On Wednesday 17 August 2016 17:54:46 Martin Dobbins wrote: > Gene wrote: > >Looking at the torque curves of the SDSK series, and the steady state > >available powers, for a spindle drive the steady state limit is the > >important one, and its much lower than its peak torque while moving > > to a fixed position. You have to get large and ex$pen$ive to get > > steady state above 250 oz/in at a usable rpm. You may want to check > > out the MC series which are brushless servos internally. I'd have to > > assume they also would overheat in a steady state run at a given > > load. You can only pack so much horsepower in that volume before > > something gets too hot. > > Andy wrote: > >Choose one with the same peak torque as the stepper that people are > >using, and you get the same as they have at low speed, and useful top > >speed. > >You still need to trade-off peak torque for spindle speed, but the > >Clearpath motors do look useful. > >However, the Clearpath motors that make 1800 oz.in don't go faster > >than steppers. But then 1800 oz-in is 12Nm, which is lots, possibly > >rather more than necessary. > > First off, what it's competing with, from: > > http://sherline.com/product/33050-dc-motor-speed-control-units/ > under the "specifications" tab > > 90 VDC motor .06 KW (60 W) at 10 oz. in. / 6100 rpm > Duty rating: Continuous-10 oz. in. at 6100 rpm, .85 amperes; > Intermittent-30 oz. in. at 5500 rpm, 1.75 amperes (5 minutes on / 15 > minutes off). I'm guessing that the 30 oz. in is the highest > (intermittent) torque in the entire speed range. The torque at the > spindle is increased by speed reducing pulleys 6100 motor rpm = 2800 > spindle rpm. > > I first saw Tecnik Clearpath servos many months ago and thought they > were DC motors with encoders, and why swap one DC motor for one the > same size or smaller? I've since realized that all of them, both SD > and MC are brushless motors internally. Nevertheless, I imagine they > do run hot given the large heatsinks on the back and the provision for > adding a fan. > > It's very difficult (for me, at least) to compare a stepper motor to > one of these because the torque of a stepper is going to fall off > rapidly with speed. The stepper used is so huge, however, how much > torque will it lose between zero and (say) 1500 rpm? > > Martin
That depends on 2 things, Martin. 1. And likely the most important by far, is the coil inductance. A motor with only 2 millihenry's of inductance can achieve full coil currents quick enough to have usable torque at 500 rpms. The same motor frame, wound for the same starting torque but has a 15 or more millihenry inductance will use less operating current to run at 5 rpm than the 2mh one, and will use perhaps 1/3rd the current. But it will be all tapped out and may not ever get to 500 rpms under zero load. 2. The supply voltage fed to the driver, raising it will get the current flowing quicker, but trying to make that 15mh motor turn as a usable high speed motor, could take more voltage to overcome the inductance than the driver can tolerate. That has a nasty tendency to break the mirror and let all the smoke out. I replaced a 1600oz/in motor used as the Z drive on a G0704, with a 950oz/in. Where the high inductance of the 1600 fell over was at about 27 ipm. The newer, smaller motor with a new self-contained psu/driver, can now run that close to 40 lb head up and down the post at 70+ ipm and doesn't break a sweat even when rigid tapping. Look at the Clearpath torque curves, they fall over at a rate virtually the same as a good stepper. The 'real' substitute will generally be a decent BLDC 1kw servo motor. That is what the much higher priced MC series is, with a 3 phase drive built in. 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> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
