Mitsubishi Mr Slim fault

I got called to a fault a few weeks back on a PUHZ P125VHA. I found the breaker tripped and no power to the condensor.
After a few checks i found continuity between live and neutral on the power board with mains power and interconnecting disconnected. I checked the compressor and got readings of 0.5 kohms from u-v, v-w, and u-w.

I spoke to Mitsubishi who advised me to not only change the power board but also the actm, rush resister and noise filter.
Another guy went back and fitted them and he said it was running fine and he recomissioned it.
However when i have gone back today to carry out maintenance, i've found the unit off and upon furthur inspection found the same problem....continuity between live and neutral.
I tried to follow the electronics but they confuse me, even the spark was lost, has anyone come across this before?

Hi Marc

I doubt the compressor to be a cause as when the units turned off the windings would be isolated from the point your testing by the contactor.

When you say continuity was it a straight short or was there resistance read on the tester.
When you tested the power board did you also remove every wired connector on it, if not then next time you go and the fault is there take every thing off the board, if it still shows continuty it must be at least the board is knacked, if it reads ok with every thing dissconected then connect everything back on one at time testing for continutity as you go in a process of ilimination, once you get continuity then trace the wire through the unit to the culprit part.

Just a thought, seen this kind of thing before on Daikin VRV2 when they had a bad batch of condenser motors that over time the insulation broke down and let moisture in when it rained and caused the fuses to pop on the main board, maybe something similar is happening here, is the unit tracking water in onto the electrics some where.

Hope that helps.

Transformer, crankcase heater ??

When i first went to the look at the fault i disconnected everything from the power board and tried plugging things back in one at a time. It looked like it was the rush resister and this is when i phones mitsubishi and they suggested to change the other parts as well.

I checked the resistance of the fan last time i was there and it checked out ok.

Next time i go back i will have to go through it again and disconnect things one at a time again.

Heya.

You've ruled out power supply problems I gather Mark?

Is it three phase or single, and have you verified that theres no phase rotation or phases down if 3ph?

Could it be that the systems on an undersized or incorrectly rated MCB.........point being it sounds to me like theres no fault on the ccu as such and that the supply could be at fault.

Therefore it's worth verifying the integrity of the supply cabling to the ccu also.

Let us know.

Don't forget your measuring across windings/capaictors so you going to get continuity across L/N when previously tested!

Regards,

J.

Hi J, i am going to change the breaker as a matter of course when i go back.

It's a 3 phase system and all three phases are balanced and it can't be reverse phase because the engineer that went back to fit the boards last time had it running and he re commisioned it.

What i did notice was that the company that used to have the contract have left an old inverter board and compressor for that unit on site. So they have also been changed, which makes me wonder how long they have had this problem for.

You are going to have continuity across a board as after all there are circuits across it filtering , diodes, chips and capacitors, just like a motor will have continuity.
Did you megger test the compressor?
If inverter did you diode check IPM? Also check IPM is cleanly secured to a heatsink?
Condenser fans often cause problems and short out boards to, I know fuji can, and I've seen/heard off daikin fans doing it too.

I had a problem with a power inverter a about six months ago. I had to change two noise filter boards, actm and two inverter boards before I found the problem to be a wire from the reactor had short too the metal casing within condenser! I'm not saying that is the problem but could be worth checking wires within condenser especially those tight to any metal casingAttachment 2693

Attachment 2694

Just an update went back to the site today. (the customer hadn't paid the previous bill so until he did we stopped all service calls).

We were working on another call out but as we were leaving decided to have another quick look at this unit.

I disconneceted everything from the power board and there was continuity from L to N on the board.
I traced every connection onto the board making sure that it was right and i found that the 3 phases to the compressor "U", "V", "W" were not wired how they should be but in fact were wired "V", "W", "U" on the power board.

Would this cause a problem? Can 3 phase compressors run backwards like a motor?

The guy who fitted the boards last time said the system was cooling whilst he was with it- indicating that the compressor must be running fine.

The compressor is not down to earth but i haven't done an insulation resistance test on it yet because i'm waiting for it to be delivered.

If it's a piston compressor then it should be able to run in either direction.

Oil pumps can be a problem on larger compressors tho'.

Just to update the thread, after changing the compressor teminals which were connected wrong on the board, i changed the board which was blown and ran the system up.

The units running fine now so whoever changed the inverter and compressor, wired it back up wrong.

Thanks for the update Marc, glad to hear that it's running again.

Just to add guys, The only reason they replace the ACTM Module on the ducted systems, Is that when a short takes place the ACTM receives a high current draw weakening the interior components and wiring the compressor will do it every time. Mitsubishi Only use scroll and rotary compressors which like to run one way. They are more efficent then other types and thats why they are used with inverter technology. The diode check on the powerboard from my experience is not an absolute check. Best way to go is to bypass the Noise filter or ACTM if possible with a 3amp (made up device and see if the 320 to 370dc is coming out. Hope this helps. Would appreciate some feedback on this. Cheers Guys.