LETTERS TO THE EDITOR
Having said that, there are two
more factors to weigh. First, if you
are considering a large, household-style
refrigerator, you can obtain a very high
quality unit for a reasonable price,
and these are universally AC powered.
These refrigerators will run only on
shorepower, an inverter, or a generator.
Second, there’s more to understanding
refrigerator compressors than knowing
whether they operate on AC or DC.
For more on this subject, I consulted
Rob Warren of Coastal Climate
Control in Bowie, Maryland
Rob sells and services a variety of
marine refrigeration and HVAC
systems. Here’s his take on AC
“A small, single-phase AC hermetic
compressor is simply an AC induction
motor that is inside a housing coupled
to a compressor, and AC power is
fed directly to the ‘run’ and ‘start’
windings. DC hermetic compressors
have motors that are one of two types:
a magnet motor with brushes, or a
polyphase induction-coupled motor.
The former have all but disappeared
now, but Danfoss versions were used
on the older Adler Barbour Combi
Cold systems, and the Colder Italia
versions were used by Frigoboat and
others for small holding-plate systems.
Their reliability was not very good.
“The polyphase induction-coupled
versions, as used on the current line of
Danfoss BD35, BD50, BD80, etc.,
units, utilize three identical windings
in the motor and a controller that
effectively spits 12 volts out to each of
the three windings in turn, so there are
no brushes to wear or fail. The beauty
of these compressors is that we can
vary the rate of delivery of the direct
current to the windings, which results
in variable compressor speed, and
subsequently variable capacity, in order
to achieve greater efficiency. The longer
and slower a compressor runs, the more
efficient it becomes—i.e., the COP
(coefficient of performance) goes up as
the speed is reduced. We now also use
electronic controllers to vary the speed
of large, three-phase AC compressors
in chilled-water systems to reduce
compressor cycling so that fewer but
larger compressors can be utilized.
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