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Variac wired for current limiting

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cbfull

Tue Apr 15 2008, 01:15PM

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

It occurred to me the other day that if I use a variac wired for current limiting (variable current, voltage stays constant at line V), to vary the power to a load, the resistive losses in the copper should be lower.

This thought is pretty fresh, I'm sure there are many factors I am missing, so that's why I brought it up.

Any thoughts?

HV Enthusiast

Tue Apr 15 2008, 05:59PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

how are you going to wire a variac for current limiting?

Not going to work unless you make major core modifications.

Dr. Dark Current

Tue Apr 15 2008, 06:20PM

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

The variac will saturate unless you make an air gap in the core. It will be like this: the current will rise very slowly, but at one point it will skyrocket. Not useful at all for current limiting.

cbfull

Tue Apr 15 2008, 07:42PM

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

Maybe I should explain this a little better. It is supposed to saturate, that's when it starts letting the current pass through.

basically the thing is wired so that hot is on an end winding, and the wiper becomes the output connection or the "modified hot" so that the knob (wiper) controls how many windings are active in the "variable inductor".

When just one or a few windings are in series with the load, the core saturates very quickly and then lets current pass. When all the windings are in the circuits, the core does not saturate and very little current gets through, although since this is an inductor and not a resistor, the voltage is not reduced. You still get your full line voltage.

This is a very common technique used by many who want a super-simple way to vary the output current of a transformer without affecting the voltage or spark length.

Of course this is for 60Hz AC.

Does that help?

Dr. Dark Current

Tue Apr 15 2008, 07:55PM

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

noo, I'm afraid it will be like this

Steve Conner

Tue Apr 15 2008, 08:34PM

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

I think the core saturation will make it work in roughly the same way, with roughly the same waveforms, as a phase angle lamp dimmer. In other words, it works fine, but it just doesn't work the way you think it does...

cbfull

Wed Apr 16 2008, 12:03PM

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

Plasmaaddict:

Although I like your graphs, that is not what I want, that is just a linear of graph of simple power resitance, with a variac wired in the traditional manner.

Your graph of "what you get" is correct if we are talking about using direct current. AC is a different story.

If you have a variac, try wiring it so that it is nothing more than a variable inductor in series with the load. As Dr. Spork says, it controls the power like a dimmer, but you guys seem to be forgetting that the voltage behaves in a very interesting manner with an inductor in an AC circtuit. Try it and you will see. Hook up your voltmeter to the line and neutral with all windings "dialed in", and then again with no windings dialed in. You will see that it reads 120VAC no matter how you set it.

You can verify this visually by hooking up a neon as your load. As you dial in more windings you see the spark shrink a little at first, because it is loosing it's current, until it eventually gets to a thin white spark resembling a small van de graff discharge. On my 15kV neon, the spark will shrink in girth, but at this low current it still maintains a strike distance of about an inch, until the current becomes so low that it can barely be seen. The spark length only shrinks because the reduced current takes away the heat that used to allow it "stretch out" to several inches.

This is basic textbook inductor behavior. Since any inductor in a live circuit at time=0 allows 0 amps to pass, it looks to the AC waveform like an open circuit, hence the voltage measured accross the inductor is 120VAC. The higher the inductance, the longer it is in this state. If the inductance is low enough to allow saturation (too few windings, not enough iron, or voltage too high), the core will quickly saturate and it can no longer block the current as the voltage drop across the inductor also drops. As this is happening, the voltage drop across the load rises inversely proportional to the voltage drop across the "choking" inductor (variac).

Sorry if I seem frustrated, I don't mean to sound rude so I apologize if it seems like I am.

As always I appreciate the opportunity to think about any of this stuff in a way I haven't before. So keep it coming if you still see something wrong with my logic.

Dr. Spook, I am seeing how this makes it act like a dimmer where the power curve is cut off, but am I correct in understanding that the variac technique cuts power at the beginning of the half-cycle, whereas a dimmer cuts it (and cuts it sharply) towards the end of the half-cycle?

Craig

Dr. Dark Current

Wed Apr 16 2008, 01:05PM

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

OK, I don't want to confuse you more, but the basis is that a mains transformer has a very high inductance value, which will limit the current to almost zero (perfect transformer has infinite inductance).

HOWEVER there is a phenomenon called core saturation. What happens is that after you exceed a certain value of volts per turn (this is a property of the core), the inductance drops dramatically as will rise your current.

My graphs are valid for AC only and I wanted to point out the phenomenon of saturation (which does not exist in an ideal inductor/transformer).

cbfull

Wed Apr 16 2008, 02:03PM

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

I see. Your second graph only seems to represent what happens during one-half cycle, showing the current rise at saturation, that's why I said it represented DC, sorry! A "half-cycle" is certainly not DC.

I don't think you are being condescending when you introduce me to the phenomenon of core saturation. I am more familiar with this phenomenon than I might be expressing here.

Another example, the series-wired variac core saturates much faster when there are only a few windings dialed in, and this results in a short delay before the load gets any current, even though at time=0 the load still saw a full 120V voltage drop.

This is where I'm fuzzy. Does this 120V that is measured across the load transformer (neon in this example) cause the neon to begin it's normal approach to saturation as thought there is no inductor in series? I believe it does.

I believe that the load transformer core follows it's usual saturation process as thought it were plugged directly into the 120V line. Afterall it is the voltage that causes the saturation phenomenon and not the current. Right?

So given this, as current rises, the neon core is already at the level of saturation that is determined by the angle of the AC half-cycle, and the output voltage of the neon is also the same. The only difference is that the external seriesed inductor has limited the amount of current that gets to the neon. The result, voltage is not affected, but current is reduced in proportion to the seriesed external inductance. It achieves exactly the same result as adding and removing shunts from the core of the neon.

Thanks for participating in this with me. My mental picture is becoming clearer.

Dr. Dark Current

Wed Apr 16 2008, 02:24PM

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

cbfull, now I'm afraid I didn't understand you well, you want to control transformers that are already internally current limited? If so, that will work perfectly fine.

If they are not limited, that is another story. If you short the transformer's output, this is like connecting your 'variable variac' directly into the line. As you will turn the variac down, less and less turns will be engaged. The current will rise very slowly. As soon as you start approaching the maximum Volts per turn for your variac core, the current will shoot up dramatically as you "remove" next few turns.
If you manage to set the variac so that the current draw with your HV transformer shorted matches your wanted maximum current, and you will start decreasing the load on the transformer (such as drawing an arc), the current will go down quickly meanin that just little power can be transfered to your load - for example you will get just very short arcs.

Hope this was clear

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