Nicholas Hall wrote:
Hello list, I have a Staco 5A variac that I've used with my 15/30 NST powered coil with no problems. I've now upgraded to a 15/60 NST. I'm wondering how Variacs fail and if it's Ok to overload them for short periods. I'm also wondering if keeping it in transformer oil would help. At this point I cannot afford a new variac (they're fairly expensive on Ebay) so if anyone has input it would be much appreciated.
If the wiper brush and contact windings are in good shape, a variac can typically be run at higher than it's nameplate rating for a specified duration of time before you need to shut it down and allow it to cool. Such an overload rating is provided in the form of an overload graph, one for Staco can be seen here:
http://www.variac.com/staco_overload_graph.htmYour 15/60 pulls (ideally) 900VA from the wall. At 120V, that is 7.5A of current, or a 150% overload condition. At the duty cycles involved with Tesla Coils, that is well within the capabilities of your unit. Additionally, if your variac is wired up as a booster (going from 0V to 140V), providing 140V (where your NST will pull 8.75A), you are still well within the units ability.
Jon
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