Variac wiper carbon brush lifts off the wiper track

• Variac wiper carbon brush lifts off the wiper track
  Posted by Faringdon on 15 Aug, 2023 14:19
• Hi,
  
  Just cleaning a three phase variac that was showing different L's and R's......i now see that the top coil's wiper comes off the  coil track completely when you get down to the lower and higher voltage levels......so the carbon doesnt even contact one of the coils for a lot of the wiper track....only the middle of the track does it contact....other two coils are OK.
  
  (ends of the coil)
  
  Do you  know what has caused this, and how to fix?....
  
  Looks like the brass brush holder is getting stuck in the metal shaft piece...will file it down inside...and apply grease.
  Any other tips appreciated?
  
  I found this out whilst cleaning it...this was going to be my question, before the  above became known....
  (Just cleaning a three phase variac...just replaced the carbon wiper...and now its squeaky and a bit stiff and unsmooth as the knob is turned....is there any  grease for this purpose?...obviously a non conducting grease.).
  
  Also, do you know where to buy the wee carbon brushs?...its just a piece of carbon (graphite?) of dims 13.4mm x 9.3mm x 2.5mm
• #1 Reply
  Posted by jpanhalt on 15 Aug, 2023 15:25
• Pictures ..... pictures..........
• #2 Reply
  Posted by AVGresponding on 15 Aug, 2023 16:30
• Sounds like the shaft is bent or somehow misaligned.
  
  Yes, it's just normal brush material. Any brush the right size will do, or if you can, machine one down to size.
  
  Don't use grease on the wiper; you'll cause connection issues. Carbon is a pretty good lubricant on its own, it'll improve as it beds in. Unless you've too much spring pressure on it.
• #3 Reply
  Posted by factory on 16 Aug, 2023 16:51
• Erm no, variac brushes are not just any brush material, in use the brush will short out turns.
  The weblink below gives examples of methods used for preventing damage from the shorted turns, either by special brush material, or separate wipers & diodes.
  
  https://sound-au.com/articles/variac.htm
  
  
  
  David
• #4 Reply
  Posted by mikeselectricstuff on 16 Aug, 2023 18:29
• I think some variac brushes use a sandwich of layers of copper and graphite, to preferentially conduct in the wire-to-base direction without shorting turns. May only be the case for higher powered ones.
  RS sell replacements
  https://docs.rs-online.com/8007/0900766b8157d4f4.pdf
• #5 Reply
  Posted by AVGresponding on 17 Aug, 2023 05:22
• Copper and graphite... exactly the same as high power brushes. Plain graphite, exactly the same as normal brushes.
  
  The brushes in my variacs are plain graphite, but then they are only 500VA and 3kVA.