On Thursday 24 December 2020 03:18:24 Chris Albertson wrote: > "All the time?" When you have a hard requirement like this is when > some detailed engineering can pay off. Assume you are reversing at > 50% duty cycle and burning 1KW 50% of the time. Where I live this > can cost 12 cents per hour. This is $89 per month if you actually do > run "all the time". Or $1,000 per year. Or maybe you are usig a > lower value resister and burning 4KW with your brake? That is $4K > per year > > A better way is to use a battery inside the power supply. a 100 > amp-hour lithium battery would cost possibly $400. Then the braking > energy goes into the battery and then a few seconds later the battery > is used to run the spindle in the forward direction. The battery > needs to be sized so that the energy transferred each cycle is small > (maybe 1%) of the total capacity. The power supply only has to > supply power to > overcome friction. Most of the energy to accelerate the load comes > from the brake. > > This is exactly how a hybrid car works. They are most efficient with > batteries that work at about 400 volts or more. > > THis said, I doubt anyone is actually going to run a 7.5KW motor at > max performance forward reverse cycles for hours on end. But if you > do running a stove element is not cheap. > > It is really not very complex to brake with a battery as long as the > battery is very large relative to the current. A double H-bridge is > all you need. > > You make a very good argument. Using my GO704 with a stock PMDC motor as an example, but running on around 127 volts instead of the nameplate 90 volts, administered by one of Jon's pwm-servo amps, with the reverse sequence controlled by some limit3 in the hal file, I block the reversal and program a quick stop by zeroing the input to a limit3, which dumps the motor back into the psu, running it up to about 170 volts as it does so. But the stop is sensed by a oneshot watching an encoder signal, and when 10 millisecs has passed, with no encoder, it finally allows the reverse to get to the controller, the requested speed input to the limit3 is restored, and that ramps it back up to the S speed set, using up that excess charge in the psu. only when the charge in their considerable number of microfarads is used up does it resume drawing power from the powerline. And the reversal time, even at 3000 rpm is less than 400 milliseconds. That peak at 170 volts is well above the voltage rating of those caps, but it is such a short duration that the average heating from their leakage has not damaged them in close to 5 years. And at tapping speeds of say 500 rpm in low gear, the overtravel at the bottom of a G33.1 is well under a full turn. Jons pwm-servo is set to limit the currant at 17 amps, not quite double the nameplate 9.7 amps, and the current limit makes the iron in the motor chirp for a small fraction of a second. NO other excitement. When I am tapping a hole big enough I have to peck the tap, I have a 4 second wait at the top of the routine, long enough to clean the tap with an air hose and give it another drop of cutting oil. I have tapped 7/16xUSS holes in 1/2" alu that way.
Pretty good for a cheap machine with plastic gears that is permanently out of tram because the post isn't vertical. The bolt holes are far enough off I can't fix it without reaming the holes thru the post to the next mm up. A reamer I didn't have the last time I had it apart. But that is a different story. The point is, that I am recovering that energy from the motor, and using it to restore its speed in the other direction, and its equally as effective at the opposite M4 to M3 reversal at the top of the stroke. I am using a similar bit of hal trickery in the rpi4 config on the Sheldon with a resistorless vfd. I rigid tap there at 100 rpms, and with a nearly 40 lb 8" chuck mounted, the over travel there is .24 turns. Much worse at higher speeds of coarse. Its a permanent part of my axis display. > On Wed, Dec 23, 2020 at 9:23 PM Scott Harwell via Emc-users < > > emc-users@lists.sourceforge.net> wrote: > > THE Huan Yang 7.5 requires 1000 watt at 75 ohms. > > > > > > > > > > On Wednesday, December 23, 2020, 8:17:03 PM CST, andrew beck < > > andrewbeck0...@gmail.com> wrote: > > > > Just to tag in. > > > > I need a braking resistor for my cnc mill spindle it's a 7.5kw vfd. > > > > Needs to rigid tap all the time etc 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
