HomeRoast Digest


Topic: OT - Hospital Grade Electrical Outlet (was (8 msgs / 201 lines)
1) From: Bob Hazen
If your line is soft enough, sure.  A constant power load results in 
increased current when the voltage drops.  Theoretically, this would result 
in runaway if the load demands increasing current at a greater rate than the 
line is dropping.  However...  Is the line truly soft?  Or is the voltage 
varying?  Constant-voltage transformers (read Sola) have a fair amount of 
loss with efficiencies in the 90% to 94% range.  They also use 
ferro-resonance to control their output voltage by keeping parts of the 
transformer core in saturation.  Both of these characteristics mean the 
transformer isn't truly a constant power load.
Constant power loads can be tricky.  In the early telecom days with 
mechanical switches, the 48V rectifiers (power supplies) had enough capacity 
to run the load and recharge batteries after an outage.  All was well, until 
they put DC-DC converters in place to run electronic telephone switches. 
DC-DC converters are constant power loads on the 48V bus.  During an outage 
with the plant on battery, the converters took increasing current as the 
battery voltage dropped.  The snag was on re-start when the battery took 
charge current and the converters took extra current at the low battery 
voltage.  The rectifiers didn't have enough capacity to provide current to 
charge the battery >and< serve the higher current to the DC-DC converters. 
So the plant got stuck at that point and wouldn't re-charge.  This is old 
news.  It was figured out in the 70's.  Before my time, but a lesson to 
learn from.  Today, I do aerospace power conversion.  We deal with constant 
power loads and the possible interaction/instability between stages.  Google 
"Middlebrook Criteria" for more than you probably want to know.
And if anybody wants a constant voltage transformer for their roaster, check 
out the link below.  A 1000VA transformer is about 60lbs.http://www.solaheviduty.com/Bob

2) From: Michael Mccandless
"Soft line" . . .
If true - ferroresonant wouldn't be a good idea
If false - you wouldn't need one - Variac would do fine.
McSparky
On Sun, Jul 20, 2008 at 8:57 AM, Bob Hazen  wrote:
<Snip>
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3) From: raymanowen
"...the battery took charge current and the converters took extra current at
the low battery voltage.  The rectifiers didn't have enough capacity to
provide current to charge the battery >and< serve the higher current to the
DC-DC converters. So the plant got stuck at that point and wouldn't
re-charge.
This is old news."
Yup, you're right- it is old.
This maxim has a reference title: Prior Planning Prevents P. Poor
Performance. Been stuck in that quagmire a few times, and I'm trying to
ameliorate the proclivity in the coffee domain...
About that Costa Rica Peaberry, FC+, roasted 09 Nov 07, stored in Mason jar
in its own CO2 puddle in my son's pantry: butter smooth shot. I botched the
grind. I noticed I still had it set on *50* from my last Steinway as it was
doing the "Ph-" of the total "Phht" grind.
The grind aroma was gorgeous- how bad could the shot be? Had a double with
light crema in 12s flat. I just used the filter basket as a measure, and
dipped the beans out of the jar with a long-handled *$ espresso spoon. Wurkt
good.
Cheers, Mabuhay -RayO, aka Opa!
Got Grinder?
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4) From: raymanowen
If I have the choice, I use a variable transformer only for test purposes.
<Snip>
Neutral return. Es Macht Nichts- still a soft circuit.
The *Variac* will be just fine. All you need to do with a variable
transformer is continuously measure the Load potential and correct the tap
setting in real time for a constant Load power. Fun + Games. You have to be
there when something changes-
Could you reset the *Variac* while the thermostat is cycling? The saturable
reactors [Sola] can.
Still wanna play *Variac*?
On Sun, Jul 20, 2008 at 11:48 AM, Michael Mccandless 
wrote:
<Snip>
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita (ex- NYC Paramount) WurliTzer- 1976
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5) From: Michael Mccandless
I set the Variac to the "heat on" load I want & don't worry about variance
in the "heat off" mode.
If fan is an issue, it can be split wired.
Don't need to be sucking power out of the neighbors' houses.
McSparky
On Mon, Jul 21, 2008 at 6:57 AM,  wrote:
<Snip>
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6) From: raymanowen
The similar assumption I've made in this circumstance is that I was the only
high-current user on the neutral line. Between 10am and 2pm, lots of
industrial users are using heavy Line currents, with minor Neutral current.
I know- if the system is balanced you won't even see Neutral current flowing
from the distribution panel to the supply. At least, that is the
ass-sumption Cal Volts made at in Oakland, for the 240 volt delta power to
1115 21st Street.
When you're flying, you aren't the only plane in the troposphere. No sucking
power out of the neighbors' houses, just keeping Neutral in the building,
not on the block.
Cheers, Mabuhay -RayO, aka Opa!
Get your own substation!
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7) From: Michael Mccandless
I'm probably blowing the current thing a little out of proportion, but in
testing those 1:1 FR transformers w-20A output, we used a Variac on the
input.
When ramping up the input, the 100A ammeter pegged in a big way, then
settled down near full supply V.
Visualizing that on a residential system just doesn't seem very functional.
Many trips to the breaker box.
Slow blow fuses?
McSparky
On Tue, Jul 22, 2008 at 12:08 PM,  wrote:
<Snip>
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8) From: raymanowen
They always ran hotter than the Place of Fabled Heat for me. Why? Can't
dissipate real power in the monster inductance- a puzzlement. -ro
I got your spare Slo-Blow fuse- Cu plumbing pipe. Blade Fuse- vise the ends
flat...
On Tue, Jul 22, 2008 at 4:43 PM, Michael Mccandless 
wrote:
<Snip>
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita (ex- NYC Paramount) WurliTzer- 1976
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