Wednesday, October 10, 2012

new video - rebuilding variable transformers

I use variable transformers* a fair amount, most often in crude high voltage power supplies, such as Tesla coil power supplies. They are expensive to buy new, so I am always on the lookout for good deals on used units. Often, an old variac will have defects from abuse and will need a bit of TLC before they can be put back into service.

The item in this video was one such. It had been left sitting in a single position for many years, while more current than it was rated for ran through it. This resulted in one area of the (brass) slip ring being badly eroded and pitted, the (copper-graphite) slip ring brush being badly pitted and eroded, and some of the winding contact areas being pitted.

The (graphite) winding brush was in good shape thankfully, as it would have been a bear to replace. This first video shows how I turned down the surface of the slip ring to remove the pitted areas. The slip ring is mounted on the underside of the rotating brush plate, so you can't get a good idea of its condition unless you take the variac apart. Before you buy an old, used variac, it would be a very good idea to examine its brushes, its windings, and its slip ring carefully. If you didn't do it before you bought it, you would be smart to do it afterward before you use it, to avoid unpleasant surprises. Even if you don't own a lathe, you can do a lot of good to the slip ring with steel wool or a Scotch-Brite pad. If you decide to clean up the contact area of the windings, beware that these are often plated with a thin layer of brass to increase wear resistance vs. bare copper. If at all possible, you want to leave as much of that brass plating on the windings as possible, to extent the life of the unit. Frankly, for occasional amateur / home use, you probably won't put enough wear on those windings for it to matter, but there's no sense wasting good workmanship if you can avoid it. Not all variacs have this feature, some just provide a bare copper surface.

* commonly called "variacs" by old timers - Variac™ was a trademark of the now-defunct General Radio Corporation. Other common brand names include PowerStat™ from Superior Electric and Staco, both of which are still making and selling excellent products. Finally, there is an importer of Chinese variable transformers who is using -- legally or not -- the name "Variac" in their company and domain names. Whether or not that company bought the trademarked brand from Teradyne (the inheritors of the Variac brand) I neither know nor care, because I have examined the build quality, wire size, and so forth of their units, and they do not seem worth the pricetag. Caveat Emptor!

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