Variac carbon brush replacement

[Mick Bailey]

My lovely old Zenith variac needs a carbon brush replacing. Its a 270v 10A unit and one of the pair of wiper brushes is no longer making contact. It still works fine, but the current capacity is reduced.

Am I correct in thinking that a re-profiled washing machine brush could be used? It presses into a brass carrier so no difficulty mechanically. All sorts of information on the web about directional properties of carbon that sounds like armchair theory to me. It's 1950s so I'm thinking it would just be a run-of-the-mill piece of carbon.

Alternatively anyone got the correct part for a Zenith Y20-HM?

[Steve Conner]

I think it would be a pretty safe bet. The required directional properties are the same for a motor brush as a variac brush anyway.

On my old 10A Foster variac, the end of the brush has been filed down to short out as few turns as possible. Looks like it was that way from the factory.

[Mick Bailey]

The remaining one is still pretty much intact and from the end looks as though it's hollow-ground like a knife. Just enough width not to drop between the windings.

My small variac has a metal roller, which seems like a much better idea.

[Leo_Gnardo]

The remaining one is still pretty much intact and from the end looks as though it's hollow-ground like a knife. Just enough width not to drop between the windings.

My small variac has a metal roller, which seems like a much better idea.

Before I ever got my hands on it, some fixit type swapped out the (presumably shot) brush carbon for a bit of bent brass sheet. Been working 30+ years without fail. What are the directional conduction properties of brass I wonder?

[Mick Bailey]

I have yet another variac, a tiny 500VA Claude Lyons school-lab type which I think is from the 70s, that has a bronze (or beryllium copper) wiper from the factory. The windings are quite fine but like yours haven't suffered at all as a result of metal-to-metal contact.

In an old General Radio Variac owner's manual it recommended lubricating the contact area with silicone grease.

[drewl]

Check if there's any motor rewinding shops in your area, they may have one. When I was doing CAD drafting and running the machine shop for one place i worked,
I used to take motors for turret presses to a little shop for rewinding and those motors used the same type brushes.