JC refrigeration AC or DC upgrade?

[woodtrucker]

Just wondering if any of you could tell me whether I should buy the AC or DC conversion for my dometic rm2652.

We are on shore power almost always except for traveling. Would the truck charge battery enough to keep the 12VDC unit cooling while on the road?
If so, I'll go the 12V route.
Thanks,
Scott

[NRALIFR]

Wiz, thank you for the detailed explanation. I appreciate you taking the time to write it.

Not sorry I asked at all! :B

:):)

[theoldwizard1]

theoldwizard1 wrote:

NRALIFR wrote:
Yeah, that would have to be a very special inverter to produce 3-phase AC from 12v DC. The curiosity in me wonders how they do it.

CORRECTION ! Some models now use brushless DC motors instead of three phase !

Quick review of how a single phase inverter works. You will be sorry you asked ! I was an EE !)

"High frequency" inverters "switch" the incoming DC ON and OFF at 40KHz-10-KHz. This "square wave" AC is the sent to one of more transformers (high frequency transformers are smaller than low frequency transformers) and the resultant voltage is >180V peak-to-peak AC. This then converted (rectified) back to high voltage DC (>150V DC). A separate circuit makes a "synthetic" 60 Hz sine wave. This sine wave is sent to a Class-D amplifier that is powered by the +150VDC.

"Low frequency" inverters skip the step up stage and rectification stages. They still synthesize a 60 Hz sine wave and send it to a Class-D amplifier that use 12VDC. The AC output has to have enough power to drive a VERY BIG transformer that steps it up to 120VAC.

For 3 phase, you do it 3 times, making sure each of the synthesized 60Hz reference sine waves is exactly 120° out of phase with its predecessor.

NRALIFR wrote:
This is a screen grab of the DC unit that’s supposed to be for my fridge. 12V 7.5A 90W

I don’t see anything that I can identify as a dedicated power conditioning device, so I assume it’s inside the sealed can???

Nope. It is not in the picture.



Danfoss is one of the few manufacturers of DC powered refrigeration compressors. Note the box on the side. That is where the magic happens !

NRALIFR wrote:
So, from a power efficiency standpoint, which would be the better way to go? AC conversion kit, or DC?

My GUESS is that the DC conversion would be a bit more efficient. 3 phase motors are always more efficient than single phase.


Last, I will through in inverter manufacturers are making a TON OF PROFIT these days. The actual cost of parts between a modified-sine wave inverter and a pure sine wave inverter is probably <$50 maybe even <$25. The most expensive part inside these things is the transformer (lots of copper and steel).

For some reason the low frequency inverters (you know they are low frequency because they weigh a TON) seem to be more "durable" but I don't know why.

[theoldwizard1]

NRALIFR wrote:
Yeah, that would have to be a very special inverter to produce 3-phase AC from 12v DC. The curiosity in me wonders how they do it.

Quick review of how a single phase inverter works. You will be sorry you asked ! I was an EE !)

"High frequency" inverters "switch" the incoming DC ON and OFF at 40KHz-10-KHz. This "square wave" AC is the sent to one of more transformers (high frequency transformers are smaller than low frequency transformers) and the resultant voltage is >180V peak-to-peak AC. This then converted (rectified) back to high voltage DC (>150V DC). A separate circuit makes a "synthetic" 60 Hz sine wave. This sine wave is sent to a Class-D amplifier that is powered by the +150VDC.

"Low frequency" inverters skip the step up stage and rectification stages. They still synthesize a 60 Hz sine wave and send it to a Class-D amplifier that use 12VDC. The AC output has to have enough power to drive a VERY BIG transformer that steps it up to 120VAC.

For 3 phase, you do it 3 times, making sure each of the synthesized 60Hz reference sine waves is exactly 120° out of phase with its predecessor.

NRALIFR wrote:
This is a screen grab of the DC unit that’s supposed to be for my fridge. 12V 7.5A 90W

I don’t see anything that I can identify as a dedicated power conditioning device, so I assume it’s inside the sealed can???

Nope. It is not in the picture.



Danfoss is one of the few manufacturers of DC powered refrigeration compressors. Note the box on the side. That is where the magic happens !

NRALIFR wrote:
So, from a power efficiency standpoint, which would be the better way to go? AC conversion kit, or DC?

My GUESS is that the DC conversion would be a bit more efficient. 3 phase motors are always more efficient than single phase.


Last, I will through in inverter manufacturers are making a TON OF PROFIT these days. The actual cost of parts between a modified-sine wave inverter and a pure sine wave inverter is probably <$50 maybe even <$25. The most expensive part inside these things is the transformer (lots of copper and steel).

For some reason the low frequency inverters (you know they are low frequency because they weigh a TON) seem to be more "durable" but I don't know why.

[time2roll]

Q. which is most the power efficient 120V or 12V A. on paper the 12V is the most efficient @ 7.5A 90W, but it runs some slower than the 120V, so in the end they are practically the same. But if no inverter is on board then the 12V is still much faster than your gas/elect.

https://jc-refrigeration.com/product-information/

Unless you already have an inverter I am thinking I would go DC. Note a bit below you will need to upgrade the DC connection with possibly a relay interface between the control board and unit.

[NRALIFR]

I only looked at the Dometic kits, and the most expensive ones are about $1100, I assume those are for the largest refrigerator models. The one for my fridge is $595 AC, $695 DC. The DC kits appear to be $100 more across the board.

JC Refrigeration HVAC Units

:):)

[pianotuna]

How much does the conversion cost? 12 volt is certainly the route I'd take.

[time2roll]

NRALIFR wrote:
So, from a power efficiency standpoint, which would be the better way to go? AC conversion kit, or DC?

:):)

What are the wattage specs? Does the DC version actually have a DC compressor or is there an included inverter? If it does have an inverter is it 3-phase or single phase? I think a 3-phase inverter driven by DC would be very efficient.

Is there a price difference? Of course with AC you would need to add an inverter (maybe 1000 watts) to power the fridge on the road.

[NRALIFR]

Thanks Drew. It sounds like there’s going to be some conversion losses in there regardless of whether you go with AC or DC.

I did just find this in the Q and A section of the JC Refrigeration website.

“ Q. which is most the power efficient 120V or 12V
A. on paper the 12V is the most efficient @ 7.5A 90W, but it runs some slower than the 120V, so in the end they are practically the same. But if no inverter is on board then the 12V is still much faster than your gas/elect.”

I’m going to keep this in mind for the day if and when my cooling unit goes casters up. Mine works great now, but that could change tomorrow.

Considering where my inverter is mounted, out in the open where it can be seen and heard easily, I’d probably go with the DC conversion kit for mine.

:):)

[DrewE]

NRALIFR wrote:

Yeah, that would have to be a very special inverter to produce 3-phase AC from 12v DC. The curiosity in me wonders how they do it. I’m familiar with rotary and static AC phase converters, but not DC.

It's not much different than a single phase inverter--there are just three sets of output transistors rather than one set. In either case, you have a DC-DC converter that produces high voltage DC rails, and then some output devices--likely MOSFETs or IGBTs--that switch them to produce an AC waveform. For an MSW inverter, these are just switched on and off per cycle to make a sort of squared off waveform; for a PSW inverter, they would be synthesizing a sine wave using a PWM drive circuit (and perhaps a simple filter circuit).

Brushless DC motors use basically the same sort of technology, just not always with a higher voltage supply. It's entirely possible that the compressor is using a brushless DC motor rather than a three phase induction motor; either one would be quite practical for the application.

[NRALIFR]

theoldwizard1 wrote:

rdhetrick wrote:
Because the conversion isn't an absorption refer - it's a compressor similar to a household refer.

NRALIFR wrote:
That’s probably what I would recommend. No DC-AC conversion loss.

Every 12/24VDC compressor refrigerator actually INCLUDES its own inverter ! The incoming DC voltage is converted to 3 phase AC and sent to the compressor motor. 3 phase is used because it is a much more efficient way of powering a motor.

Just to be clear, this "built in" inverter is specially designed just for this application. The AC output voltage does not have to "match" any "standard". It can be whatever voltage the inverter and motor designers choose ! Also, many (most ?) of these inverter are variable frequency meaning that the compressor motor itself runs at different speeds as the required.

If I lost you, a DC compressor refrigerator is likely more efficient than a straight up residential AC refrigerator powered by a separate inverter.

Yeah, that would have to be a very special inverter to produce 3-phase AC from 12v DC. The curiosity in me wonders how they do it. I’m familiar with rotary and static AC phase converters, but not DC.

The OP is trying to decide between two conversion options from JC Refrigeration. They offer a an AC kit and a DC kit.

This is a screen grab of the DC unit that’s supposed to be for my fridge. 12V 7.5A 90W

I don’t see anything that I can identify as a dedicated power conditioning device, so I assume it’s inside the sealed can???



And this is the AC conversion kit. I’m assuming these are pictures of the actual units, but they look identical to me. 120V .8A 96W



So, from a power efficiency standpoint, which would be the better way to go? AC conversion kit, or DC?

:):)

[Housted]

I converted our last may. Would never have any thing else.My primary reason is that I did not have to rebuild the cabinet as you would for a residential fridge. The doors and such are the same as the old cooling unit.

YMMV

Housted

[woodtrucker]

I’m converting because my domestic rm2562 started spewing ammonia.
Yuck. I would have gone residential route but I literally only have 55” of height.
So I opted to pull the cooling unit on the domestic.
It’s out and in the dumpster minus the control boards and fins.
So in will go the dc compressor. Sounds like even if the fridge runs batter down even in a couple hours, that would help cool on a long travel day say 8-9 hours until I get to shore power.
Boondocking is not in our future until our kids are gone and it’s just two of us.
Thanks for the info. 12vdc is what I’ll buy!

[theoldwizard1]

woodtrucker wrote:
We are on shore power almost always except for traveling. Would the truck charge battery enough to keep the 12VDC unit cooling while on the road?

The good news is that if the refrigerator and its contents are cold, it will likely keep them cold for a good 8 hours, if you don't open the door !

A typical 7 pin 12V battery charge connection should keep the battery from discharging and keep the refrigerator running as long as the vehicle is running.

If you are boondocking/dry camping, things are more complicated. You will likely need to re-charge your house battery. 3 way to do that :

• solar
• generator and charger
• DC-DC charger from the tow vehicle

I would also recommend a pair of 6V golf cart batteries. They store more energy than 12V batteries and may even cost less !

[theoldwizard1]

rdhetrick wrote:
Because the conversion isn't an absorption refer - it's a compressor similar to a household refer.

NRALIFR wrote:
That’s probably what I would recommend. No DC-AC conversion loss.

Every 12/24VDC compressor refrigerator actually INCLUDES its own inverter ! The incoming DC voltage is converted to 3 phase AC and sent to the compressor motor. 3 phase is used because it is a much more efficient way of powering a motor.

Just to be clear, this "built in" inverter is specially designed just for this application. The AC output voltage does not have to "match" any "standard". It can be whatever voltage the inverter and motor designers choose ! Also, many (most ?) of these inverter are variable frequency meaning that the compressor motor itself runs at different speeds as the required.

If I lost you, a DC compressor refrigerator is likely more efficient than a straight up residential AC refrigerator powered by a separate inverter.

[rdhetrick]

enblethen wrote:
Why convert is my question!

In my dad's case, it is for performance reasons - his current absorption unit doesn't work great, can't get his ice cream cold enough!