Help Needed Bad

Hello All

I need help, I am very new to the PLC world, we are experiencing some difficulty on a project. When ever take power away from a PLC box we sparious activation of valves causing unexpected actions.

Question:
Do PLC have a default setting, that when power is loss for a period of, power being unstable, that cause the PLC to drive a valve open?

This problem has occur when we power back up after an extended down period.

We dont perform a power sequence, we just cut the power. When we bring power back on line this when the problem occurs.

mainly Plc do not have a defalult settings but some istructions that you use in the program can come back in the same state they wore whan the power was shut down, upon power up.So for instance if your instruction was true, and the valve was open while the power was shut down, than upon power up this istruction can be still true causing your valve to open.

So say i do some maintance and open a valve, thats is normally closed, when power is applied the plc would continue to open the valve

or does it work the opposite way if the was closed and the instruction is true. Power is lost then restored, the valve will open?

that would be some what correct, check the instruction that tells the valve to open or close. but you have to be more specific to what type of plc you are using and the instructions that are controling the valve open close cycle. always be specific to the bone whan you are asking for help that gives everyone a better understending of what the cause of the problem could be.

I dont know enough to get very specific, this is the problem the system was shut down due modification to another system. When power was reapplied the sprinkler system activated due the the logic being something like this if the valve open/no faults start pump. So software perform correctly, probelm is the pump sprayed water and no real condition was present. How does this happen? We had this problem twice. Only after a power outages.

You are asking questions that needs more data.
I am guessing you can't see the plc code.? If you can we can ask more questions there
So lets talk about the valves. Air?
2 position, 3 position open center or closed , detented ?

N2001 Motor Operated Valves

and siemans plc

need code but I don't have Siemans.

It will depend on many things, the type of valve (already covered), the type of PLC (Siemens S7-200 or S7-300 ??) and the PLC Program.
Have you a copy of the code that you can post here?

moggie said:

It will depend on many things, the type of valve (already covered), the type of PLC (Siemens S7-200 or S7-300 ??) and the PLC Program.
Have you a copy of the code that you can post here?

Click to expand...

or S7 1200 or S7 400

NotSMART said:

Question:
Do PLC have a default setting, that when power is loss for a period of, power being unstable, that cause the PLC to drive a valve open?

This problem has occur when we power back up after an extended down period.

Click to expand...

When a PLC first powers up, all outputs should be off. It won't turn on valves until it initializes. The PLC can remember values and status, or forget. This is called Retentive and Non-Retentive memory. It's up to the programmer. A PLC also knows when a power failure occurs, and the programmer can write code to properly handle a power fail recovery.

You said this happens after an extended shutdown. A power supply with bad capacitors can do this. Let's use a 24DC linear supply for example. The main filter capacitor is bad. When cold, the output could be 18 volts, and 6 volts of AC ripple. A bad capacitor usually starts getting hot. As the capacitor heats, it gets better. After an hour or so, things might look fine. The only clue is that the capacitor is hot. If you checked, the output might be a bit low, and ripple out of spec.
Shut down for an hour, and it might start at 22 volts, with 2 volts ripple. You never notice.

Whenever there's a difference between long and short shutdowns, the problem is always related to heat. Something works hot - dead when cold.