On Tuesday 19 May 2020 15:44:51 John Dammeyer wrote: > Hi Gene, > Since I'm running DC Servos I wanted the ability to clamp the back emf > from a decelerating motor. I built a small circuit that sensed both > high voltage and AC power. It puts a resistor across the DC rail to > pull it down and the resistor is connected with both a relay and a FET > in parallel. > > When power vanishes the relay is de-energized and the 105VDC power > supply is quickly drained through the NC connection on the relay. The > FET does the same but at high speeds to avoid that over voltage > condition. > > Since the little PIC processor senses AC voltage it can also run a > relay on a second small board that switches across an inrush resistor. > The advantage over a high power thermistor is no heat and you can > cycle power without worrying about the inrush thermistor still being > way too hot and therefore too low a resistance. > > Schematics attached. > Both would seem to be more complex than a few lines of hal code I can easily write. The only time energy recovery is/is not a problem is during a full song reverse, where the full 4 quadrant pwm-servo runs my 126 volt supply up to about 170 volts by the time it has bought that big motor to a full stop, which is well above the 65V x 2 of the filter caps, but its used up respinning the motor in the other direction in another 200ms, so cap heating from the caps leakage is not a problem. Essentially the same condition exists for TLM's reversal since It also uses Jon's pwm-servo, but there the caps are rated at 150 working, 175 surge, and the surge is of such short duration its a never mind. With a 5" chuck for inertia to reverse, its obviously longer, around a second, but I don't tap at more than 200 revs anyway. More revs=more overshoot, hard to guess so I don't. > John Dammeyer > > > -----Original Message----- > > From: Gene Heskett [mailto:ghesk...@shentel.net] > > Sent: May-19-20 10:49 AM > > To: Enhanced Machine Controller (EMC) > > Subject: [Emc-users] hypothetical, maybe not, switching psu question > > > > Greetings all; > > > > Having destroyed one 40 amp SSR by turning on the switching psu;s > > for the two running the X&y motors and the builtin supply of the > > even bigger drive running the Z axis, already, I am now > > contemplating how to do a soft start to limit the initial in-rush of > > charging up those 3 power supplies. > > > > And doing it in a lower powered version of how I am doing the huge > > analog supply that runs the spindle motor. > > > > There, I have two timers which are controlling two 40 amp SSR's with > > a 50 ohm 200 watt resistor, so it starts charging that supply with > > its thousands of u-f's by applying the power thru that big resistor, > > then the 2nd SSR comes on applying it directly 3 seconds later. > > This has worked flawlessly for about 4 years now. > > > > Now I have obtained two more used genuine crydom 40 amp 480 volt > > SSR's, and am considering doing the same thing in essence, only with > > a much smaller ohmage R to limit the in-rush this batch of switchers > > in a similar manner. > > > > But switchers, I have heard, need that initial bump to get started > > correctly. To that end I've also ordered a pair of 10 ohm 70 watt > > resistors, which if need be can be paralleled for 5 ohms in series > > with these supplies for the first 1/2 to 1 second. > > > > But I intend to use one of these in another analog supply, starting > > the spindle psu in The Little Monster too, putting all its heavy > > power under LCNC's control. But that is a different project. > > > > These switchers are, or s/b all fused to protect them in the event > > they don't start. So I don't expect a start failure to do more than > > blow the fuse. > > > > Has anyone else any experience with this, that can add gotcha's to > > watch for in such a current limited startup? 10 ohms in series with > > the power to protect the SSR's is the basic idea. 10 ohms would > > limit at 12.5 amps of in-rush, which seems reasonable. > > > > Thanks all. > > > > 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 > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users
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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
