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Ballast for dual MOT tesla coil power supply?

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profdc9

Fri Nov 24 2017, 11:01PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Hello,

I was thinking of how to ballast a dual MOT tesla coil power supply, and the very large inductor that might be required. I have not removed the magnetic shunts from the MOTs, so that in part helps to limit the current, though the MOT seems to run pretty close to saturation.

Is there an alternative to getting two more MOTs, shorting their secondaries, and using those as ballasts? I am currently using a 3 kVA variac, but using this lowers the voltage of the MOT output as well. Isn't part of the voltage drop in the primary across the ballast and therefore it has about the same effect as the variac?

Any ideas on how to ballast a MOT supply?

Thanks,

Dan

paulj

Sat Nov 25 2017, 05:10PM

Registered Member #59353 Joined: Sun Apr 17 2016, 02:08PM
Location: France
Posts: 84

Hello,

I believe it's possible to limit the current by adjusting the power factor with decoupling capacitors.

I don't have enough knowledge, but look here:

personally, I make a tesla coil with a 4 MOT power supply in series
I think I use these capacitors to limit the reactive power.

Sulaiman

Sat Nov 25 2017, 08:29PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have not tried this but I think that it may work,
keep the magnetic shunt(s), and somehow* reduce the airgap between the shunt(s) and the main core,
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.

* e.g. transformer steel shims into to the gap(s) would work,
but 'eddy current' losses/heating could be large.

hen918

Sun Nov 26 2017, 11:38AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Sulaiman wrote ...

...
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.
...

I think you mean that the effective leakage inductance would halve.

Benjamin

Wed Nov 29 2017, 09:24PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

I use microwave capacitors in series on the high voltage side for mine, although I do have a voltage doubler in the circuit.

Sulaiman

Thu Nov 30 2017, 08:59AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

hen918 wrote ...

Sulaiman wrote ...

...
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.
...

I think you mean that the effective leakage inductance would halve.

I think that I meant exactly what I wrote.

hen918

Thu Nov 30 2017, 12:57PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Sulaiman wrote ...

hen918 wrote ...

Sulaiman wrote ...

...
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.
...

I think you mean that the effective leakage inductance would halve.

I think that I meant exactly what I wrote.

Sorry, yes, magnetising inductance increases, leakage inductance decreases.

profdc9

Thu Nov 30 2017, 06:04PM

Registered Member #58522 Joined: Tue Mar 15 2016, 08:33PM
Location:
Posts: 50

Thank you all for your answers.

I'm currently using a voltage doubler with my 2 MOTs. I am using two 0.95 uF microwave capacitors with it in a voltage doubler configuration. I suppose I could reduce the capacitance to reduce the current. I am trying to figure out the capacitance to use to reduce the current to draw a maximum of 20 A at 120 VAC. I would think a crude estimate of current is I=2 pi f C V would work, but with f = 60 Hz, V = 2100 volts, C = 0.95 uF, this is 0.75 A. This seems much too low. Also, based on charging/discharging the capacitor C V^2 f = 250 W. Is there something I'm missing here?

Dan

Vigh holtage

Sun Dec 10 2017, 02:02AM

Registered Member #61739 Joined: Wed Aug 23 2017, 04:43PM
Location:
Posts: 44

I balast mine with a mot, but ... i cut the I beam off of the EI cor and punch off the secondary and save it for wire exe... then you are left with a E core and its primary as an inductor, without the I section on the E core it reads 6mh open and 40mh closed. This is for aluminum wire primarys, with copper wire its about 10mh open and 60mh closed you can add plastic shims under the I section to fine tune anywhere in between, i find a large core with a copper mot primary open all the way will balast 2 mots just fine at 10mh and it wont even over heat, an aluminum primary mot core needs 15mh of inductance so a 1/4 spacer between the EI works pretty good on the mot cores with Al primarys

I am balasting a quad mot box by useing a large mot core i cut the center leg off of on my band saw to make it a C I core, i then wrap a crap ton of 12awg thhn wire in it till the C core is full, makes about 15 mh open C and 80-100 mh closed and depending on your spacer between the C and I any inductance in between

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