15 kW variac

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ipi31415
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15 kW variac

Post by ipi31415 » Mon Aug 06, 2012 4:34 am

I recently acquired a 66kV 230 mA transformer (pic attached) and was wondering where a good place to look for a matching 15 kW variac would be. The transformer input is 300V at 30 kHz. I believe this transformer fits the requirements for fusion but I also wanted to check to make sure the current isn't too high. Thanks in advance!
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Tyler Christensen
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Re: 15 kW variac

Post by Tyler Christensen » Mon Aug 06, 2012 4:41 am

Are you sure it's not 30kHz? Sure doesn't look like a 15kW mains transformer, and 30hz makes no sense anyways.

Assuming it's 30kHz, you will need an inverter not just a variac.

ipi31415
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Re: 15 kW variac

Post by ipi31415 » Mon Aug 06, 2012 5:42 am

Yes I meant 30 kHz sorry. Here's a diagram that was part of the documentation.
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Tyler Christensen
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Re: 15 kW variac

Post by Tyler Christensen » Mon Aug 06, 2012 5:48 am

Well, first off you don't get a 15kW variac, since you're not actually going to put 15kW into your fusor. Get whatever variac you want.

You'll need to build or find an inverter capable of generating the 30khz. There is quite a bit of info on this in the forum's history.

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Richard Hull
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Re: 15 kW variac

Post by Richard Hull » Mon Aug 06, 2012 2:47 pm

A fabulous find and all you will ever need to do significant fusion.

All are correct in that you must now hand assemble a 30khz driver system for the primary. No small task, but quite doable.

As noted, you will never actually utilize more than 10 ma at the higher voltages so 1kw is all you will demand of this transformer. A 240 volt variac with a rating of 10 amps is all you will need to feed the inverter electronics. The output driver semiconductors on most of these electronic inverter supplies are modern 3rd generation IGBT's.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Rich Feldman
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Re: 15 kW variac

Post by Rich Feldman » Mon Aug 06, 2012 4:07 pm

Good acquisition, Ryan. I'd noted the same unit on ebay. Does the seller have more? [edit] I see the answer is yes.

Now as others have said here, you will need to practice some electrical engineering. Did you realize that your fusor power supply project would become an exercise in inverters, HV-HF rectifiers or doublers, and dielectric oil potting?

[edit] Be sure to get the "full specs available as a .pdf" from your ebay seller. Some details might be over your head, but very helpful to anyone helping you to design the ancillary electronics.

You don't need a 240 volt wallplug and 240 volt variac to feed your 30 kHz inverter.
It's easy to get 1 kW from a 120 volt circuit and 120 volt variac. Without any further transformation at mains frequency, you could have DC rails at +/- 150 volts going into your inverter. Starting with a split 240V AC source, your DC rails would still be +/- 150 volts, but would need half as much capacitance for the same ripple voltage.

I think that fine transformer would be seriously at risk if you run it under oil without using proper vacuum processing. Look for an oil tank big enough for the transformer and HV rectifiers, and strong enough to hold a vacuum.
All models are wrong; some models are useful. -- George Box

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Re: 15 kW variac

Post by George Dowell » Tue Aug 07, 2012 1:41 am

As mentioned you won't need a 15 kW Variac, but if you ever do, the practical method is to stack two 28 Amp, 240V Superior Powerstats, and equalize them with a purpose made reactor.

That's what I did in this mid 70's "Big Rig".

Geo
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Doug Coulter
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Re: 15 kW variac

Post by Doug Coulter » Tue Aug 07, 2012 4:50 pm

I've had pretty reasonable luck with variations on the circuit depicted here.
http://www.coultersmithing.com/forums/v ... p?f=9&t=96

I pretty much just used the example circuit from the IRF datasheet. A little tweaking and fine tuning should get you there, or at least, you'll learn what you really need at pretty low cost upfront.
It does take good circuit layout to have high fet currents near high sensitivity chip inputs.

Like anything YMMV on this one - it's the simplest thing that can work. A little tweaking is required
to make it work great, which depends a lot on the transformer you're driving.

I use a 15v dc supply to run the chip, then a variac to control a conventional bulk DC supply for the fets. I've wrung many changes on this basic circuit with some success. More a quick and dirty approach than the best you can do, but cheap, and I've not blown one out in a long time now. It's crucial to tune the frequency right for the transformer, and that will change (dramatically perhaps) when you put it under oil and add parasitic capacity to the secondary windings (oil has a much higher dielectric constant than air).
Why guess when you can know? Measure!

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Rich Feldman
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Re: 15 kW variac

Post by Rich Feldman » Wed Aug 15, 2012 5:12 am

Rich Feldman wrote, on August 6:
> Good acquisition, Ryan. I'd noted the same unit on ebay. Does the seller have more? [edit] I see the answer is yes.

This thread started when Ryan posted a picture (from ebay listing) of a transformer.
In follow-ups, R. H. called it a "fabulous find" and I alluded to "that fine transformer".
Do our forum rules permit a delicate hint at a crude expression?
That that transformer might inspire a certain temporary physiological reaction at first sight, if any transformer could?

Anyway, apparently the last of four specimens was sold the very day it was mentioned here as being available. Reminded me of a recent lot of 3He neutron detector tubes. http://www.ebay.com/itm/251122519300
http://www.ebay.com/csc/i.html?_sacat=0 ... te=1&rt=nc

I wonder if any other registered fusor.net member would own up to being one of the buyers?
This would be a great place for users to connect, to share their information and experiences.
All models are wrong; some models are useful. -- George Box

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Richard Hull
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Re: 15 kW variac

Post by Richard Hull » Wed Aug 15, 2012 1:48 pm

Interesting point Rich.

A prewound HF, HV transformer is a true find for sure, provided one has the knowledge and fortitude to build the circuit to make it perform. The great parts is that with the right trasformer you need not drive it to its design power if it is in the KW range. Most fusors can get by with 500 watts of true, delivered juice. Certainly, a 1kw PWM circuit is easily fashioned with modern IGBT components in either a half or full bridge design.

I figure if any others here bought the transformer, we would hear about it in time, but only after the supply came on line or was in the process of being assembled and de-bugged.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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