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?
How does a variac affect watts consumed?
Mar 12, 2007
5 Replies
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.
You are also losing power in the variac.
I suspect its a large loss too, and will ask a goiod buddy with a electrical engineering degree
Variacs are transformers and are very efficient. Rheostat dimmers have high losses.
-- bud--
There's probably little loss in the Variac. It's a transformer, and probably 90% efficient or better.
Dave
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