How does a variac affect watts consumed?

[ls1mike]

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?

[tnom]

As others have said it is a squared function. What you should really be asking is your variac big enough to handle the wattage that you are attempting to control. Not all variacs are large capacity.

[Deke]

You are also losing power in the variac.

[hallerb]

I suspect its a large loss too, and will ask a goiod buddy with a electrical engineering degree

[Bud--]

Variacs are transformers and are very efficient. Rheostat dimmers have high losses.

-- bud--

[Dave Martindale]

There's probably little loss in the Variac. It's a transformer, and probably 90% efficient or better.

Dave