More beginner questions!!

MasterBri

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Join Date
Mar 2018
Location
Ohio
Posts
12
Im looking into parts to buy for a project I have on my hands, I think I have somewhat of a handle on things but I want to be sure before I start ordering stuff.

First the what: My first step in this process is to get a plc and some parts in hand to make a simple circuit work. I currently have a fairly large machine with 2 electrohydraulic actuators centrally located on either side of the machine. Controlling the actuators I have 2 up/down pendant stations on opposite corners of the machine so it can be operated from either side. This system is run with 24V and the EHAs pull around 40A when maxed out. I have 24v running into the machine and a mess of wires running from side to side of this machine with a couple relays controlling the motors. The system is wired up and running currently but I would like to put in a PLC so that I can add some logic points in at a later date.

My first goal is to get a PLC and some contactors to recreate the current situation: push up on either side and it goes up, push down and it goes down..

I think I have the contactors down to a couple Allen-Bradley 100-C72DJ10, one for up and one for down. I am thinking it could be wired as follows:
Contactor_Wiring.jpg


One thing that I'm having trouble with is figuring out witch type of PLC to get.. Sinking vs. Sourcing.
I believe that I want sinking in order to keep amps away from the PLC?? Or would it not really matter at this point because the only circuit that would really be running through the PLC is the pushing of a button in order to trigger the contactor coil?.
 
I am not worried about having an Ethernet connection at this point..

Unless you give me some upsides that could change my mind..
 
it looks like the Micro820 is a sourcing PLC then?
I think this brings me back to one of my original questions... Does it only source the small amps related to operating a push button that would trigger the contactor and then the brunt of the amps would run through the contactor to the motors?

Also if I wanted more I/o I would just add in an I/o module then?
 
First, here are just a few reasons why you may want Ethernet.
Modbus TCP - HMI - Internet.

If you need more I/O just add module.

As for as the outputs goes it is "Relay" it just closes a set of contacts.

Inputs can be wired to sink/source.
 
I would ditch that wiring, because it can be simplified a lot:

Inputs:
Down PB1
Down PB2
Up PB1
Up PB2

Optional Inputs:
Feedback from K1
Feedback from K2

Outputs:
K1
K2

Do all your interlocking and whatnot in logic. As for sink/source. I would use sourcing outputs and interface relays, or relay outputs.

**EDIT

As for safety, I would completely segregate your safety and control. Depending on the PL required, I would use something like an E-Stop with a relay and drop the contractors, or even the power source out. Depending on your architecture, you may want to add an input for the Estop/Relay status.
 
Last edited:
I would ditch that wiring, because it can be simplified a lot:

Inputs:
Down PB1
Down PB2
Up PB1
Up PB2

Optional Inputs:
Feedback from K1
Feedback from K2

Outputs:
K1
K2

Do all your interlocking and whatnot in logic. As for sink/source. I would use sourcing outputs and interface relays, or relay outputs.

**EDIT

As for safety, I would completely segregate your safety and control. Depending on the PL required, I would use something like an E-Stop with a relay and drop the contractors, or even the power source out. Depending on your architecture, you may want to add an input for the Estop/Relay status.

I like this kind of info!!
Im still kind of confused as to the use of contactors vs relays? Are you suggesting to use an interface relay or output relay instead of a contactor?
 
I like this kind of info!!
Im still kind of confused as to the use of contactors vs relays? Are you suggesting to use an interface relay or output relay instead of a contactor?

It does depend what you are switching, if you are turning a light bulb on then use a relay, if you are running a motor or heavy load then you will need a contactor, however in this context i would think he's suggesting the use of an interposing relay between the PLC and end device, this is common as the contactor coil burden may be above that of what a PLC can switch, unless you use a PLC with relay outputs.

NOTE: You can actually get low burden contactor coils specifically designed to be switched from PLC transistor outputs
 
Last edited:
No he's not, the use of an interposing relay between the PLC and end device is common as the contactor coil burden may be above that of what a PLC can switch, unless you use a PLC with relay outputs.

NOTE: You can get low burden contactor coils specifically designed to be switched from PLC transistor outputs

I see..
Im looking at a contactor now and under the Input data specs it says: Rated actuating current ~12mA.
This would be the current it would take for the PLC to trigger the contactor correct.
 
I see..
Im looking at a contactor now and under the Input data specs it says: Rated actuating current ~12mA.
This would be the current it would take for the PLC to trigger the contactor correct.

Sounds like it is but without seeing the spec i can't say for sure
 

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