inverter showing wrong voltage output

FabianFabian Posts: 60Registered Users ✭✭
I recently got a used 24v 3000watt 120vac 60hz reliable pure sinewave inverter. It has a 2 display on the front that shows the dc voltage input and the output voltage reading. In normal operation it always shows about 120 -122vac output on the display.

Recently i was pulling about 1100watt from it constantly and I realize that the case was getting very warm to a point that i assume the fans should have chip in by now which they did not so I assume maybe it wasn't time yet/not hot enough or probably the fans or not working. After a while I heard a poof sound and the inverter was still on but no output. I turn it off same time. I waited a while and turn it on again and it was working normally but this time the output voltage now drop from 122vac to 110vac.

What could have gone bad in terms of when I heard the poof sound? what could have cause this to happen is it because it got too hot and the fans did not chip in to cool it down. What would cause the voltage to drop so low.

I have not open it as yet to check I am just waiting to hear someone's input on the matter first so I can now what to do when i open it to repair it. It is still working now but just with a much lower voltage output and I really want to get it back to the 120- 122vac reading please.
So please help me out in this regards.
I anxiously await your responses.


  • mcgivormcgivor Posts: 2,201Solar Expert ✭✭✭✭✭
    Any magic smoke smell?
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • FabianFabian Posts: 60Registered Users ✭✭
    no smell. as i said before its working still. It comes with 8 fuses so I am wondering if one of the fuse burn and cut off that section of the circuit thus still working but with a decrease voltage output. I am planning to open it as I reach home today and check. I was just hoping to get some replies from u guys before i do.

    If the fans are working but the trigger part of the circuit to let them chip in when the thermal is hot is not working, what do u suggest i do to let them work? I was thinking about adding a different heat sensing circuit to let them turn on at a preset temp range. any ideas or just connect them straight to the battery bank and let them run constantly?
  • mcgivormcgivor Posts: 2,201Solar Expert ✭✭✭✭✭
    Can you post the battery capacity used to feed the inverter, was the ambient temperature high, size of the conductors battery to inverter and dose the manual mention how the fans are controlled? Some cheaper inverters, which Reliable is, use a load factor rather than temperature monitoring circuitry to control the fans. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • FabianFabian Posts: 60Registered Users ✭✭
    the battery size is 24v 390ah, the wire size is 3 feet of 2/0 awg battery wire to feed the inverter. I don't have the manual but recently i open the inverter and i saw a temp sensor with some thermal paste on it connecting to one of the heat sinks so i figure that is that sense when the temp gets to a certain preset value which would trigger the fans.
  • PhotowhitPhotowhit Posts: 4,701Solar Expert ✭✭✭✭
    There is the possibility that your fan has died, mine did several years ago. Perhaps it doesn't have or has a less robust thermal shut down. I replaced the fan and it has been put back in service about 5 years ago and still 'runs my life'.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • mcgivormcgivor Posts: 2,201Solar Expert ✭✭✭✭✭
    edited May 27 #7
    Some inverters which are larger capacity, use 2 smaller inverters stacked with a sync connection to provide the final output power rating. A while back I had such an inverter fail due to a gecko finding a home inside but the circuit board was burned, the unit would not work, after removing the sync cable it was half the inverter it once was, until lightning took care of the good half. You  could check the fuses, if half are bad, you might get lucky, assuming a 1500/1500W synced unit. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • FabianFabian Posts: 60Registered Users ✭✭
    When I open it i notice 4 small transformers inside. 2 at the top and 2 at the bottom. So maybe one of them got cut off when the fuse(s) blow. The circuit appears to be 2 in one as u stated because the bottom of the circuit shows that it is so as u can see in the pic. and also when you test the ground and the neutral you get about 61vac and the ground and the live you get about the same.
  • FabianFabian Posts: 60Registered Users ✭✭
    I open it today and discovered that one of the fuse was burn and one of the fets got damage.  The inverter has four small transformers and each transformer has 2 fets assign to each of them because I notice that once one of the fets got damage the fuse for that section blow and therefore  that section got cut off from the inverter so you are still getting power from the inverter but with decrease output voltage pulling it down from 120vac to 114vac. 

    The inverter uses (8) big fets. IRFP4368.

    When the fet got damage i was pulling about 2000watt and I realize that the case was getting quite hot and the fans did not chip in and suddenly i heard a poof sound. So after i open the inverter i replace the blown fet with a slightly smaller one IRF3705 as i did not have any of the big one. I replace the blown fuse. After testing the inverter i realize that I was getting back the 120vac for the output. I was not sure if the temp sensor on it was working so i connect the fans straight so as the inverter turn on the fans came on also.

    So i went ahead and did back the estimated 2000watt test and after just a short time i heard the same poof sound again and when i open it the other irfp4368 on the same section was blown and the fuse blown also but the small irf3705 that i just put in was still good. I am kinda confused because the fans were running constantly and the case was not hot so how comes the other big fet blown and I was not anywhere near the max 3000watt rating for the inverter. And how comes the big one blown and the smaller fet did not blow too?

    I am wondering if is just a bad batch of irfp4368 came pre-install on the board? Would it be best to replace all 8 fets with 8 irf3705 instead of using the big fets that came in it? Using (8) irf3705 would still give me back the 3000watt rating or which other big n channel fet would you suggest i use to replace the irfp4368(s) that i have now?

    I am thinking about connecting back the fans to the sensor because maybe the temp needs to reach very high before they chip in because I was thinking that it was heat cause the big fet to blow but after running the fans constantly and drawing back the same load without the case getting hot and the big fet still blow i get to realize maybe it wasn't heat cause it to blow.

    So any ideas guys as to the questions that i asked?
  • BB.BB. Posts: 27,882Super Moderators, Administrators admin
    What is the DC input at that 2 kWatt load?

    What is the rating of the fuse?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FabianFabian Posts: 60Registered Users ✭✭
    The dc input at the 2 KWatt load was about 25.1v and the fuse rating is 40amp. Each section is equip with (2) 20amp fuse so making the total 40 amp for each section and 160amp overall for all 4 sections.

    I realize is just that section the problem keeps on happening on. So i am wondering if is because maybe that section is the first entry to pick u the dc input or the first path or current flow?
  • mcgivormcgivor Posts: 2,201Solar Expert ✭✭✭✭✭
    edited May 28 #12
    Personally I can't comment on the use of different mosfets other than to say they probably should be replaced with like components. Are the mosfets clamped to the heat sink with screws and heat transfer paste? What is the 2Kw load, sometimes a poor power factor can cause issues, motors and microwaves are examples. Does it work with lower loads? 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • FabianFabian Posts: 60Registered Users ✭✭
    yes the fets are clamped to the heat sink with screws and paste. They work with lower loads.
    The 2kw load consists of a new tech inverter fridge, a clothes iron and a very modern digital washing machine, few lights, one fan and a led 35watt tv.
  • mcgivormcgivor Posts: 2,201Solar Expert ✭✭✭✭✭
    Nothing outrageous with the loads, could  be something in that part of the circuitry, check all solder points for dry joints, a magnifier is helpful, but in reality one failed diode or resistor could cause a cascade effect. Sorry can't be of much help.  
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 

  • FabianFabian Posts: 60Registered Users ✭✭
    but if its a failed diode or resistor so why it  is still working with lower loads?
  • FabianFabian Posts: 60Registered Users ✭✭
    The inverter came with (8) irfp4368 and now i have (6) irfp4368 and (2) irf3705. Will this cause any issue with the fet mismatch setup?

    So what else could have cause the problem because if it was a shorted cap it would have cause 1 or 2 of the fets that i just put in to blow but so far everything working perfectly.

    And i don't think it is a shorted transformer winding either as i don't hear any pitch sound and I examine the windings and they look quite fine to me.

    I got some irf4242 would those me a great replacements for all 8 that i have installed now?

    I connected the fans straight would it be ok to connect them back to the temp sensor as i see on the net where it stated that the inside temp have to reach 45 degree celsius before the fans come in so I am wondering if the 2 irf3705 that i put in would handle the heat until the fans come in and start working?

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