MGL6973 (Customer) asked a question.

October 27, 2023 at 3:34 AM
How to stop an off delay timer before the timer has elapsed

Hi,

I'm using an off delay timer to run a motor for 10 min. The motor is controlled by a typical start stop switch. I'm trying to make an emergency off to the motor (using the stop switch) before the timer has elapsed. I understand that the timer off delay is starting the timer once the rung changes from true to false. The software doesn't allow me to use a reset on T1. If I use a reset or latch that I can open on my Y1 it will only temporarily stop the motor while holding down the stop switch. Am I understanding this correctly?

 

Thanks

  • 7 answers
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  • Todd Dice (Customer)

    I agree with RBPLC that for safety, you SHOULD NOT rely on the PLC to act as your Emergency Stop. You should have a hard wired Master Control Relay circuit powering the output common to safely drop out power to the motor circuit. That said, a simple coding of what you have described:

     

    Screenshot 2023-10-27 080145

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  • Tinker (Customer)

    While I'm not 100% sure, I think one can directly modify the timer data. For your example of T1, TD1 stores the current time and if one forces that to be equal to or greater than the set point the timer should turn off (for an off delay, or on for an on delay) . One could use COPY or MATH to change the value.

  • PouchesInc (Customer)

    You shouldn't use an off delay and a signal dropping to start your motor. It is unsafe to run something like a motor based on a signal dropping out, because if a wire breaks or becomes loose then it can start running the motor when the PLC sees the signal drop. Using a standard timer and a true signal from a switch to start something is not only safer, it solves the problem you are having of not being able to stop the timer.

  • RBPLC (Customer)

    An emergency off switch should not be dependent on the PLC to stop the motor. The emergency switch should disable the motor regardless of what the PLC is doing.

  • Todd Dice (Customer)

    I agree with RBPLC that for safety, you SHOULD NOT rely on the PLC to act as your Emergency Stop. You should have a hard wired Master Control Relay circuit powering the output common to safely drop out power to the motor circuit. That said, a simple coding of what you have described:

     

    Screenshot 2023-10-27 080145

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  • MGL6973 (Customer)

    That does make sense. Fortunately there are multiple emergency stop switches around the facility that will kill all of the control circuits as well as the line voltage to the motors in this area. The stop function I'm looking to use would be used if the user didn't want to run the full time limit.

     

    How would I start a motor that needs to run a specific time-frame without using a timer? Or are you saying that I should use the TOF but without a dropping signal?

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  • MGL6973 (Customer)

    I just saw your design. I'm obviously new to this... I never even thought of using a on delay to make this work. Thanks for all of your and advice.

    • Todd Dice (Customer)

      It is just easier to stop an on-delay timer via power loss than having to code a forced value to the "current" of an off-delay timer. Off-delay's have their place but is not the right choice for your need. An example of a good use of an off-delay is wanting a light to illuminate for a time after a button push. New or old, people on this board are always happy to provide advice/help. Good luck!

 

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