Wayne, thanks for sharing. There aren't all that many guys that have actual experience with DC contactor's, not just theory. My experience was limited to their sales, applications, and support. Because the were just a small portion of what my responsibilities were I still had to get up to speed on them in order to be a viable support resource as well as good support from my engineers.
I have worked on DC cranes before this job at the steel mill, but I'm a little rusty and get some things switched around in my mind as I did in post 16 which for some reason I was thinking a shunt motor:ashamed1:
But being at the mill for over a year now and getting back into the swing of trouble shooting the many DC motor applications we have out there has been a learning experience as some of the things I have seen are all new to me.
I do know that you can design the circuit to use the lower cost AC contactors to swap the polarity and use a single DC contactor to remove the motor load before each time the polarity is changed, the circuit drops out the motor then it is reversed then you can plug the DC contactor to start the motor in the reversed direction, this way the reversing set of contactors never break under a load and the arc problem is eliminated on the AC contactors, it could save a little cost on the price of 4 DC rated contactors but will add to the complexity of the control system and the trouble shooting, to maintain the battery back up but allow the control to use 120 volt AC coils or even 230 volt AC coils you can use a low cost DC to AC inverter that run on 250 volts DC and output 120 or 230 volts AC, this will allow you to design most of your control system around an AC circuit with only switching the DC contactor through a series set of tips from a control relay that only has to be sized for the coil load of the DC contactor, installing aux contacts on the AC reversing contactors will prevent the DC contactor from pulling in if the contacts on the AC reversing contactor have not yet made, this will prevent the AC reversing contactor from ever switching under a load, if a DPDT contactor rated for the motor load could be found the circuit could be made even simpler with only one coil to switch between forward and reverse, the common of each pole would be to the armature common 1 would go to A1, common 2 would go to A2, NC on pole 1 would connect to NO on pole 2 and connect to the DC contactor which the other side of the DC contactor would come from the positive feed, the NO of pole 1 would connect to the NC of pole 2 then feed S1 of the field winding, S2 from the field would go to the negative return of the circuit, with the reversing DPDT coil not energized the motor would run CW, energize the reversing contactor coil and the motor will run CCW.
Let me see if I can draw this up in paint to which I'm not very good at:happyno:
Ok heres a stab at it: