Hi James,Surplus US-manufactured variable autotransformers are significantly more robust and reliable than newer Chinese autotransformers. For example, surplus Staco, Superior, Ohmite, General Radio, and GE are built to run at 100% of their face plate ratings continuously, and at 2X their rated current for up to 20 minutes. Most 115-120 volt units are rated at full current for the full output range of 0-130 or 0-140 volts. For example, here are Staco's overload curves. The overload ratings for the other manufacturers are similar:
http://www.capturedlightning.com/photos/P-Chokes/staco_Overload_graph.jpgUnfortunately, Chinese autotransformers are made using smaller cores, fewer turns of smaller diameter wire, and smaller sized brushes. They use every trick in the book to minimize manufacturing cost, just like hot-running microwave oven transformers. As a result, these units tend to overheat when operated for any length of time at the "rated" output current.
Also, look carefully at the specifications - some Chinese manufacturers specify rated output current at only 100 volts, not up to the maximum output voltage. Because these units have fewer turns and smaller core cross-sectional area, they operate closer to core saturation, causing them to draw more no-load current. The smaller core and wire mass result in less thermal mass, which when combined with lower operating efficiency, they run hot, and quickly overheat when stressed.
So, as long as you don't abuse them and let them cool down periodically, Chinese variacs can be made to work for Tesla Coil use. However, if you're like most coilers, you'll need the reserve power handling of a solidly-made variac when you start cranking up the power to "11" or "12". I'd recommend getting a surplus US-made variac off eBay or at a hamfest if you plan to do any serious coiling.
Bert James Janota via Tesla wrote:
I was looking at various Variac models and kind of concerned about what I should be looking at. Any advice from you all would be appreciated. Regards James Janota (727)512-7112 Phone (727)595-6625 Fax _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla
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