Re: gEDA-user: [OFF] high current amplifier

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Wednesday 20 May 2009, DJ Delorie wrote:
>> Not even a 40 horse compressor in a 22 ton (rated, yeah sure) Lennox
>> will draw that much for that long.
>
>Gee, you guys are making me feel bad.  Now I have to go out and
>research air conditioners :-P
>
>Anyway, I know I have a 60 amp circuit for it, and it hits 123 amps
>long enough for my DVM to stabilize at 123 amps.  How long it stays
>there depends on when it last ran (the first unit broke because it ran
>too often, and would lock - 123 amps until it overheated and shut down
>- hence the furnace controller).  When it's in a good mood the surge
>lasts just under a second or so.  During hot days it could be longer
>because it cycles more (minimum 30 minutes "off" time or it has a hard
>time restarting).
>
Then put a time delay to off of at least 3 to 5 minutes into the condensor fan 
circuit only, its killing them both simultainiously, leaving a severe overtemp 
condition that is falsely holding up the head pressure, or even running it up 
over 400 psi.  There must be enough fan running to liquify the remaining gas, 
bringing the head pressure down to something the compressor can get restarted 
against in a timely manner, which should be under 1 second.

By the same token, if the evaporator fan in the furnace is being stopped at 
the same time as the compressor, you have a virtual guarantee that the 
evaporator will turn into a solid block of ice on a warm muggy day when you 
will really notice it.  It must have enough after-run to vaporize all the 
refrigerant in the evaporator coils so that they will warm up, thawing any ice 
that formed when it was running, and running with what may be a borderline low 
charge, often the case even for new installs.

On a long run cycle, long enough to reach steady state conditions, gas charge 
can be somewhat judged by inspecting the big line where it comes back out of 
the furnace.  It should have a coat of sweat on it, not frost.  If no sweat 
and no frost, (and the air is moving 100% normally) its quite likely down to 
half charge or less.  As you add refrigerant, it will first frost, then 
eventually clear to sweat, and this is pretty close to the ideal charge level 
for the conditions that exist that day.  Low side pressure/temp should be held 
not lower than 34F (for whatever gas is in it), and high side pressures/temps 
to 275F or so maximum.  This high side is for older F12 systems of course, I 
believe that R-134 will run a high side somewhat above that.  R-134 is also a 
much smaller gas molecule and will leak from systems that can hold an F12 
charge for decades.  That higher high side temps for R-134 also translates to 
failure of the lube oil that circulates with the refrigerant at a higher rate.  
This can lead to plugging of the capillary tube used as the expansion 
restriction in the entry to the A coil in the furnace with flakes of varnish 
from the overcooked oil too.  The tech will generally want to replace the 
whole coil because of the leak possibilities with even well done silver solder 
repairs on site.

>Note that I do *not* have the "hard start" (aka "soft start") kit for
>this, although I've been thinking about getting it.

Those can be very hard on the compressors, prolonging the startup overcurrent 
phase.  Giving it a good stiff supply so it gets started as quickly as 
possible is actually the easiest on them, far less of an instant overtemp 
surge.  Again I'm referring to multiphase motors of course.  Single phase 
stuff can sometimes be optimized for the starter coil efficiency with a minor 
change to the quality and size of the starting capacitor.  The keyword there 
is often ESR, and the usual non-polarized electrolytics can be quite poor.  
Not knowing the phase angle the starter coils are inserted into the stator can 
make that approach a cuss and cry method though.

>The 10 kVA transformer wasn't enough for this.  At startup, the mains
>voltage would brown-out to 190 vac.  They swapped it for a 25 kVA
>transformer, and upped my wiring from 00 to 0000 coming in to the
>house.

25kVA, still small.  How many houses are they running from it?  I think the 
can on the pole I'm fed from is now a 50kVA, but its feeding 4 or 5 houses.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
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