1. why do you switch the dc with the estop relay? (opening contacts with dc on them realy eats contacts. wouldn't it be better to cut the ac before the bridge?)
2.why a four pole relay for the estop what else should be run through this? (incoming vfd power?)
3. should the estop drop all incoming power?
4. should i power the dc3iob & mpu11 through a computer ups? how much power do these boards take? would be realy thankful in a blackout
1. Switching the DC ensures that the supply is cut immediately. If you interrupt the AC supply power will continue to be available for a short time while the capacitor discharges. How long depends on how much load is on the drive. You are right that it is hard on contact points if you interrupt it under load.
2. Flexibility. Sometimes 3-phase power was routed through there, either on the way to an inverter, or on the way to other motor starters (e.g. flood coolant pump etc.). In recent years the common recommendation has been just to use the additional poles to interrupt the coil power to the other motor starters, and the inverter's sequence common that is used for run enables.
3. That is a subject of debate in the industry. It is arguably safest to interrupt all power to anything that moves (servo drives, spindle drive, accessory motors, hydraulic pumps, etc.). However, frequent cycling is hard on the bus charging circuits of the servo drives and spindle drives, and cutting off line power on E-stop prevents a drive from actively decelerating its motor. The spindle will stop much quicker if you only interrupt the enable, than it will if you cut off power. A more complete solution to the deceleration problem (at the expense of more complex wiring) is to interrupt the enables immediately, and use an off-delay timer to interrupt the line power a few seconds later.
4. That would not be a bad idea. The logic power consumption is very low (probably less than 100W for the two units combined). Note that, if you shut off power to your computer, you also need to shut off power to the MPU11. I would just put the computer, MPU11 logic supply, and DC3IOB logic supply all on the same UPS. Even a small one would be more than enough.
thanks for the intelligent reply
more Questions
1. if the dc is cut will the dc axis drives decelarate or coast to a stop?
2. if they coast, would cutting the ac give a few miliseconds of decelaration? (i relize that if they are not moving it will take a lot longer to discharge the capacitor)
I doubt switching the AC will help. Yes, there is power in the cap and the drive could conceivably use it and/or regenerative braking to slow the motors but I believe that the drive shuts down the FETS as soon as it recognizes and estop condition.
