Dometic Air Conditioner Cycling Fix??
David Lip
http://home.pacbell.net/jaybo/dometic.html about installing a time delay
relay in the control circuit of a roof top Dometic AC to prevent the
compressor from stalling if the power glitched. Link is dead. Anybody
have a lead or a copy of the article they can post?
Thanks,
David
news
David . . .
I don't know of a "thread," but I used to add time delay relays to air
conditioner compressors that we were installing on building standby
generator systems. If we were installing in a building that didn't have a
UPS (uninterruptible power supply) capable of running the air conditioning,
then obviously there would be "at least" a 30 second delay while the
generator sensed the outage, started, warmed up and finally came on line.
Back then, some air conditioning systems would hold a "stalling head
pressure" for up to 3 minutes. We would simply install a small time delay
relay that was across the supply side power (the coil of the relay was
across the input or AC power to the unit) and in series with the main
compressor relay. (contactor coil) These relays had a variable timer knob
(we used Potter Brumfield but there are many other mfg's) and the relay
would delay closing back the coil circuit to the main compressor relay for
whatever period of time it took the compressor to reduce the head pressure
to an acceptable level following a loss of AC input power. In other words,
the relays we installed, didn't close for up to 3 minutes following any loss
of supply power, preventing the compressor from coming on even it there was
demand from the thermostat. The sensing period for these relays varied, but
most easily sensed a loss and started the time delay in under 250 ms (1/4
second) or less.
The relays are in the $17.00 to $39.00 range and be sure that you purchase a
relay base to plug the relay in to. (don't even think of trying to attach
to the relay "pins." I've actually seen people try to do this.) Don't
attempt this unless you understand safely working with what can be, "lethal"
currents or AC power, wire chafing, working in closed appliance enclosures,
and a number of other issues I haven't mentioned.
Workmanship is also extremely important. Even where you put the relay can
be important, because you might not always be willing to tolerate the delay.
In other words, you might want to turn the relay's knob to "zero" when you
know you have no head pressure and you want AC instantly, so it would need
to be reasonably accessible in this case.
Working with AC power can cause death, fire, injury or a number of other
things I haven't mentioned. (like, but not limited to, voiding the
manufacturers warranty)
I don't recommend that you do this.
I am simply discussing what we did some time ago in the distant past while
installing emergency generators.
Please contact a professional if you intend to pursue an installation of
this type.
Don . . . Nashville, TN
WA4ZFN
"David Lip" <rvne...@hotmail.com> wrote in message
news:3C9971ED...@hotmail.com...
ww...@earthlink.net
which means that there will be a three minute delay for the compressor
to come on whenever you turn the switch on. I changed mine to a
"delay-on-stop" relay which starts the time running whenever it is
turned off. That way, the compressor starts when it is turned on and
you get the cold air faster.
George
On Thu, 21 Mar 2002 13:33:13 GMT, "news" <don....@nashville.com>
wrote:
Vince Wirth
>Most of the RV A/C's that I have seen have a "delay-on-start" relay
>which means that there will be a three minute delay for the compressor
>to come on whenever you turn the switch on.
George,
I thought that the delay was to let the compressor head pressure
decay so that the start would not be under load. This is implemented
so that when you have a momentary power off condition, the delay
will keep from overloading the unit. Stall currents on an inductive
motor can be very high. By rewiring, you have lost that protection.
Vince
John W. Pinkham
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.
John Pinkham
"Vince Wirth" <vince...@earthlink.net> wrote in message
news:3c9a1a08...@news.earthlink.net...
Vince Wirth
>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
John,
The normal starting surge of the compressor is because of the inductive motor.
The head pressure is low or non-existent at startup unless it was just turned
off. The pressure slowly rises as the refrigerant is moved to the expansion
valve so you don't need all the complexity of a three-way valve.
Vince
JDavis1277
start the compressor and minimalize the inrush.
However, as most modern RVs are equipped with microwave ovens and lots of
lighting it is probable that most folks would continue to opt for a 4kw+
genset.
IMO, there is minimal value for the majority of RV users to reduce air
conditioner starting load. Consequently, the low sales demand for a slightly
less powerful genset would doom it from the start.
YMMV.
Butch
ww...@earthlink.net
Not at all. The delay is still there, and in the case of a momentary
lose of power, is really no different from the "delay-on-start"
relay. The difference is that with the delay-on-start relay, the time
delay begins when power is applied. With the "delay-on-stop" relay,
the time starts when power is turned off. With a momentary power loss,
the actions is the same. However, when you first turn the unit on, it
will start putting out cooling air fasted though only the three
minutes faster. In actual practice the relay plugs into the same
socket and looks the same. The only difference is what triggers the
start of the time delay.
George
Vince Wirth
>On Thu, 21 Mar 2002 17:44:04 GMT, vince...@earthlink.net (Vince
>Wirth) wrote:
>
>>On Thu, 21 Mar 2002 16:36:15 GMT, ww...@earthlink.net wrote:
>>
>>>Most of the RV A/C's that I have seen have a "delay-on-start" relay
>>>which means that there will be a three minute delay for the compressor
>>>to come on whenever you turn the switch on.
>>
>>George,
>>I thought that the delay was to let the compressor head pressure
>>decay so that the start would not be under load. This is implemented
>>so that when you have a momentary power off condition, the delay
>>will keep from overloading the unit. Stall currents on an inductive
>>motor can be very high. By rewiring, you have lost that protection.
>>Vince
>
>
>Not at all. The delay is still there, and in the case of a momentary
>lose of power, is really no different from the "delay-on-start"
>relay. The difference is that with the delay-on-start relay, the time
>delay begins when power is applied. With the "delay-on-stop" relay,
>the time starts when power is turned off.
George,
Interesting but how does the relay count down with the power off? Does
the logic screw up with reapplication of power?
Vince
ww...@earthlink.net
Wirth) wrote:
Not sure aboutthe internal workings. So far it has worked fine in our
coach for about 8 years now.. The relay is a standard A/C part which
is available at any appliance parts store.
George
Neon John
<jwpi...@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 viola! 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
---
John De Armond
johngdDO...@bellsouth.com
http://personal.bellsouth.net/~johngd
Cleveland, Occupied TN
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