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PLC Ladder Logic help needed!

Discussion in 'Homework Help' started by ruzfactor, Jul 23, 2009.

  1. ruzfactor

    ruzfactor New Member

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    I'm preparing for mu exam. I'm stuck with ladder diagram of the following problem for a PLC. Can any one check whether my Ladder Diagram is correct or not!

    Problem: When the start button is pressed, motor 1 is started and motor 2 is started 15 seconds later. Both of these motor will stop if any of these is subjected to overloading.

    Here's my rough ladder diagram. Plz correct me if i'm wrong:
     

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  2. JimB

    JimB Super Moderator Most Helpful Member

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    As drawn it will sort of work, however there are a few problems.

    1 The coils should be at the right hand of the diagram.

    2 There is no way to manually stop the motors, you need a stop switch.

    3 The overload contacts, in a practical situation there will be an overload contact for each motor. Use both contacts to stop both motors, you could use an extra rung of logic to combine the overload stops and the manual stop.

    JimB
     
  3. ruzfactor

    ruzfactor New Member

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    Thanks. I've made some correction. Can you plz check it? Will it now work for the problem stated before?

    one more question: After start sw is pressed timer will be enabled and after 15 sec it will match the preset value. The start sw will be pressed momentarily and that's why a latch is used for both motor to keep them running. but what happens for timer? will it stop at the false condition of rung3 or it will only stop at preset value no matter what happens in rung3?
     

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    Last edited: Jul 24, 2009
  4. dave

    Dave New Member

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  5. JimB

    JimB Super Moderator Most Helpful Member

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    You still dont have a manual stop switch.

    There are various different types of timer function block in most PLCs. You will have to read the manual for the PLC and understand the operation of the different types before you can make a decision as to how it should work in your application.

    JimB
     
  6. ruzfactor

    ruzfactor New Member

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    I'm using Allen-Bradley as reference. THough it's only rough diagram.

    Plz check this one. Hope this will work.:eek:
     

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  7. SPDCHK

    SPDCHK Member

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    Your 15 seconds TON timer needs the input held high for the full 15 seconds before the output will go high. Are you planning on keeping the start button pressed for that whole period?

    If not, use the Motor1 feedback to trigger the TON timer.

    The rest looks fine.
     
  8. ruzfactor

    ruzfactor New Member

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    no, the start sw will not be pressed whole time. It will be pressed once to start motor1.

    I hope this will work:
     

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