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VFD Braking Resistor

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  • #16
    Sorry for coming into this late but I happen to have a Clausing 5914 lathe with 2 HP motor that is powered by a Freqrol FR-A220E-2.2K VFD. So far as I know the VFD has no braking resistor and I've had no problems running the lathe & VFD over the past two years. The lathe still has the OEM vari-drive system.

    What would the braking resistor be useful for?

    Mike
    Mike Henry near Chicago

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    • #17
      Mike, the braking resistor is helpful when crisp decels to a stop is desired. Most machine tools have enough power robbing clutter (gears in mesh, belts etc) to stop the spindle in an unsassisted coast in a reasonable time.

      As ususal I got off on a purist rant over VFD resistors which led us into another debate that served us poorly as practical points for the home shop machinist to consider.

      If you set your VFD to decel in two seconds or less from full spindle speed chances are you'll get an "OV" trip. This means the kinetic engegy of the rotating parts was regenerated as electrical power by the switching ctansistors in the VFD and stuffed back into the DC buss. The VFD has a protective circuit that senses this excess voltage and shuts down the drive.

      If your machine works fine as is, don't fix it.

      [This message has been edited by Forrest Addy (edited 08-13-2005).]

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      • #18
        However, do not be surprised if someday when you have a heavy thing on the faceplate, or the like, you do get an OV fault.

        It is the energy received from the load (the motor) that determines the bus voltage, and how fast it arrives. If you have low mains voltage, you may NOT get an OV fault when if you had higher voltage you would.... if you start low, you have "farther to go".

        For the technically inclined, the energy returned from the motor will appear as excess charge (and thus voltage), on the bus capacitors. The bus voltage varies as the square root of the received energy, since energy in a capacitor (Joules, or watt-seconds) is (C* V^2)/2 where C is the capacitor value.

        So it is perfectly possible to never have an OV fault and still have braking happening, if you don't need to remove much energy. The excess charge is soon used up by powering the motor.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #19
          Well, after searching the internet, Ebay and local electronics shops, I ended up with a 50 OHM 220 Watt wirewound resistor. Not 40 OHM like the manufacturer recommends but I hope it will work. I did have a choice of a 33 OHM 500 watt resistor but it was about 1 foot long.

          Since the 2HP VFD required a 60 OHM and the 5HP VFD required a 40 OHM I figured that I'm only using a 3HP Motor a 50 OHM was probably OK - do you agree?

          Thanks,

          Jeff

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          • #20
            Sorry about the late reply,
            I was eyeballing the inner workings of a Haas
            toolroom lathe and there was what I believe to be an electric stove element in a metal box with air holes punched in it, I dont think the element will ever get hot enough to significantly alter it's resistance from coast down current, used as a stove then yes.

            opinions are like _ _ _ holz we all have one, unless we have children or ex wives then maybe more than one!

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            • #21
              Just as a reminder.........

              The switch element in the VFD is designed to carry the braking resistor current, at the relatively low duty cycle required.

              Since they are designed to a price, and the market is extremely competitive, they are no bigger than they have to be, and have no more heatsink than required.

              If you use a resistor lower in resistance than the manufacturer recommends, OR it is temperature variable, and is too low when cold, you may blow the braking switch element (probably a lower current mosfet). Then you have NO braking resistor, and many VFDs are not very repairable.

              The stove element may be perfect, IF you know the resistance, and that resistance is right for your VFD and load.

              But, it won't be right for a small VFD, almost certainly.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #22
                Forrest,

                OK - sounds like I am fine on the lathe. A partially operational horizontal mill with VFD has been giving me periodic faults, though so now I have a clue on what to check on that one. The VFD was a used model, so it probably has the wrong settings for the mill motor.

                Mike
                Mike Henry near Chicago

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