It is very easy to compute the rotational momentum of a complex-shape flywheel. Let’s use a simplified case as an example: You have a 100 mm diameter steel disk with an 80 mm hole. This looks like a ring made of 10 mm thick metal.
First compute the momentum as if there was no hole, for a solid 100 mm disk, Next compute the same for an 80 mm disk. Subtract the 80mm disk momentum from the 100 mm disk momentum. If you want to account for the spokes, figure out their “average thickness” as if the spokes were replaced by a thin sheet of metal and add that back in. There is a more complex way to do this but it requires Calculus. I think they showed us the hard way just so that they could come back and show us students that you could decompose any complex wheel into a set of simple disks and then add and subtract them. +++++ Stepper motors CAN work. But not if you use the simple Step/Direction interface most drivers offer. The step/dir convention is NOT a function of the motor. It is a function of the motor driver. The motor itself as A+/A-, B+/B- leads and takes analog voltages. It is nothing more than a two-phase BLDC motor with many pole pairs. You can drive a stepper in “continuous and smooth non-stepping” mode if you like, if you get a smarter driver that can continuously vary the input voltages. The more sophisticated controller can driver the motor in “torque mode” so that it supplies a specified torque. > On Feb 23, 2024, at 10:05 AM, gene heskett <ghesk...@shentel.net> wrote: > > On 6/16/22 21:57, John Dammeyer wrote: >> Hi Gene, >> Quite right. That link I posted used a table while in fact, as you pointed >> out, the mass is mostly on the outside of a flywheel with spokes. I would >> imagine at there is some average where if it's a 300 lb disk that is 24" >> might be the same as a 36" disk that is 400 lbs. >> Think of a fly press for example with a clutch that engages the tooling. >> Even if it does take 5 seconds to get up to speed, the clutch engages, the >> tool moves down and punches and moves up and the clutch releases. Even if >> the speed slowed down by 20% when the clutch released then assuming linear >> acceleration now only 1 second is required to bring the speed back up. At >> 50 RPM (0.83 seconds per rev) then you could do another punch stroke 1 >> second later and so possibly run 30 strokes per minute. >> That jpg chart I included suggests with 100% efficiency and no real friction >> that 45 oz-in are required. Seems very low to me hence the questions. >> Even if I did use a stepper motor and went 16:1 to bring the RPM down to 800 >> RPM the motor could easily be a size 23 300 oz-in. >> Could that actually bring a flywheel up to that speed in 5 seconds? > > The closest I could come, assuming no frictional losses, would still be just > a SWAG. But it sure seems to me a decimal point got moved or left out > someplace. > > A stepper would be a poor choice of power unless the stepper drive also > started at zero. A stepper unable to stay synced with the incoming step rate > has next to zero torque. A vfd makes far more sense as you could set it for 2 > or 3x the motors FLA and the vfd would then throttle the current, using > seriosly more drive currant immediately after a strike to get it back to > speed, but the average would still be only maintenance unless it was striking > with every revolution. EG 50 strikes a minute. Given the time to extract and > replace the next work piece is going to be at least a second, that would be > one heck of a busy machine. Much the same could be said of a hirez encoder > whose output was compared to the desired speed and a 1 horse treadmill motor > being run by one of Jon's pwm-servos. Both solutions would need far less > electrical power to get the job done than a steeper could do. >>> -----Original Message----- >>> From: gene heskett [mailto:ghesk...@shentel.net] >>> Sent: June-16-22 6:34 PM >>> To: emc-users@lists.sourceforge.net >>> Subject: Re: [Emc-users] Acceleration question. >>> >>> On 6/16/22 20:54, John Dammeyer wrote: >>>> OK. I realize this will be a dumb question but please bear with me >>>> especially since I've included the ability >>> to accelerate in my Electronic Lead Screw project. >>>> >>>> A friend and I were discussing bringing a 300 pound flywheel up to speed. >>>> Vz=0 RPM, Vf=50 RPM. Reduction drive to the flywheel shaft is 32:1 so >>>> final speed of motor is 1600 RPM. >>>> >>>> Assume we're happy with 5 seconds to accelerate for Tz to Tf. Motor >>>> voltage is 12V. >>>> >>>> We have the mass, we have the velocity, we have the time and motor >>>> voltage. The question is what are >>> the calculations to determine how much current the motor will require to >>> create this acceleration? >>> Assuming of course the motor is 100% efficient. >>>> >>>> We're getting all confused with F=ma and 1/2*a*t^2 etc. >>>> >>>> What size motor is actually needed to do this? >>>> >>>> Thanks. >>>> John >>>> >>> That John, is going to be determined by where that weight is. >>> If 270 lbs of it is in a rim 4 feet in diameter and the other 30 >>> is in the spokes supporting that rim, its going to take a lot >>> more torque to get it up to speed in 5 seconds than it would >>> take if its only 2 feet in diameter, its the linear speed of the >>> outer diameters major mass that has to be moved to twice >>> as many feet per second needing 4x the torque to do it for >>> the 4 foot example, and Einstiens E=m*v*v comes into the >>> picture, cuz v=2*2 is 4, but v=4*4 is 16, not 8. >>> >>> That's as close as I can get to the math, sorry. I'd have to >>> ask someone else for a SWAG or more knowledgeable >>> answer too. This is a case also, of doing a bit of cheating >>> with a bigger vfd running at a higher voltage and the low >>> speed current boost could, if enough line voltage is present, >>> bang a 1 horse motor hard enough to natch a 3 or 4 horse >>> motor, knowing the overdrive will only last a few seconds. >>> >>> But, if going to machine cut with that motor, I'd have an >>> amprobe or equ watching the motor currant to make sure >>> the steady load is within the FLA on the motors nameplate. >>> >>> I hope the real answer means you've a motor and vfd in >>> stock that will do it. >>> >>> Cheers, Gene Heskett. >>> -- >>> "There are four boxes to be used in defense of liberty: >>> soap, ballot, jury, and ammo. Please use in that order." >>> -Ed Howdershelt (Author, 1940) >>> If we desire respect for the law, we must first make the law respectable. >>> - Louis D. Brandeis >>> >>> >>> >>> _______________________________________________ >>> Emc-users mailing list >>> Emc-users@lists.sourceforge.net >>> https://lists.sourceforge.net/lists/listinfo/emc-users >> _______________________________________________ >> Emc-users mailing list >> Emc-users@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/emc-users >> . > > Cheers, Gene Heskett, CET. > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author, 1940) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
