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  • VFD Braking Resistor

    Hello,

    I'm in the processing of converting a Clausing 5904 lathe to a VFD since the Vari-drive was shot and once I fixed it had a lot of vibration. I have aquired a few 2HP VFD and one 5HP VFD.

    2HP - Mitsubishi Freqrol Z024-1.5K 240VAC
    5HP - Mitsubishi Freqrol Z024-3.7K 240VAC

    I plan to use a 3HP Baldor Motor that my son gave me so do you have any help on what and where to get the braking resistor? I read that Forrest said that these are rip off if purchased from the manufacturer.

    The manual says for the 2HP unit MRS 120 W 60 and for the 5HP unit MRS 120 W 40

    Thanks,

    Jeff

  • #2
    Use Ohmite or equivalent power resistors from Newark or DigiKey for braking resistors. They're a little more of a PITA to install but at $10(?) for a 50 watt resistor it beats the $150 factory ones hollow. If need be, get an adjustable resistor a bit too large and adjust it with the slider to the factory reccommendations.

    "The manual says for the 2HP unit MRS 120 W 60 and for the 5HP unit MRS 120 W 40" puzzles me. 120 W = 120 Watt? 40 = 40 Ohms? for a 5 HP VFD? Sounds close. A call to the maker's customer service/tech support desk will clear that up.

    Comment


    • #3
      Is a braking resistor always required? I have an AC Tech VFD on a 20" bandsaw, and I was going to implement the braking protocal, but haven't gotten around to it.

      Maybe that's a good thing?

      Ed
      Ed Bryant

      Comment


      • #4
        Apparently it's not required unless you want to enable DC braking on the drive, which you'd want on the lathe (although mine has a threaded spindle,) but on a bandsaw I guess it's not as important.

        I've got a Delta VFD drive and tried braking without the resistor. Of course, once there was enough braking force (it would take a little bit,) it errored out on the DC bus exceeding the configured maximum, which I had tweaked even lower than the default just out of paranoia. Thank goodness for internal protections/shutoffs.

        Who was it, if I recall, someone on here (or there) mentioned that a heater coil for an electric water heater could do the trick in a pinch. I was looking twice at them in the hardware store, and wondering how close each one was to the properties required in the VFD specs. It definately would be nicer to just go and pick one up rather than having to order it and wait for the shipping you just paid for. But of course, when you need the right part, you gotta get what you need.

        Anything I've seen listed very specifically as DC braking resistors for VFD drives is very expensive. Is there anything special about these that adds to their performance in a VFD application, or would any standard component do the job to equal satisfaction? If there's any difference in the components, I'd hope the expensive one would have a longer lifetime.

        Comment


        • #5
          Try it first.
          Most VFD's have a programable braking cycle built in, usually from 0.1 to 10 seconds that will do what you need.
          Braking resistors usually are only needed f the time has to be short or the load is larger than the VFD can handle within a time limit.

          From experiance and we have fitted literally 100's of these thing to our machines don't go below 1.0 - 1.5 seconds on the braking.
          If you do there is a very good chance that back EMF will cause a trip error and require a reset.
          Stayig about 1.5 seconds will save a lot of hassle.

          [EDIT]
          Riobyguy posted whilst I was writing this with the same results.

          Also remember that sudden stops under a braking resistor will subject the machine to loads it was never designed to accept and will cause wear on keyed shafts and splines.

          Do a web search and look for the case where the H&S made a lumber company fit braking to a big bandsaw. This was a company with a 100% safety record. The machine had run fine for years. After the braking module was fitted against the wishes of the company it broke a 4" drive shaft and killed the operator.

          John S.

          [This message has been edited by John Stevenson (edited 08-10-2005).]
          .

          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



          Comment


          • #6
            Don't just "decide" what size (i.e. value in ohms and wattage) resistor to put on....

            The whole purpose of the resistor is to remove the "pump-up" charge from the rectified DC bus in the VFD. It is only connected during braking, and ideally only if the DC bus voltage gets too high.

            IF you use too low a value, you just add load that draws current from the power line thru the VFD rectifiers. As we all know, those are sized just right, because the thing is a commercial product that has to be made to a cost.

            Too much current, and you fry them. I believe someone here did that.

            There is a "best value" (ohms) based on the size of the motor load. Normally that will be given somewhere in the unit manual. With that known, a required wattage can be figured out.



            [This message has been edited by J Tiers (edited 08-10-2005).]
            1601

            Keep eye on ball.
            Hashim Khan

            Comment


            • #7
              <font face="Verdana, Arial" size="2">Originally posted by Ryobiguy:
              Apparently it's not required .....

              Anything I've seen listed very specifically as DC braking resistors for VFD drives is very expensive. Is there anything special about these that adds to their performance in a VFD application, or would any standard component do the job to equal satisfaction? If there's any difference in the components, I'd hope the expensive one would have a longer lifetime.
              </font>
              I suspect that the OEM parts are exactly sized for the specific VDF/motor. Most generic high power resistors come in a few standard sizes that are not going to hit that spec very closely. So if the manufacturer buys a resistor of some in between value, he has to pay for a production run. Then he has all the costs of stockage for a period of years and the costs of processing orders. Their prices may just reflect all those legiminate costs.

              A resistor is a heating element and a heating element is a resistor. Pure and simple. Absolutely no difference electrically. Only the external configuration is different.

              I doubt that there is anything else that is "special" about them. As others have stated, you can buy a variable resistor and adjust it to the correct value. But remember that when you do this, you are reducing BOTH the resistance and the power that it can handle. The power derating is linear. So a 100 Ohm, 100 Watt resistor that is adjusted to 80 Ohms will also be reduced to 80 Watts. And if it was adjusted to 1 Ohm, it would only be capable of dissipating 1 Watt.

              For some values a series or parallel combination of fixed resistors may be a better way to go. But the power calculations can be more difficult in these cases if the resistors are not equal in value.

              Paul A.
              Paul A.
              SE Texas

              Make it fit.
              You can't win and there IS a penalty for trying!

              Comment


              • #8
                YES! You want breaking. You will love it.
                The factory units are nothing special and extremely over priced.
                I made my own out of [email protected] of 12 Watt resistors, all wired in parallel and I was able to nail the exact value needed.
                If I remember... I needed about 350 watts.
                Used a couple of pieces of un-etched circuit board material about 3" square, drilled 30 holes in each (5x6 about 0.5" apart) made a resistor sandwich. Added a couple of aluminum angles for mounting. Made a nice compact unit that stays cool.
                This was a lot cheaper than the wire-wound Vitreous Power Resistors.
                Don't forget, if you use the adjustable wire-wounds, derate the wattage by the amount of adjustment.

                Tom M.

                Comment


                • #9
                  I belive J Tiers is speaking of resistance.

                  The VFD need to "see" sufficent resistance to keep current low enough.

                  Wattage only protects the resistor.

                  I tried using a 1500 watt water heater element without thinking it thru. 1500 watts @ 240 volts is under 7 ohms. Element turned red and the VFD's transister is shot. No more external BR for that unit :-(

                  [This message has been edited by joahmon (edited 08-10-2005).]

                  Comment


                  • #10
                    At Forrest's request I talked to the manufacturer. They said I need and 40 OHM 300 Watt for the 5HP and a 60 Ohm 300 Watt for the 3HP VFD. He said that the Watts are not that important if you using occastionally but I've found a 40 OHM that is only 25 watts.

                    Any other help.

                    Comment


                    • #11
                      I like the idea of using multiple standard resistors, maybe I'll go that way.

                      Lets see, who's in my surplus electonics bookmarks... $4.50 for a 125 ohm 160W resistor:
                      http://www.goldmine-elec-products.co...em=10&mitem=11

                      Is power dissipation additive when several are put in parallel? Geez, maybe I should go read my basic electronics books again.

                      Comment


                      • #12
                        <font face="Verdana, Arial" size="2">Originally posted by Ryobiguy:
                        I like the idea of using multiple standard resistors, maybe I'll go that way.

                        Lets see, who's in my surplus electonics bookmarks... $4.50 for a 125 ohm 160W resistor:

                        Is power dissipation additive when several are put in parallel? Geez, maybe I should go read my basic electronics books again.
                        </font>
                        It is an excellent idea, if you have room and can make the resulting thing look decent and be safely covered up (it's at line voltage, never mind the 380V DC bus)

                        The power is additive among a connection of identical value parallel resistors, yes.

                        Since teh resistor SHOULD only be connected whe braking is operating, it does not have to handle as many watts as if it were always connected to the bus. BUT that is already taken into account in teh wattage.... so don't cut it very far....

                        A 60 ohm resistor on a 360V bus draws around 2100W, so using a 300W part is already assuming a derating for "duty cycle" (short time on, long time off).

                        [This message has been edited by J Tiers (edited 08-10-2005).]
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          For them that's unfamiliar with the workings of VFD's, the dynamic brakes of a VFD (as was pointed out earlier) is a gadget for dissipating the charge the switching transistor dumps unto the DC buss as heat through a resistor. The regenerated power charges the filter sapacitors raising the voltage across their terminals.

                          The dynamic brake has a transistorized chopper that senses the DC buss voltage and cuts the braking resistor in and out of the circuit. The chopped DC energises the resistor to burn up the excess charge as it builds up in the filter caps keeping the buss voltage to a safe level.

                          Sizing this resistor is like selecting a lever and placing a fulcrum to move a heavy weight. It's a matter of some judgement (actually calculation) to get the value right. Fortunately the factory had to figure all this out when they designed the VFD so go by theuir values.

                          If you have the right resistance and enough wattage for your machine's braking duty cycle it really doesn't matter what you use for a resistance; light bulbs, stove elements, ceramic coated wire wound resistors, or something inventive so long as it's safe.

                          Comment


                          • #14
                            <font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:

                            If you have the right resistance and enough wattage for your machine's braking duty cycle it really doesn't matter what you use for a resistance; light bulbs, stove elements, ceramic coated wire wound resistors, or something inventive so long as it's safe.
                            </font>
                            True....mostly.....but......

                            One possible issue.... non-resistors, i.e. things that are not intended to be resistors of a certain value, meaning light bulbs, stove elements, etc, may have a variable resistance.

                            Their resistance 'cold" may be quite different from their resistance "hot". Usually they are HIGHER resistance when hot than when cold.

                            This may be a problem for the switch elements in the VFD. They are designed for a certain current, and are made to a price....meaning they didn't put in much extra capacity.

                            A light bulb, for instance, may have a resistance "cold" which is 1/10 (or less) of the resistance when fully lit up. So when the internal switch component turns on the current, there may be a LOT more than expected.

                            I don't know about stove elements, I hate electric stoves.... I would imagine they do change resistance somewhat, no reason they shouldn't, its cheaper not to make them have a stable resistance, and they are made to a price......

                            If your "resistor" is supposed to draw 5 A, and it actually draws 50A as a starting surge, things in the VFD can be messed up.

                            I'd recommend using the right stuff, namely a resistor or an array of resistors with a known resistance value, and suitable wattage........ you have been warned.....

                            You can get resistors from DIGI-KEY, or Mouser electronics, google them for the URLs.

                            EDIT:
                            A "real resistor" is made of materials with a very stable resistance over temperature. They have to be, a 10% tolerance resistor can't vary more than that with temperature or any other factor.


                            [This message has been edited by J Tiers (edited 08-11-2005).]
                            1601

                            Keep eye on ball.
                            Hashim Khan

                            Comment


                            • #15
                              Jerry is right about hot resistance (I hate it when Jerry's right). It is a factor if the resistor develops significant heat in service, particularly if the resistor in question has a low time constant as would an array of light bulbs.

                              Most machine tools have little more kinetic energy of rotation than the armature, pulleys, and input shaft except when running in their higher gears. My 5 HP 17" engine lathe's worst case is when I have the 10" three jaw mounted. A single three second stop from 2000 RPM causes the pair of 100 watt ceramic wire wound resistors on its VFD to warm up about 80 degrees over ambient. Obviously YMMV.

                              All common resistance materials have a positive temperature coefficient; their resistance goes up with temperature. Most VFD makers offer several Watt ranges of dynamic braking resistors depending on the braking service required by the application.

                              An extreme case would be a centrifuge in a sugar mill, a water extractor in a laundry, or a long speedy conveyor loaded with weighty bulk material. Here the dynamic brake may have to dissipate over a span of minutes whatever kinetic energy the motor's accelleration has built up in the load. So a VFD dynamic braking resistor in a 5 HP machine tool could be safely rated at 300 watts whereas an extreme application might require a 10,000 watt resistor. Same resistance; mucho more kinetic energy to dissipate.

                              Some fancy VFD's offer a regeneration to line option but I'm not smart ehough on that refinement to do more than mention it.

                              [This message has been edited by Forrest Addy (edited 08-11-2005).]

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