wg48 - 5-3-2017 at 07:46
I won a none working 240V 25A variac. It was supposed to only have a seized shaft and be in good working order. No it was not.
I thought corrosion had seized the shaft but it appears that the phenolic shaft had expanded. Possible from lubrication. What lubrication would do
that? I have managed to dismantle it and having sanded the shaft down and the holes it fitted in, it now turns.
Unfortunately the commutator and brush assemble were corroded. I have lapped the commutator, its now shiny again but it has one wire pitted, its near
the end (20V) so I can mange with it. I managed to extract the brush assemble but damaged one of the two brushes @&!! There about £100 for a
replacement (£35 for exchange) more than the cost of the variac.
I have wondered why the brushes don’t burn out when they short adjacent wires of the commutator.
The brushes hold a small rectangular block of graphite about 4x4x2 mm. The 2mm face slides on the commutator with the 4mm length parallel to the wires
of the commutator. but its wide enough to bridge adjacent wires !. Actualy I think they are at a slight angle to the wires so they can ride the gaps
between the wires.
I discovered that each block consists of two smaller blocks separated by a thin mica sheet. I assume this is to make a longer path for the short
between adjacent wires. A diagram of a similar brush assemble is shown below. Each block has a screwed clamp (not shown in the diagram) that holds the
graphite block. So I will have to try to make a replacement including the mica.
When it done it will be perfect for regulating the power of heating elements and other stuff, 6kW max output.
[img] http://www.carbonbrush.com/variacvb8.jpg[/img]
Oops sorry I could not get the image to insert
[Edited on 5-3-2017 by wg48]
[Edited on 5-3-2017 by wg48]
yobbo II - 6-3-2017 at 05:01
THAT's a variac and a half!
The was a discussion about this subject here
http://www.sciencemadness.org/talk/viewthread.php?tid=12351
It is interesting to note what you observed.
Yob