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  • VFD Braking Resistors - another question.

    There was a recent thread on VFD braking resistors, but I’ll start a new one.

    I’m probably going to reveal a whole lot of ignorance of VFD’s by asking this question, but here goes.

    Using a VFD, you can slow and then stop the motor by setting the ramp down duration, or use a braking resistor or use DC injection. Let’s ignore the DC injection method. If I set up the VFD to use a braking resistor (assuming I have set the parameters correctly for the breaking resistor use), does this override the ramp down settings? By this I mean, will it stop the motor by using the braking resistor only, or does it wait until the ramp down period has finished and then switch in the braking resistor?

    Ian.

  • #2
    That sounds like a question for the manufacturer.
    Paul A.
    SE Texas

    And if you look REAL close at an analog signal,
    You will find that it has discrete steps.

    Comment


    • #3
      Originally posted by IanPendle View Post
      There was a recent thread on VFD braking resistors, but I’ll start a new one.

      I’m probably going to reveal a whole lot of ignorance of VFD’s by asking this question, but here goes.

      Using a VFD, you can slow and then stop the motor by setting the ramp down duration, or use a braking resistor or use DC injection. Let’s ignore the DC injection method. If I set up the VFD to use a braking resistor (assuming I have set the parameters correctly for the breaking resistor use), does this override the ramp down settings? By this I mean, will it stop the motor by using the braking resistor only, or does it wait until the ramp down period has finished and then switch in the braking resistor?

      Ian.
      No and No.

      It will stop the motor according to set ramp "rate" by dumping the "braking" energy to the brake resistor. Without brake resistor its not possible to use as fast ramp as you would be able when braking with resistor.
      (Motor will act as a generator during dynamic braking and the generated energy has to go somewhere. Either brake resistor, backfed to utility network(only fancy expensive models) or used up by VFD losses and partly stored on DC bus capacitors)
      Last edited by MattiJ; 04-16-2018, 04:09 AM.
      Location: Helsinki, Finland, Europe

      Comment


      • #4
        Hello Matti,

        OK, understood.

        "......Without brake resistor its not possible to use as fast ramp as you would be able when braking with resistor......"

        That sentence makes it all clear.

        Thanks.

        Comment


        • #5
          That's pretty much it- you need a resistor for either a heavy rotating mass, or to stop very quickly (or both.)

          For example, my little Logan and Sheldon both are set for sub-2-second ramp down. I don't need a braking resistor, they seem to tolerate those speeds just fine, even with decently sized chucks and workpieces.

          My big Springfield, however, if or when I convert that to VFD, will likely need a big braking resistor if I wanted to stop it under 2 seconds- especially if I had a big workpiece in it. But, if i dialed that back to a 3 or 4 or 5 second ramjp-down, said resistor likely wouldn't be needed.

          Doc.
          Doc's Machine. (Probably not what you expect.)

          Comment


          • #6
            My BP would error out on 2K rpm and above with a four second stop without a resistor. I just ignored it and pressed reset. With the resistor, I can't get below two seconds at 1200 rpm. I still have to tinker with the settings a little after understanding the language. By the way, my resistor didn't even feel warm to the touch after 7-8 timing experiments. But the weather has been cool lately.

            If DC Injection is so bad, why are all the major YT players using it? I see (and hear) it used a lot.

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            • #7
              DC injection works very well.

              The energy from the stop stays in the motor, it appears as heat in the rotor.*. so if you use it to stop very often, the rotor, and eventually the rest of the motor, will get considerably hotter than usual, which is generally bad.

              * During the stop, large currents are induced in the rotor, which is essentially a short circuited winding. The heat comes from the resistive loss in the rotor, due to the (small) resistance of the rotor "windings". The currents are larger than the normal running current.
              CNC machines only go through the motions

              Comment


              • #8
                Originally posted by CCWKen View Post
                My BP would error out on 2K rpm and above with a four second stop without a resistor. I just ignored it and pressed reset. With the resistor, I can't get below two seconds at 1200 rpm. I still have to tinker with the settings a little after understanding the language. By the way, my resistor didn't even feel warm to the touch after 7-8 timing experiments. But the weather has been cool lately.

                If DC Injection is so bad, why are all the major YT players using it? I see (and hear) it used a lot.
                Downsides of DC injection is possible overheating of the motor and limited braking torque. Good VFD +resistor set up correctly should be able to slow down the motor with dynamic braking in same time as it takes to speed it up = pretty fast.
                I think DC braking is perfectly fine for applications with low/limited inertia and limited amount of starts per hour. Like a mill or most lathes. High rpm lathe with large steel chuck could be pushing your luck.
                Dynamic braking excels when you have 60 ton load on a crane and you need to lower it 30 meters down.
                Location: Helsinki, Finland, Europe

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                • #9
                  Is there any problem using the manual brake to assist stopping at higher rpms or with a heavier tool such as the Suburban fly cutter? Or will that foul the VFD's logic?
                  Last edited by CCWKen; 04-16-2018, 01:43 PM. Reason: Corrcted the "fowl" language. :)

                  Comment


                  • #10
                    The VFD slows by reducing the frequency at the selected rate. That makes the motor "generate", because it is then turning above synchronous speed, and as a result it puts energy back onto the DC bus, raising the voltage, and removing the energy fom the rotating load, slowing it.

                    All the VFD is doing is reducing frequency, and consequently, motor speed. If you use another means to slow the machine faster than the VFD does, the VFD may actually start to DRIVE the machine against the braking, which defeats the purpose.

                    Use one or use the other.
                    Last edited by J Tiers; 04-16-2018, 12:27 PM.
                    CNC machines only go through the motions

                    Comment


                    • #11
                      I installed a bicycle disc brake on my motor pulley and setup a pedal actuator with the vfd set to freewheel stop. Normal stop I just lightly press the pedal to actuatte the stop microswitch and the motr coasts to a stop.
                      Helder Ferreira
                      Setubal, Portugal

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                      • #12
                        With many VFD you can program them to use dynamic braking (resistor or external braking unit) AND foot brake. When you press the foot brake it overrides the dynamic braking and simply freewheels (coasts) to a stop under mechanical brake control.

                        My prior big lathe had an electro-maganetic brake - superb, and stops on a dime (unlike a vfd without DC injection). My current lathe has a foot pedal bar. I like the later but miss the electric brake. When I get around to converting this lathe to VFD, I'm going to implement both, and if I can get a vfd that will allow it, add DC injection at the last part of the dynamic braking cycle to make for a positive stop. The trick is to turn OFF the DC injection when rpm falls to zero, not just inject for defined fixed period. And... thermistor sensing (inside the motor) via the VFD to avoid overheating of the motor.

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                        • #13
                          If you can set an over-ride, then obviously there is no issue, you never have both at one time.

                          DC injection is stupidly effective at high speeds. At lower speeds it becomes less effective, but despite common warnings, it is pretty effective even at speeds that you can turn the motor just with your hand on the shaft. Try it sometime, and you will see.

                          the one thing it really does not do, though, is a "positive stop". Zero speed is zero braking.

                          And the DC that is injected is generally not enough to do much heating. The heating is actually of the rotor conductors, which heats the rotor, and indirectly, the stator and frame. I have never even tried to see what FLA as DC would do in terms of braking.... certainly a heck of a lot... Even a few hundred milliamps of DC does a very significant job of braking.
                          Last edited by J Tiers; 04-16-2018, 10:31 PM.
                          CNC machines only go through the motions

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                          • #14
                            Hmm... if you have a VFD available, play with DC injection. Unlike dynamic braking, DC injection is a superb at low speeds (and yes, can be very aggressive at any speed) - a very positive gear slamming stop - and it literally locks the rotor in place if left applied (bad idea). I'd have been happy if it "gave up" at zero speed, but without feedback it doesn't know what zero is.

                            My usage is all on various (three) Hitachi VFD. Hitachi specifically recommends that you use a motor-mounted Thermistor in conjunction with DC braking, so they are concerned. The problem I ran into was that it was not possible to define that the DC injection finished at zero rpm, only for a defined time and braking time varied, so often the rotor was "locked" for a short (or long if programmed as such) time after stopping. Maybe the latest VFD have more control.


                            For the rest of the audience,
                            http://electrical-engineering-portal...-braking-motor

                            http://www.machinedesign.com/technologies/give-me-brake

                            https://en.wikipedia.org/wiki/DC_injection_braking

                            Toshiba note for DC injection implementation

                            https://www.toshiba.com/tic/datafile...aking_6551.pdf
                            Last edited by lakeside53; 04-16-2018, 11:40 PM.

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                            • #15
                              Originally posted by lakeside53 View Post
                              Hmm... if you have a VFD available, play with DC injection. Unlike dynamic braking, DC injection is a superb at low speeds (and yes, can be very aggressive at any speed) - a very positive gear slamming stop - and it literally locks the rotor in place if left applied (bad idea). I'd have been happy if it "gave up" at zero speed, but without feedback it doesn't know what zero is.
                              This is where different manufacturer literature (and probably actual implementations) seem to differ. (and I can't try it out on my VFD's as ABB DTC drives have only dynamic braking and flux braking(what yaskawa probably calls high slip braking))

                              Yaskawa is talking about less than 100% nominal torque during dc braking
                              https://www.plantservices.com/assets..._SlipBreak.pdf

                              But Google finds me for example this:



                              So the most of the dc injection braking torque is available at relatively low speed, around 10-20% nominal and full speed braking torque is much less.
                              Location: Helsinki, Finland, Europe

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