Announcement

Collapse
No announcement yet.

VFD (Solving DC bus issue)

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    #16
    Way back in the dark ages when I got my feet wet with VFDs, my mentor had made an application manual for me and page one, in giant letters, said only “No Contactors, Clutches, or Crankshafts!” The reason for the No Crankshaft rule was precisely this issue, it’s very common that this would cause VFD over voltage stresses. Since then VFD technology has improved to allow for reciprocal load adaptation for sure, but the “No” is only modified to “Beware of” because not all drives are created equal.

    In my experiences with this issue and having to use resistors, I have used the resistor sizing criteria that assumes the “braking duty” is 25%, meaning that in 75% of each revolution the motor is either motoring, neutral or only slightly regenerating. That has worked for probably 90% of these applications I have come across (prior to the advent of the reciprocal load algorithms that come in better drives now). The other 10% were drives in which the braking TRANSISTOR was under sized (i.e. 10% duty cycle max, common in cheaper drives) or the application was so severe that I recommended using line regenerative drives rather than resistors. Now that Regen drives are becoming more readily available and less expensive (than they were), I haven’t used braking resistors in a few years. Even when sized right they are still a high failure point in a system.
    __________________________________________________ ____________________________
    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


      #17
      Originally posted by Jraef View Post
      Way back in the dark ages when I got my feet wet with VFDs, my mentor had made an application manual for me and page one, in giant letters, said only “No Contactors, Clutches, or Crankshafts!” The reason for the No Crankshaft rule was precisely this issue, it’s very common that this would cause VFD over voltage stresses. Since then VFD technology has improved to allow for reciprocal load adaptation for sure, but the “No” is only modified to “Beware of” because not all drives are created equal.

      In my experiences with this issue and having to use resistors, I have used the resistor sizing criteria that assumes the “braking duty” is 25%, meaning that in 75% of each revolution the motor is either motoring, neutral or only slightly regenerating. That has worked for probably 90% of these applications I have come across (prior to the advent of the reciprocal load algorithms that come in better drives now). The other 10% were drives in which the braking TRANSISTOR was under sized (i.e. 10% duty cycle max, common in cheaper drives) or the application was so severe that I recommended using line regenerative drives rather than resistors. Now that Regen drives are becoming more readily available and less expensive (than they were), I haven’t used braking resistors in a few years. Even when sized right they are still a high failure point in a system.
      We've put regen VFDs on paper machines where there can be an overhauling section or two but mostly we fed them from a common DC buss and the net power requirement was positive or out if you like. We did have dynamic braking for use in the event of an emergency stop.

      We need more information from the OP before we get into too much speculation here...............

      Comment


        #18
        I suspect the problem was solved three days ago. Another job well done!!
        Tom
        TBLO

        Comment


          #19
          Originally posted by ptonsparky View Post
          I suspect the problem was solved three days ago. Another job well done!!
          someone likely changed a bunch of parameters and it stopped tripping. no one will ever know why it stopped tripping though.
          Bob

          Comment


            #20
            Originally posted by ptonsparky View Post
            I suspect the problem was solved three days ago. Another job well done!!
            Maybe the OP would have the good grace to confirm that......

            Comment

            Working...
            X