Problem in Analog Output To Open Control Valve

neerajpuranik

Member
Join Date
Feb 2006
Location
Bharuch, Gujarat
Posts
87
Dear Friends,
We are commissioning Siemens S7-400 series PLC with analog output module 432-1HF00-0AB0. I am attaching the schematic. The problem is when we are connecting it to control valve it is not operating although we are getting 4-20mA from AO card. On further checking we found that in open loop the voltage in the output is 18 V & current 20mA but when we connect to I/P converter the voltage drops to 5.1V, currents remains at 20mA (at 100% Set Point). At 0% SP it is 1.6V. We have tried making Negative & earth common but there is no change. Can any body give any suggestion?
Regards,
Neeraj
 
Well, that is how a current output is supposed to work, (except of course that you cannot get 20mA when the looop is open).

If the valve is getting it's 4-20 command, you probably have to either supply it with it's motive force (Pneumatic? Electric? Hydraulic? Bicycle?), and span the valve.
 
Ya your observation is correct. I am measuring the current by using multimeter in series. The problem is dropping of voltage from 18 volt to 5 volt. We are not able to understand what is causing it. In the control valve we have given 1.4 Bar pressure which you call as motive force. After receiving variable current of 4-20mA the I/P converter should give 0.2 to 1 bar corrosponding pressure provided the loop voltage is 18 to 24 V. Pls correct me if I am wrong anywhere.
 
No, a current loop will maintain whatever voltage it needs to, in order to control the loop current. 5VDC is fairly normal for 20mA on a single device loop, as most devices have a 250 ohm resistance.

So, one device, with 250 ohms gives 0.020A * 250 Ohms = 5VDC.

Put three devices in series, all with 250 ohm resistances...
0.020A * 750 Ohms = 15 VDC. The loop is controlling current, not voltage.
 
The manufacturer is Samson Controls Pvt.Ltd ( India ). I am attaching the Technical data sheet for the Electropneumatic Positioner. We don't have only one valve but the 7 valves connected to 8 - point AO module ( 432-1HF00-0AB0 ) with one output spare. The Observation is same for all the valves/ outputs.
 
neerajpuranik said:
The manufacturer is Samson Controls Pvt.Ltd ( India ). I am attaching the Technical data sheet for the Electropneumatic Positioner. We don't have only one valve but the 7 valves connected to 8 - point AO module ( 432-1HF00-0AB0 ) with one output spare. The Observation is same for all the valves/ outputs.
The line size is 2"
 
I can't find the installation instructions for your I/P on the internet, although I think it is a model 3763 from your attachment.

Is this correct?

Are you sure your wires are hooked up correctly at the I/P end? The I/Ps I use do not work if you have the wires reversed.

Also, as rdrast mentioned, you might have to span the I/P and the valve. Here is a troubleshooting guide from Swico Technologies, Inc. for Samson valves/positioners. Page 5 has some instructions for adjustments of some Samson positioners listed.

Let us know what you find...
 
Tnx Kent & rdrest for paying attention to my problem. The positioner we are using are 763 i.e for non hazardous area. We have checked the positioners & they seems to be OK. I am a little confused as our commissioning engineer ( who has commissioned same setup at many installations ) is saying the he usually gets 18V minimum when voltage across the output is measured. Is it possible we are making any wrong connections in Module. The connections at positioner are correct as I already verified it.
 
Dear Friends,
Thanks for your support & Kent tnx for the manual. We were finally able to solve the problem. As per the explaination & advice of you guys, we tried Zero & Span the positioner event though in the back of our mind we had a feeling that 5V is not sufficient. But the calibration proved successful & we are now getting repeatibility too.
Once again thnaks.
Regards,
Neeraj
 

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