Variac faults: what to look for

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Scott Hanson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <huil888-at-surfside-dot-net>

A few comments about variacs, what can go wrong, and what to look for:

Yes, variacs (variable autotransformers) can be damaged. It's been my 
experience that many variacs that have had at least some smoke let out of 
them show up at swap meets and on eBay, so the buyer of a used unit has be 
careful. I would be VERY careful of buying anything that I couldn't examine 
first-hand unless the seller provided a money-back guarantee.

The very first "test" is to simply run the variac through its entire range 
of rotation. There should be no rough spots or binding. If there is, this 
is an indication of a problem which should be identified and fully 
understood before considering the purchase of the unit.

Probably the most common variac fault is worn-out brushes. This can be a 
serious problem, as replacement brushes from the manufacturer are quite 
expensive, and may be simply unavailable for very old (discontinued) units. 
Unless the variac is almost free, avoid units with badly worn brushes. 
Anything with less than 3mm worth of brush projecting from the brush holder 
is very near end-of-life. Depending the variac design, sometimes the 
rotating member that carries the brushes can be re-positioned on the shaft 
to provide a little more contact pressure.

The second most common fault is overheated windings, usually shown by 
discolored or darkened enamel on the magnet wire. In very severe cases, the 
enamel will be literally burned away or flaking off.  Variacs showing this 
fault need to be checked very carefully at the commutator surface. A common 
cause of burned windings is worn brushes, which permit arcing at the 
commutator surface and erosion of the wire. Depending on severity, this may 
be a fatal flaw, as the erosion causes the surface of a few windings to be 
lower than the surrounding turns, and arcing will always occur as the brush 
passes over this point, which further erodes the windings, which worsens 
the arcing, etc, etc. This is why the initial "smoothness" test is such a 
good indicator of variac condition. If a variac has minor commutator 
erosion, it can be disassembled and the entire commutator surface carefully 
dressed with a large fine-cut file (or a piece of 320 grit silicon carbide 
paper mounted to a dead-flat backer), but you can really remove only a 
limited amount of material. Keeping the entire commutator surface flat 
(co-planar) is important; you can't just file away at the eroded spot.

A subtle problem that is sometimes found in variacs that have been 
overloaded is brushes that have been overheated. Typically, the brush is a 
graphite composition bar that is soft-soldered into a brass housing (the 
graphite is first copper plated to make it solderable). Gross overheating 
will sometimes melt this solder, or overheat the brush spring causing it to 
lose tension. Variacs with any indication of overheated & discolored 
windings should be checked for brush and brush-holder problems.

Less frequently seen problems are bent brush-carrier plates or bent shafts. 
If the brush-carrier plate wobbles, brush contact pressure will vary, and 
in extreme cases the brush will lift off the windings at some point, or 
reduce contact pressure to the point that arcing may occur. This type of 
problem is strictly mechanical, and can usually be corrected by 
straightening or re-aligning the offending part.

Finally, remember that variacs actually have TWO brushes, one which 
contacts the windings, and the other which contacts a slip-ring concentric 
with the shaft. The slip-ring brush is usually solid brass or 
copper-graphite composition, and the slip ring is brass. Occasionally the 
slip ring surface will be found to be pitted or burned, but this can 
usually be cleaned up with 400 grit silicon carbide abrasive paper.

Very old ('40's vintage) variacs may be found that did not use a slip ring 
arrangement, but instead used a heavy asbestos-insulated cable that wrapped 
around the shaft two or three turns and connected the wiper terminal to the 
moving brush-carrier plate. I replace this cable with a piece of 
fiberglass-jacketed high temperature aircraft wire, exercising the usual 
cautions with handling of any asbestos-containing material.

Regards,
Scott Hanson