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Does a variac have enough inductance to current limit a X-Ray transformer ?

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sparky99

Mon Jun 23 2008, 05:29PM

Registered Member #1159 Joined: Fri Dec 07 2007, 02:10AM
Location: Hudson Valley of NY State
Posts: 84

I have a autotransformer that came out of an X-Ray controller, big & heavy. I'd rather use a beefy 240 Volt, 30 Ampere variac. Am I barking up the wrong tree with the variac ? The transformer I'm trying to ballast is a 125 kV @ 300 mA,

Thanks,
Sparky 99
Bob

[Edit: Corrected the units.]

Dr. Dark Current

Mon Jun 23 2008, 05:58PM

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

You cannot ballast a transformer with a variac because variac has very large inductance which drops to almost zero after saturation (this has been discussed here). Inductor must have an air gap to function properly as an inductor.

Steve Ward

Mon Jun 23 2008, 08:26PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

And furthermore, ive heard of variac mod's going bad. People try to mill out a gap in their variac and the thing goes "sproing!" and they lose the whole thing.

Sulaiman

Mon Jun 23 2008, 09:44PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

125kV x 300mA = 37.5kW !!!
37.5kW / 240V = 156 Amps !
Are the figures correct?

Anyway, the magnetizing current of a variac is several times less than the rated current
to maintain efficiency and minimise heating.
When your 240 Volt 30 Ampere variac is connected directly across the mains with no load it will probably draw a few amps (reactive) . The wiper can be used as one of the two terminals for a variable inductor as you thought, BUT
do not exceed the rated current of the wiper/brush.

The volts-per-turn of the variac cannot be increased or the core will saturate,
so if the wiper is used then you need to ensure that the voltage across the variable inductance is no more than it would be for the used portion of the windings when used as a variac.

Cutting a slot/gap in the variac core would allow more amp-turns / lower inductance
but the volts-per-turn will not be affected.

Dr. Dark Current

Tue Jun 24 2008, 06:26AM

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

Sulaiman wrote ...

Cutting a slot/gap in the variac core would allow more amp-turns / lower inductance
but the volts-per-turn will not be affected.

If you cut a gap in the core be sure to insert something tough and non-conductive into the gap otherwise your variac will be damaged by the core trying to slam together (the forces in magnetic cores are respectable).

Steve Conner

Tue Jun 24 2008, 09:36AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That's if it hasn't already sproinged to its doom like the other Steve hinted at. The core of a variac is a coil of steel tape that will want to unwind like a clock spring the instant you cut it.

When I was a kid, we made ballasts by getting a big spool of insulated copper wire from an electrical store, then stuffing a bunch of welding rods into the hole in the middle. Sometimes the spool works fine by itself with no added iron. Or cutting the cores of scrap transformers and putting them back together with an airgap. Or getting a bunch of mercury vapour lamp ballasts, electric heaters, toaster ovens, and putting them in parallel, etc. Some guy in here made a water ballast from a plastic 55 gallon drum full of salty water with electrodes.

X-Ray transformers aren't ballasted in the X-Ray machine, because the tube isn't a discharge tube. They run as voltage sources.

cbfull

Tue Jun 24 2008, 11:52AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

Sulaiman wrote ...

The wiper can be used as one of the two terminals for a variable inductor as you thought, BUT
do not exceed the rated current of the wiper/brush.

This is more accurate. To say that a variac "cannot" be used as a ballast is not correct. True, the variac you describe cannot be used, but as Sulaiman points out, the power/voltage ratings of the variac must match/exceed that of the transformer you are trying to control.

If you follow the above rules, you can safely dial down the number of windings in the variac (reduce inductance), which WILL result in saturation of the core, and this is how current is allowed to pass through to your load transformer. As long as the load transformer does not draw more power than the variac could handle in normal operation, the wiper and windings (or even a single winding) will be okay.

I have a 1.4VA variac that I have used for many years on a 15kV 60ma neon and have never had a problem with a damaged brush, overvolted turns, or overheated winding.

J. Aaron Holmes

Tue Jun 24 2008, 03:48PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Dr. Conner wrote ...

Some guy in here made a water ballast from a plastic 55 gallon drum full of salty water with electrodes.

I'll bet you're referring to me, although it's a plastic garbage can (not quite 55 gal) and the water gets doped with baking soda. Works great for short runs up to 15kVA (so far). The electrodes are 1" copper water pipes spaced about 1/4". A piece of PVC water pipe over one of the electrodes is raised/lowered to vary the resistance. Anyway, as the topic is variacs, I'll let somebody start a separate topic if they want me to keep rambling

Cheers,
Aaron, N7OE

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