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Anyone want to help me with my 5-10hp Eddy current dynamometer project?

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  • Anyone want to help me with my 5-10hp Eddy current dynamometer project?

    I am looking for some parts to build a small eddy current dynamometer.

    Eddy current dynos are pretty simple machines. It involves spinning a metal disc in a magnetic field.



    Typically, electronic components are laminated to reduce the eddy currents, but in this situation the eddy currents are desirable. If the disk is spinning at a constant speed, then the heat produced by the magnetic field is in equal power to the power driven into the input shaft of the dyno. Heat is produced by the magnetic field when the individual electrons of the metal disk are spun and moved by the field. Think of it as an electronic version of riding a speed boat across a lake and dipping your hand in the water. The water pushes back on your hand because it is viscous. The electrons in the metal disk do something similar.

    What is interesting about this type of setup is that the energy of the magnetic field does not have to equal the energy going into the dyno. At work, we have dynos which absorb 500hp when are excited by only 1hp of magnetic power. However, the torque going into the shaft of the disk is equivelent to the force acting on the magnet times the distance from the center of the disk to the approximate center of the magnet. So we have an accurate way of measuring torque on a spinning dyno. As long as we monitor the speed, we can easily calculate power.

    I would like to build a 5-8hp air cooled dyno. This dyno would be for a local high school to test the engines of their Wisconsin supermileage go-carts.

    For the spinning disk I have a brake rotor from a Ford Mustang. It is hand because it is already machined, cheap, and has air cooling fins built right into it. I will build a shaft and trunnion setup for the disk with my new purchase of equipment. (I just need a welder)

    Some things I have to answer before everything goes live:

    How much magnetic power do I need?

    Unfortunately, most electrical engineers are taught how to minimize eddy currents. I have talked to quite a few EE's, and no one so far knows how to calcualte the eddy current heat produced by a specified magnetic field moving past a metal material. I have no idea if the 1hp used to absorb 500hp is scaleable, or if I will still need 1hp of magnetic field even when absorbing 5hp. (We controll our dynos by controlling magnetic power, but my brake rotor is a different diameter and I don't know what my efficiencies are going to be)

    Can an aircooled mustang brake rotor reject 5hp of heat constantly?
    I know that a brake rotor can reject quite a bit of heat. A 3600lb mustang can go from 70mph to 0mph in 160ft. (http://www.caranddriver.com/reviews/...02-test-review) If I assume the car had a steady deceleration, the front of the car handles 70% of the braking power, and each front rotor handles 50% of the front then each front rotor is producing over 200hp. It could actually stop faster if the tires could handle the force created by the rotors. However, any trucker driving in the mountains knows the rotors will over heat if you use the brakes too long. I have experience measuring heat rejection, but my assumtions about airflow through the rotor at speed may be sketchy. I am coming up with about 10-15hp continous heat rejection above 900rpm if I want the rotor to remain under 350 degrees F.

    What is the burst speed of a mustang rotor?
    At 200mph the rotor of a mustang is spinning 3000rpm. I would prefer the dyno to be able to handle between 5,000 and 6,500 rpm. This would accept all standard speed electric motors and utility small engines. However, if a disk ruptured at that speed it would be pretty dangerous. I need to prove the capability.

    So, the short version... anyone know where I can get a decent 1kW DC power supply and some magnets? I am thinking about building my own magnets with copper wire and some steel bobbins, but I need to know the voltage and current of my power supply first.

    Anyone know where I can get a 1kW DC power supply? I'd prefer an adjustable voltage. Maybe something that plugs into 110AC and puts out 48 volts or 220AC and 90 volts.
    If the women don't find ya handsome, they should at least find ya handy

    https://www.facebook.com/WDHSTechClub

  • #2
    You can greatly simplify the project by using samarium cobalt permanent magnets in a rotor that interacts with a stationary aluminium disk. The amount of drag may be controlled by changing the distance of the magnets to the disk. If you need to actually calibrate it to an absolute value then an electric motor of known hp can be used to drive the dyno. Really though, the focus will be on relative engine output rather than actual calibrated output. As long as the unit can show changes in dyno input accurately that is all you need to know.

    The power can be measured by reading the temperature rise of the aluminium disk or you can directly measure the torque produced and obtain an actual torque measurement that together with rpm will give actual horsepower. Measuring torque is as easy as restraining the rotation of the aluminium disk with a spring scale such as may be used for weighing fish.

    Samarium cobalt magnets must be used because of the heat. Neo magnets won't withstand high temperatures.

    This isn't just theory. I built a magnetic transmission for my 3 wheel cart using an aluminum disk based eddy current torque converter. It worked fine except for the amount of heat generated. I didn't have samarium cobalt magnets so the efficiency fell quickly as the magnets heated up.
    Evan
    Senior Member
    Last edited by Evan; 01-21-2012, 12:37 PM.
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    • #3
      You can also use a perm. mag motor and turn it into a genset - then dump the heat/current into something stable.

      use the voltage reading to figure out the power output - if u want to get finicky install a temp gauge on the Gen to keep track of the variances because they do matter, very basic system but I don't know if you can find a perm mag motor large enough for what u want...

      I built a little twin perm mag motor/gen dyno for my experimental bike cranks and it works very well - but I use it to power a ford tuarus V6 rad. fan to help keep me cool and keeping track of ambient temps is a big deal cuz that's the drag medium that the fan has to work against (and therefore the gen) - barometric pressure is wrapped up in there too but Im not getting that finicky with it... not to mention all these variables have an effect on the human machine to boot.





      I just wing it but when it comes to this stuff Id listen to Evan on this one as he knows allot more as to where the changes occur due to heating and such...
      A.K. Boomer
      Senior Member
      Last edited by A.K. Boomer; 01-21-2012, 01:09 PM.

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      • #4
        Originally posted by Evan
        This isn't just theory. I built a magnetic transmission for my 3 wheel cart using an aluminum disk based eddy current torque converter. It worked fine except for the amount of heat generated. I didn't have samarium cobalt magnets so the efficiency fell quickly as the magnets heated up.

        Evan, do you have any engineering calculation to show how many magnets and at what strength they have to be for the disk to absorb 5hp? Are you willing to share any details for your three wheel cart project?

        What size magnets?
        Diameter of the aluminum disk
        Thickness of the disk
        Magnets on both sides of the disk?
        Do you have an estimation of the magnetic power of the magnets?
        Estimation of the power absorption by your system?
        etc..

        Our projects are similar, but I am looking to adjust power by varying the voltage to the electromagnets. If I can get stage one done, stage two involves programing a microcontroller to control the dyno. In the end I want to be able to measure brake specific fuel consumpion of an engine, and other things we do with our dynos at work. The micro is needed because the heat is constantly changing, so the dyno has to adjust the field to maintain a constant power absorption. Constant power for up to 20 minutes is required to get an accurate BSFC measurement.

        Further, the air cooled 5hp dyno is for the local highschool...I plan to build a water cooled twin-rotor 200hp dyno with the same parts for my garage
        Ggerg1186
        Senior Member
        Last edited by Ggerg1186; 01-21-2012, 01:20 PM.
        If the women don't find ya handsome, they should at least find ya handy

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        • #5
          I don't have the math skills or the related knowledge to calculate the magnetic field strength in a particular configuration required to produce a particular amount of eddy current drag in a particular amount of aluminum at a particular distance etc. It is a very complex problem. I approached it empirically and simply tested different configurations.

          The generator approach as suggested by Boomer is probably even easier to implement and you can obtain a direct readout of the power generated. Allowances will need to be made for the loads and using water cooled resistors is a good way to insure relatively constant load.

          Some dynos use a water tank that is stirred by an agitator to provide load. Measuring the temperature rise gives a very accurate reading on power generation. Water is very well characterized and the appropriate curves for viscosity vs temperature are widely available.


          The engine I used produces a few horsepower at most. There are several threads regarding the trike although I don't recall if I posted information re the magnetic torque converter. The magnets I used didn't come with specs. It was a test of the concept and it proved that it can be made to work well with the right components. I do a lot of experimenting with these sorts of ideas. Some of it is just empirical testing with little to no design engineering in advance. I also design projects with great attention to the engineering with everything carefully calculated and quantified but that wasn't one of them.

          When using an eddy current load the better the conductor the more effective it is. A copper disk will produce greater mechanical resistance for a particular size since there are less resistive losses but of course it will be more expensive. A motor/generator is really just a more complicated eddy current device.

          Since you would like to be able to dial in the load electrically you could use an alternator and simply adjust the field strength. A truck alternator rated at 350 amps at 14 volts will handle up to 7.5 hp. The amount of load can be directly varied by changing the field coil voltage (and therefore current).
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          • #6
            Yeah, I think I am going to have to do empirical testing too. The best information I could find on the subject were some engineering books written in the 20's that cannot be checked out of the Detroit public library. I made copies of the books, but the engineering language from the 20's is just different enough that I cannot make heads or tails of what they are trying to accomplish. The books were about metling iron ore for smelting steel with eddy currents. I've talked to a multitude of EE grads from many different schools, as well as several professors of engineering and have gotten no further information.

            Can you tell me the size of the aluminum disk (diameter and thickness), how many magnets you used (and their physical size), and where the magnets were placed relative to the disk?

            I'd use the alternator for the local high school, but there aren't many cost effective electrical solutions to absorbing 200hp. The empirical testing on the 5hp air cooled rotor will give me insight for the 200hp version.

            I guess I am kinda stuck on the design. Been day dreaming about the project for 5 years now. Anywho, any pitcures or info on your magnetic clutch would be very helpful.
            If the women don't find ya handsome, they should at least find ya handy

            https://www.facebook.com/WDHSTechClub

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            • #7
              Originally posted by Evan

              When using an eddy current load the better the conductor the more effective it is. A copper disk will produce greater mechanical resistance for a particular size since there are less resistive losses but of course it will be more expensive.

              You have confused me there Evan. I thought the idea was to creat an eddy current brake? The mechanical instruments that use eddy current drag to move an indicator needle use an alumin(i)um disk.

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              • #8
                The lower the resistance of the eddy current disk the stronger the magnetic fields produced by the eddy currents. The induced currents in the conductor are in opposite phase to the fields that induce them. That is what produces the drag. The energy is still dissipated in the conductor but lower resistance results in greater amperage and stronger fields.

                The only reason to use aluminum is cost. Silver will work even better than copper.
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                • #9
                  I'll take a stab at that - When it comes to instruments any conductive material will create resistance -- so why not go with the cheapest and then just adjust your resistance springing accordingly.

                  but when it comes to eddy braking there's a multitude of variables to consider size for effect, heat handling (meltdown or warpage temps)
                  I would think silver to be yet better than copper - and gold better than silver - but cost and meltdown temps of course need to be considered...

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                  • #10
                    Can you tell me the size of the aluminum disk (diameter and thickness), how many magnets you used (and their physical size), and where the magnets were placed relative to the disk?
                    I will dig around in my archives and see what pictures I may have. That will take a while so keep an eye on this thread for a day or two. ( I have thousands of pictures)
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                    • #11
                      and gold better than silver
                      Gold isn't a particularly good conductor. Copper conducts about 35% better than gold. Gold is slightly better than aluminum. Gold is used on contacts because it doesn't corrode.
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                      • #12
                        Telma retarder brake

                        Eddy current brakes are widely used in trucks and trains. The big name in the industry is Telma (see http://www.telmausa.com/telma_htm/default.htm). Perhaps you can get some information there. You can occasionally find telma retarders on Ebay and elsewhere. They tend to be a big larger than what you need.

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                        • #13
                          Originally posted by Evan
                          Gold isn't a particularly good conductor. Copper conducts about 35% better than gold. Gold is slightly better than aluminum. Gold is used on contacts because it doesn't corrode.


                          wow --- I did not know that - I assumed it was superior to all not only in corrosive but conductive, thanks...

                          so what was all the hype awhile back about gold plated speaker wire ? no need to stop corrosion in insulated wire except at the connections right? or was it just that - hype,,, ?
                          A.K. Boomer
                          Senior Member
                          Last edited by A.K. Boomer; 01-21-2012, 04:21 PM.

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                          • #14
                            Originally posted by A.K. Boomer
                            wow ---
                            so what was all the hype awhile back about gold plated speaker wire ?
                            Sucker factor.
                            Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                            ~Practitioner of the Electromechanical Arts~

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                            • #15
                              By the way, the on-topic discussion of this thread is of current interest to me as I am incorporating a low speed eddy current damper in a clock design. I will be watching with interest and will report my own findings.
                              Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                              ~Practitioner of the Electromechanical Arts~

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