On Thursday 14 June 2018 19:40:49 Chris Albertson wrote: > This is a general motion controller question.... > > A couple days ago I blew up a pair of mosfets What I think happened > was I had a motor going forward then switched to reverse. Basically > slammed over. The motor likely developed it's full stall current of > 21 amps. This blew the fuse I had on the power supply. Then I'm > guessing the motor finds itself inside a powerful magnetic field and > has open circuit leads and puts some large voltage across the mosfet > H_bridge letting some smoke out. > > I think that is the physics. basically a motor and some attached > rotational mass trying to act as a generator powering a high impedance > circuit. Not 100% sure I understand this well enough to calculate > voltages. > > Question: This must be a common problem. Is it common to use some > kind of diode voltage clamp to limit the volts across the controller? > I've also heard of placing a reversed diode in parallel with the power > supply fuse to basically sort the back EMF. > > What are people doing with DC brushed motors that work with currents > up to a couple tens of amps? > I'm doing that twice, once on TLM, my 7x lathe, and once on the g0704, both with 1 hp at the label motors, and both with psu's bigger than the motors nameplate voltage and capable of 20+ amp surges, 15 till the cows come home.
I trip the safety shutdown latch on one of Jons pwm-servo amplifiers occasionally on a hard turnoff, requiring I climb the short stepstool an poke a plastic tool thru a hole over the reset button on Jons amp. It was often enough that I got into the habit of writing my gcode to slow the motor to 20 revs for about half second before issuing the m5 to stop it. However, it is not a problem with rigid tapping as the motor is never out of control at high rpms unless you issue an m5 while its running wide open. That is where you'll trip out if you are going to trip out. Jons pwm-servo's also have a current limiter that is non-disabling, and if you listen carefully you can hear a short chirp from the motor when the m3-m4-m3 reversal is taking place, but I pretty much minimized that with a limit3 in front of the PID.command in order to slow the ramping of the signal to something the amplifier can handle. ISTR thats a setp of 400 for both the voltage and the current. > What is the maximum back EMF current? I'm guessing that a motor can > only generate current up to it's stall current. No, it can generate both a higher voltage and a higher current, higher voltage into an open circuit, and a higher current into a short circuit. I have seen the psu, running at 126 volts under normal load, peak above 160 at turn around time when using Jon's amps because they absorb the energy when the amp is slowing the motor by dumping it back into the psu. Thats also above the surge rating of that huge bank of caps used for a filter, but its also not for long enough to heat the caps since the accel loading in the other direction pulls it back down to normal in maybe 200 milliseconds maximum. Basically, if you are going to do spindle motor control, you have 2 choices. A vfd with its input profiled to be brutal, but not destructively so, or a dc controller that does a full 4 quadrant control. There are others I'm sure but in the 1 or 2 horse field, Jon's pwm-servo is king of the crop. Properly programmed, its a hocky puck, you can't make it hurt itself. For instance, at 2900 revs in high gear, I can reach up and change the head to low gear. The secret? Two switches and a notch in the edge of the knob. The pid gets normal drive if either switch is closed, meaning the back gearing is fully engaged. But if I grab the knob and start to turn it, within 4 or 5 degrees of rotation the notch is no longer allowing the switch to be closed. That steers a mux4 whose inputs are for the no switch closed condition are setp'd to about about 20 rpm, so in a few milliseconds, much faster than I can turn the knob, the motor is down to creep speed, and stays there, facilitating the gear engagement and doesn't restore full speed until the knob is firmly seated in the other gear, allowing the other switch to close. These same signals also switch the tach range so the tach remains extremely accurate. As does all the signals needed for rigid tapping. I obviously don't mind hacking around in hal files. :) > > Why did the motor go from full forward to full reverse? In this case > it was dutifully following a hand controller but it could result from > other dumb things like an error in programming. > > I can think of all kinds of solutions but I figure some have been > tested by time and are well known, The limit3 module is tailor made for gentling the sequencing this stuff, give it a shot. -- 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) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
