Portable aircon blowing the compressor again and again?

I have my sisters protable aircon here and with the summer on the way I am now finally required to see if I can fix it.
But I am somehow lost with this one...

Problem:
The aircon runs perfectly fine as it is supposed to for about 3 - 4 weeks with usage depending on how hot it is between 4 and 6 hours a day.
But without any warning, noise or things like that the compressor simply stops running - or does not start.
Noone really noticed what happened but noticed it no longer cools.

Fixes so far:
During the warranty period, which is over now, the unit was replaced twice by the shop.
After that it had to be dropped off at a repair center where the compressor was replaced twice.
After each compressor replacement we were told the compressor failed because the unit was not used for weeks and then it had to run for too long - total bogus if you ask me....

Checks from my side:
Twice I measured the failed compressors and there was no resistance in the motor anywhere, totally burnt out or a thermo fuse blown I would say.
Even after 3 hours of ongoing use the compressor won't get any warmer than my fridge or freezer compressor.
So I rule out overheating.
Pressure levels seem to be fine too although I can only measure from the service port and can't check hot and cold side seperatly.
However:
The unit seems to have a timing/safety problem.
On my aircons it is impossible to make the compressor start again once it completed a cycle, only once the gas pressure has equalised it will turn on again.
If, for example, the aircon just shut off and you set a lower temp on the dial the compressor will jump in right away.
Even worse if an additional fan is blowing in the wrong direction, then the unit might turn on again seconds after it shut off.

Conclusions so far:
I am at the point where I say the starting under still high pressure is causing the motor to stall and then burn out.
Only other thing I can think of would be a very badly designed compressor....
Since the compressor is still working (so far) I am now tempted to add a resettable fuse that is just a bit higher than the nominal motor current.

Questions:
A fuse might help to protect the motor but certainly is not the best solution.
Am I correct to assume that the motor will need far more power to start if the system is still under nominal pressure and not equalised yet?
If all my assumptions are more or less correct then what would be the best place to add a timer that prevents the compressor from starting again too early?
From what I can understand from the electronics inside it seems a set of triacs of thyristors is switching the compressor, but all marking on them are removed ROFL
Would have to do some more digging to check for they are driven to draw a better conclusion on the type, might even be tiny SSR's !? ;)
At this stage I would consider a simple 555 timer that activates once the compressor is turned off by the electronics.
Once running the timer will block the signal to the triacs or whatever is used until the time is run out.
Might even go for a simple timer relay if I can find one in my spre parts boxes.
If I go for a 555, how can I integrate it ?
Can I just keep the start signal for the timer high as long as the compressor runs?
How do I "block" the start signal from the electronics to simply reset the timer before it runs out?
Currently confusing myself here with the starting, stopping, timing and getting it between electronics and compressor :(


iceng1 month ago

You are correct in every statement here..

It boggles my mind that a manufacturer would put out a crappy unprotected air conditioner product.. And that repair guy saying "the unit was not used for weeks and then it had to run for too long" is a totally false for such a product !!

Every air-conditioner, I have come across will time out if for some reason the compressor is stalled by un-equalized gas pressure..

One that I had, some times would need three restart attempts..

My best machine will turn on the blower to measure the room temp and only activate the compressor if it detects a degree or two warmer otherwise it turns off for several minutes before testing the room again..

I'm surprised the resistance of the failed sealed compressor motor was zero because the if it was long enough to short the coils it would more likely burn them open..

I including some fridge start circuits which should be similar to air condition compressors.

If I had your problem, I would try to make a power cut out relay with two timers

A 3 to 8 second time to ignore start current and a 3 minute power lock out whenever the compressor gets stopped..

To do this, you might look at overall motor current... After 8 seconds kill power for 3 min If motor current is too high, or If motor current is zero..

Refrigerator-starting-relaysm.jpg
Downunder35m (author)  iceng1 month ago

My bad with not thinking straight - you are of course correct, no short but open circuit for the compressor winding.
And I guess if you confirm my fears then it might be best to first program a spare Arduino to replace the crappy electronics and add some safety.
Guess easiest solution is to seperate it all.
Electronics do the temp checks and the Arduino between them and compressor will have some counter to prevent a re-start unless the timer has run out.

Yonatan241 month ago

TLDR, a leak? Maybe water got in the circuit board?

I see I should have read.