power factor correction cap on the primary side?

thank you much!!

Dr. Dark Current

Fri Dec 18 2009, 07:02PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi,
1. The caps resonate with the leakage inductance, not magnetising inductance. This is because the primary is driven from a low-impedance source.
2. Yes it is, you can connect your secondary winding to mains and short the primary, then from inductive reactance you can calculate the inductance.
3. With the secondaries in series, the resonant capacitor is half the size (double the inductance).
4. Not at all from my experience, the resonant action actually corrects the primary power factor pretty nicely (the leftover reactive energy gets "recycled" back to the primary and mains).

For your average (unballasted) MOT, a good start is two microwave oven capacitors in parallel as a series resonant capacitor.

lokeycmos

Sun Dec 20 2009, 05:27PM

Registered Member #2553 Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97

thanks for the info it helped me understand more. can you please elaborate on #2 about calculating inductance? also i was hoping some one can give me some more advice about resonance caps.

my first try was 2 mots with primaries in parallel and secondaries in series. i started out with 2 caps in series with the output and it actually cut the output so much that it would hardly arc. in a nut shell i tried 3 in series all the way up to 7 in series. i also tried 2x2 and 3x3. all the combinations where not nearly impressive as no caps. does it matter that all the MOT caps i have include an internal resistor? thank you for all your help!!

Antonio

Sun Dec 20 2009, 05:57PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

I would not expect significant resonance with a MOT. They are not current limited. The leakage inductance is small, and resonating it with a series capacitor would eliminate just a fraction of the output impedance of the transformer. A MOT in short-circuit draws a huge primary current and self-destructs quite rapidly.
See the comments here, for example:

Dr. Dark Current

Sun Dec 20 2009, 07:14PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Antonio, the capacitors do indeed resonate with the leakage inductance. The inductance of the secondary winding with the primary shorted is around 4-5H for a typical MOT. With 2 microfarads of capacitance (two microwave caps in parallel), the resonant frequency calculates to 50.3 Hz with 5H.

From my experience, larger than resonant capacitor produces bigger arcs, the resonant voltage rise is still apparently enough, and the current is higher. The arcs stretch out to something like 30cm with a single MOT, vs. 12cm without the caps.

lokeycmos, you short the primary winding, plug the secondary into mains and measure current. Then Xl=Vmains/I, but also Xl=2*pi*f*L, from here you can calculate the L.

lokeycmos

Mon Dec 21 2009, 01:47PM

Registered Member #2553 Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97

holy Sh*t! i put 2 parallel caps in series with the output and it is absolutly insaine on the jacobs ladder.

Henry H

Mon Dec 21 2009, 07:32PM

Registered Member #2298 Joined: Sat Aug 15 2009, 08:16PM
Location: ex UK, now Santa Cruz, CA
Posts: 35

Are you using a primary-side ballast? What kind of current draw are you getting?

lokeycmos

Mon Dec 21 2009, 08:15PM

Registered Member #2553 Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97

no, im not using a ballast. when i arced the MOT by itself it would trip the breaker quite easily on a 20 amp circuit. now with the caps i can arc all day and not trip it. what would be the benefit of a ballast? i would imagine it would cut the ouput considerably.

Zeus

Mon Dec 21 2009, 11:24PM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

I am assuming you are on 110VAC so I would recommend that you put a 4mH inductor in series with the MOT and a 80 microfarad PFC in parrallel with it. Also try 2 MOT caps in parrallel and 3 in series like this.

|-II-II-II-|
|-II-II-II-|

lokeycmos

Tue Dec 22 2009, 12:25AM

Registered Member #2553 Joined: Fri Dec 18 2009, 01:36PM
Location: St Cloud Minnesota
Posts: 97

yes, im on 110. how would i benifit with the 4mh inductor and 80uf cap do? primary side or secondary side?

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