I need an education on electric brushes for a variac - AKA electric motor brush? - Page 1
Forum section: electronics / repair

author: t1d
author_rank: Super Contributor
timestamp: February 18, 2021, 09:58:51 pm
content: I am rounding up backup parts for a new BK Precision 1655A AC Power Supply that I am buying. I have learned to buy spare parts as soon as I buy equipment that has a long life, because it is likely that the unit will not be manufactured forever and parts may not be available, in the future.There are two consumable parts that are listed in the manual, with an explanation of how to change them, but they are not listed in the parts BOM. One is the fuse in the leakage probe. It has no stamp on the end cap, but I was finally able to get the manufacturer to find out that it is a 500mA/1KV ceramic safety fuse. The lack of the stamp is common to all of these fuses and a known problem, to the owner community.The other item is the brush for the coil. The manufacture tells me that you must buy a whole new coil assembly, even though the manual explains how to replace the individual brush. So, I am hopeful that you can tell me a work-around, as I know very little about such brushes. Might I buy a larger brush and file it down to size?Here is a good YT video made by an owner. It shows the brush at 8:40.I have done an exhaustive search on the web. I am very much looking forward to your replies. Thank you for your help.

author: jmelson
author_rank: Super Contributor
timestamp: February 18, 2021, 10:05:05 pm
content: It is unlikely the Variac brush will wear out except in heavy industrial use. If it does, you can probably buy a replacement brush at a hardware store and file to fit.Jon

author: bob91343
author_rank: Super Contributor
timestamp: February 18, 2021, 10:17:37 pm
content: I think it's not like a motor brush. One of the patents for the original Variac states that the brush must short adjacent turns, but with a low resistance to minimize circulating current. That way there are no sparks as you turn the knob and not much wasted power in the brush. A motor brush is more likely lower resistance. Maybe a brush for a dc/commutator vs one for slip rings.However I concur that it's highly unlikely that the brush will wear out. I have never had to replace one. I currently have several variacs, some built into equipment, and they get used so little that I cannot imagine wearout will occur in my lifetime.

author: Circlotron
author_rank: Super Contributor
timestamp: February 18, 2021, 10:19:12 pm
content: Just think how many motor rotations it takes to wear out the brushes! If you do get a spare, put it in a small bag inside the unit, don’t just put it on the shelf and try looking for it twenty years later.

author: Ian.M
author_rank: Super Contributor
timestamp: February 18, 2021, 10:35:57 pm
content: There's a good article on Variacs that goes into considerable detail about the specifics of the brush design here:https://sound-au.com/articles/variac.htmDrussell posted this link for a Variac brush supplier last year:http://www.carbonbrush.com/variacbrush.htmThey may well have something that will work off-the-shelf or be able to help with a custom one.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: February 18, 2021, 11:52:40 pm
content: I inherited a pretty old Variac about 20 years ago with broken brushes after a fall. Found a local brush manufacturer who was very helpful with advice on best composition for that application. Simple job to provide them details of dimensions and curvature required and they custom made a couple of sets for me. Not expensive. Been working fine ever since on one set. I think I taped the spare set inside the cover but guess I'll find out if/when I need them.

author: t1d
author_rank: Super Contributor
timestamp: February 19, 2021, 12:28:27 am
content: Thanks, folks! All really great information.I would have never thought of it having a special composition.Good link.Good tip, on keeping them inside the unit.I was not necessarily buying it for just wear, but for the unexpected. But, the point is well taken.