Connect with us

Controlling a DC motor with a PLC

Discussion in 'Sensors and Actuators' started by BHEKI, Nov 13, 2014.

  1. BHEKI

    BHEKI

    24
    1
    Aug 28, 2014
    Im working on small project using PLC SIEMENS S7-200, I want to driver two DC motors but im not sure which motor to have and how I will connect it to my PLC. This PLC is having 8*24DC DI and 6*RELAY outputs. Im only having 12DC motor with - & + terminal and im not sure if It can be suited to be interfaced with my PLC or maybe there are special type of DC motors that are connected to PLC's.
     
    Last edited: Nov 13, 2014
  2. Bluejets

    Bluejets

    3,452
    698
    Oct 5, 2014
    Normally, the relay outputs on PLC are "dead contacts" or "voltage free" meaning it operates as a switch.
    You supply the voltage you need from outside the PLC , in your case 12Vdc.
    You will need to make sure the relay contacts are rated to run whatever motor you are planning to connect beforehand.
    One would need to check the above against the data sheet for the particular PLC unit though so be warned.

    Edit:- I just took a quick look at the unit you showed above and it appears that only the "add-on" modules may be relays with the stand-alone unit being digital i/o's so you will have to look into this.
    Have you read the data on this unit?
    Quite a complex piece of equipment for someone asking how to connect a dc motor to it.
    Just wondering if it is beyond your capability.
     
    Last edited: Nov 13, 2014
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,214
    1,994
    Jun 21, 2012
    The CPU222 version has eight 24 V DC inputs and two sets of three SPST relay contact outputs, each capable of switching 2A in a voltage range of 5 to 30 V DC or 5 to 250 V AC. Each group of three relays has one contact of each relay in common, connected to the DC or AC supply (external) for that set of three relays.

    So, if you are driving two DC motors, and not wanting to reverse their directions, and they each draw no more than 2 A, you are all set: just add an external 12 V DC power supply capable of supplying at least 4 A.

    If you want to reverse the motors, you must use an external reversing relay for each motor and dedicate two of the relay outputs to drive these reversing relays, using four of your six available relay outputs. Don't forget to add diodes for relay contact protection.

    If the motors require more than 2 A, use external relays with appropriate contact ratings, in which case you may choose any DC or AC coil voltage you desire for those relays, replacing the CPU222 external relay power supply with whatever is appropriate for your external relays. I would take this approach even if the DC motor current is within the 2 A range of the on-board PLC relays because it is easier to replace an external relay that fails due to a fault condition than the relays in the PLC.
     
  4. BHEKI

    BHEKI

    24
    1
    Aug 28, 2014
    CIRCUIT.png
    I tried to build a circuit which will be driven by the output of my PLC and I tested the circuit using an external 24VDC AS input to my circuit and the motor was able to run forward and reverse and I used 5VDC to trigger my relay and to drive the 12V motor, I only supplied 5V to my motors since they will only be opening and closing a boom at very low speed. But when I connected the circuit to the outputs of the PLC the relay didn't even close as to drive the motors. So I was not sure whether the problem was on the interface or what.

    This is the circuit that I come up with......
     
    Last edited: Nov 14, 2014
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,214
    1,994
    Jun 21, 2012
    Well, except for the missing connection from cathode of D2 to +5V, it looks okay. I would have expected to use a somewhat larger value for R1 and R2, but that depends on the voltage of the external power supply you use to energize the PLC relay contacts.

    Did you leave the 24 V DC supply connected to the common contacts of the PLC relays? Why not use the +5VDC supply instead of the 24 V DC supply, and drive the relay coils RLY1 and RLY2 directly from the PLC relay output contacts, Q0.0 and Q0.1?
     
    KrisBlueNZ likes this.
  6. Bluejets

    Bluejets

    3,452
    698
    Oct 5, 2014
    So which output unit are you using?
     
  7. BHEKI

    BHEKI

    24
    1
    Aug 28, 2014
    Excuse my ignorance but I dnt understand your question. Are u asking the output terminals of a PLC?
     
  8. BHEKI

    BHEKI

    24
    1
    Aug 28, 2014
    Im using the PLC that is already installed and connected from my school, they were connected as to teach the learners and I will be using those PLC for my project. Im not sure about other pins of the PLC like I have L+ bridged with 1L and 2L. I also have terminal M connected to M1 & M2. Can you also help me the use of those terminals.
     
  9. Bluejets

    Bluejets

    3,452
    698
    Oct 5, 2014
    I looked at the unit you quoted "S7-200" and as I said previously, it appears to be digital i/o for the basic unit.
    So I therefore asked which expansion module i/o unit were you using to get the "8*24DC DI and 6*RELAY outputs".
    I do not see any combo expansion units with the numbers of i/o's that match what you have quoted.
    I see individual expansion units with 8-24v in but none with 6 relays.
    This would be the starting point to determine what you can drive at the output.
     
  10. Kiwi

    Kiwi

    313
    71
    Jan 28, 2013
    BHEKI.
    I use Omron PLC's, so not too familiar with Simens.

    Looks like you have a CPU222 module with 6 relay outputs. The outputs are split into two sets of three with a common for each set. Each output is rated at 2A, with a total maximum of 10A for the module.

    Please advise the full part number of your module, so that I can be of more help.

    What is the current of you motor?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-