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Mounting of VFD Braking Resistor

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  • Mounting of VFD Braking Resistor

    My braking resistor arrived today so I plan on installing it for the mill VFD tomorrow. My control box is PVC. Can I mount it to the outside of the box with a couple of metal washer standoffs?


  • #2
    A little strip of aluminium with a small gap from the enclosure will help heat transfer and save the pvc
    Helder Ferreira
    Setubal, Portugal

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    • #3
      Yup.
      If you want to be code compliant you will have to cover up the open wiring too, easiest way is to use 1/4 or 3/16" expanded metal to enclose the entire thing.
      Better to mount it vertically too, then dust wont build up on it.
      Cheers,
      Jon

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      • #4
        Originally posted by Noitoen View Post
        A little strip of aluminium with a small gap from the enclosure will help heat transfer and save the pvc
        Thanks, I have just the piece--About 2" x .080" and enough for the length of the resistor. I just used a piece of it to make a new band saw table plug.

        ADDED:
        He, he, he... We don't have no codes out here. But I'll run the leads through a length of fiber covering and through the box.
        Last edited by CCWKen; 04-11-2018, 06:28 PM.

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        • #5
          What does the drive manufacturer say?

          Mounting it to a strip of Ali is OK, but if you actally want it to do anything as far as heat dissipation etc, it needs to be bigger than the resistor. Many of the resistors like that one are almost 2" wide already.. if it is the same width and length, there is nearly no need to bother, its net effect is only to add a little mass to be heated up.

          Some VFD makers provide a spot on the heatsink for the braking resistor. If they do, and that one fits the location, that is the best place for it.
          CNC machines only go through the motions

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          • #6
            The MFG only gives the resistor values for the models. I couldn't find anything else.

            The plan was to stand the strip off the box with a couple of washers, add two more pairs of washers then the resistor. As I understood Noitoen's post, the strip will not be a heatsink but merely a shield.

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            • #7
              Originally posted by CCWKen View Post
              The MFG only gives the resistor values for the models. I couldn't find anything else.

              The plan was to stand the strip off the box with a couple of washers, add two more pairs of washers then the resistor. As I understood Noitoen's post, the strip will not be a heatsink but merely a shield.
              Yes, it may or may not even be necessary to do that. A single level of standoffs might have done as well The resistor will only get hot when there is heavy active braking, i.e when the DC bus goes up above the trip point. Without having some understanding of the braking duty cycle, which you probably have no way of estimating, it's guesswork.

              You probably have a setting to vary the allowed duty on the resistor, and with the resistor sitting out in the air alone, I'd keep that at the minimum you can.
              CNC machines only go through the motions

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              • #8
                There's nothing called "braking duty cycle". I can set several parameters used for deceleration such as DC Braking Frequency, Current, Wait Time (delay) and Time. I can choose the basis of the braking frequency using maximum, display setting or a percent of maximum frequency. This is not a production machine so I'm setting all the values to maximum frequency and 100% with no delay. I'll experiment with the brake time initially set to .5 seconds.

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                • #9
                  Are you confusing DC INJECTION braking with DYNAMIC braking? The former injects DC in the motor to stop it; nothing to do with the braking resistor.

                  With dynamic braking the time it takes to stop will depend on the inertial mass, the brake resistor ohms, and what else your vfd has to do to keep the regenerative voltage in check. 100 ohm resistor is likely not as aggressive as you could have gone, but duty cycle comes into play not so much as to what you "set" but how much you use it and from what speeds. I use 35 ohm on 3hp motor and on my mill I cannot brake quickly (2-3 seconds) above 4000 or so as the cycle has to be extended to to keep the buss voltage in check.

                  I can stop it on a dime with DC injection, but that's a real good way to fry a motor unless you have thermister integrated with the motor and VFD (I don't).

                  Exactly what model VFD do you have?
                  Last edited by lakeside53; 04-11-2018, 09:22 PM.

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                  • #10
                    Keep an eye on the temperature of the PVC box and the joining screws/washers. If it gets hot enough the screws may pull through.
                    At the end of the project, there is a profound difference between spare parts and left over parts.

                    Location: SF East Bay.

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                    • #11
                      Originally posted by danlb View Post
                      Keep an eye on the temperature of the PVC box and the joining screws/washers. If it gets hot enough the screws may pull through.
                      Which is why to keep the resistor duty low, if the setting exists.

                      Choice #2... use a bit longer bolt, and put a fender washer or two right in the middle, with a nut on each side.... heatsink for the bolt.
                      CNC machines only go through the motions

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                      • #12
                        I don't know what it is. I was getting an error code at higher spindle speeds with a 4-second deceleration and the manual says I need a resistor. I bought a resistor. The VFD also has DC braking options. I assumed it was one and the same. I'd like to get the stop time down for power tapping.

                        I was just looking at the manual and all the DC braking options have default settings of "0". So I guess it's currently set for dynamic braking. I never tried setting any of those. Is it either exclusively or can both be used?

                        Oh yeah, it's a KOC100 Model 1R5S2.

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                        • #13
                          Parameter bb-30, page 54
                          CNC machines only go through the motions

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                          • #14
                            How did you find that so fast? The default is 100%. (With or without the resistor.)

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                            • #15
                              Originally posted by CCWKen View Post
                              How did you find that so fast? The default is 100%. (With or without the resistor.)
                              I knew exactly what I was looking for. I've programmed VFDs and designed them.

                              Hint: I also downloaded the PDF, and used the advanced search on the word "brake".... MUCH easier than slogging through the chinglish.

                              I am not certain, but it sorta looks as if that company may be the new incarnation of "Huanyang".... Maybe they have cleaned up their act.
                              CNC machines only go through the motions

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