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    4160 Isolation Transformer

    I have a question regarding a 4160 isolation transformer.

    The current setup is 13.8kv to 4160 step down transformer feeding a VFD. The VFD is then fed to an isolation transformer, then to a 4160V motor. There is also a bypass for the VFD but i am not sure how it is connected.

    The issue is, i looked at the plate of the transformer and it said 4160-2300.... I am 99% sure it is feeding 4160.

    Also i believe we are hooked up to BOTH the secondary feeds off this transformer. It has a primary feed delta, then one Delta and one Wye secondary. I believe both are hooked up and then only one feed hits the motor.

    Has anyone seen something like this?

    I am replacing the VFD and will be removing the isolation transformer as it is now not needed. I am removing the bypass switch and just going 13.8kv to 4160 to VFD to Motor. I am concerned that i plan on putting 4160 on the motor. The motor is rated for it but could be wired differently.


    Transformer plate
    https://i.imgur.com/ImAw46Z.jpg
    Last edited by spikes2020; 11-29-17, 11:07 AM.

    #2
    I believe the 2 secondary feeds are going into some starter/bypass that we have never used. So if the vfd took a dump we could bypass it and still start the motor.

    I am just looking for if anyone knows kind of what is going on or seen something like this and could explain it better.

    Comment


      #3
      sometimes xfmrs like that are used for vfd isolation
      I've seen one like this:
      prim 4160 delta
      sec 3 x 1400 deltas (wire to supply 4200)

      I guess yours are wired for the sec delta/wye to produce ~ 4 kv
      it's done for harmonics

      I can't see the nameplate pic, it would help

      Comment


        #4
        Originally posted by Ingenieur View Post
        sometimes xfmrs like that are used for vfd isolation
        I've seen one like this:
        prim 4160 delta
        sec 3 x 1400 deltas (wire to supply 4200)

        I guess yours are wired for the sec delta/wye to produce ~ 4 kv
        it's done for harmonics

        I can't see the nameplate pic, it would help
        The nameplate is there, just not very clear.
        But you may well be correct about harmonic mitigation.
        It could be a Ddyn11 to eliminate lower order harmonics.
        Si hoc legere scis nimium eruditionis habes.

        Comment


          #5
          I found some drawings, this thing is from like 1990 so the drawings are a bit old and were hard to find. It all makes sense now!

          It appears that there is a bypass 4160 strait to the motor. The "isolation transformer is to generate 2 feeds at different phase angles from each other that's why one is 'wye' and the other is a 'delta'. Then the VFD uses these 6 feeds to build a stair step wave form to the motor.

          This VFD and transformer is specialy made for each other. My new VFD uses pulse witdth modulation, instead of the stair step method. Thus i wont need this transformer.

          I guess you learn something new every day~!

          Edit

          Here is the drawing i found
          https://i.imgur.com/hoHYh6i.jpg
          Last edited by spikes2020; 11-29-17, 02:33 PM.

          Comment


            #6
            Originally posted by spikes2020 View Post
            I found some drawings, this thing is from like 1990 so the drawings are a bit old and were hard to find. It all makes sense now!

            It appears that there is a bypass 4160 strait to the motor. The "isolation transformer is to generate 2 feeds at different phase angles from each other that's why one is 'wye' and the other is a 'delta'. Then the VFD uses these 6 feeds to build a stair step wave form to the motor.

            This VFD and transformer is specialy made for each other. My new VFD uses pulse witdth modulation, instead of the stair step method. Thus i wont need this transformer.

            I guess you learn something new every day~!
            The wye and delta windings are to reduce input harmonics. Your "new" PWM output won't change that.
            You need to understand this important point before any money goes into the project.
            Si hoc legere scis nimium eruditionis habes.

            Comment


              #7
              Originally posted by Besoeker View Post
              The wye and delta windings are to reduce input harmonics. Your "new" PWM output won't change that.
              You need to understand this important point before any money goes into the project.

              I see what your saying the wye/delta wingdings are to reduce the harmonics interceded on the input side of the transformer. I would assume that isn't as bad if the drive is designed correctly with internal capacitors and filters. Also we have an 35MW electric arc furnace so it wont be the largest generator.

              Comment


                #8
                Originally posted by spikes2020 View Post
                I see what your saying the wye/delta wingdings are to reduce the harmonics interceded on the input side of the transformer. I would assume that isn't as bad if the drive is designed correctly with internal capacitors and filters. Also we have an 35MW electric arc furnace so it wont be the largest generator.
                I don't know what your drive configuration is. Most VFDs comprise an input rectifier, six pulse for the most part, twelve pulse for larger units, a DC link filter, and a PWM output stage.
                Si hoc legere scis nimium eruditionis habes.

                Comment


                  #9
                  What your drawing is showing there is an old Current Source Inverter (CSI). In that technology, the GTO SCRs on the front-end rectifier would have created a lot of line notching and noise, so the isolation transformer was crucial in the design. The 2 offset secondaries was for harmonic mitigation, what's called a "12 pulse" rectifier.

                  Most modern MV VFDs are now going to be either Active Front End CSI drives, which don't need the input transformer, or Cascaded H Bridge Voltage Source Inverters (VSI), which will have a 24 pulse transformer built-in. There are still a few people using VSI designs with passive rectifiers and 12 or 18 pulse front-ends, but they are falling out of favor now. but even those will often use an autotransformer type design, the isolation is no longer as important as it was in those old designs.

                  But regardless, if your feed is 13.8kV, you will end up with an isolation transformer no matter what.
                  __________________________________________________ ____________________________
                  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


                    #10
                    Originally posted by Besoeker View Post
                    I don't know what your drive configuration is. Most VFDs comprise an input rectifier, six pulse for the most part, twelve pulse for larger units, a DC link filter, and a PWM output stage.
                    I do understand how it works, and i am saying that 1MW motor harmonics don't compare to a 35MW arc furnace. But also the DC bus on both the old drive and the new drive are designed completely different. Our old drive did not have any capacitors, while the new drive will. This will reduce the harmonics generated by the drive. Also Jraef response about how they work kind of lines up with my thoughts that it is more of an active front end to control harmonics better.

                    Comment


                      #11
                      Originally posted by Jraef View Post
                      What your drawing is showing there is an old Current Source Inverter (CSI). In that technology, the GTO SCRs on the front-end rectifier would have created a lot of line notching and noise, so the isolation transformer was crucial in the design. The 2 offset secondaries was for harmonic mitigation, what's called a "12 pulse" rectifier.

                      Most modern MV VFDs are now going to be either Active Front End CSI drives, which don't need the input transformer, or Cascaded H Bridge Voltage Source Inverters (VSI), which will have a 24 pulse transformer built-in. There are still a few people using VSI designs with passive rectifiers and 12 or 18 pulse front-ends, but they are falling out of favor now. but even those will often use an autotransformer type design, the isolation is no longer as important as it was in those old designs.

                      But regardless, if your feed is 13.8kV, you will end up with an isolation transformer no matter what.

                      Ah thanks! yes this is what was thinking after i found the drawing. Yes we have 2 transformers for each motor. 1 goes from 13.8 to 4160 and then the second one, 4160 to 2 feeds of 2300. I am happy to remove 1 of the transformers to just leave the 13.8/4160. This means one less transformer to test, keep a spare and drain its secondary containment.

                      Comment


                        #12
                        Originally posted by spikes2020 View Post
                        I do understand how it works, and i am saying that 1MW motor harmonics don't compare to a 35MW arc furnace. But also the DC bus on both the old drive and the new drive are designed completely different. Our old drive did not have any capacitors, while the new drive will. This will reduce the harmonics generated by the drive. Also Jraef response about how they work kind of lines up with my thoughts that it is more of an active front end to control harmonics better.
                        Will your new drive have an active front end?
                        Si hoc legere scis nimium eruditionis habes.

                        Comment


                          #13
                          Originally posted by Besoeker View Post
                          Will your new drive have an active front end?

                          I sure hope so . Rockwell is the one supplying it, they understand what is supplying the drive and what the drive is feeding. So they didnt seem to have any concerns.

                          Comment


                            #14
                            Originally posted by spikes2020 View Post
                            I sure hope so . Rockwell is the one supplying it, they understand what is supplying the drive and what the drive is feeding. So they didnt seem to have any concerns.
                            Are you the project manager?
                            Si hoc legere scis nimium eruditionis habes.

                            Comment


                              #15
                              I am a little confused; so there is a 13.8kV to 4160V transformer feeding a VFD and the output of VFD is connected to a phase-shift transformer powering a "motor"? Shouldn't the phase shift transformer be connected to the 12 pulse rectifier and not its inverter?
                              "Because it's there!"
                              George Mallory

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