On Wednesday 22 September 2021 13:42:00 John Figie wrote: > John Figie > > But if it overshoots the > > > commanded position and goes into reverse to bring the motor back by > > more than a 1 or 2 % drive while the motor is still coasting fwd, it > > will crowbar the psu, getting it hot instantly, causeing the psu to > > do a shut down until it has cooled for 2 or 3 minutes. Even then, no > > heat in the BTS IC's. > > So if the motor is rotating forward and has enough inertia then when > you try to slow it or reverse > it it will become a generator and try to increase the output voltage > on the PSU. Is the > crowbar circuit in the PSU an overvoltage protection crowbar? If so, > then maybe that is the issue.
I think, from what I have seen on the scope, that yes, its a generator, but with the bridge switched in the reversal, its the equ of hooking a -24 volt generator to the psu. Equals instant, many amps drawn crowbar. Up to that point the psu is running dead cold, but when it shuts down, the hot transistors in the psu are hot enough that the radiated heat says that corner of the psu is burn your hand hot, in milliseconds. There is no damage, and normal operation is resumed if time to cool has been given and power is then cycled off & hack on. That FWIW, is a feature of F2 on all my machines. I have not hooked that up in hal, mainly because that would kill the power instantly, stopping the cooling fan, and extending the enforced stop from 2-3 minutes to around 10 minutes. On the scope, the dc has no up-spike, it just drops like a rock, taking around 10 micro-seconds to get down to 1 volt. That holds for several milliseconds while the psu is deciding to shut down from the short on its output. Could the BTS 7960 bridge itself actually turn into a short under those conditions?. It has a good sized heat sink that never has a detectable heat. With single digit milliohms of R when on, thats is not beyond imagining. A subtrate scr perhaps? There is a one paragraph description buried in technical gobbledegook in the manual. And there is an INH input but driving it would take external full time logic in order to meet its reaction time. > Maybe you need some way to get rid of the stopping energy with a shunt > resistor and switch that activates > before the crowbar trips. Matches my thinking but I haven't pursued it into visualizing the logic to do it with. Said another way, I am a C.E.T., getting ancient and rusty but w/o enough data to make sense of it. And being a CET, I'd druther fix it with finesse, not by making a lot of heat dumping power brute force. > In industrial motion systems a shunt resistor and switch is used to > regulate the DC bus that feeds > all of the servo axis. This is not a precise voltage regulator, it > just turns on the shunt as the voltage gets > above a set limit and then turns off again as the voltage returns to a > lower limit. > I have thought of doing something like that for the spindle psu, which shoots well above the surge voltage rating of its filter caps when doing a spindle reverse, however its only for a few milliseconds before the controller uses that recovered energy to accelerate the spindle in the new direction, doing it for free, no noticable surge in the wall current draw at all, the surge is of such short duration that there is no detectable heating of the caps during the few milliseconds of the overvoltage duration. And its been doing it for 5+ years with zero failures. From 3k revs fwd to 3k revs reverse is 350 to 400 milliseconds. > Regards, > > John > Thanks John. 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
