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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Anisotropic resistance just means that the brush is processed to have a higher resistance to currents flowing between turns, than to currents flowing in the wanted direction. The same trick is used in DC motors, where the brushes are always touching two or more commutator segments at a time. I've also seen variac brushes filed down so they don't short any more turns than they have to! (two) They are purely resistive, no diode action that I know of.
The required turns per volt depends on the core cross-section, so 0.8v/turn is not always the answer.
If you decide to use multiple switched windings, surely a binary progression makes more sense: 1v, 2v, 4v, 8v, 16v etc.
Finally the poor man's toroid winder: prepare a flat wooden stick, like a popsicle stick or a wooden ruler for a larger job. Cut notches in the ends and wind the required length of wire onto it. You now have a long flat "spool" of wire that you can pass through the centre hole easily.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Anisotropic resistance just means that the brush is processed to have a higher resistance to currents flowing between turns, than to currents flowing in the wanted direction.
To what magnitude/factor does it work? (Because by normal resistance you couldn't achieve better 'anisotropy' than a factor of 2) How is material (graphite) manufactured to do that?
Or is it purely ohmic resistance as others say?
If somebody has a variac handy, try counting the turns on it. Looking at the volts-per-turn, you can calculate what the dissipation will be with e.g. a turn "shorted" to an adjacent turn through brushes with a resistance of e.g. 0.1 ohms. Then you can look at the dissipation when an e.g. 5A, load is drawn between the tap and ground, and the dissipation that would cause in the brushes.
Unless there is anisotropic effect really taking serious place here I would expect this to fundamentally limit power pushable through any variac.
If you allow your brush to dissipate 10W max in any case, and 2W in stand-by, (which is lot), and variac is 20A, then it's resistance must be lower than 0,02ohms for 8W dissipation.
Now if we consider that graphite has insulating layers and anisotropy of 2 then it could be 0,04ohms at most and in order to fulfill power dissipation requirements you need less than 0.28V/turn. Somewhat too small for variac of that size.
So if I want a huge (like 500A ) variac I need to stack a large number of small cores?
Still this explains smooth and sparkless contact, since it's never actually broken for more than a fraction of volt.
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DIY variac
