HomeRoast Digest


Topic: 10 amp variac for an I-Roast (10 msgs / 290 lines)
1) From: Mike Koenig
Has anyone successfully used a 10 Amp variac on an Iroast? (got my
hands on one for free)  If I remember correctly, my I-Roast (an
original, not an IR2) has a wattage rating of 1500W,  but I'm not sure
if it truly draws that much current.
I'm not too worried about blowing fuses, just concerned about wasting
coffee if it stops mid-roast.  I'll probably try it empty when I get
home today.
--mike

2) From: miKe mcKoffee
Not on IR2, but have been using 10A variac heater side of Frankie with my
Rostos for 5 years. Sometimes doing dozens of back to back roasts no
problem. (like for Holiday gifting) Norm is 4 or 5 back to back roasts
weekly.
Remember variac ratings are for 24/7 continuous duty. You can calculate the
safe duty cycle easily for any given variac and load. Here's a 2/25/06 post
cut & paste from the SM Archives (also re-posted in recent 3/07 Controlling
air popper heat thread)
"From: miKe mcKoffee
Yes, 10A is fine for heater side control. The US made variable transformers
10A rating is for 24x7 continuous use. Mine doesn't even get warm running
Rosto roasts back to back. Using a 10A Powerstat no problem. From the
Superior Electric Company spec sheet for my 10A the rated *safe* overload
use formula rating is: actual current divided by rated current squared minus
one equals off time divided by on time. For only a 5A rated under 120v
roasting with a 1100w FR (pulling 9.166A @120v) with 10 minute roast time
the *safe* operational off time would be: (9.166/5)(9.166/5) - 1 = X/10min
with X being safe time off between roasts. Solving for X=.236min = 16sec for
1100w FR on 5A variac. For a 1300w Rosto 52.85sec. off time same 5A. Much of
the above I cut & pasted from my List posts discussing use of 5A variacs and
over fusing from April '03. "
End cut & paste
Searchable archives a wondrous resource...
Kona Kurmudgeon miKe mcKoffee
URL to Rosto mods, FrankenFormer, some recipes etc:http://mdmint.home.comcast.net/coffee/Rosto_mod.htmUltimately the quest for Koffee Nirvana is a solitary path. To know I must
first not know. And in knowing know I know not. Each Personal enlightenment
found exploring the many divergent foot steps of Those who have gone before.
Sweet Maria's List - Searchable Archiveshttp://themeyers.org/HomeRoast/
<Snip>

3) From: Jeffrey Pawlan
Why use a variac with an i-roast?  I thought they were programmable (sort-of).
If you muck with the line votage to the entire i-roast and not just the heater
which is what poppers and also Mike does, then you seriously run the risk of
destroying the electronics. Besides, you have just downgraded your i-roast to an
old Hearthware Precision. If you really like air roasting, it would be best to
do the popper route or the Mike Rosto. (I don't like air roasting myself).

4) From: Mike Koenig
The main reason is for voltage regulation, more than control.  I have
two places at home where I can roast outside (I currently lack a good
kitchen vent hood).  One is in the front of the house, which is the
closest outlet to my electrical panel,  the other is my back deck,
which is the farthest from the panel.  There is a significant
difference in how my IRoast works between the two locations.  In the
cold weather, I can't even roast on the back deck, since my roasts
will stall (even putting the roaster in a box).  I have no problems
with stalling in the front outlet,  rather the exact opposite,  I'd
like to slow things down a bit.
I don't plan on being able to tweak the voltage to the point of
totally managing my roast profile with the variac (that's a project
for another day) but I'd like to get to a happy medium between the two
locations.
I'm learning that I don't really like air roasting either...
--mike
On 3/28/07, Jeffrey Pawlan  wrote:
<Snip>

5) From: Mike Koenig
The main reason is for voltage regulation, more than control.  I have
two places at home where I can roast outside (I currently lack a good
kitchen vent hood).  One is in the front of the house, which is the
closest outlet to my electrical panel,  the other is my back deck,
which is the farthest from the panel.  There is a significant
difference in how my IRoast works between the two locations.  In the
cold weather, I can't even roast on the back deck, since my roasts
will stall (even putting the roaster in a box).  I have no problems
with stalling in the front outlet,  rather the exact opposite,  I'd
like to slow things down a bit.
I don't plan on being able to tweak the voltage to the point of
totally managing my roast profile with the variac (that's a project
for another day) but I'd like to get to a happy medium between the two
locations.
I'm learning that I don't really like air roasting either...
--mike
On 3/28/07, Jeffrey Pawlan  wrote:
<Snip>

6) From: Dean
I think your math is a little off:
For a 9.166A load on a 5A gadget:
 actual current				off time
[-------------- ] squared minus one =  ---------- 
 rated current 				on time 
[9.166/5]*[9.166/5]-1= [1.83*1.83]-1=2.36  so off time/on time = 2.36  
and if on time = 10 minutes, then off time = 23.6 minutes  --> 23.6/10=2.36
Another, simpler way on looking at it is duty cycle is 1 divided by 
(overload factor squared) =>
1.83x overload ^2=3.36;  1/3.36= .30  or you can run up to 30% of the 
time safely.
There is also a maximum on-time that depends on the overload--at 2x it 
seems to be in the area of 15 min.
followed by 45 minutes of rest
If you look on the Variac website, there are lots on manuals with 
diagrams, etc.
For the other example given, the calcs above say give it 104 sec off 
between10 minute roasts for a 10A Variac
and 47 min off (a 21% duty cycle) on a 5A.
Dean
in da weeds in Iowa
miKe mcKoffee wrote:
<Snip>

7) From: True, Dennis W. FC1 (CVN69)
OWWWW your making my head hurt I haven't used those formulas since "A"
School....
ACK that was 17 years ago
Dennis
AKA
FC1(SW) Dennis W. True
Safety Dept
USS Dwight D. Eisenhower (CVN 69)
FPO AE 09532-2830
HG/DB and Z&D roasting in the Indian Ocean
 "On station and on point 54 and counting down..." 
I think your math is a little off:
For a 9.166A load on a 5A gadget:
 actual current				off time
[-------------- ] squared minus one =  ---------- 
 rated current 				on time 
[9.166/5]*[9.166/5]-1= [1.83*1.83]-1=2.36  so off time/on time = =
2.36  
and if on time = 10 minutes, then off time = 23.6 minutes  -->
23.6/10=2.36
Another, simpler way on looking at it is duty cycle is 1 divided by 
(overload factor squared) =>
1.83x overload ^2=3.36;  1/3.36= .30  or you can run up to 30% of =
the 
time safely.
There is also a maximum on-time that depends on the overload--at 2x it 
seems to be in the area of 15 min.
followed by 45 minutes of rest
If you look on the Variac website, there are lots on manuals with 
diagrams, etc.
For the other example given, the calcs above say give it 104 sec off 
between10 minute roasts for a 10A Variac
and 47 min off (a 21% duty cycle) on a 5A.
Dean
in da weeds in Iowa
miKe mcKoffee wrote:
<Snip>
<Snip>
time
<Snip>
5A 
<Snip>

8) From: Ross
Jeffrey,
The Variac makes the I-Roast work the way it was supposed to.  You can set a 
solid 110v and get come reasonable consistency.  You can also make some 
small adjustments to a ongoing program that is not doing what you had 
intended but the main reason is to bring your line voltage to a value that 
the IR will work with, for me that is 110v.  The addition of a Variac turned 
my I Roast from a flaming speed demon to a nice controllable roaster.
Ross

9) From: miKe mcKoffee
Thanks for the correction Dean! I can believe:
A) I made such a stupid error dividing 2.36 by 10 instead of multiplying
B) It took FOUR years for someone to catch the error!!!
So re-running the math for a 10A variac with 1500w @120v (IR example asked
about) load on for 12 minute roast yields 6.75 minutes "safe" off time
between roasts. If cooling roast in the IR then nominal current load during
cooling so pretty much part of cooling time.
Kona Kurmudgeon miKe mcKoffee
URL to Rosto mods, FrankenFormer, some recipes etc:http://mdmint.home.comcast.net/coffee/Rosto_mod.htmUltimately the quest for Koffee Nirvana is a solitary path. To know I must
first not know. And in knowing know I know not. Each Personal enlightenment
found exploring the many divergent foot steps of Those who have gone before.
Sweet Maria's List - Searchable Archiveshttp://themeyers.org/HomeRoast/
<Snip>

10) From: Mike Koenig
OK, I stuck a 15A fuse in my 10A variac last night, and did two back
to back roasts (with ~ 6 min between).  Worked fine,  the variac
wasn't even warm at the end of either roast.  And I was able to use my
normally "hot" outlet without the normal raging inferno.
Now to explore what sort of tweaking I can do during the roast.
Thanks to all..
--mike
On 3/29/07, miKe mcKoffee  wrote:
<Snip>


HomeRoast Digest