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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:
Unfortunately, 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
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