HomeRoast Digest


Topic: My poppery II modding experience (6 msgs / 157 lines)
1) From: Dan Zwell
The curious may like to know how this went--to summarize, the results 
were great, but I don't entirely understand why it works...
Rationale:
I had a WBP-II that didn't finish roasts very well. First crack was weak 
and hard to hear, and it didn't get to second crack unless I put in a 
lot of beans (3/4 cup). When it did get to 2nd crack, it took 12 minutes 
or so. That's too long, from what I've read. I have tried 3 poppers, and 
this one had the strongest fan (and slowest/coolest roast). I noticed a 
correlation between fan speed and heat, so I thought I could fix this 
roaster by slowing down its fan.
Procedure:
I took it apart (not difficult) and attached a 600W dimmer to the motor 
circuit. I did /not/ split the motor's circuit from the secondary dimmer 
as others have done.
Result:
I do not engage the dimmer at all. Perhaps just having it connected adds 
enough resistance. The fan seems slightly less strong (I can only roast 
2/3 cup at a time, and now I need to stir the beans for the first 
minute). First crack ends at 5:30, and second crack starts a couple 
minutes later. I set my smoke detector off :)
This mod would probably enable a walgreens popper to work better--the 
walgreens (Kitchen Gourmet) unit I tested had a very strong fan, and I 
am convinced that it could not roast coffee, as a result. (It blew too 
much of the coffee upwards, and the beans would not spin.)
I may put this information on my web site--as though there weren't 
enough WBP-mod pages already!
Happy roasting
-Dan
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2) From: Phil Palmintere
How many watts does the WBP-II draw?
<Snip>
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3) From: Dan Zwell
The WBP-II (mine, at least) draws 1200 watts. The 600W dimmer is only 
connected to the fan/secondary heater, so I think it will be all right.
Phil Palmintere wrote:
<Snip>
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4) From: Allon Stern
On Jul 13, 2009, at 11:17 AM, Dan Zwell wrote:
<Snip>
Are you using a dimmer designed for motor control?
Most dimmers are designed for resistive loads, such as a heater or  
light filament, not for inductive loads, such as a motor. Inductive  
current lags behind the voltage, so you will not get the desired  
effect with an inductive load. There are dimmers that are designed  
for inductive loads, but they're usually labeled as "motor controllers"
<Snip>
Dimmers don't work by adding resistance - they work by chopping the A/ 
C. And unless a dimmer has an end-of-travel switch that shorts across  
the circuit,  it is *always* engaged, even when turned all the way up.
I'm sure RayO will be speaking up soon.....
-
allon
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5) From: Dan Zwell
Thanks for the explanation! I didn't take enough electronics classes to 
learn about all this. From what I've read/inferred, motors (inductive 
loads) want a constant current, and the effect of this is that their 
resistance may become negative at the low points of the voltage wave. By 
adding a component that enforces a type of PWM, I'm preventing the motor 
from getting its current. (This seems bad for the motor because it's not 
getting constant current, and bad for the dimmer because the voltage 
across the dimmer will spike.)
Is this mostly right? I wonder whether I'm going to cause my popper to 
fail prematurely. It's done maybe 6 batches since I modified it, and 
it's still working. Would it be safer to replace the dimmer with a 
(low-ohm) resistor that can handle handle enough current?
Thanks again,
Dan
Allon Stern wrote:
<Snip>
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6) From: raymanowen
Know what is the difference between:  (a) motor speed controls, and (b)
light dimmers?
(a) is connected to motors (series-wound), (b) is connected to light bulbs.
(incandescent)
The W-B PII motor is a low voltage dc, permanent magnet field motor with
brushes. The motor is low-powered and narrow-minded and doesn't know that it
has to have constant current LOL.
Feed the motor 0 volts, and it will draw 0 amps current; [Lionel trains had
a a liitle Variac transformer that put out something like 0 - 26v ac.]
Apply 100% voltage, and it will still draw very low current until you
actually load the motor and make it supply rotational torque to some load
other than the friction of the bearings. The current load increases with
increasing torque output.
As the armature spins in the field magnetism, it generates a counter emf.
With no shaft load, the rpm of the armature will generate an emf equal and
opposite to the applied emf.( i --> 0) Brush and bearing friction have a
finite drag and cause finite current flow even with no other load.
The thyristors are generally regarded to alter the phase angle of the power,
although PWM is an instructive analogy that can be demonstrated on a graph.
Back in the day, the latest thing was a 2D21 thyratron that I used to
control a magnetic amplifier for mobile arc welding control instead of a
position-sensitive Ignitron, that contained a puddle of Mercury.
With a 600watt dimmer, the low power motor will probably have only two
speeds unless you readjust the tap of its ballast resistor as a voltage
divider and feed only the ballast resistor with the dimmer.
Cheers, Mabuhay -RayO, aka Opa!
"I taught them everything I know and they're *still *stupid!"  - - Al Weber,
KL7AG
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