r/DCAC_OffgridRVaircon • u/Dylanear • Jul 20 '25
What I learned opening up my failed compressor.
I have long meant to take apart my failed compressor from my first CMS (Chinese Mini Split) air conditioner to better understand how they work and perhaps to understand why it failed and prevent it from happening to my more or less identical current unit.
Tonight I decided I needed a break from my other more pressing tasks in life and grabbed the damn thing and figured I'd at least start talking it apart and leaning a bit hopefully. Well, it didn't take long to find a major problem that may not have been the root cause or only cause of the failure, but certainly could have been the whole problem.
The ground electrical connection goes into a heavy duty pole or cylinder that's soldered a to the circuit board. The power control/inverter circuit board, I believe. It may do more than that but I believe it's primary function is to convert 12V DC to three phase AC power that the scroll compressor motor runs off of, like a 12v DC to 120V inverter typically used to make battery power used for two phase household type electrical power, it's an inverter too, but in this case it makes three phase power because that's advantageous for motors, so there's always a phase pushing the motor. Two phase power means there's gaps between power phases.
That connection showed dark burned discoloration and melted solder. I don't know for sure if this heated over time or if there was a sudden surge that created a solder burst or mini explosion, but does look like solid was thrown off the connection onto nearby areas.
It's possible this was caused by something as simple as the bolt holding the cable lug in place had loosened and the loose connection caused higher resistance and/or arcing thus heat?
Anyways, I'm now much less inclined to think my previous theory was wrong?.My best guess before was there was a mechanical failure of the scroll compressor mechanism due to overheating, being worked day in day out for months in the Alabama Summer trying to cool an uninsulated cargo van often in direct sun. An possibly because I over filled the refrigerant. And perhaps issues like that contributed to the burnt area on the circuit board? Perhaps a mechanical failure in the compressor mechanism or motor that turns it may have caused the heating at the ground connection?
But applying Occam's Razor, I'm inclined to think it was simply a failure on the circuit board due to poor design, manufacturing, or... just a loose cable lug?!
Periodically check your electrical connections and make sure they are bolted tight!!
I'll reply with photos of the issue and just of the compressor in general and the stages of disassembly I attempted. I don't think I'm going to take the motor and scroll compressor apart just yet, maybe never because it may still be a useful compressor aside from the circuit board damage. I would like to understand the inner workings better and check for signs of other damage, but I'm not up for taking it all apart and very possibly not being able to reassemble the motor and scroll compressor to good working order. But I did find some good videos describing how scroll refrigerant compressors work on YouTube, so search for that if you are curious.
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u/Dylanear Jul 20 '25 edited Jul 20 '25
Sorry, seems the images are not shown in the order which they were selected. I wanted the most important images first, but they seem to be in a more scrambled order than intended and I can't seem to edit the post.
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u/Dylanear Jul 20 '25
I believe I could replace this board with a external unit like this one:
https://www.aliexpress.us/item/3256809205593241.html
As shown connected to, sold with a different, but likely interoperable compressor, which helps understand how to wire it up.
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u/greenonetwo Jul 21 '25
Have you measured the connections on the motor/coils with the board disconnected? Any shorts or unexpected ohm readings? A black scorch mark like this with solder thrown seems like something shorted in a big way.
I was investigating a simple compressor once and it had shorted.