Delays in the restarting of Freon compressors is common and generally quite
necessary. Restarting any motor is difficult if it has to start under full
load. If starting under full load is necessary, the motor has to be sized
larger, and that costs more to build and also more to operate.
We don't start our lawn mowers in high grass, especially if you are using
your own arm to pull a rope starter.
The high-pressure output line of an air compressor typically has a one-way
valve at the tank entrance, and a relief valve in the line (or a small
intentional leak in the line) so the line running from the compressor output
to the storage tank can depressurize before the compressor has to restart.
But you can't do that with a Freon compressor because you don't want to lose
Freon each time the compressor turns off. You could install a bypass valve
around the needle valve such that the bypass valve opens when the compressor
shuts off, and closes after the compressor turns on. But that costs extra
money to install, and it just constitutes another part that can fail.
Instead, you can just wait for the high pressure in the line between the
compressor and needle valve to lower its pressure by letting the
high-pressure Freon (in that section of tubing) move through the needle
valve and equalize with low pressure side after the needle valve.
Some small inexpensive window air conditioners don't have a built-in time
delay, and the directions tell you to wait three minutes (or so) before
restarting the unit. If you do not heed this warning, of if you cut the
delay too short, the compressor motor will fail to start, will overheat, and
the thermal-overload circuit breaker will trip. These generally reset on
their own, and the compressor will attempt a restart in perhaps 30 seconds.
If that it not successful, the thermal breaker will trip again... etc. This
isn't the greatest thing to do because it is essentially a "fail-safe"
mechanism rather than a "normal operation" mechanism. So it's best to wait
the required time for the high-pressure line to depressurize before trying
to restart the compressor.
Larger and/or better-designed Freon systems have a timed delay in addition
to the thermal-overload breaker. I'm used to seeing a three minute to five
minute delay. If your refrigerator has a ten-minute delay that seems a
little long to me, but it could be part of how a high-efficiency unit is
designed. That is, you would want to use the minimum-sized motor that
starts reliably. If you wait long enough to make sure the Freon pressure
between the compressor and needle valve has reached its minimum pressure,
the motor can be sized smaller than if you only waited for the pressure to
drop part way to its minimum value. Still, I'm not used to seeing delays
longer than about five minutes. The newest large A/C units I have worked
with are very high efficiency and only have a delay of three minutes.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270 (office)
edmiston@bluffton.edu
-----Original Message-----
From: Donald Polvani
Sent: Tuesday, August 30, 2011 3:46 PM
To: 'Forum for Physics Educators'
Subject: [Phys-l] Freezer Delay
During Hurricane Irene, I used our generator to keep the freezer going.
Unfortunately, the generator (for reasons still unknown) decided to run
smoothly for the first 15 minutes, or so, and then slow down and almost
stop, then recover itself and run for 5 or more minutes smoothly again, then
slow down and almost stop, etc. until after a number of such cycles it
finally would stop (maybe after 45 minutes total). For the first 15
minutes, the generator would bring the freezer temperature down. The
freezer has a digital temperature display on the outside of the front door,
so it's easy to see what's happening. However, after the generator started
the above cyclic behavior, the freezer would not come back on during the
smooth running portion of the generator cycling and the temperature would
continue to climb until the generator finally stopped.
Now that our power is back on, and the freezer had had enough time to reach
thermal equilibrium at 0 deg F, I shut the freezer off and quickly turned it
back on. After turning it back on, the digital display returned but the
compressor stayed off. After about 10 minutes, when the temperature had
climbed to 8 deg F, the compressor came on and started to bring the
temperature back down.
Can anyone explain to me why the freezer compressor seems to have a built in
delay after the power is shut off and then turned quickly back on? Is this
a deliberate attempt to prevent the compressor (or delicate electronics)
from experiencing too many switching transients from a power source which is
turning on and off, or is it in the nature of compressors to not be able to
rapidly start and start? Was I hurting somehow the compressor, or
electronics, by letting the compressor cycle in the above manner?