Variac power ratings

Hello list.

There has been some discussion recently about variac power/current ratings. 

I just want to remind coilers that variac kVA ratings don't really mean
anything except when the variac is delivering max current at the max output
voltage i.e. max(V)*max(A). Even then its complicated by the nature of the
load. Because, depending on the setting, the output current flows through
different lengths of the winding. 

This means the max output current is higher for a constant impedance load
(i.e. when current drawn is proportional to applied voltage) since the max
load current will flow through only a very small length of the winding.
Unlike a constant current load (i.e. when output current is carried despite
the output voltage setting) where the max current will flow through a much
longer length of the winding at low output voltages.

The longer the length of the winding, the greater the power dissipated by
the variac (from I^2*R losses). If too much power is used to heat up the
winding...you will let the smoke out! 

So, if you want to overload your variac to control a low impedance load
like an pole pig, avoid long run times at the lower voltage settings. Your
variac will thank you for it.

Safe coiling,

Gavin Hubbard