Hugh Currin wrote: > DIY-CNC, EMC & Gecko Groups: > > I have nearly decided to upgrade my controller to Gecko drives. But > I'd like to run this by the group before I spend the $. I'd appreciate > any feedback anyone has. > > I have a Bridgeport size knee-mill retrofit for CNC. It has a Rockford > rolled ball screw kit on X and Y and a rolled ball screw on the > quill. This all works fine with smooth motion and little backlash. So > far so good. > > It uses stepper motors with 2:1 timing belt reduction between the > motors and the lead screws. > > I have size 34 654 oz-in Motion Control Group steppers on each > axis. These are speced at 4.9A/phase (parallel) and 3.7V. The > drivers are 5A Chopper kits from Camtronics and use a 36V supply and > are set for 4.1A. I bought the motors and controllers from Camtronics > in 2000.
The usual recommendation for supply voltage is 10 to 25 times the motor rating. Using a 36V supply for 3.7V motors is about 9.7 times rating. That is on the low end, and will show up as limited high speed torque. 4.1A actual vs 4.9A motor spec means you will get 4.1/4.9 = 83.6% of the nominal motor torque (at standstill). With 654 oz-in motors, that means you will get about 547 oz-in. I don't know how the Camtronics drives set the current, but if they are rated at 5A, you should set them up to give your motors the 4.9A that they want. > I've never gotten better than 15-20 in/min without loosing steps. I > also tend to loose steps under heavy load, this noted only while drilling > with the Z axis. I measured the torque on the X and Y axes with a fish > scale and get about 540 oz-in at the motor. These speeds are OK for > cutting but a little slow for rapids. I really don't like loosing > steps though. I could increase the Z axis reduction from 2:1 to 3:1 > which would probably fix the Z axis lost steps. The 540 oz-in measurement matches up with the prediction I calculated above. Get that current up to 4.9 amps to improve your slow speed torque. Increasing the ratio will help at very low speeds. But it will might get worse, not better. > I understand steppers should run hot but these run quite cool. Never > uncomfortable to hold ones hand on. Would this indicate the current > could be increased? Would the difference between 4.1A and 4.9A rated > account for this? Yes. The copper loss in the motor (heating) is proportional to current squared. You are running at 83.6% of current - squaring that says you are producing only 69% of the heat that the motors would produce at their rated current. So they will run cool. > Has anyone else run a similar combination? If so was your experience > similar? > > I'm thinking of using Gecko drives at 70V. Will nearly double the max > speeds? Would the Geckos increase low speed torque? Doubling the supply voltage will certainly increase the max speed. Not sure if you will get double, but you should see a significant increase. As for low speed torque, either the Gecko or the Camtronics driver will deliver more torque IF you adjust it to give the motor 4.9 amps instead of 4.1. Regards, John Kasunich ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
