On Thu, 2008-06-05 at 21:19 -0500, Jon Elson wrote: > Kirk Wallace wrote: > > A while back I played with a KBIC speed controller which demonstrated > > fairly good torque at high and low speeds. I was curious about how a PWM > > amp would do, so I set up a Pico amp to a parport PWM signal at about > > 25kHz and was surprised to find that low end torque was much less. I > > thought the PWM frequency might have had something to do with the torque > > issue, so I made a similar setup with a 50kHz PWM signal from a Pico PWM > > controller and got similar results. Then I got to thinking, the Pico amp > > is a PWM to voltage converter. The motor torque will be the torque for > > the voltage that the motor happens to be driven with at the time. I > > thought the KBIC had feed-back during the zero crossing period ,but I > > didn't realize how important it is. So, in order for the PWM amp to have > > good torque and speed control, feedback needs to be set up. Does this > > seem correct? > > > Yes, you have it exactly. My PWM servo amps are truly PWM => > voltage output, with no compensation for the motor resistance or > back-EMF. You could, of course, use some kind of feedback. If > you have a spindle encoder, you could make HAL read the encoder > velocity and add a correction to the PWM output. > > Jon
What are the issues with choosing a PWM frequency? I would think that efficiency would be one, where you would balance switching losses and iron or copper losses? Evidence of this might be seen as higher motor or amp temperatures? Lower frequencies might produce noise. I believe the Pico PWM amp has a 25kHz to 100 kHz range? What would happen with lower or higher frequencies? What are the symptoms of not setting the bootstrap parameter, like for my parport setup? Does the current limit resistor affect the amp performance or just set the current at which the amp alarms out? -- Kirk Wallace (California, USA http://www.wallacecompany.com/machine_shop/ Hardinge HNC/EMC CNC lathe, Bridgeport mill conversion, doing XY now, Zubal lathe conversion pending Craftsman AA 109 restoration Shizuoka ST-N/EMC CNC) ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
