Variac nonsense

Just so it's clear the reason why it isn't interpolation is because you move in discrete steps. If V is the voltage at some position then V + dV is the voltage when you turn one step... you can't get anything lower than dV. There is no interpolation... has nothing to do with the contact or anything. How do you expect to get dV/2?

Note V is linear w.r.t to the turns so dV is constant. This means as you turn the rotor you increase V by discrete steps. Simple as that. Now how's the moron? Should name names?

"Jon Slaughter" a écrit dans le message de news:fw9ll.10418$ snipped-for-privacy@flpi150.ffdc.sbc.com...

The one small thing your narrow mind don't get is that the Rs change, depending on the driving points position.

Should you have checked, since you have one, you'd have noticed continuity and (maybe) thought before spouting all this non sense.

-- Thanks, Fred

Your model contains non-interpolation as an assumption, because of the way you've placed one resistor per turn.

Model the brush with twice the number of resistors per turn, and allow it to be moved a half turn at a time, and in three consecutive positions you get

Voltage A +-----+-----+-----+-----+-----+ | | | | | | R R R R R R | | | | | | +--+--+ +--+--+ +--+--+ | | | V------L1--+-----L-----+-----L-----+-----L-----+---L2---GND V1 V2 V3 V4

and

Voltage B +-----+-----+-----+-----+-----+ | | | | | | R R R R R R | | | | | | +--+ +--+--+ +--+--+ +--+ | | | | V------L1--+-----L-----+-----L-----+-----L-----+---L2---GND V1 V2 V3 V4

and Voltage C +-----+-----+-----+-----+-----+ | | | | | | R R R R R R | | | | | | +--+--+ +--+--+ +--+--+ | | | V------L1--+-----L-----+-----L-----+-----L-----+---L2---GND V1 V2 V3 V4

It should be reasonably clear that Voltage B will lie midway between Voltage A and Voltage C, so the number of discrete voltages has been doubled (ignoring end effects).

Model with three resistors per turn, and the number of discrete voltages is tripled.

The closer the model gets to being representative of a continuous resistive contact, the closer you get to having a continuously variable voltage.

Sylvia.

Funny, so your contact changes size as you turn the rotor?

Hilariously, that's not what the model implies.

If it makes it clearer, think of the horizontal lines beneath the resistors as representing the surface of the wires, thus

Voltage A +-----+-----+-----+-----+-----+ | | | | | | R R R R R R | | | | | | +--+--+ +--+--+ +--+--+ +--+--+ | | | | V------L1--+-----L-----+-----L-----+-----L-----+---L2---GND V1 V2 V3 V4

and

Voltage B +-----+-----+-----+-----+-----+ | | | | | | R R R R R R | | | | | | +--+--+ +--+--+ +--+--+ +--+--+ | | | | V------L1--+-----L-----+-----L-----+-----L-----+---L2---GND V1 V2 V3 V4

and Voltage C +-----+-----+-----+-----+-----+ | | | | | | R R R R R R | | | | | | +--+--+ +--+--+ +--+--+ +--+--+ | | | | V------L1--+-----L-----+-----L-----+-----L-----+---L2---GND

Sylvia.

Obviously you don't own a Variac, or you could do the experiment yourself. My variac has about 200 turns and can be set, with care, to

1 part in 3000 of full-scale output. The limit seems to be stickiness of the mechanics, my ability to hit a desired angular position.

Explain that. Show your work.

John

[snip]

"Show your work" was the operative rule at MIT.

If you didn't show your work, but only the answer, at the very least, no credit... at the worst, called on the carpet if it were suspected you were cheating.

That happened to a friend. Exam question was about VSWR, which he knew right off of the top of his amateur radio head. He explained himself easily to the professor and walked away with credit, but in a substantial sweat ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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^^^^^^^^^^^^^^

No.

Look inside your Variac. The guy who invented the idea, circa 1930, was a lot smarter than you are.

John

Maybe this will help you see what Sylvia is saying:

The contact area between the wiper and the winding changes.

| Wiper } | ----------------------------}-------- __________ ___________ ______} Winding N Winding N+1 Winding N+3

Move it a little:

| Wiper } | ----------------------------}-------- __________ ___________ ______} Winding N Winding N+1 Winding N+3

Move it a little more:

| Wiper } | ----------------------------}-------- __________ ___________ ______} Winding N Winding N+1 Winding N+3

Ed

Yup, that's how it works. The windings aren't little cylinders, they are machined flat. The ratio of exposed contact width to gap is roughly what you've drawn... more copper than gap.

The brush contacts, at most, the full width of three wires; at least, two.

The brushes on mine are also rounded on the ends, so they sort of creep up on a contact...

instead of making a hard on/off sort of connection. I suspect they naturally wear in this shape, rounded on the corners.

I have read that the brush is made of anisotropically conductive graphite, with the better conduction along the vertical axis. This reduces the direct-turns-shorting resistance.

I've observed no change in primary idle curent as the wiper is moved, so the turns-shorting transition is very smooth.

Pretty clever, overall.

John

--
Hmmm...

news:jmgbp4hcq8dhcmf1qe82ts3237bbu2456o@4ax.com

JF

Mine's different. See abse.

Or google stepless variac

John

Way back in ancient times, philosophers used to gather at the local Greek pub and vehemently argue about important questions of the day, such as how many teeth a horse had.

Euripedes would argue that it was self-evident-- just as there were

6 main deities, each with four limbs, a horse must have 24 teeth. No argument possible there. Everyone else must be a moron. Right after that, Bellerophon belched loudly and went into a rant as to how everyone other than him was a moron, because it was obvious that if a lizard has 72 teeth, and a chicken has none, you can with straightedge and chalk construct a 3/4/5 triangle whose perpendicular quadriceptor gives you a projection of length 28.

Silly them, if they'd just have looked at the butt of the slave boy they were passing around, he'd been bitten by a horse and you could count the teeth marks. But nobody did.

------------------ If you don't get the drift, if you're going to use a model, it has to be a realistic model. Or you could look at the real thing.

Your basic carbon contact is not a bunch of R's standing on end, it's a bunch of vertical and horizontal R's. If you try that model, you'll see the vertical resistance near the edges is higher, as there are half as many horizontal R's paralleling the vertical ones. So there is going to be a continuous sweep of voltages averaged by the contact.

More important, the tops of the wires are often machined flat, so they don't act like point contacts.

You could actually look inside a Variac and see this, instead of theorizing.

John

Edges make no difference. Your model should be about the line of tangency that CONTACT is defined by. THAT mating surface, and the electron flow interface into or out of it is all that matters. The area near the corner or face of the brush rarely even gets utilized, much less plays into the physics behind the operation of a carbon based high amperage brush against a copper face. The mating face is rarely a 'full face' contact, so think about a railroad wheel and the rail. All that weight rests on a strip of steel as narrow as a dime. A quarter inch wide brush likely has less than an eighth inch of strip used as a calculation for its capacity by its designers.

Nope. More often, they are flattened without removing any of their cross sectional area, then lightly surface finished to yield a coplanar face. They *still* do act like a point contact as full face mating is never possible due to they way the brush "wears". One gets about half the "thickness" of the brush as a 'mating line'.

Or, you could look, and thinking that you know all, guess at how it is made, and even assume that all are made 'your way', as you have done here.

Yes, John... exactly that, f*****ad.

Point contacts would produce voltage steps.

I've seen references to both machined and ground winding surfaces in variacs. I suspect that are made all sorts of different ways.

Some people even use roller contacts.

John

Neat - I wonder how they do that.

I like your diagram - much better than mine. Also I noticed that I made an error - should be winding N+2, not N+3. Still, the concept remains the same.

Yes! :-)

Ed

Graphite is an inherently anisotropic material having far different properties parallel to the strongly bonded hexagonal planes than perpindicular to them in the weakly bonded inter-plane direction. Electrical resistivity within the hexagonal planes is about 2.5 x

10**-6 to 5.0 x 10**-6 ohm*meter, and about 3000 X 10**-6 ohm*meter in the perpindicular direction.

Real polychrystaline graphite used as brushes is only partially ordered (else it would shear too easily between the planes) so the ratio of conductivities is probably less than 10:1. (Thermal conductivity varies similarly, with about a 300:1 ratio in single crystal graphite and much less in practical semi-ordered polycrystaline graphite.) The best ordered readily available graphite is graffoil (tm), but it is much too weak to use as electrical brushes. Graphite flakes pressed into sheets tend to order themselves with the strong planes roughly in the plane of the sheet if the flakes are large enough relative to sheet thickness, and the sheets can be stacked and pressed at high pressure and temp to form semi-ordered blocks suitable for brushes.

Ref. "Handbook of Carbon, Graphite, Diamond and Fullerenes" by Hugh O. Pierson