On Monday 27 November 2017 01:26:12 John Dammeyer wrote: > I recall reading somewhere, and I can't remember where, that it's > possible to damage a stepper motor by running too much current through > them. Not because of overheating, (although that's a possibility), > but because the larger magnetic field produced by the increased > current demagnetizes the magnets. > > In fact, as I recall, removing the rotor from the motor also results > in a loss of magnetism and once re-assembled the motor no longer has > the same torque. > > It's possible, although I've no direct evidence for this, that once > the motors are assembled the motor shaft is locked is a specific > position and then a large pulse of current through the windings is > used to magnetize it. The motor frame then acts as a 'keeper' for the > magnets. Remove the rotor and some of the magnetism is lost. > > But I'm not sure about that. > As a C.E.T., I am sure. And I've seen instruction sheets that come with some motors advising against ever removing the armature from the frame. That the increased heat might hasten to magnetic degradation was also somewhat in the back of my mind when I left it running for over an hour yesterday. But between the current turndown when its not moving, and the actual duty cycle of a typical move, heat will not be an in-service problem.
> For this rotary table I'd do what I did for my lathe. Create an > adaptor to hold an arm of say 12". Set it horizontal and then hang > weights from it until the arm starts to move. That's the amount of > torque required to overcome static friction. Say it takes a 1 pound > weight. Then you know you need a 1 ft-lb motor. Look at the torque > curves for motors and pick one that maintains the torque you need up > to the RPM you want to run. Kinetic Friction is always less than > Static Friction so the motor will be correctly sized. > > I did that for my Milling Machine Knee. With a Gecko and the 1200 > oz-in motor and 3:1 reduction the motor moves the knee with both vice > and rotary table weight up to the point where the torque curve on the > motor starts to drop off. As long as I stay below that speed the > Gecko runs the knee very nicely. I did cheap out and try a 600 oz-in > motor with series winding arrangement in order to stay under 3A but I > could never get the speed I wanted. Hence the Gecko, the 1200 oz-in > and 6A. Makes perfect sense. Thanks John 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) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
