From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Dometic Air Conditioner Cycling Fix??
Message-ID: <t87t9u42vgv8ihduc0492sjkgd88rdssap@4ax.com>
Date: Sun, 24 Mar 2002 22:56:09 -0500

On Thu, 21 Mar 2002 20:02:09 GMT, "John W. Pinkham"
<jwpinkham@attbi.com> wrote:

>Since RV a/c units have a large starting amp requirement, is there any way
>to reduce this with some kind of unloader that would gradually increase head
>pressure?  I'm thinking along the lines of a gradually closing three-way
>solenoid bypass valve
>connecting the high and low sides. If feasible, this would allow the use of
>smaller generators to power a/c units. Another approach would be to use an
>inverter a/c compressor, such as Mitsubishi makes, which have considerably
>less starting amp draw than conventional compressors.

Sure.  Easy to do.  Simply install a solenoid refrigeration valve from
somewhere on the high side to the compressor suction.  If you tap the
high side before the condenser and feed it into the evaporator inlet,
you can also use the valve for rapid defrost in the event the unit
freezes up.  This is exactly what I do with restaurant equipment.
Used refrigerators are MUCH cheaper than freezers and the only real
difference is the capability of freezers to defrost.  I simply add the
solenoid (implementing what is known as hot gas defrost), plumbing and
a defrost timer and voila!  I have a freezer.

For compressor unloading, if you use a normally open valve, you don't
even need any control circuitry.  Simply hook the coil in parallel
with the compressor power terminals.  When the compressor is
energized, the solenoid closes and the unit works normally.  When it
is de-energized, the compressor is bypassed.  Of course it makes a
little racket as the high pressure gas rushes over to the low pressure
side.  Easy enough to add some cap tubing or other restriction to slow
it down.  If you put a little time delay relay in series with the
valve, you can give the compressor a few seconds to come up to speed
(and the generator recover from the starting inrush) before applying
load.

I implemented this exact setup on my apartment central AC that I built
from scrap parts.  I put an expansion valve in the evaporator to
improve efficiency and dynamic response.  The old condensing unit I
found is designed for cap tubing systems that eventually equalize
themselves.  It lacked sufficient starting torque to start against the
head that an expansion valve maintains. (yeah, I know there are
equalizing valves available but not usually as parts salvaged from
other machines).  I installed a hot gas bypass valve inside the
condensing unit cabinet so the racket would be outdoors and wired it
just as described above (without the time delay relay.)  With this
valve installed, I can turn the unit off and right back on again and
have it start with no strain.  I also installed a delay-on-make TDR in
the main 24 volt lead to protect the system from power chugs.  When
the utility chugs the power, the relay holds the system off for 3
minutes, giving the equalizing valve time to do its thing.

John