plchacker
Member
While modifying a couple of my trainers I ran into an interesting situation. The trainer is for the AB SMC 180 drives. This little drive is similar to the 1305 VFD. For controls, it uses its own sinking 12VDC power source. In three wire controls the stop (like always) should have normally closed contacts, while Run and Rev use normally open contacts. Fine and well. Try to run one with a PLC and the obvious solution is to use relays.
So.... I ordered four DIN mounted SPDT relays for each drive. The first to select between the panel mounted PB's or allow the PLC to take control, and the other three for three wire controls from the PLC.
Remember the Stop contacts, yep, they will allow the voltage to feed back to the power side of the other switches/relays, and allow the drive to be started or reversed from the wrong control (either PLC or PB's). Stopping can only be accomplished from the selected control, PLC or PB's. BAD NEWS~!
In a mill, I would use the PB's as inputs for the PLC, and only use three relays. In our lab, with the nifty (and expensive) little trainer, this is not an option. The other thing is that in a mill I would have plenty relays on hand, and adding either a DPDT relay to control the stop, or a fifth SPDT to do the same would be no big deal. In my lab, that means jumping through the money hoops to get the extra relays. Again, this is not a mill. The trainer must function with or without the PLC.
So, I used Diodes. To keep my wiring clean I used two per trainer, isolating the stop contacts (relay and PB). This works like a champ.
At long last my question. Is this acceptable? It works fine, and will soon be documented. The students will not see this until after Christmas. I just want to cover my bases and not present my students with bad practices. In the real world I have used diodes/MOV's to contain spikes from inductors like sol. valves and the like. So I don't see any real issue with this, but above all, I want to produce students that understand good practices.
Thanks for any comments.
So.... I ordered four DIN mounted SPDT relays for each drive. The first to select between the panel mounted PB's or allow the PLC to take control, and the other three for three wire controls from the PLC.
Remember the Stop contacts, yep, they will allow the voltage to feed back to the power side of the other switches/relays, and allow the drive to be started or reversed from the wrong control (either PLC or PB's). Stopping can only be accomplished from the selected control, PLC or PB's. BAD NEWS~!
In a mill, I would use the PB's as inputs for the PLC, and only use three relays. In our lab, with the nifty (and expensive) little trainer, this is not an option. The other thing is that in a mill I would have plenty relays on hand, and adding either a DPDT relay to control the stop, or a fifth SPDT to do the same would be no big deal. In my lab, that means jumping through the money hoops to get the extra relays. Again, this is not a mill. The trainer must function with or without the PLC.
So, I used Diodes. To keep my wiring clean I used two per trainer, isolating the stop contacts (relay and PB). This works like a champ.
At long last my question. Is this acceptable? It works fine, and will soon be documented. The students will not see this until after Christmas. I just want to cover my bases and not present my students with bad practices. In the real world I have used diodes/MOV's to contain spikes from inductors like sol. valves and the like. So I don't see any real issue with this, but above all, I want to produce students that understand good practices.
Thanks for any comments.