EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: BravoV on December 11, 2016, 02:36:42 pm
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Just bought an used variac (Matsunaga brand, made in Japan), as its so dirty when I got it, decided to clean and complete tear down.
As I never owned a variac, and never seen up close at it's part details, spotted interesting part about the wiper.
My question is about the wiper contact material, is it graphite as like graphite brushes in AC motor ?
Can't be metal right ? As it will scratch the copper wires, cmiiw.
My worry if it will wear down after intense used, as the shape is quite unusual, it will be hard to find as mine has weird shape encased in brass tube. Or I'm worry too much ? :-//
Close up photos if the variac's windings at the contact points and the head of wiper attached below.
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That looks like graphite to me, possibly with copper particles embedded in it for lower resistance. The "weird shape" is so it makes contact only on one or two windings at most, if you need a new one it won't be that hard to grind it down to that shape.
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Yes, it's the same stuff as carbon (graphite) AC/DC motor brushes. It's important that it is resistive enough that it doesn't cause excessive loss by shorting adjacent turns on the winding. That's also why it's shaped to minimise the number of turns that it spans. They wear down very slowly in practice, not really something to worry about (just think about the number or rotations a motor armature does on a set of brushes!)
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Yes graphite brush, and they typically last decades in use, even one used daily. If you do wear one down there are many brush manufacturers who will make one for you, given the dimensions of the original, or you just buy a brush larger in diameter and length and use a metalworking file to profile it, which is easy as the material is softer than steel. It however in use works well, as it is self lubricating, and the film it leaves provides lubrication of the wire surface.
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Thanks guys, for re-assuring the wiper graphite will not wear like those motor's brushes do. :clap: Btw, its probably aged more than 30 years.
Yes, it's the same stuff as carbon (graphite) AC/DC motor brushes. It's important that it is resistive enough that it doesn't cause excessive loss by shorting adjacent turns on the winding. That's also why it's shaped to minimise the number of turns that it spans. They wear down very slowly in practice, not really something to worry about (just think about the number or rotations a motor armature does on a set of brushes!)
Thanks, for the info about the wiper contact arrangement at the windings, just noticed this too as the contact point is positioned abit slanted, so its touching only 2 or 3 windings, attached photos below, also show the wiper and arm construction, as you can see the graphite is customed made, a graphite rod inside a brass tube.
Yes graphite brush, and they typically last decades in use, even one used daily. If you do wear one down there are many brush manufacturers who will make one for you, given the dimensions of the original, or you just buy a brush larger in diameter and length and use a metalworking file to profile it, which is easy as the material is softer than steel. It however in use works well, as it is self lubricating, and the film it leaves provides lubrication of the wire surface.
Thanks, also another question on cleaning and maintenance, all I've done is just cleaning all parts using vacuum cleaner sucking all those dirts, webs and dead spiders :-\
I have not touch anything yet at the exposed windings, do I need to do something about it ? Clean ? How ? Close up photo at 1st post should show clearly it's condition.
About the graphite contact surface, it doesn't need to be smooth like ? How bout mine ?
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No need to clean the brush itself, just run around a dozen times from end to end, use a dry cotton cloth to remove the dust produced from the wires and then it will be fine for years. Centre contact clean with some IPA and then apply a thin film of white lithium grease to the surface to reduce wear. A dab on the bushes as well, or a drop of SAE30 engine oil applied with a thin wooden skewer to them to reduce wear and it will be fine.
Remember the standard usage area for a variac is in an industrial panel in plant, and there it has zero maintenance for decades.
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About the graphite contact surface, it doesn't need to be smooth like ? How bout mine ?
Nope, it has spent the last 30 years bedding in against the copper winding (and the winding against it!). Personally I would leave the brush and winding dry to avoid any chance of clogging up with dust. Whatever you do, don't touch the winding with anything that could possibly raise a burr. I'd leave it at a dry paint brush (brush outwards to avoid trapping any bristles).
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I successfully replaced the missing brush in one of my larger variacs by cutting and filing the carbon rod from a D cell carbon-zinc battery after cleaning it. while not ideal, it has been working fine for 10+ years.
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Centre contact clean with some IPA and then apply a thin film of white lithium grease to the surface to reduce wear. A dab on the bushes as well, or a drop of SAE30 engine oil applied with a thin wooden skewer to them to reduce wear and it will be fine.
Do you mean the two rings shape brass ? Grease or oil in between ? Isn't that create more contact resistance ? No, I don't have any experience on this, just asking clarification.
Remember the standard usage area for a variac is in an industrial panel in plant, and there it has zero maintenance for decades.
Noted, thanks SeanB. :-+
I successfully replaced the missing brush in one of my larger variacs by cutting and filing the carbon rod from a D cell carbon-zinc battery after cleaning it. while not ideal, it has been working fine for 10+ years.
Thank you, now that is a really good idea, noted. :-+
Damn, I didn't even think about it, as you can see the brush is cylindrical inserted inside a brass tube.
Carbon zinc cell ... can't remember when the last time I bought those, must be decades. :-DD