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 > JC refrigeration AC or DC upgrade?

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theoldwizard1

SE MI

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Posted: 01/30/21 08:00pm Link  |  Quote  |  Print  |  Notify Moderator

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.

[image]

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.


NRALIFR

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Posted: 01/30/21 10:08pm Link  |  Quote  |  Print  |  Notify Moderator

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

Not sorry I asked at all! [emoticon]

[emoticon][emoticon]


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billyboy

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Posted: 01/31/21 08:33am Link  |  Quote  |  Print  |  Notify Moderator

Will a cheap harber freight modified sine wave 1700 watt inverter run the fridge


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woodtrucker

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Posted: 01/31/21 08:57am Link  |  Quote  |  Print  |  Notify Moderator

Just ordered 12v cooling unit from jc refrigeration! Thanks for the great info all!


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goducks10

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Posted: 01/31/21 09:30am Link  |  Quote  |  Print  |  Notify Moderator

When I was looking to replace my POS Norcold I checked out JC fridges. It was $695 and $280 for shipping to the west coast. $975 total.
I then looked at 12V specific RV fridges. I found a 12V RV/Off grid GE 9.8 cu.ft that was a direct replacement for my Norcold N8X. (Reall 7.5 cu.ft)
IIRC $899 at the local appliance store. Added a 3 yr warranty for $50. Total $949.

Even though I already have a 2000W inverter I didn't want to have to run the inverter 24/7 when dry camping. Having a 12V only allows me to run directly off my four 6V GC batteries. When on shore power the batteries are getting charged.
I also have 400W of solar. I also gained 2.3 cu.ft.

Only got to use it at the end of last season. So far we love it. Way more room inside. It cools down super quick. Freezer gets almost too cold but the temps stay more constant than with the Norcold. It's also quiet. We can hear the compressor run when we're laying in bed but nowhere else. I thinks it's more of a vibration as I pushed on a wood panel in the bedroom closet and the noise quit. Other than that we never hear it during the day.

theoldwizard1

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Posted: 02/01/21 03:32pm Link  |  Quote  |  Print  |  Notify Moderator

billyboy wrote:

Will a cheap harber freight modified sine wave 1700 watt inverter run the fridge

$35 difference between the HF Jupiter 1500 watt modified sine wave inverter and the HF Jupiter 2000 watt pure sine wave inverter.

A residential refrigerator runs better on pure sine wave.

pianotuna

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Posted: 02/01/21 03:41pm Link  |  Quote  |  Print  |  Notify Moderator

Billyboy how long do you want the fridge to last? PSW is better.


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wborst

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Posted: 02/04/21 07:48am Link  |  Quote  |  Print  |  Notify Moderator

theoldwizard1 wrote:

billyboy wrote:

Will a cheap harber freight modified sine wave 1700 watt inverter run the fridge

$35 difference between the HF Jupiter 1500 watt modified sine wave inverter and the HF Jupiter 2000 watt pure sine wave inverter.

A residential refrigerator runs better on pure sine wave.


The Jupiter 2000 watt inverter from HF is a modified sine wave inverter. Just checked their web site.


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n0arp

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Posted: 02/04/21 08:37am Link  |  Quote  |  Print  |  Notify Moderator

I'm late to the thread, as it looks like you've already made your purchase.

I have the 120V version in my truck camper, which uses 1.0-1.3kWh per day. I chose it because I have a native 48V bank, so it's being converted one way or the other, and I wanted the slightly extra cooling capacity.

The following is directly from their FAQ:

Quote:

Q. what is the duty cycle (run time) A in our testing @ 80F the AC compressor will run approx. 56% and the DC approx. 64%, that is not opening and closing the doors. So, this will vary some according to your usage
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.


Our compressor pulls 90W on the AC side (not 96W). At the 56% duty cycle mentioned above, that works out to 1.209kWh/day, which is in line with what we've observed.

The DC compressor is advertised to pull 90W. At the mentioned 64% duty cycle, that's 1.382kWh/day.

By those numbers, the AC version is 13% more efficient. Inverter losses are usually a little under 10%. That indicates the AC version may actually be more efficient overall, even accounting for the inverter, but just like their FAQ says, it's more or less a wash.

* This post was edited 02/04/21 08:53am by an administrator/moderator *


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rdhetrick

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Posted: 02/04/21 09:08am Link  |  Quote  |  Print  |  Notify Moderator

jshupe wrote:

I'm late to the thread, as it looks like you've already made your purchase.

I have the 120V version in my truck camper, which uses 1.0-1.3kWh per day. I chose it because I have a native 48V bank, so it's being converted one way or the other, and I wanted the slightly extra cooling capacity.

The following is directly from their FAQ:

Quote:

Q. what is the duty cycle (run time) A in our testing @ 80F the AC compressor will run approx. 56% and the DC approx. 64%, that is not opening and closing the doors. So, this will vary some according to your usage
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.


Our compressor pulls 90W on the AC side (not 96W). At the 56% duty cycle mentioned above, that works out to 1.209kWh/day, which is in line with what we've observed.

The DC compressor is advertised to pull 90W. At the mentioned 64% duty cycle, that's 1.382kWh/day.

By those numbers, the AC version is 13% more efficient. Inverter losses are usually a little under 10%. That indicates the AC version may actually be more efficient overall, even accounting for the inverter, but just like their FAQ says, it's more or less a wash.


I think that's true if the inverter only has the losses when the compressor is running, but there is a dead load to the inverter even when it's doing nothing. I admit I haven't ran the numbers, but I've got to believe that unless you're turning the inverter off when the refer doesn't need it and on when it does, it's got to use more power over the course of a day.


Rob
2006 Mandalay 40E Full Time
2015 Jeep Grand Cherokee

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