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500 HP on soft start trips out intermittently

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    500 HP on soft start trips out intermittently

    What I have gathered so far;
    500 HP motor trips out at the soft start with no rhyme or reason.
    POCO serves the mill with a 2500kVA pad, 480Y/277 to an 800 amp main. That panel has a breaker feeding an AB soft start. When the soft start does trip out, the only way to get it back on is to kill the circuit and wait 15 minutes. This circuit was built 4 months ago and will run weeks at a time with no problems.
    Today we connected a PQA at the soft start with the 500 HP motor running doing his job and recorded for 20 minutes. It did not trip today.
    While we were connecting the meter, the drive rattled for a couple seconds like an old contactor failing. Nothing happened and it was just a couple seconds.
    The first thing I see in the recording graphs is the currents are not close.
    In the currents minimum column, L1-378, L2-464, L3-434.
    In the currents maximum column, L1-462, L2-546, L3-514.
    Voltage measurements look very good.
    In the old days I would lean toward the motor.
    Can a soft start do this to the currents? AB has been difficult to work with so far. This is a $20K+ drive.
    Looking for a direction.
    Thanks,

    #2
    Some questions,

    does soft start control just two or all three lines during starting?

    Does it go into full bypass (via contacts) after starting or is there still solid state components in the circuit during run time?
    I live for today, I'm just a day behind.

    Comment


      #3
      I can tell you exactly what that is. Your motor load is exceeding the rating of the soft starter. On an SMC Flex (assuming that’s what it is), if your motor current exceeds the rating of the bypass contacts, or they over heat, it switches back to the SCRs. Then if the current drops OR the current is too high for the SCRs too, it goes back to the bypass contacts. If you are grossly exceeding the ratings of both, it chatters back and forth, which is that sound you heard.

      What is is the nature of the load / machine? The spikes in current may be happening faster than your meter can see them. I’ve seen this happen on a 500HP rock crusher application, where bigger than normal chunks would drop into the crusher and spike the current really high and fast, but it would not show on a standard meter. We could only see it with a scope using a high speed trigger. But the electronics in the Soft Starter could see them and react to them.

      There is a parameter in the SMC Flex that lets you adjust the response to this situation. That didn’t work in my situation because the spikes were way too high and it would still trip on OC. But it did help it not to chatter back and forth on the bypass so much.

      The other option on this is to add a full blown Across-The-Line rated bypass contactor that closes and stays closed once it’s running. I didn’t have room in the box for that option, I had to put in a 600HP rated Soft Starter.
      __________________________________________________ ____________________________
      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


        #5
        You mentioned good voltage measurements but how close were the line voltages? Your current unbalance might be caused by as little as ~2% voltage unbalance.
        If practical in this application, shifting the three line inputs to motor and re-checking the currents could help determine whether motor is causing observed unbalance.
        ​​​​​​

        Comment


          #6
          You mentioned good voltage measurements but how close were the line voltages? Your measured current unbalance might be caused by as little as ~2% voltage unbalance.
          If practical in this application, shifting the three line inputs to the motor and re-checking the currents could help determine whether motor is causing observed unbalance.
          ​​​​​​

          Comment


            #7
            synchro
            PQA results show voltage diff less than 1%.
            Moving phases over one spot is tried and true. I have used on across the line starts, but wanted input from this forum before I asked for a clearance.

            Comment


              #8
              Originally posted by synchro View Post
              You mentioned good voltage measurements but how close were the line voltages? Your measured current unbalance might be caused by as little as ~2% voltage unbalance.
              If practical in this application, shifting the three line inputs to the motor and re-checking the currents could help determine whether motor is causing observed unbalance.
              ​​​​​​
              500 HP on soft start trips out intermittently UPDATE.
              In the last 9 weeks, the POCO has monitored the voltage with a PMI recorder and believes they are in the clear.
              AB tested everything and agreed to replace the starter. That was done and the 500HP ran with no problems for 3 weeks. Everything from starter to motor was megged by AB.
              Now the pattern is the same as before. Some days the soft start will trip, some days no trip.
              Today a new complaint in the same mill with the same POCO supply.
              Ten motors, computer controlled with starters that have thermal trip heaters. When the mill computer turns those ten motors OFF, the starters trip 30-45 seconds later. The starters have no load.
              Voltage was checked at the 500 starter, 277-279, 484 across all three.
              When the voltage was checked at the ten little starters, five of ten had a high B phase. The other five did not and had balance voltage.
              I plan to be there tomorrow with everything I own.
              I have not seen a starter trip with no load before but I haven't been everywhere and seen everything.

              Comment


                #9
                Do you or the POCO have power factor correction capacitors in use?
                Tom
                TBLO

                Comment


                  #10
                  Originally posted by ptonsparky View Post
                  Do you or the POCO have power factor correction capacitors in use?
                  When the mill is running, the POCO primary is lag 98%. When the mill load falls off, the POCO static capacitors take line to lead 20-40%.
                  I have not seen PF correction in the mill.

                  Comment


                    #11
                    Originally posted by 11bgrunt View Post

                    When the mill is running, the POCO primary is lag 98%. When the mill load falls off, the POCO static capacitors take line to lead 20-40%.
                    I have not seen PF correction in the mill.
                    A leading PF of 20 - 40%? Doesn’t that create considerable overvoltage?
                    Tom
                    TBLO

                    Comment


                      #12
                      Originally posted by ptonsparky View Post

                      A leading PF of 20 - 40%? Doesn’t that create considerable overvoltage?
                      An overvoltage on the primary has not been reported. The POCO does have primary regulators maintaining voltage for the main line the mill is on.

                      Comment


                        #13
                        Just a thought based on a couple of other experiences I’ve had with soft starters. On the ten “little” starters that turn off and cause the soft start to trip, are they using 480V coils by any chance? I have seen soft starters falsely triggered to fire their SCRs at the wrong time (called “self commutation”) by the inductive kickback of 480V contactor coils even on Size 1 starters. When the SCRs self commutate like that, they fire at the wrong time respective of each other, which causes a spike in current and the soft starter electronics react to it, often even if the soft starter is not running the motor. The solution was simply to add surge suppressors to those other starter coils.
                        __________________________________________________ ____________________________
                        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


                          #14
                          Originally posted by Jraef View Post
                          I can tell you exactly what that is. Your motor load is exceeding the rating of the soft starter.
                          If so, would it run for weeks at a time with no problem? Anyway, at that sort of rating (375kW) I would have expected it to have a bypass contactor.

                          Comment


                            #15
                            Originally posted by Jraef View Post
                            Just a thought based on a couple of other experiences I’ve had with soft starters. On the ten “little” starters that turn off and cause the soft start to trip, are they using 480V coils by any chance? I have seen soft starters falsely triggered to fire their SCRs at the wrong time (called “self commutation”) by the inductive kickback of 480V contactor coils even on Size 1 starters. When the SCRs self commutate like that, they fire at the wrong time respective of each other, which causes a spike in current and the soft starter electronics react to it, often even if the soft starter is not running the motor. The solution was simply to add surge suppressors to those other starter coils.
                            All holding coils for these starters are 120 volts from a large dry type delivering 208-120 Y.
                            The soft start does have a bypass contactor.

                            Comment

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