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500 HP on soft start trips out intermittently

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    #16
    Originally posted by 11bgrunt View Post

    500 HP on soft start trips out intermittently UPDATE.
    ...
    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.
    Perhaps the voltage drop across the starter from line-in to load-out on each phase could be measured when it's running to see if there's any noticeable drop from the bypass contactor. Hopefully the contacts can handle the current reliably but it could still be worth checking out.

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      #17
      Originally posted by 11bgrunt View Post

      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.
      Maybe something else is being turned on or off by the computer these 30-45 seconds later, and that causes a transient on the 480V lines with a high enough dV/dt to trigger the starter SCRs (the self-commutation effect that Jraef mentioned).

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        #18
        Originally posted by synchro View Post

        Maybe something else is being turned on or off by the computer these 30-45 seconds later, and that causes a transient on the 480V lines with a high enough dV/dt to trigger the starter SCRs (the self-commutation effect that Jraef mentioned).
        Self commutation will cause an SCR to go into conduction regardless of the hate so it shorted out the soft start. It does not trip it out, it blows it up! So it will do far more than just randomly trip.

        Hence all soft starts have or should have MOV surge arresters in them somewhere. They usually only have 2 or 4 SCRs per phase, a temperature sensor, the MOV, and some RC snubbers in the power stack side of things. Not uncommon for the snubber cap to fail over time either, You can easily ohm & megger a MOV to test it for failure as well as the caps.

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