JC refrigeration AC or DC upgrade?

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.