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Triac has peak current rating, rating of a 12A triac for 10 mains cycles is typically >50 Arms. As discussed before, the NTC doesn't reduce the inrush current if it's already hot, you must be prepared for the full motor inrush current in any case.
I did discover that the firmware of the conytoller prevents the compressor cycling faster that 60seconds. So that seems ok for NTC cooling time and the Triac.
I found a suitable NTC with a 200J rating. It will do the job.
But, it is not available. In fact, none of any kind are available and the lead times a shocking.
It appears I cannot solve this problem immediately with a different NTC.
I am hoping to rework existing boards in the short term.
I will consider a different Triac next with a higher current rating. It is Dpak so choices are again limited.
And others keep asking how it can last for 18monts if it is as underrated as I say. I do not know.
I / we have performed several tests since.
Cycled the compressor through the NTC at short intervals for 3000 cycles. Whilst not cooling down and not limiting much, the NTC has not even discolored. I measured 134dC on the NTC during that time.
The inrush was around 12-13A RMS for these cycles
Cycled it at longer intervals but with more operating current (not more inrush). NTC not as hot.
Now, we lowered the mains voltage to a point where the compressor is struggling to start and we get the full starting/inrush current of 14A rms for almost 2 seconds!!
The NTC shoots up to 130dC but is just laughing it off.
Right now, I am not able to blow one up and I now do not know how this can ever happen.
They are really tough.
One point could be the series impedance of mains.
When you are located far from the distribution transformer maybe the high series impedance of the wiring may help to keep the inrush current low.
The one I am testing is from the same batch we made. But that sits on my mind too.
We are still trying to estimate how many cycles they mights have gone through. There is an unknown lag between leaving our factory, shipping to zUS, sitting in warehouses, being sold, installed... and then also, how used by tge end customer. But what I am doing here with this cycling is MUCH worse than it could ever be under normal conditions. But, who knows.
@KLAUS, I have considered this because I had a similar issue some time ago with almost all units from a very renowned speaker supplier blowing up on us.
We are currently powering this from a variac as we have 240V here and these units are for export. I did already mention to the boss that the impedance could be a problem.
I am not sure how to work around that?
We also have an auto transformer but I can tell from the size that it can't cut it.
In desperation to at least see something happening, We hit is last night with 14A Rms for over 2seconds (the max is 4A). It shot to 130dC again but did not blow up, no discolouration, nothing. This must be one of the toughest components I have seen in a while. Yet, they are failing in the field.
I did discover that the firmware of the conytoller prevents the compressor cycling faster that 60seconds. So that seems ok for NTC cooling time and the Triac.
Manufacturers' advice tells consumers to let a compressor (example, air conditioner) sit for a few minutes after shutoff, before turning it back on. The reason has to do with cold liquid refrigerant which remains in the chamber, and which needs time to return to gaseous condition.
The incompressible liquid resists the motor's effort to turn. The stalled motor draws extreme current, as much or more than plain startup surge, and for a longer time than startup surge. Unless power is quickly shut down the result can be to burn out the motor or circuitry.
One minute cycling time may not be enough for liquid refrigerant to turn to gas. Or, consumers may get around the minute delay, resulting in failure of units in the field.
I am not 100% sure if the are all form the same batch. I am looking into that but it appears our manufacturer does not keep records to such detail (unless specified prior).
It is still impossible for us to blow this thing up.
We hammered it with everything we can think of within the constraints of the peripherals fitted.
The external temperature went up to 220dC (!), we could smell it but it shrugs it all off and will not blow up.
We have tried even crazy conditions such as:
Stalled the refrigeration compressor by lowering the mains voltage so when it tries to start, it stalls.
It sits with the almost full starting current or ~10A through the NTC. But after some seconds, the compressor overload trips and everything is fine.
And so, it either has to be a component problem with some of these NTC parts or there is still a condition that happens that we are not able to imagine.
It would either blow the fuse or the fuse + NTC or plus triac. we have not actually done that.
Reason is that the compressors are still operational after the NTC failures so we ruled out any short.
I will re-evaluate that.
My final statement: NTC are inappropriate as compressor inrush limiting device. In case of refrigerator compressors, they'll even fight with the built-in starter circuit (either PTC or mechanical contact). Other than rectifier circuits with filter capacitor, motors are already limiting the inrush current by their winding resistance.
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