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Old 04-18-2018, 09:25 PM   #99
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Never looked at it that way. By all rights, my 22 cuft Frigidaire is a DC refrigerator! I charge my 12 volt batteries and it gets cold.

Simple...no not that simple.

The issue here is not a DC refrigerator, it is a DC compressor. You all have experience with DC refrigerators but I doubt you ever went beyond the power connections.

First off, the most efficient compressor is the slowest compressor. Higher throughput means lower efficiency. That is why modern Energy Star reefers have inverter driven compressors so they can run as slowly as needed but not for something like 25% of the time like your father's reefer, but at variable speeds, including a very, very slow speed. They always run and never draw zero amps. Mine runs at it slowest speed for 6 hours then fast for 15 minutes and then mid for an hour or so. Then it starts the whole thing again.

Now, nothermark, you can't run a DC motor at these variable speeds, certainly not a permanent magnet one. These units use AC since they want to maintain torque on a large speed range.

ScoobyDoo, those neat truck reefers use Secop (formerly Danfoss) compressors. They accept 12 and 24 volts as well as 120 to 240 volts AC and require an auxiliary DC control subsystem that is separate from the compressor that runs at a variety of RPMs. The compressors ain't DC, even though the reefer is!

twinboat

Same here. The most popular compressor for marine reefers is the same Secop/Danfoss, etc. (AC/DC) unit.


Today, if you skip the cheap dorm refrigerators, the only way to get a competitive Energy Star rating with a unit over 15 cuft or so...is to use an inverter powered, variable speed compressor.

I could get into the brushless DC vs commutator DC, the latter actually being an AC motor...but well it ain't worth it.

Where did all these inverters come from...why the explosion in solar energy has made them cheap and much more efficient.

Dig a little deeper than the terminal specs, you just might be surprised as to what is inside...and why.
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Old 04-19-2018, 08:07 AM   #100
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Quote:
Originally Posted by FatChance View Post
How can one determine if a residential refrigerator has a simple motor driven compressor or a rectifier/inverter to drive the motor? I ask to know if it can be run with MSW or requires PSW inverted power? We have a Haier (GE) 15cf residential refrigerator and no one on their customer service line has a clue or can find anyone who can help.
Check the literature. An inverter driven setup can accept dirty power and often a much wider range of input voltage and frequency so a smart advertising department will tell the consumer. If they cannot tell you assume AC motor and compressor.
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Old 04-19-2018, 08:16 AM   #101
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RE: "Now, nothermark, you can't run a DC motor at these variable speeds, certainly not a permanent magnet one. These units use AC since they want to maintain torque on a large speed range."

We do it all the time! Many variable drive systems use DC motors because they are easier to control than AC. AC was a real problem to vary until the Variable Frequency Drives came out. That was fairly recently by motor standards.
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