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quad mot ballast help

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teslacoolguy

Wed Mar 05 2008, 09:19PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
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Posts: 792

I just bought 4 matching 1200W mots for a large tesla coil i am building and the problem is, my variac is rated for 15A and i emadgine that the mot's will draw a minimum of 25A. What do you think i should use to make a 15A ballast?. Also what are the pros. and cons. between a inductive ballast and a resistive ballast and what would be better for my purpose.

Kolas

Wed Mar 05 2008, 10:08PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Resistive literally burns off power, so it's typically not used because the heat must be dealt with. Inductive is a good option, it doesn't dissipate much heat, but it is typically very heavy. There is a third option: Phase angle firing. This is used all around the electronics world, especially for controlling 50-60hz. It uses SCR's or triacs, and chops the sine wave. This is a more complicated approach, but it's ultimately the lightest and most variable solution.

hope that helps.

Shaun

Wed Mar 05 2008, 10:19PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Check out Steve Connor's Solid State Variac, it's known to be a very good circuit and employs SCR phase-angle firing. Basically you can control as much voltage and current as your SCRs can take, and even small SCRs on a good heatsink can do 40A continuous.

His site is scopeboy.com, check there.

teravolt

Thu Mar 06 2008, 05:31AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

4 1200 mots is 40A @ 120Vac, 20A @ 240Vac. if you had a dryer outlet with 240 you won't need one. if you can find a saturable transformer on ebay. a thougt ocured to me and that is could you use a ordinary transformer say 2kw like an 120 to 240 with variac configuration

teravolt

Thu Mar 06 2008, 05:32AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

like this

teslacoolguy

Thu Mar 06 2008, 04:16PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

OK i think i will probably go with a inductive ballast. I was thinking of going with steve conners solid state variac but i am not good with the TL494 PWM chip yet so troubleshooting it could be a challenge. Now does anybody know what approxamite resistence and inductance that a coil has to be to draw around 15 amps shorted beause it may not be hard to find one like that at the scrap metal place that i go to every week and they have a dumpster full of inductive products.

Dr. Dark Current

Thu Mar 06 2008, 04:20PM

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

teslacoolguy wrote ...

OK i think i will probably go with a inductive ballast. I was thinking of going with steve conners solid state variac but i am not good with the TL494 PWM chip yet so troubleshooting it could be a challenge. Now does anybody know what approxamite resistence and inductance that a coil has to be to draw around 15 amps shorted beause it may not be hard to find one like that at the scrap metal place that i go to every week and they have a dumpster full of inductive products.

Assuming 120V mains:
X=120/15= 8 ohms
Now plug the values to

and the result is 21.2 mH.

I would like a dumpster full of inductive products too

Are any big transformers/cores in it? You'll need a fairly large core for 2kVA.

vasil

Thu Mar 06 2008, 04:26PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Or some smaller wound cores, connected in parallel.

teslacoolguy

Fri Mar 07 2008, 03:16AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

jmartis wrote ...

Assuming 120V mains:
X=120/15= 8 ohms
Now plug the values to

and the result is 21.2 mH.

I would like a dumpster full of inductive products too

Are any big transformers/cores in it? You'll need a fairly large core for 2kVA.

every so often they will get a very large core and when i find one large inough i am going to jump on it because scrap iron sells for like 20 cents per lb so it wont be to expensive either. and i will try to hunt down a 21.2mh choke rated at least 20 amps and if i cant ill worry about that later.

teravolt

Fri Mar 07 2008, 06:01AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

the reson i set it up that way is that you may have some adjusibilty over the power. i dont know how big the variac needs to be maby 10A. another option is to use capacitors in series with your mot's say 330uf @ 440VAC. you can make this with 7 47uf caps. if you drop out caps you can reduce power. this can be done with inductors as well. more power increases spark length so another option is to remove a set of mots so you will not draw the power in the first place. be careful mots are LEATHAL. like poisonous snakes they need respect

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