# How does a variac affect watts consumed?

• posted on March 12, 2007, 7:36 pm
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?
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• posted on March 13, 2007, 8:25 am
On 12 Mar 2007 12:36:46 -0700, snipped-for-privacy@gmail.com wrote:

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.
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• posted on March 13, 2007, 9:32 am
On 12 Mar 2007 12:36:46 -0700, snipped-for-privacy@gmail.com wrote:

You are also losing power in the variac.
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• posted on March 13, 2007, 2:01 pm

I suspect its a large loss too, and will ask a goiod buddy with a electrical engineering degree
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• posted on March 13, 2007, 3:10 pm
snipped-for-privacy@aol.com wrote:

Variacs are transformers and are very efficient. Rheostat dimmers have high losses.
-- bud--
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• posted on March 14, 2007, 5:14 am

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