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  • Hysteresis magnetic brake?

    Oddball question, but has anyone ever done this yourself? I was looking at Magtrol HB-450 magnetic brake and saw the picture of its inner workings and basically it is only a metallic cup that spins freely on ball bearings in an air gap and in this air gap you can generate a magnetic field so that it induces a drag on the cup portion, meaning braking action. And by varying the current to the coil the amount of drag can be controlled very precisely.

    Here is a nice PDF about it, cutaway model on page 2: http://www.magtrol.com/datasheets/hb-mhb.pdf

    The only thing I'm missing probably would be the bearings, everything else can be found from scrap box, but is there any reason for not trying to do this? Accidental discovery of nuclear fusion springed to my mind...

    Nah, meaning how would one determine the maximum braking force (Newton meters) once it is complete? Wind a cord around the brake and just add weight on it until the brake slips?
    Amount of experience is in direct proportion to the value of broken equipment.

  • #2
    I have a larger version of an electrmagnetic brake on one of my companies trucks. It is a Telma Driveline Retarder, slick as greased $hit. Prolongs brake life by a 5X factor with no real maintenance. As you said, very simple.

    P/R

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    • #3
      Most brakes of this kind are "geared up" because the braking force depends on the angular velocity - something to keep in mind during the design phase.

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      • #4
        Originally posted by Bruce Griffing
        Most brakes of this kind are "geared up" because the braking force depends on the angular velocity - something to keep in mind during the design phase.
        "My engines so small, if I need to stop all I need to do is turn on the AC and headlights."
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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        • #5
          Originally posted by Bruce Griffing
          Most brakes of this kind are "geared up" because the braking force depends on the angular velocity - something to keep in mind during the design phase.
          The brake will be adjusting wire tension and one of the pulleys will have a tension sensor on it. This sensor gives feedback to a small comparator that controls the amount of current to the brake to maintain the wire tension at a set level Easy, but will cost 150 for the sensor and who knows what for the brake. Haven't yet found a Finnish supplier for Magtrol brakes, so that is why I'm thinking if one could be easily made.

          The tensions are quite small, say 100 N at maximum, so a brake with for example 3 Nm maximum torque would provide that if the brake pulley diameter would be about 60 mm or smaller.

          Edit: Oh and the wire speed is a constant, so only thing that varies is the tension.
          Amount of experience is in direct proportion to the value of broken equipment.

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          • #6
            Originally posted by Bruce Griffing
            Most brakes of this kind are "geared up" because the braking force depends on the angular velocity - something to keep in mind during the design phase.
            I needed to incorporate a magnetic brake in the Magnetic Clock to damp out oscillation in the seconds rotor. The brake, or damper, is just an aluminum disk running between a pair of Neodymium magnets. The damper is effective within the six degrees of angular rotation each second.
            Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
            ~Practitioner of the Electromechanical Arts~

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            • #7
              Originally posted by Weston Bye
              I needed to incorporate a magnetic brake in the Magnetic Clock to damp out oscillation in the seconds rotor. The brake, or damper, is just an aluminum disk running between a pair of Neodymium magnets. The damper is effective within the six degrees of angular rotation each second.
              Fast and easy Eddy current brake, nice job!

              Finally found a Finnish rep for Magtrols brakes, sent them a request for quote for one brake. Shall see how many souls they want or will I just fire up the lathe and try making my own...
              Amount of experience is in direct proportion to the value of broken equipment.

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              • #8
                I've seen friction brakes that take their input from a lever arm which rides on the wire. If the tension is high, the wire straightens somewhat, and the arm moves, lowering the braking force. Tape decks often used something like this. Comes to mind that you could control the position of magnets surrounding the 'hysteresis' disc, so you could still get frictionless braking with this very basic mechanical method.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Originally posted by darryl
                  I've seen friction brakes that take their input from a lever arm which rides on the wire. If the tension is high, the wire straightens somewhat, and the arm moves, lowering the braking force. Tape decks often used something like this. Comes to mind that you could control the position of magnets surrounding the 'hysteresis' disc, so you could still get frictionless braking with this very basic mechanical method.
                  Very neat idea and would be quite easy to do, but hard to control as there is no kind of sensing of how much the tension is in Newtons on the wire at any given time.

                  This is the reason for a tension sensor and control feedback electronically back to the brake.
                  Amount of experience is in direct proportion to the value of broken equipment.

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                  • #10
                    I'm not too sure of the application you're referring to but it's a great opportunity fo a quick and dirty tutorial on electrical motor braking.

                    DC injection to the stator of an induction motor is a very old trick to decellerate connected loads before the days of VFD's.

                    All you needed was a DC power supply capable of full load motor current at about 1/4 the line volts and an adjustable resistor to trim to final current. It's controlled by relay logic with either a timer or a zero speed switch for automatic shut off. As was said the retarding torque of DC injection falls off as the motor speed drops below two or three times the slip RPM from zero. If a dead stop for an overhauling load is required a separate mechanical brake may be required.

                    The beauty of DC braking an nduction motor is there are no friction mateials or braking surfaces to wear and large inertia loads can be controlled at a low installed cost. The energy absorbed from the inertia load accumulates as increasing temperature in the armature. Since the armature has considerabe mass, area, usually is fuurnished with aluminum fan on both ends, and is subject to cooling air from its motion a fair amount of kinetic energy can be dissipated.

                    VFD's when so equipped have a mode of operation where they "motor" or dynamic brake the kinetic energy converting it into leading power factor fed into the line thus recuperating a share of the kinetic energy. Otherwise this energy is dissipated in a braking resistor to some fraction of the motors synch RPM, DC injected to near zero, and if equipped with an encoder, the motor may be electrically braked to zero and the armature held stationary by electronic dynamic positioing. While electronic hold is stationary as if the shaft was restrained with a brake it requires energy from the line to do so. A mechanically applied brake requires zero actuating energy and power can be safely disconnested from the motor without fear of movement or over-running from the connected load.

                    Hystersis brakes implies to me that there is an increasing braking characteristic on slowing, quite the opposite of DC injection braking. IS that so?
                    Last edited by Forrest Addy; 04-27-2012, 05:51 AM.

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                    • #11
                      The application will be the controlling of wire tension in a wire EDM. One pulley has the brake, one pulley has the tension sensor and these have been connected through a feedback loop electronically. First you set the wire travelling speed, then increse braking force from a potentiometer or with a computer control until the wanted tension in the wire is achieved. The speed is constant and if the sensor feels too much tension in the wire, it lowers the current going to the brake, thus relieving the tension.

                      Quite easy to do with electronics, just hook the sensor input to the comparator inverting input and the potentiometer (or other controlling voltage source) to the non-inverting input and connect the output to a transistor or similar to control the current to the brake. This ensures that the tension will remain at the set value all the time.
                      Amount of experience is in direct proportion to the value of broken equipment.

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                      • #12
                        Jaakko, typically in these situations a "torque" motor is used. These are a limited torque motor that can be stalled out without damage. Pull feeders on aluminum mig machines use them to help feed the wire up the liner without pushing too much on the wire. Same type of motors are used for reel to reel tape players.

                        A lot of servo drives have a torque mode where you send it a +/-10v analog signal and the torque of the motor is varied in proportion to the signal.

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                        • #13
                          Originally posted by macona
                          Jaakko, typically in these situations a "torque" motor is used. These are a limited torque motor that can be stalled out without damage. Pull feeders on aluminum mig machines use them to help feed the wire up the liner without pushing too much on the wire. Same type of motors are used for reel to reel tape players.

                          A lot of servo drives have a torque mode where you send it a +/-10v analog signal and the torque of the motor is varied in proportion to the signal.
                          So how would you incororate one to the WEDM machine? Because if you pull the wire with a certain torque motor, nothing assures the tension in the wire remains the same. But if you constantly pull the wire and brake it from the supply side, the tension in the wire can be easily controlled from practically 1 - whatever Newtons needed.

                          Or did I misunderstand something?
                          Amount of experience is in direct proportion to the value of broken equipment.

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                          • #14
                            Well, if you feed it with a constant speed from the supply side, you could pull on the wire with a torque motor at the other side.
                            Voila, constant tension.
                            You could use two identical motors and just make different drive electronics. From a control point of view it would probably be easier though to get them rated at different voltages or gear down the constant speed motor.

                            Your solution would however be intrinsically safe against a big mess when the wire breaks. Yours would stop the advance of the wire while the other will continue to pile new wire onto the workpiece.

                            For an easy smooth braking device, get a brushless DC. That's a lot quicker than building a few iterations of your own eddy current brake. If you run the wire only at a few different speeds, you could switch between a couple of resistor values to short the windings. For more precise control it's better to get some PWM controller in there.

                            Igor
                            Last edited by ikdor; 04-27-2012, 08:49 AM.

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                            • #15
                              The problem of "pushing" the wire is that 0.25 mm diameter brass wire is just stiff enough for that one maybe, but try pushing a 0.1 mm brass wire It is like a cats hair, bends and kurls like it wants and VERY much fun trying to put that through a diamond wire guide (Spent 7 hours doing that once)

                              So that is the main reason I want to just pull the wire from the end of its travel side and just brake it from the supply side to get tension.

                              Thanks for the brushless DC motor idea, but it has the same problem than with stepper motors: cogging. The magnets will give a jerky motion in low to medium speeds and that means huge ripples in tension (like tension - no tension - tension - no tension).

                              That's the reason for the hysteresis magnetic brake, as it is smooth from the beginning up to what RPM the parts can handle without disintegrating. But you can get it cogging too and that is if you let the current run through it while stopped. This magnetizes the cup and thus gives cogging effect and it needs to be demagnetized before it works again.
                              Amount of experience is in direct proportion to the value of broken equipment.

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