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Old May 26th, 2008, 02:50 AM   #1
JEldreth
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HOA with Start / Stop

So I'm installing 4 pumps (in a rather large system) and each will have an HOA switch as well as remote start/stop. I'm torn as to whether I should or shouldn't eat up 4 PLC inputs per station. 1 for Manual, 1 for Auto, 1 for start PB and one for stop PB. I know I can get away without using so many I/O, but what is best practice/Industry standards. Any help?
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Old May 26th, 2008, 06:17 AM   #2
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personally i would use all 4 per station. That will make it much easier in the future if you have to modify any single station.



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Old May 26th, 2008, 07:35 AM   #3
randylud
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Try this approach!

Quote:
Originally Posted by JEldreth
So I'm installing 4 pumps (in a rather large system) and each will have an HOA switch as well as remote start/stop. I'm torn as to whether I should or shouldn't eat up 4 PLC inputs per station. 1 for Manual, 1 for Auto, 1 for start PB and one for stop PB. I know I can get away without using so many I/O, but what is best practice/Industry standards. Any help?
You do not need two inputs for the Auto/Manual function. A single selector switch input, either NO or NC will suffice. When you program, it will reflect the condition of the selector as either being in Auto or not and if it is not, by default it is in manual if you write your program properly. As for the the start stop inputs for those PBs, you should have them. If you need further help, just show some of your work and I am sure many will be willing to help you.
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Old May 26th, 2008, 08:01 AM   #4
krk
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Quote:
Originally Posted by randylud
You do not need two inputs for the Auto/Manual function. A single selector switch input, either NO or NC will suffice. When you program, it will reflect the condition of the selector as either being in Auto or not and if it is not, by default it is in manual if you write your program properly. As for the the start stop inputs for those PBs, you should have them. If you need further help, just show some of your work and I am sure many will be willing to help you.
He's using 3 position selector switches ....
HOA = Hand - Off - Auto
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Old May 26th, 2008, 08:15 AM   #5
randylud
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Makes do difference, the hand or manual position is still only one input! If it is on, it could be in Auto, and Hand when it is not on. The off function could be hard wired into the output to prevent the coil from being energized and would not require an input to the PLC at all. Just takes two contact blocks on the operator for the selector.
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Last edited by randylud; May 26th, 2008 at 08:18 AM.
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Old May 26th, 2008, 08:34 AM   #6
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All that you should need is the Auto. The hand position should go directly to the hard wired circuit, and the off should be off.

So, the only input that the controller needs is Auto - any other position should be manual.

Then have a running feedback to confirm operational status.

Now if you want to have a start stop auto from your SCADA system, the switch should be in auto for it to work.
Reviewing the modes with this setup:

Off
Hand
Auto - Auto
Auto - Start
Auto - Stop
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Old May 26th, 2008, 08:45 AM   #7
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I've seen "hand" used with minimal engine-damage type protection, so it's not out of the question to use the input. Sometimes you want to know when your switch is "OFF", as well. In that case, you do need both the H A positions wired.

JEldreth - which is safer - H and O being unwired, or knowing their positions?
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Old May 26th, 2008, 09:06 AM   #8
Ken Moore
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For mission critical applications, where you need to run something even if the PLC is off line, we do the following:

H-O-A

In auto the PLC output goes to the motor starter, in hand the local start/stop buttons control the motor starter, in off, neither go to the starter.

The Auto position has two contacts, both N.O. when in Auto the 1st N.O. tells the PLC that it has control, the other then allows current to flow from the PLC output module to the starter. The starter has feedback to the plc as well. If the plc is trying to start the pump and it is not in auto, an alarm is generated to the operator.
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Last edited by Ken Moore; May 26th, 2008 at 09:09 AM.
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Old May 26th, 2008, 09:08 AM   #9
Stationmaster
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I concur with Oakley, as your question was about the "industry standard" for this situation.

The pump stations I have seen, and built, use only the "AUTO" position of the H-O-A switch as an input to the PLC. When the switch is in the "OFF" position or "HAND" position the PLC program translates the input as "NOT AUTO". The "OFF" position has no connection to anything. The "HAND" position is intended to run the pump even if the PLC has failed, has no power, or has been removed from the picture entirely.

That is: "HAND" is a completely independent circuit, separate from the PLC. You may consider including some safeties in this "independent" circuit by running "control power" through a set of contacts on the phase monitor and through a set of contacts on the low level sensor before it goes to the N.O. "HAND" switch contacts on its way to the contactor (or VFD).

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Old May 26th, 2008, 06:03 PM   #10
JEldreth
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Thanks for all the input. After reading the GDS it is critical to know all positions of all switches because there are a few valves that have to cycle before the pump can be started, even in the Hand position. I also need feedback to the HMI to know if it is Auto, Off or Hand so it looks like I will eat up 4 I/O for each station. Again, thanks for all the ideas and comments.
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Old May 26th, 2008, 08:49 PM   #11
Mickey
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What is/a "GDS"
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Old May 26th, 2008, 09:31 PM   #12
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Quote:
Originally Posted by Mickey
What is/a "GDS"
Gosh Darn Safety
Georgetown Day School
Global Delivery System
Geriatric Depression Scale
Global Dialing Scheme
.....
.....
need more?
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Old May 26th, 2008, 09:35 PM   #13
JEldreth
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Sorry...A company acronym, General Design Specifications. It is a written synopsis of how the entire system should operate, what parts to use, what methods and software to use. About 300 pages for a small system.
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Old May 26th, 2008, 10:25 PM   #14
Mickey
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Quote:
Originally Posted by geniusintraining
Gosh Darn Safety
Georgetown Day School
Global Delivery System
Geriatric Depression Scale
Global Dialing Scheme
.....
.....
need more?
That one I understand.
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Old May 27th, 2008, 12:29 AM   #15
elitheei
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In some places I have seen the Hand circuit used as a "jog" for field control. So in one aspect you actully hand a momentary (jog) - off - auto. I think this is quite common actually.
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