On Wednesday 13 February 2013 11:18:51 Stephen Dubovsky did opine: > My 1996 baldor dc servo drives use contactors on the outputs (Supermax > mill.) The limit switches open the contactors, which disconnect the > drives from the motors, and put a power resistor across the motor leads > for braking. You have to manually move the drives off the limit > switches. There are small degreed wheels to do this. Sort of like the > degree wheels on a bridgeport but w/o the handles. Since it should > never hit the limit switches this isn't a big deal. > > Disconnecting a motor from drive/vfd isn't the problem. Reconnecting it > while its still powered is what typ lets the magic smoke out. > My point was, and still is, that relays & switches, unless mercury wetted, are generally dirty at both operations due to contact bounce or whatever its called in your neighborhoods, so opening the circuit with them will be seen on the scope as several open & re-close operations over a period of about 8-10 milliseconds. The open shouldn't hurt, but the re-close while the back emf and any inductive spikes are present at the motors terminals, can let the magic smoke out even on an opening.
Morale of the story is: if the drive has an enable function, use it, then kill the power to the driver in however long it takes the relay to do it, its much safer. Killing the power, due to the capacitance used for filtering in the driver, absorbs the effects of the contact bounce but will still render it pretty powerless in 50 milliseconds, with minimum stress on the driver since it will have spent that 50 milliseconds trying to stop the motor. Ideally, re-closing that safety open should involve powering down the power supply long enough for it to decay to only a volt or so before re-closing those contacts. The reasoning there is to prevent the contacts from micro- welding themselves together again from the extremely high currents that would flow at the instant of contact, between the filter caps in the power supply, and the filtering in the driver brick. I would use the backside contacts of a relay, the NC contacts for this because it would be easy to 'seal' the relay in the on state, so the NC contacts remain open until such time as the PSU itself was turned off and the relay looses its seal in voltage. > On Wed, Feb 13, 2013 at 9:37 AM, Dave <e...@dc9.tzo.com> wrote: > > On 2/13/2013 12:05 AM, Gene Heskett wrote: > > > On Wednesday 13 February 2013 00:00:37 Dave did opine: > > >> On 2/12/2013 10:47 PM, Gene Heskett wrote: > > >>> On Tuesday 12 February 2013 22:36:22 Przemek Klosowski did opine: > > >>>> On Tue, Feb 12, 2013 at 10:57 AM, Dave<e...@dc9.tzo.com> wrote: > > >>>>> I have a large CNC lathe that has contactors wired in between > > >>>>> the servo drives and the motors and on an Estop, the contactors > > >>>>> drop out and the 3 phase servo motor windings are shorted > > >>>>> together to stop the motors. This could also work as a short the coils only to finish stopping the motors, but should be delayed until until the driver is without power, say 100 milliseconds after that relay above has opened and removed power from the drivers. But I'd think the control sequencing complexity to do that would be more trouble that its worth. The might involve some braking resistors also as a safety measure for PM motors, which can if directly shorted as speed, actually generate enough current to damage the PM's. I do that even on my toy mill, using 'suicide brakes' on its spindle to stop it, but with a 20 ohm 20 watt resistor as the braking resistor. It turns a bit stiff when stopped, but still stops from 2500 revs in a little over a second. > > >>>> > > >>>> I thought this is a no-no---opening of the circuit causes the > > >>>> servo driver stage to abruptly change from high-current to zero > > >>>> current flow, bound to cause transients in every inductance in > > >>>> the system. THere's a standard warning for the people rewiring > > >>>> their equipment with VFDs to take the reversing drum switch out > > >>>> from next to the motor, and replace it with something that > > >>>> commands the VFD to reverse. > > >>>> > > >>>> Is it one of those things that shouldn't be done routinely but is > > >>>> OK in an emergency? > > >>> > > >>> Generally its a big "no" on that. The VFD probably assumes there > > >>> is a motor out there, and using a switch to interrupt would be a > > >>> bit hard on it because the average switch breaks dirty, going on& > > >>> off for 5 to 10 milliseconds, and re-closing the switch at an > > >>> unknown position in the VFD's output sequence stands a very good > > >>> chance of letting the magic smoke out of it, and we all know > > >>> things don't work without that magic smoke. Do ALL your starting > > >>> and stopping via the input controls on the VFD, so that it can > > >>> handle the sequences properly. > > >>> > > >>> This is also true for stepper drives. The most solidly connected > > >>> wires in the system should be between the motor and the driver. A > > >>> flaky connection there will blow the tops off the chips in the > > >>> driver. Instantly in terms of human time. > > >>> > > >>> Cheers, Gene > > >>> > > >>>> I thought this is a no-no---opening of the circuit causes the > > >>>> servo driver stage to abruptly change from high-current to zero > > >>>> current flow > > >>>> > > >>>> Generally its a big "no" on that. The VFD probably assumes there > > >>>> is a motor out there, and using a switch to interrupt would be a > > >>>> bit hard on it > > >> > > >> That is what I thought also.. however that is the way that lathe is > > >> wired. It uses Siemens servo drives for the feeds and the spindle > > >> and the drives are original. > > >> > > >> I would not recommend doing that to any random servo drive, but it > > >> seems to work with these Siemens drives. > > >> > > >> Dave > > > > > > And they were designed which side of Fred& Wilma Flintstones > > > wedding? > > > > > > Sorry, just couldn't resist, sometimes this stone age stuff turns > > > out to > > > > be > > > > > pretty tough. > > > > > > It had to be when Barney Rubble was turning the cranks. ;-) > > > > > > I'll get me pj's& toddle off to bed now... > > > > > > Cheers, Gene > > > > They aren't That old! 1992 Vintage. Not too long after the wheel > > was invented and well after the creation of "dirt". > > > > So they aren't older than dirt. ;-) > > > > Siemens made very similar servo drives up until about 2000, but they > > changed the packaging. > > > > Dave Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! My views <http://www.armchairpatriot.com/What%20Has%20America%20Become.shtml> Do not use or store near heat or open flame. I was taught to respect my elders, but its getting harder and harder to find any... ------------------------------------------------------------------------------ Free Next-Gen Firewall Hardware Offer Buy your Sophos next-gen firewall before the end March 2013 and get the hardware for free! 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