Heat Pump repair (outdoor pcb, mhi) - Page 1
Forum section: electronics / repair

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 12:05:24 am
content: HiJust at the end of winter my heat pump failed. SRK71ZRA-W. Just stopped working, it may have been a bit stormy but I didn't notice any strikes nearby.It had been running for 6 months happily but I can't seen to organise warranty, unhappily.Two cascaded breakers had been tripped. 32 amp and a 20 amp.After reset the error lights started messaging. Green run LED 5 flashes, yellow timer LED always on. This decodes toQuoteThought i might document my attempted repair.https://mhiaa.com.au/document/service-support-handbook/

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 12:24:55 am
content: So opened up the outdoor unit an checked every fuse i can find.found 4, F1, F3, F4, F8 all were good. Circuit breakers must have broken first.They have a nice little block diagram inside the outdoor unit.On the diagram it seems one of the PAM circuits for PFC has failed.There is a relay before the recitifier which now no longer turns on, so the fault is detected before it trips the breaker again.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 12:25:47 am
content: some photos

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 12:30:32 am
content: I have found a short on an IGBT. and have removed it.Single sided board, in an enclosure that seems to give good access to ants and a moth that died.Conformal coated.there are two parrallel active filters, but only one has failed. They consist of a boost converter driven by apparently a PAM algorithm.10mH inductor, shorting switch (igbt) series switch (diode) and caps.the igbt is marked KGF 50N60KDA is driven by TLP351H,i have ordered IKWH60N65WR6XKSA1 to replace at AU$7 each. Hopefully it is equivalent.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 12:47:18 am
content: I was wonder what would happen if I just disconnected one of the Active filters ( the failed one) or both?But anyway I have a probable fix (i hope) which I will try first.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 01:03:46 am
content: Annotated the pcb for those interested.

author: ace1903
author_rank: Regular Contributor
timestamp: October 10, 2023, 07:56:43 am
content: Could you please pictures of the inductors?Usually I see iron core inductors and that makes me wonder what the operating frequency is.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 10, 2023, 09:46:12 am
content: When i reassemble I will take a picture.I assume they are steel core, Im fairly sure I saw laminations but wasn't paying attention.

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2023, 11:26:10 am
content: Possible Gate driver damage should be checked for. Assuming 2 separate drivers, Digital multimeter resistance and diode type comparisons can be made.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 15, 2023, 11:22:01 pm
content: Here is one of the 10mH inductors.Looks like silicon steel.I am not sure on the pfc frequency but im pretty sure it is 20khz to 100khz and im guessing probably around 25khz.I am not sure how well these work at DC and at these frequencies but i'm guessing its a pretty standard component.

author: wraper
author_rank: Unknown
timestamp: October 15, 2023, 11:26:26 pm
content: Quote from: HackedFridgeMagnet on October 10, 2023, 12:30:32 amLooks like flux residues from wave soldering.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 15, 2023, 11:30:49 pm
content: Yeah good call on the gate drive. I was concerned about it.Once I had remove the IGBT then both gate drives exhibited the same resistances in every place i could test. So I assumed it was good.I did further unpowered testing of all the other components nearby in the circuit. All good.My IGBTs arrived Friday and the unit is all back together now.I ran the heat pump last night and it worked fine. Yay.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 15, 2023, 11:34:10 pm
content: It definitely was conformal coated. When I unscrewed the bolts for the heatsink it lifted up a waterproof film with the bolt.Also had to pierce the coating to take any measurements.Probably flux residue too.

author: johansen
author_rank: Super Contributor
timestamp: October 16, 2023, 02:39:39 am
content: Some of these thing use a much lower frequency for CCM boost conversion, i have seen iron cored pfc inductors in a number of them.The high frequency drives can fit a 3 ton system through 2, inductors about 7cm outer diameter and 3 thick. That's the finished dimensions of the toroidal inductor.I have 2, 5 ton boards and the two inductors are Krispy creme donut sized.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 16, 2023, 04:30:01 am
content: Interesting that they go lower than audio frequencies.I never suspected they would, but I don't just know much about applied magnetics.I wonder what frequency the do use then.I guess they also multiphase the two switches too.