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?

wrote in message
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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?

Generally, power varies as the square of the voltage. This is not exactly
true in those cases where the load is non-linear (resistance is a function
of temperature).

1.1^2 = 1.21 ( you get 1.21 x the power with a 10% increase in voltage ).

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?

Get a Kill-A-Watt. Like this:

http://cgi.ebay.com/Kill-A-Watt-LCD-...QQcmdZViewItem

On Mar 12, 3:36 pm, 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?

Resistive load (heater) P=IxE double the voltage means double the
watts (directly proportional)

Resistive load (heater) P=IxE double the voltage means double the
watts (directly proportional)

Well let's see. If you double the voltage on a resistive load you
would also influence a doubling of the current in that same load.
Last time I multiplies 2x2 it equaled 4

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?

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.

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.

*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

wrote:
�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--

" writes:

*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

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

Dave