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inverter showing wrong voltage output

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  • inverter showing wrong voltage output

    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.

  • #2
    Sounds like a capacitor. Time to get another Reliable Inverter.
    MSEE, PE

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    • #3
      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?

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      • #4
        I will suggest that when the fuse blew, one IRFP4368 had failed, but the other one was also damaged.
        You might have better luck replacing them in pairs, the part is available and not expensive. Bruce Roe

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        • #5
          Sounds like you have a SMP Inverter from the description of the transformers. Most common problems you will see and give you the exact same symptoms you are seeing is 1 of 2 things.

          1. One of the Transformer windings has shorted. This is caused from the windings vibrating and beating the insulation off of the wire. If you ever noticed a high pitch noise is a dead give away and a symptom of a very poorly built unit.

          2. Filter/Storage cap shorted. Your FET is feeding a shorted cap.

          What I can tell you it is not a heat problem. The excess heat is a result of another problem like a short circuit.
          MSEE, PE

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          • #6
            I don't think its none of the above. Reason been is that i replace the 2 fets and put in (2) irf3705 fets and I have been running loads on the inverter for about 14hrs today and even at a one point i was drawing close to a 1000watt for about 1hr and 15mins and so far so good.

            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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            • #7
              Unless you know how to design a grid tie inverter, I suggest you stick to the original type parts
              in this critical circuitry. Fans, OK. Bruce Roe

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              • #8
                so i should reconnect the fans to the original fans slot instead of letting them run constantly?

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                • #9
                  Originally posted by fabieville View Post
                  I don't think its none of the above. Reason been is that i replace the 2 fets and put in (2) irf3705 fets and I have been running loads on the inverter for about 14hrs today and even at a one point i was drawing close to a 1000watt for about 1hr and 15mins and so far so good.

                  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?
                  IRFP4368: 75V 1.46mohms
                  IRF3705: no such thing. Maybe you meant IRL3705? If so that's a logic level FET. 55V 10mohms.

                  They will not share at all; the 4368's will take all the load. Might as well just take out the logic level FETs and either leave out or replace with correct parts.
                  I got some irf4242 would those me a great replacements for all 8 that i have installed now?
                  IRFP4242 are 300V 49 milliohm parts. That would be a terrible replacement.
                  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?
                  You have to do the work. Most FETs cannot handle a junction temp above 175C. You have to work backwards through thermal impedances to see what sort of cooling/heat transfer you will need.

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                  • #10
                    Originally posted by jflorey2 View Post
                    IRFP4368: 75V 1.46mohms
                    IRF3705: no such thing. Maybe you meant IRL3705? If so that's a logic level FET. 55V 10mohms.

                    They will not share at all; the 4368's will take all the load. Might as well just take out the logic level
                    FETs and either leave out or replace with correct parts.

                    IRFP4242 are 300V 49 milliohm parts. That would be a terrible replacement.
                    I would say in this situation, the potential for letting more magic smoke out of the wires
                    is so high that I would not be giving much specific advice. Bruce Roe

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                    • #11
                      Can you say Smoke on the Water? I agree with Bruce the OP should not be giving anyone any advice.

                      MODs I suggest closing thread as being dangerous.
                      MSEE, PE

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