Ballast for dual MOT tesla coil power supply?
profdc9, Fri Nov 24 2017, 11:01PMHello,
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
Re: Ballast for dual MOT tesla coil power supply?
paulj, Sat Nov 25 2017, 05:10PM
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.
paulj, Sat Nov 25 2017, 05:10PM
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.
Re: Ballast for dual MOT tesla coil power supply?
Sulaiman, Sat Nov 25 2017, 08:29PM
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.
Sulaiman, Sat Nov 25 2017, 08:29PM
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.
Re: Ballast for dual MOT tesla coil power supply?
hen918, Sun Nov 26 2017, 11:38AM
hen918, Sun Nov 26 2017, 11:38AM
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....
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.
...
Re: Ballast for dual MOT tesla coil power supply?
Benjamin, Wed Nov 29 2017, 09:24PM
I use microwave capacitors in series on the high voltage side for mine, although I do have a voltage doubler in the circuit.
Benjamin, Wed Nov 29 2017, 09:24PM
I use microwave capacitors in series on the high voltage side for mine, although I do have a voltage doubler in the circuit.
Re: Ballast for dual MOT tesla coil power supply?
Sulaiman, Thu Nov 30 2017, 08:59AM
I think that I meant exactly what I wrote.
Sulaiman, Thu Nov 30 2017, 08:59AM
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. ...
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.
...
I think that I meant exactly what I wrote.
Re: Ballast for dual MOT tesla coil power supply?
hen918, Thu Nov 30 2017, 12:57PM
Sorry, yes, magnetising inductance increases, leakage inductance decreases.
hen918, Thu Nov 30 2017, 12:57PM
Sulaiman wrote ...
I think that I meant exactly what I 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. ...
approximately; if you halve the nett airgap you will halve the short-circuit current,the effective output inductance would double.
...
I think that I meant exactly what I wrote.
Sorry, yes, magnetising inductance increases, leakage inductance decreases.
Re: Ballast for dual MOT tesla coil power supply?
profdc9, Thu Nov 30 2017, 06:04PM
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
profdc9, Thu Nov 30 2017, 06:04PM
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
Re: Ballast for dual MOT tesla coil power supply?
Vigh holtage, Sun Dec 10 2017, 02:02AM
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
Vigh holtage, Sun Dec 10 2017, 02:02AM
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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