Variac nonsense

Yeah but then what would I read on a rainy afternoon?

And they hadn't even met Slaughter yet.

Doesn't look like it. But that would be another way to smooth the interpolation. Or use a round or diamond-shaped contact. Or use two offset contacts, as one pic in abse hints at.

John

I can verify that the brushes are made from grain-oriented graphite, at least in the units made by Staco. BTW, Variac is a trademark of General Radio:

Superior Electric uses the trademark Powerstat, and I think GE had a "Volt-Pac". Staco's units are properly called variable autotransformers. There are models that have a two-layer insulated winding which provides isolation, at the cost of larger size for the same kVA.

We had problems with one of the Staco models, a 1520, which is nominally rated 9.5 amps at 240 VAC. We use them at currents up to 10x their rating for short bursts, and although they are rated for such use, we had problems with the brushes and windings failing catastrophically. Upon testing, we saw a lot of arcing at the higher levels, and also noticed that the brush got very hot at moderate levels of overload, where we might need to draw power for up to 3 minutes. We noticed that this model had a very thin, flat brush, rather than the wider tapered version illustrated above. It seems that the thin brushes overheated and cracked to the point that the brush holder landed on the windings and shorted them out, causing a major overload and burned windings. We reduced the problem by substituting a tapered brush assembly, and also adding a stronger spring to maintain more pressure on the windings.

There is or was a company in Italy that makes a variable autotransformer with a carbon roller for a brush, but I have never used them. I would think it might cause more problems than the usual design.

There is also the unique Peschel variable transformer made by Hipotronics. It uses a sliding copper brush and diodes to eliminate shorted turns and provide higher power capability. They have units up to 1.6 MW! I worked with an engineer from Hipotronics and he showed me some of the units. But I'm not convinced they are all that much better than the toroidal design.

What I really want to do is design a solid state "variac" using PWM technology.

Paul

What he has been saying is funny, as fits a name that can be rearranged to make Jons Laughter...

Paul

Whatever happened to "if you short a turn, it will fry..." declaration, John? Make up your mind, boy.

Wrong. Machining would reduce the ampacity of each turn.

The wire faces are pressed flat and minimal machining follows.

That way, the cross sectional area of the wire is not diminished appreciably, which with machining, it would be.

The graphite brush has a lot of resistance, especially in the lateral (turns-shorting) direction. Graphite is an anisotropic conductor, so one can arrange the good conduction direction to be perpendictular to the windings, and the high resistance between the shorted windings. I think Variacs also run low turns-per-volt to keep the shorted-turns current down.

If you used a copper brush, it would probably toast the shorted turns. Try it.

After all, it's no different from any other transformer with a shorted turn. The math is the same. Toroids have low leakage inductance, so the shorted-turn current depends mostly on the turns/volt and the ohms/turn. Adding the graphite brush resistance greatly reduces the shorted-turn current. One wire turn is probably milli-ohms; the brush adds hundreds of milliohms to the shorted-turn circuit, reducing the current in proportion. Pretty simple stuff, actually.

I measured 116 mA primary excitation current on my 5-amp, 120V variac; Phil measured 20 mA on his 240 volt unit, both independent of wiper position. The shorted-turn current can't be very high.

You've never measured anything. Numbers scare you.

John

Only a tiny bit, since a fraction of the length of the turn is reduced in area.

And isn't pressing "machining"? As usual, all you can argue about is words. Not volts, amps, ohms, conductivities, all those hard "number" things. Just words.

Minimal or none? Make up you mind, boy.

John

[snip]

Yes, "Boy", you certainly impressed AlwaysWrong ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

--
If none were done, then the wiper plane would contain some detritus from
the pressing and it certainly wouldn\'t be as flat as it should be.


 
JF

Wow, should I get some popcorn now?

Don't forget the cattle prod, elephant gun and troll proof gloves. Their crooked teeth leave nasty wounds.

--

Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in your account:

"John Larkin"

** Toroidal TRANSFORMERS have low I leakage from primary to secondary ( being as the two are overwound right round the core ) - but a variac is NOT one of them.

Magnetic coupling from one part of the winding on a variac to non-adjacent parts comes via the wound iron strip - and that means HIGH leakage inductance.

Only reason there is not a lot of voltage loss due to this is the fact it is all one winding and hence directly coupled by the resistance of the copper wire.

..... Phil

--
Then you probably didn\'t measure it right. 

I did it this way:


MAINS>------+
            |        
            P||S------+

The transformer is a current transformer or any garden-variety
transformer with the low voltage winding connected in series with the
transformer.

.
.
.

>I measured 116 mA primary excitation current on my 5-amp, 120V variac;
>Phil measured 20 mA on his 240 volt unit, both independent of wiper
>position. The shorted-turn current can\'t be very high.

I put a Fluke TRMS dvm, on its current range, in series with the input. You don't trust Fluke?

That top setup has a gain that's proportional to frequency, so any harmonics you see on the scope are magnified compared to the fundamantal.

Then you probably didn't measure it right.

John

Looks like one or other of us got through to him.

Sylvia.

OK, assume a toroidal transformer has low pri-sec leakage inductance because the secondary is very close to the primary.

And magnetic coupling from one winding of the primary to a non-adjacent part of the secondary is through that same core.

What winding could be closer to the primary than a turn of the primary itself? Can a secondary turn be magnetically coupled to the primary better than a primary turn? Cute trick if so.

Can anybody explain that last one to me?

John

"John Larkin" "Phil Allison"

** Not interested in your f****it verbal games. ** False logic.

A secondary IS wound all over the primary - essential for minimising I leakage.

This is NOT the case with a few turns in the middle of a variac.

** It's sooo simple.

A variac can work with DC.

..... Phil