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Old 21-05-2022, 07:36   #1
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Does a 220 AC fridge draw less than 12v DC?

Greetings,

I'm refitting a 53 ft. Royal Cape Catamaran. We are based in South Africa and our systems are 12 Volt DC and 220 Volt AC. I will be replacing all my chillers. My craftsman advised me that a 220 volt AC fridge or freezer / chiller will draw less power than a 12 Volt DC chiller when both units are drawing power through an inverter. So his suggestion is that I must buy 220 V AC units and not 12 volt units in order to achieve better power utilization. Is that assumption correct?
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Old 21-05-2022, 08:13   #2
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Re: Does a 220 AC fridge draw less than 12v DC?

That has always been what I've remembered from my formal education, and that a 3-phase motor was even smaller and more efficient.


And of course the higher voltages can use smaller wiring which helps with the expense of longer runs and subsequent power loss.


If I get a chance I'll look back through my documentation. Victron may have some good reference materiel on their website. Also Reinhout Vader's book on energy aboard.
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Old 21-05-2022, 08:15   #3
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Re: Does a 220 AC fridge draw less than 12v DC?

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Originally Posted by YPSILANTI View Post
Greetings,

I'm refitting a 53 ft. Royal Cape Catamaran. We are based in South Africa and our systems are 12 Volt DC and 220 Volt AC. I will be replacing all my chillers. My craftsman advised me that a 220 volt AC fridge or freezer / chiller will draw less power than a 12 Volt DC chiller when both units are drawing power through an inverter. So his suggestion is that I must buy 220 V AC units and not 12 volt units in order to achieve better power utilization. Is that assumption correct?
Does the 12V unit needs to draw its power "thru" an inverter rather than operate directly from battery power?
Perhaps I misunderstood.
Any time we insert some device to change the relationship between amps and voltage their will be some losses in overall efficiency, primarily due to heat.
In the formula for heat the value for currant is squared.
Modern inverters have much lower losses than old units, but some loss is still present, even at idle/no load.
A 220AC unit will consume less watt-hours when operated directly off of the shore mains than it will when operated thru a battery-to-inverter system.
The flip side is that the newer 12VDC units can operate at multiple speeds which reduces total watt-hour usage over a given time frame whilst AC units are always running "full speed".
It would be interesting to compare graphs that show BTU heat removal vs. watt-hour power consumption of each type.
Some variables would include the size of the battery bank, (stable voltage with little drop under load,) as well as length of run of 12V cables.
Perhaps Richard or Oz will chime in with a more substantive answer from their field experiences.
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Old 21-05-2022, 08:24   #4
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Re: Does a 220 AC fridge draw less than 12v DC?

Variable speed AC motors are available...


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Old 21-05-2022, 08:36   #5
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Re: Does a 220 AC fridge draw less than 12v DC?

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Variable speed AC motors are available...
Of course they are, quite common in machine tools of all sorts.
We are talking about hermetic refrigeration compressors of small fractional HP.
VFD units themselves consume power, no free lunch.
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Old 21-05-2022, 09:34   #6
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Re: Does a 220 AC fridge draw less than 12v DC?

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The flip side is that the newer 12VDC units can operate at multiple speeds which reduces total watt-hour usage over a given time frame whilst AC units are always running "full speed".

The OP did not state hermetic, but okay if you want, and you said the above.


How about these hermetically sealed, AC variable-speed, and less than 746 W input power compressors?


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Old 21-05-2022, 09:42   #7
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Re: Does a 220 AC fridge draw less than 12v DC?

Hermetic refrigeration and air conditioning compressors comprise of an electric motor and a compression device inside a sealed housing. One way to vary the speed of an electric motor is to make it operate from Direct Current (DC) and then vary the operating voltage or use pulse modulation.

But that would be impractical for many reasons, the least of which would be the need to employ a commutator and brushes, as a permanent magnet motor’s speed cannot be varied. This dictates that the motor must be powered by Alternating Current (AC), as is already common, but to somehow be able to vary the speed.

Standard AC powered compressors operate at a speed that is synchronous to the frequency, or Hertz (Hz), of the power supply. A typical motor running on 60Hz American power will spin at close to 3,600 RPM, whereas the same motor operating on the same voltage but on 50 Hz European/rest-of-the-world power will run slower, at around 3,000 RPM.

For a well-traveled vessel this presents a conundrum, because while this variation in speed from different power supplies may not be problematic for many types of application, i.e. fans and pumps, it can cause big problems in refrigeration and air conditioning compressors, where lubrication, overheating, and other issues can initiate compressor failure. Single-phase air conditioning and refrigeration compressors are designed to operate safely only on either 50Hz or 60Hz power, but not both. There is one way we can safely operate a standard compressor within a limited range of speeds, and that is with a compressor built to operate from three-phase power. This is common practice with the large compressors used in chillers for super-yachts, where they can be safely operated between about 2,500 RPM and 4,000 RPM and ramp up slowly from 0 RPM at start-up to eliminate starting surge.

It’s amazing to note that we have this same clever technology in the small variable-speed refrigeration compressors mentioned earlier.
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Old 21-05-2022, 10:02   #8
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Re: Does a 220 AC fridge draw less than 12v DC?

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How about these hermetically sealed, AC variable-speed, and less than 746 W input power compressors?
Good to know, now I'm more "up to speed".
Have these systems/compressors found general/accepted usage in cruising boat applications?
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Old 22-05-2022, 19:47   #9
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Re: Does a 220 AC fridge draw less than 12v DC?

Have you compared the insulation of the two systems or is it the otherwise exact same fridge that can be purchased with either type compressor?
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Old 22-05-2022, 22:11   #10
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Re: Does a 220 AC fridge draw less than 12v DC?

That depends on the type of motor the fridge has. Many refrigerators now use the Sawafuji type compressors which use very little power and will run on either.

I used two Engel fridges both small, but there are larger refrigerators using the same technology--they just add more units--and I had one set to deep freeze and the other to ordinary refrigerator. I ran them both off solar with no problems and I also ran a cheap K-Mart refrigerator through an inverter--but only when in port unless I had guests aboard. It was amazingly efficient too--

But to answer your original question--it depends on the kind of compressors you have in your systems. DC or AC--it is the POWER that matters, not so much how it is delivered, and the thermal and electrical and mechanical efficiency of the unit. A linear or swing compressor will always be better for small units, a rotary sealed unit type probably better for larger installations.
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Old 22-05-2022, 22:43   #11
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Re: Does a 220 AC fridge draw less than 12v DC?

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Originally Posted by YPSILANTI View Post
Greetings,

I'm refitting a 53 ft. Royal Cape Catamaran. We are based in South Africa and our systems are 12 Volt DC and 220 Volt AC. I will be replacing all my chillers. My craftsman advised me that a 220 volt AC fridge or freezer / chiller will draw less power than a 12 Volt DC chiller when both units are drawing power through an inverter. So his suggestion is that I must buy 220 V AC units and not 12 volt units in order to achieve better power utilization. Is that assumption correct?
In South Africa? No question, buy SnoMaster. They will run on AC as well as 12V and 24V DC.
https://acdirect.co.za/product-categ...8aAnB4EALw_wcB
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Old 23-05-2022, 14:44   #12
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Re: Does a 220 AC fridge draw less than 12v DC?

In a previous incarnation I was caravanner and we had a three way fridge, gas, 12VDC and 230VAC. I wanted to fit the same unit when I bought a boat for completion. I found out that Gas was not a runner and I spoke to a few boaters and the information I received from the Canal guys made sense. Dont spend £'000's on a special fridge. Buy a domestic fridge and run it on the mains. Switch it to an inverter when under sail. I have a motor sailer so I do get plenty of amps from the engine and I have 16W of solar into 270A AGM batteries and a 3000W inverter.

TI fitted a small domestic fridge, a narrow one not 600mm. That has served me well for years. When I go away I have lots of frozen food in it. I then use the inverter with the engine running while I sail out of port. I then sail to my destination which is usually all day. On arrival the frozen items are still frozen, other items are chilled as they should be.

At the destination port we go on main power, consume the contents and restock with chilled. The shore power is enough to keep it chilled and the inverter keeps it so until we return to our home port or stop over.

I did all the maths when I started and decided 12v or 230v didn't make much difference so I decided on the cheap domestic fridge. It was the best choice and Ive never had to worry. The downside is my home port has no mains power so when Im on board for the weekend I use it like a cooler box.

A fridge uses most power getting down to the right temperature, keeping it there is an occasional burst unless you leave it open or add loads of warm stuff.
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Old 24-05-2022, 11:06   #13
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Re: Does a 220 AC fridge draw less than 12v DC?

Wow, this forum is amazing. Thanks so much for the immensely informative & helpful replies. The Snowmaster suggestion is one that I will certainly pursue.
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Old 24-05-2022, 11:48   #14
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Re: Does a 220 AC fridge draw less than 12v DC?

generally speaking the 12v version is more efficient when it matters most (no shore power) simply because there are no inverter losses. Your results will vary depending on the specific hardware.
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Old 24-05-2022, 12:30   #15
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Re: Does a 220 AC fridge draw less than 12v DC?

The critical point is when you are away from power:
- 12v DC fridge will use 12v directly with negligible losses.
- 220v AC fridge will need an inverter to convert the battery 12v DC power to 220v AC power. That conversion comes with some losses.

So even if the 220v AC fridge motor is slightly more efficient than the 12v DC motor, you will lose most of it and you will add a complex item in the system that can fail.
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