Just as an aside, a refrigerator is the one thing that buying a dc
version doesn't gain you much. The compressors are all (as far as I
know) ac motors. So the dc refrigerators have a built in inverter to run
off dc. You will have about the same efficiency buying an ac unit and
running it off your ship's inverter.
Russ

"Philip J. Rosch" wrote:

|----------------------------------------------------------------------|
|    Russ and Donna Sherwin                                            |
|      "Four Seasons"                                                  |
|    1981 Marine Trader DC44                                          |
|    Sunnyvale, Ca 94087                                              |
|----------------------------------------------------------------------|

At 06:25 PM 02/24/2000 -0800,Russ Sherwin  wrote:

Arild replies:
A slight correction regarding DC fridges.  They do have  some advantages.

First of all,  there is a backup built in because the  way a  two  way unit
is set up  gives preference to  120V AC and  does an automatic  transfer to
12 V DC if the AC fails.  Many inverters do not have  automatic transfer
swithces built in.

A vessel  left unattended and connected to dockside power  will not have
the contents of the  fridge spoil if the cord is  dislodged by someone else
using the  outlet  in the  owners absence.  Nor will a prolonged utility
power outage  cause problems unless the  battery also runs  down.

It  is my understanding that  Norcold fridges have 24 Volt DC compressor
motors.  The " inverter" is actually a DC - DC converter changing the  12 V
to 24 V.    The reason for this is that  wire gauge can be reduced greatly
when running 24V compared to 12 Volts.
When  running on  120V AC  the  voltage is reduced with a transformer and
rectified to suit the 24V DC motor.

Speaking of inverters, I would like to point out that  inverter brands
vary.  Some of the  early models put out a square wave. Then came  modified
square wave and finally true sine wave which  is closest to  what  you get
from the  power utility grid.
The  efficiency  of  the inverters will also  vary from brand to brand.  In
addition the efficiency varies with load.  So depending on the size of your
inverter and what  % of full rated output your fridge represents, you may
not be at optimum efficiency.

I have discovered that  motors in fridges and air conditioners run cooler
on  sine wave  inverters as compared to  modified sine/square or square
wave inverter outputs.  I have asked one manufacturer to  conduct some
tests to  document  and quantify exactly what  and how much difference
there is.

Recently, I spoke to someone who  had to replace their  AC fridge which was
running off an old square wave inverter. The fridge failed much sooner than
expected and it is my opinion that  internal  heating of the  compressor
motor due to the inverter waveform was a major contributing factor.

The conversion efficiency of the  inverter must also  be considered. Not
all inverters are equal.  For that matter not all gensets are putting out
pure sine waves. This was a surprise to me.  While attending boat shows I
discovered that many  smaller gensets  have a high  harmonic  distortion
content.  There is a new technology available which  uses variable speed
control and a complex  alternator/inverter setup to produce  60Hz  120V power.

The  variable speed  genset runs at  a power level  to match the demand.
The  output from the alternator is  then fed into an electronic control box
which  operates much like a switching  mode inverter to  create 120/240
AC regulated  at a fixed 50 /60 hertz.    As load  increases the throttle
is advanced  to produce  more elecrtical power from the alternator and thus
more  inverter output.  The problem  is, some of these elecrtonic control
boxes  do not put out pure sine wave.  One manufacturer was  specifying  a
22.5 % harmonic  distortion  content for their smallest model.
Guess what that  level  of distortion  does to the efficiency of your
fridge or air conditioner motors.  How much hotter does it make the motor
run?  How much does it  shorten the motor life?
No one  seems to have an answer.
At least with  a purpose built  DC fridge you have the manufactures
guarantee that the  unit will perform  to design spec  for the planned
lifespan  of the  appliance.

Just  my two cents worth.

Cheers

Arild