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Topic: variac basics (9 msgs / 321 lines)
1) From:
After a good number of years I today tried roasting with an air popper once again.  I came across a 1250 Watt Proctor Silex one that was left in a rental property and couldn't resist the temptation of trying it.  I was amazed at how quickly it heated up and how powerfully it kept the beans --86 gramms, same thing I used in my I-roast--  swirling around and ultimately how evenly it roasted.  
is it overkill to use a vaciac with this kind of roaster?  if it is not, can someone tell me about vaciacs; I have the scantest knowledge of what they are (I was just reading a bit online).  I take it that if I were to buy one i should get one that plugs into a whall outlet and that has an outlet into which I plug the popper.  Then there is typically a dial that you turn in order to set and  control the wattage of the popper and that by doing this you can carefully control the heat.  I assume that after using the popper enough you will become accustomed to what point on the control keeps the popper at a given temperature.  Is all fo this correct?
Tne next question is what the capacity should be for a variac that will control a 1250 w device.  if i understood correctly I would divide 1250 by 84.5, which suggests a 15 AMP variac.  But that seems like a lot doesn't it?  
Then the next question, if it is permitted in this forum; where do you purchase a variac?
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2) From: raymanowen
Let me think about this...
You said:
"I was amazed at... how evenly it roasted."
[Sounds like you got this tiger by the tail already. What more do you want?]
For the formula-challenged, Power = Volts x Amps
Suppose the nameplate on the PS popper says: 1250 watts  120 volts [1250/120
= 10.4A]
Suppose: 1250 watts  12 amps [1250/ 12 = 104 V] Impossible! 104 v does not
exist.
"divide 1250 by 84.5, which suggests a 15 AMP variac" [84.5v x 15a = 1267=
w]
1250 watts by itself is meaningless, and from where did you get 84.5? 84.5
what?
Variacs are variable auto transformers normally used to lower the input
voltage to as low as 0 volts, or raise it to as high as 1.4x the input
voltage.
With an essentially resistive load like a popper, if you raise its input
voltage by the 1.4  factor, the current goes up by the same 1.4 factor and
the power about doubles. 1.4 x 1.4 = 1.96 (2!)
If you need 15 amps for sure, you can use a Variac rated at 10 amps. It will
develop heat faster than convection air currents can remove. It will get
warm. A little fan can be added to completely eliminate heat problems.
If your device- popper or whatever, normally draws 15 amps at its rated
voltage, and you jack it up by 1.4, now it's drawing about 20 amps. You can
still use a 10-amp rated Variac, but it will just get hot faster. Bigger fan
and don't keep doing it very long.
Let the Variac cool completely after you run it at 100% overload. No formula
here, just don't spike your Edsel with hydrazine very often.
From Wikipedia:
Operation
 
 
Single-phase tapped autotransformer with output voltage range of 40%–115%=
 of
input
As in an *ordinary transformer*, the ratio of secondary to primary voltages
is equal to the ratio of the number of turns of the winding they connect to.
For example, connecting the load between the middle and bottom of the
autotransformer will halve the voltage. Depending on the
application,
that portion of the winding used solely in the higher-voltage (lower
current) portion may be wound with wire of a smaller gauge, though the
entire winding is directly connected.
[edit
] Limitations
An autotransformer does not provide electrical isolation between its
windings as an ordinary transfomer[sic] does. A failure of the insulation of
the windings of an autotransformer can result in full input
voltageapplied to the output.
This is an important safety consideration when
deciding to use an autotransformer in a given application. Furthermore, if
the "neutral" side of the input is not at ground voltage, the "neutral" side
of the output will not be either.
Note- Variac autotransformers normally don't have taps as shown in the
drawing. Usually it is a single winding on a circular toroid-shaped coil
form. The "wiper" can contact any turn of the winding.
Cheers, Mabuhay -RayO, aka Opa!
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4) From: silas coelho
Variac is also know as variable auto transformer, and basically what does is
to adjust the output voltage as a PROPORTION of input voltage, and I
enphasize proportion, because if your input voltage (115 Volts/127 Volts or
even 220 Volts) changes, the output is also going to change.
When connected to our popcorns it will control the total power of the
popcorn maker,
The issue is, unless your input voltage is constant (seldom the case) your
output power is going to change over time (means your temperature + motor
speed for the fan as well). But it does the work for  most of cases, giving
you a bit of control of the temperature for the poppers. If you want to go
even further, a suggestion will be to adapt a switch to turn on/off the
heater on the popper allowing you to have a 'cool down fan' (depending on
the popper it will require much more then just switch)
Hope will not be against the list police (no competition here) Radio Shack
and similar stores would have variacs, I will  divide 1250w/114 Volts giving
around 11 A, but going to the safe side stick to the 15 A (it is a lot)
regards
Silas
2010/11/13 
<Snip>
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5) From:
Thanks a lot.  I realized that I no doubt have transformers in devices all around my house ("sure got the tiger by the tail").   I discovered Tom's video on using a popcorn roaster and there is information there also about the switch.   I imagine that what you really will end up doing is "working the switch," turning it on and off to try to keep a constant temperature and ending up with profiles within a range (first stage between 220 and 250 degrees, for 4 minutes, second stage between 300 and 350, etc.),  I am not sure the I-roast was really any different in this.  Thank you for the information.
---- silas coelho  wrote: 
<Snip>
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6) From: Dean
Couple of good responses so far, let me add.....
There was a thread related to sizing and ratings compared to duty 
cycle a couple of years ago.
Look in the archives--it was miKe & me, et al.
The short answer to sizing is if you're running continuously get a 
12-15A rated unit.
If you are running batches with some cool-down in between, 8A should 
be OK, a 5A might be enough.
Do the sizing math from the old thread.
Tom used to sell a Variac--a bigger one.  Not sure if it's still 
offered......
Otherwise, and I hate to let the cat out of the bag here, but when it 
comes to finding stuff online  "Google is your friend" .
'bout to fire up the Behmor in da weeds
Dean
On 11/13/2010 12:15 AM, msmb wrote:
<Snip>
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7) From: Edward Bourgeois
I like using a variable transformer/variac over going PID. Been using
one with my roaster for 5 years. From all I've heard over the years
there have been many failures with the models out of China. Unless
there have been recent improvements in these I highly recommend
looking for a a US made model Staco, Superior etc., in good condition
used. Many are priced high on ebay but if your patient they can be
found for $50.-$100.
On Sat, Nov 13, 2010 at 1:15 AM,   wrote:
<Snip>
ce again.  I came across a 1250 Watt Proctor Silex one that was left in a=
 rental property and couldn't resist the temptation of trying it.  I was =
amazed at how quickly it heated up and how powerfully it kept the beans --8=
6 gramms, same thing I used in my I-roast--  swirling around and ultimate=
ly how evenly it roasted.
<Snip>
, can someone tell me about vaciacs; I have the scantest knowledge of what =
they are (I was just reading a bit online).  I take it that if I were to =
buy one i should get one that plugs into a whall outlet and that has an out=
let into which I plug the popper.  Then there is typically a dial that yo=
u turn in order to set and  control the wattage of the popper and that by=
 doing this you can carefully control the heat.  I assume that after usin=
g the popper enough you will become accustomed to what point on the control=
 keeps the popper at a given temperature.  Is all fo this correct?
<Snip>
ontrol a 1250 w device.  if i understood correctly I would divide 1250 by=
 84.5, which suggests a 15 AMP variac.  But that seems like a lot doesn't=
 it?
<Snip>
rchase a variac?
<Snip>
mariascoffee.com
<Snip>
ee.com/gallery/main.php?g2_itemId=7820
<Snip>
-- =
Ed Bourgeois aka farmroast
Amherst MA.http://coffee-roasting.blogspot.com/Homeroast mailing list
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8) From: raymanowen
Oh, Hey- How stuff works on Green network- Barista competition. Looks
delicious -ro
On Sun, Nov 14, 2010 at 8:59 AM, Edward Bourgeois wrote:
<Snip>
-- 
Persist in old ways; expect new results - suborn Insanity...
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9) From: Allon Stern
On Nov 14, 2010, at 7:59 AM, Edward Bourgeois wrote:
<Snip>
Just curious, what are your reasons for going with a variac versus a PID?
They do different things - a variable transformer gives you control over the heat applied, but where you set it is entirely up to you.
A PID runs a feedback loop with input from a thermocouple to achieve the same end - variable control of the heat, but instead of giving you control of the heat applied, it gives you control over the temperature that the heat applied gives you.
A variac is like holding the throttle on a car in the same position regardless of speed; it gives you the same power applied, but you'll go faster downhill than uphill. A PID is more like a cruise control (in fact, cruise controls incorporate PID algorithms) - you set the speed you want, and it asks the motor for more power when climbing hills and less power when going downhill, to keep the speed the same.
Roasting is anything but flat open road; there are sivitz bumps at various stages, and the roast builds inertia; knowledge of these bumps and other aspects will let you adjust your variac to attempt to manage them for smooth acceleration, but it's a battle.
Also, many PIDs will let you do more than fix a setpoint; you can program a profile to run, and that programming is real control over the roast.
I'd rather adjust a PID profile and watch it run, maybe be able to tweak it on the fly, than have to babysit and tweak a variac and watch a thermometer and try to hit specific points at particular times.
-
allon
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