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    Choosing the right VFD protection

    Hi,
    I need to choose the right protection for VFD and motor according to UL standard.

    Motor: 30kW, 480VAC, In=46.8A

    Drive: Sinamics G120P with Power Module PM230 (6SL3223-0DE33-7AA0) - LO 37kW (output 75A, input 70A), HO 30kW (output 60A, input 56A).

    I have two questions:

    1. According to the manual, Siemens recommend 100A Type J fuses for UL. Is it ok to use 80A protecion? Input current (HO) 56A x 1.25 = 70A.

    2. Can I use UL Circuit Breaker instead of Type J Fuses?


    Appreciate your help,

    Thanks

    #2
    We typically use what the manufacturer recommends. Why do you want to use 80 amp fuses instead of the recommended 100 amp fuses?
    Rob

    Moderator

    All responses based on the 2017 NEC unless otherwise noted

    Comment


      #3
      Because the main CB is 100A, if I use 100A for VFD I will need to change the main breaker to 125A min.

      Comment


        #4
        Originally posted by Alexal View Post
        Because the main CB is 100A, if I use 100A for VFD I will need to change the main breaker to 125A min.
        if you have a 100A main, you don't need one on the VFD as well.
        Bob

        Comment


          #5
          Originally posted by Alexal View Post
          ...

          1. According to the manual, Siemens recommend 100A Type J fuses for UL. Is it ok to use 80A protecion? Input current (HO) 56A x 1.25 = 70A.

          2. Can I use UL Circuit Breaker instead of Type J Fuses?


          ... Because the main CB is 100A, if I use 100A for VFD I will need to change the main breaker to 125A min.
          Couple of questions:
          What is mandating changing the feeder CB to 125A if you use 100A fuses?

          Type J fuses are available in both fast acting and time delay? Which is Siemens specifying?

          I understand you don't want to change out the feeder CB. That requires time and money. And I think I understand (but, maybe not) you want the CB/fuse to coordinate.

          So, recommend: Personal opinion (see signature line)

          Lay out the TCC curves see exactly where they overlap. You don't have to have ETAP or SKM. Just tape them up on a window. This way you will know what you are dealing with as far as the coordination goes.

          It wouldn't matter which opened first, the VFD fuse or the feeder CB. If either open, you have a problem to T-Shoot. Ease of resetting a CB over replacing $150 in fuses doesn't matter. With the information we have, I would tend to leave the feeder at 100A, and use the 100A type J ( TD or fast acting, as specified)


          Without data you’re just another person with an opinion – Edwards Deming

          Comment


            #6
            But the main breaker is for the panel (besides VFD I have other loads / branch circuits). The main breaker is part of feeder circuit... am I wrong?

            Comment


              #7
              Originally posted by Alexal View Post
              But the main breaker is for the panel (besides VFD I have other loads / branch circuits). The main breaker is part of feeder circuit... am I wrong?
              Okay, you left me confused.
              Draw out the one line.

              Panel with 100A main
              Panel CBs feeding other loads on panel
              Panel CB - feeder to VFD (we don't know what this is)
              conductors to VD
              Type J fuses in VFD (MFG specified as 100A type J)

              You are adding 50 to 75A load on a 100A panel
              Highly recommend a load study on the panel loads, panel feeder. From what you are saying, there may not be enough headroom to feed the VFD.

              Last edited by iceworm; 08-23-19, 12:03 PM. Reason: clarify reason for load study
              Without data you’re just another person with an opinion – Edwards Deming

              Comment


                #8
                Originally posted by iceworm View Post

                Okay, you left me confused.
                Draw out the one line.

                Panel with 100A main
                Panel CBs feeding other loads on panel
                Panel CB - feeder to VFD (we don't know what this is)
                conductors to VD
                Type J fuses in VFD (MFG specified as 100A type J)


                You are adding 50 to 75A load on a 100A panel
                Highly recommend a load study on the panel loads, panel feeder. From what you are saying, there may not be enough headroom to feed the VFD.
                Changed main CB to 125A
                Attached Files
                Last edited by Alexal; 08-23-19, 12:31 PM.

                Comment


                  #9
                  That looks pretty good
                  Curiosity Q: Does the drive get the Type J fuses? If so, does Siemens specify fast or TD? Doesn't matter much - just curious.

                  Caution note - if you are under the NEC:
                  430 Part X, 430.122, conductors supplying power to the drive have to be sized at 125% of the rated drive input. Even if the motor load is smaller that the drive rating - size the conductors for the drive.
                  Without data you’re just another person with an opinion – Edwards Deming

                  Comment


                    #10
                    Originally posted by Alexal View Post
                    Hi,
                    I need to choose the right protection for VFD and motor according to UL standard.

                    Motor: 30kW, 480VAC, In=46.8A

                    Drive: Sinamics G120P with Power Module PM230 (6SL3223-0DE33-7AA0) - LO 37kW (output 75A, input 70A), HO 30kW (output 60A, input 56A).

                    I have two questions:

                    1. According to the manual, Siemens recommend 100A Type J fuses for UL. Is it ok to use 80A protecion? Input current (HO) 56A x 1.25 = 70A.

                    2. Can I use UL Circuit Breaker instead of Type J Fuses?


                    Appreciate your help,

                    Thanks
                    I'd have been happy with 80A fuses.

                    Comment


                      #11
                      Originally posted by iceworm View Post
                      That looks pretty good
                      Curiosity Q: Does the drive get the Type J fuses? If so, does Siemens specify fast or TD? Doesn't matter much - just curious.

                      Caution note - if you are under the NEC:
                      430 Part X, 430.122, conductors supplying power to the drive have to be sized at 125% of the rated drive input. Even if the motor load is smaller that the drive rating - size the conductors for the drive.
                      Siemens does not specify FA or TD, just type J and value.
                      Anyway, I will use circuit breaker for VFD protection instead.

                      Regarding conductors size, I chose 2AWG for VFD LINE and 4AWG for VFD LOAD.
                      Thanks

                      Comment


                        #12
                        With an input current of 46.8 amps you could use #6 conductors to feed the drive. (46.8*125% = 58.5amps)
                        Rob

                        Moderator

                        All responses based on the 2017 NEC unless otherwise noted

                        Comment


                          #13
                          Originally posted by infinity View Post
                          With an input current of 46.8 amps you could use #6 conductors to feed the drive. (46.8*125% = 58.5amps)
                          46.8A is a motor's rated current.
                          Input current of the VFD is 70A (70*1.25=87.5A) - 2AWG

                          Comment


                            #14
                            Originally posted by Alexal View Post

                            46.8A is a motor's rated current.
                            Input current of the VFD is 70A (70*1.25=87.5A) - 2AWG
                            Oops, yes you did say that in the OP. Sorry for the mix up.

                            #3 THHN is good for 100 amps.
                            Rob

                            Moderator

                            All responses based on the 2017 NEC unless otherwise noted

                            Comment


                              #15
                              If the drive says that you need Class J fuses per their UL listing and does not say you can use circuit breakers as an alternate, using the CB alone means you are not using it in the manner intended by the manufacturer and therefore violating the NEC.

                              Looked at the other way, the only way you can use a CB alone is if the drive mfr specifically states that this is an approved option. Some drive mfrs do, some do not. Almost any drive will pass the UL SCCR listing requirements when protected by current limiting fuses, but not all designs will pass with CBs alone, and some mfrs are unwilling to do the expensive destructive testing only to find out they will not pass. So they “bury the lead” in their installation/technical documents by simply stating that fuses work. As a general rule, if a mfr has gone to the risk and extreme expense of getting their drive UL listed behind CBs alone, they make that very clear because it gives them an advantage in the market over those that do not.

                              If you don’t feel this is stated clearly enough for you in the Siemens instruction manual, you should ask them for a specific response in writing to clear it up.
                              __________________________________________________ ____________________________
                              Many people are shocked when they discover I am not a good electrician...

                              I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

                              Comment

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