Fridge 12v vs AC?

[RaymondR]

I have doubts regarding the statement "Those "Danfoss" compressors do run on AC."

The last one I cut open had a cylindrical. permanent magnet on a shaft as the rotor and a single winding for the stator. A standard DC motor generally has either permanent magnets or electromagnets as a stator and windings on the rotor which are wired through a commutator on the shaft. The purpose of the commutator is to serially switch on the windings to produce a magnetic field which is rotationally stationary in respect to the stator field.

To me AC is a nice smooth waveform of varying polarity which represents as a sine curve in cartesion coordinates. I don't know if Danfoss takes the trouble to include the circuitry to form the nice smooth wave form or just switches the polarity as a square wave but I'd prefer to conceptually keep the rotating field current of an induction AC motor and the commutated stationary fields of a DC motor separate in my mind.

[Statistical]

Originally Posted by RaymondR

I have doubts regarding the statement "Those "Danfoss" compressors do run on AC."

The last one I cut open had a cylindrical. permanent magnet on a shaft as the rotor and a single winding for the stator. A standard DC motor generally has either permanent magnets or electromagnets as a stator and windings on the rotor which are wired through a commutator on the shaft. The purpose of the commutator is to serially switch on the windings to produce a magnetic field which is rotationally stationary in respect to the stator field.

To me AC is a nice smooth waveform of varying polarity which represents as a sine curve in cartesion coordinates. I don't know if Danfoss takes the trouble to include the circuitry to form the nice smooth wave form or just switches the polarity as a square wave but I'd prefer to conceptually keep the rotating field current of an induction AC motor and the commutated stationary fields of a DC motor separate in my mind.

Curious. That does sound like it could be a BLDC motor. My refrigerator compressor is (knock on wood) over 20 years old at this point so maybe they have improved things.

Side note yes the question of is BLDC "alternating" is one with no good answer. It isn't AC like we usually think of AC but it certainly does alternate.

[Pizzazz]

I have an 85W AC fridge on the boat. The Victron 500W inverter would not start it, goes into safe mode. I agree that modern AC fridges are very efficient but we need to find inverters specifically designed to start fridges - not easy. Some work, some do not.

[Bowdrie]

Originally Posted by Statistical

It isn't AC like we usually think of AC but it certainly does alternate.

In the old days it was common to use a, (ahem,) "vibrator" to produce pulsating DC that a transformer could operate on.
Perhaps now they use electronic switching?

Originally Posted by Pizzazz

I have an 85W AC fridge on the boat. The Victron 500W inverter would not start it, goes into safe mode. I agree that modern AC fridges are very efficient but we need to find inverters specifically designed to start fridges - not easy. Some work, some do not.

85 Watts? jeepers, no matter the voltage and not counting losses that's only about 1/9th of a HP.
That's hardly enough power to run a pissant's motorcycle.
Something doesn't seem right.

[wholybee]

A brushless DC motor is considered DC because DC voltage is applied to the driver/controller. What is actually applied to the motor is PWM. The driver, in the case of some Danfoss compressors, could have a range of voltages, AC or DC applied to it, and the driver does what it needs to get the PWM. It could be described as an AC/DC motor, because it will run on both.

[Wandering1]

Does anybody have hard facts/evidence that AC fridges are more efficient than the marine DC powered variety installed in just about every production boat on the planet?

[rslifkin]

Originally Posted by Wandering1

Does anybody have hard facts/evidence that AC fridges are more efficient than the marine DC powered variety installed in just about every production boat on the planet?

As a general category, I wouldn't say they're any more efficient. It's down to the individual fridges in question. Either type can be made extremely efficient, but in the interest of maximizing food storage in a given space, many fridges have sub-optimal insulation (both AC and DC). There are also inverter losses to worry about with an AC fridge, so which option comes out better really depends on the boat (as far as whether you're running an inverter 24/7 already and what fridges fit in the available space).

[cram]

Originally Posted by Xavierp

Hi all.

My new fridge can be powered with AC or DC
I am off the grid
I to save any power I can

which is best AC or DC connection for the fridge

Thx all!!

It might be helpful to provide advice to understand the vessel its being installed in and what it has for an AC electrical system.


32 ft sailboat vs 150 ft cruiser will have very different power systems....

[Bowdrie]

Originally Posted by rslifkin

As a general category, I wouldn't say they're any more efficient. It's down to the individual fridges in question. Either type can be made extremely efficient,

Yes.
I've often told people the "1:10" ratio.
$50 of insulation/gaskets = $500 in refrigeration machinery.

[wholybee]

Originally Posted by Wandering1

Does anybody have hard facts/evidence that AC fridges are more efficient than the marine DC powered variety installed in just about every production boat on the planet?

You generally can't compare them, unless the to units in question are identical except for the compressor motor. Insulation, or even sizing of the compressor and evaporator will make a larger difference than AC vs DC. To move a given amount of heat from inside the box to outside requires a certain amount of energy. There is no way around the physics of that.

With that in mind, in an earlier post in this thread was a factory datasheet, comparing AC vs DC in the exact same fridge. DC was significantly more efficient. I suspect the efficiency loss was due to first converting the AC power to DC to run the BLDC motor.

[wholybee]

Originally Posted by Bowdrie

Yes.
I've often told people the "1:10" ratio.
$50 of insulation/gaskets = $500 in refrigeration machinery.

You could also say 50 hours of rebuilding the box with more insulation = 1 hour of installing a new compressor. About how many hours I spent in both cases on my rebuild.

[Statistical]

Originally Posted by wholybee

You could also say 50 hours of rebuilding the box with more insulation = 1 hour of installing a new compressor. About how many hours I spent in both cases on my rebuild.

Then there is just a lack of space. If you have a certain volume of space you have to decide how much of that will be insulation and how much storage volume.

[wholybee]

Originally Posted by Statistical

Then there is just a lack of space. If you have a certain volume of space you have to decide how much of that will be insulation and how much storage volume.

And that balances with volume of space in the battery compartment. A larger space with less insulation will require more money spent on batteries, and more space for them.

Such are the dilemmas of small boat ownership.

[Statistical]

Originally Posted by wholybee

And that balances with volume of space in the battery compartment. A larger space with less insulation will require more money spent on batteries, and more space for them.

Such are the dilemmas of small boat ownership.

Yeah boats are compromises all the way down. The good ones are just compromises you can live with.

[ranger58sb]

Originally Posted by Wandering1

Does anybody have hard facts/evidence that AC fridges are more efficient than the marine DC powered variety installed in just about every production boat on the planet?

I don't, but the Energy Star ratings people might. I've not ever noticed an Energy Star rating for an AC/DC "marine" fridge, but I haven't really been looking... and they might be out there somewhere.

Whereas an ES rating for household AC fridges are common as dirt... although the cost of inverter usage would have to be added, assuming periods away from shorepower.

If a particular unit is absent an Energy Star rating, I'd guess less efficient.

A point we've come to appreciate with AC units is self-defrosting, and a built-in icemaker in the freezer. That introduces some oranges amongst the apples, but...

-Chris