Hi Ray, > So let's pin you to the point. Do you see large current surges between > the power supply and drive during acceleration of a stepper motor? > Would the extent of those surges cause you to choose one kind of supply > over another? >
Yes. You do see some quite big swings in current. With high inertia machines you can get current going backwards into the power supply as the motor decelerates. The motor turns into a generator as it tries to absorb the momentum. This can be a problem in some cases as it will pump the supply voltage up. In extreme cases it can push the voltage up high enough to damage the drive. This is why Mariss recommends that you put in some sort of current dumping circuit if you are running near the edge with a big machine. I have a suspicion that was a contributing factor to my spate of exploding Geckos when I had steppers on my Bridgeport. Bigger capacitors help but they will put more strain on the transformer and rectifier. Actually this brings up an important point with stepper power supply design. Your safety emergency stop circuit needs to break the power to the drives. However it should be _before_ the capacitors. This way the caps stay in circuit and absorb the big surge you get from trying to suddenly stop the machine. For a heavy machine you want to switch in a big power resistor across the caps at the same time to discharge them quickly. Don't use an ordinary relay for this job. High DC voltages tend to weld relay contacts - BTDT. Use a contactor. They are designed to safely switch high voltages and currents. Internally they use two contacts in series so even if one welds the other will still break the circuit. I have seen contactors fail open circuit but I have never seen one fail closed. I do repairs on CNC machinhes so I get to see a lot of contactors. Actually, I find that same power supply design works well for servos. If you hit emergency stop while the machine is moving, the motors will power the drives while they are trying to bring the machine to a halt. As soon as the machine stops, the motors stop generating so there is no more power available. This approach unfortunately doesn't work with Gecko servo drives as they fault and free-wheel the motor when you e-stop. It does work with Rutex drives and it should work with most analog drives controlled by EMC. Theoretically if you have a runaway and hit a limit then you would be better off without the caps but TBH in that case you are in pretty deep trouble anyway. Switch mode power supplies tend to be very well protected. This is good in that they will shut down if they detect a problem but it is bad for the very same reason :-). Switch mode supplies prefer a steady load and really don't like having power shoved back into them. For this application I think a linear is the best bet. Simple and rugged. > BTW I enjoy the 42 reference. I think I've figured out one possible > reason for his choice of that number. It's the age when far sighted > folk need glasses or eye surgery. > LOL. That isn't an explanation I have come across before :-) Douglas Adams RIP. So long and thanks for all the fish... Les ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
