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Old November 19th, 2019, 02:38 PM   #1
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sombra is offline
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Help with Lead Lag/rotation ladder logic

Im currently working on programming a Modicon TM221 PLC.
I'm using Machine expect Basic/Somachine, I'm having a hard time programming lead lag with pump rotation.

I have 4 vacuum pumps, If lead pumps run continuously for more then 5 hours then it will automatically sequence the pump attempting to evenly distribute the run time among all available pumps.

Any help will be greatly appreciated!
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Old November 19th, 2019, 04:17 PM   #2
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Well first thing you will need to do is establish the order the pumps will run. So you will need to decide which one is Lead, Lag1, Lag2 & Lag3. Do you have a HMI that you can use to select which one is which?

Once you have that established you can start on Lead when it reaches 5 hours of run time switch to Lag1. Then when it runs 5 hours switch to Lag2 and so on.

You may need to add logic for alarms and no start situations but this should get you going.
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Old November 20th, 2019, 08:09 AM   #3
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No HMI, but they are on a Switches that has 3 positions, Off,Hand,Auto.
all in Auto position so that the PLC can control the system.
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Old November 20th, 2019, 11:32 AM   #4
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One thing often overlooked on any "rotation" scheme is that you are balancing the run-hours.

If a pump requires some form of maintenance at XXXX hours, say shaft seal replacement and the like, then they will all come up for maintenance at roughly the same time, which the maintenance team are not going to like.

Whenever I do any rotation scheme I usually introduce an initial run period for the lead pump, a shorter initial run period for lag1, etc., so that the maintenance requirements are spread out. Once you've got them "out-of-sync" cancel the initial run periods, and then go to a true "rotation" scheme.
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