On 12 Mar 2007 12:36:46 -0700, wrote:

I'm tweaking an old coffee roaster using a variac. Originally, I used
it simply to compensate for variations in the line voltage at a
particular power outlet, but I have since started to deliberately
adjust the voltage up or down to control the temperature, usually in a
range between 110-135 VAC. Essentially, the roaster is nothing more
than a hot air popcorn popper with a motor driven fan and a heating
coil. I was wondering - if I increase the voltage by, say, 10%, how
does that affect the power consumption? Since the coil gets hotter,
it's clear that it's consuming more watts. Would total watts also
increase by 10%, or does the efficiency of the heater coil change?
The motor speed changes with the voltage, as well. If watts increase
at a faster rate than voltage, then, at some point, I will probably
trip the breaker (although the heater coil probably would have already
failed). In general, how tolerant are heater coils and electric
motors to higher voltages?

You are also losing power in the variac.