On Saturday 26 June 2021 12:52:26 John Dammeyer wrote: > > Also, I'm looking for the secret sauce to get full closed loop from > > my vfd to LinuxCNC if anyone has pointers on that. I have encoder > > feedback to LinuxCNC, but i feel like I'm missing a key element to > > get the pid working properly. Most of the adjustments I've made to > > pid never result in LinuxCNC compensating for the error (i.e. G97 > > S200 = S210 actual with p =0, changing P results in a runaway in > > most cases). I'm wondering what the magic net command might be to > > get everything working so I could thread on my lathe. > > > > Phil T. > > The Feral Engineer
Generally speaking, using the hz from the vfd is a lost cause. Put an encoder on the spindle, but be aware that the 3 phase induction motors commoly used on lathes and higher powered mills all suffer from 2 effects at the high hz settings. At 60 hz, the slip angle between the rotating field and the armature will give you about 1750 rpm from the common 4 pole motor, not 1800. Step the hz up to 120 hz will reduce the coil currents because of the motor finite inductance, which combined with the higher friction load, will only get you around 3200 revs at the motor shaft because the currents are reduced, increasing the slip angle, and above 120 hz you will hit a point where no further increase in the frequency will gain you speed, and it may well slow down. That occurs at less than 180 hz on mine. The pid can tell it to go faster, but the coil currants are so low, less than an amp/phase, that its out of torque This is true for any "induction" motor. The 2 pole for the 24k revs motors is more than likely a hard steel armature running synchronously with the magnetic fields rotation, but inductance still causes a loss of torque at the higher speeds, but if its running, it is probably at the right speed. But those motors don't have the cajones to pull a tap anyway. I have an encoder on the Sheldon and I can even do rigid tapping on it, but the encoder is NOT feeding back to the PID, its only function is reporting the spindle's rotational position for G76 and G33.1 functions. That it does well given the sloppy machining and uneven wear on the tooth edges of the 60 tooth bull gear on the spindle. Now, if the PID could be used to raise the voltages out of the vfd at the higher speeds in order to keep the coil currents at the FLA value on the nameplate, we'ed have something great, but bring wagon loads of money for the switching devices in the vfd to achieve that. To my knowledge, no one has done that. To make it work at 400hz would take well over 1 kilovolt from the vfd. Tain't gonna happen. The motors winding insulation would fail, sooner rather than later. 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) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
