S7 300 analogue output problems

[Hartman 0]

Hi, I've recently come across a problem with one of our plc's its a siemens s7 300 and the problem comes from 2 analogue output cards (6ES7 332-5HD01-0AB0). These cards have 4 outputs each of which we use 3 channels on each. These 3 channels control 3 hydraulic motors (3 seperate conveyors) via proportional valves which are powered by vickers power plugs which are supposed to get there 4-20mA signals from the analogue output cards-problem is for some reason the output range from each channel for all six conveyors is 6.23-8.79mA? The first problem is i find it very difficult to interogate siemens programs as i'm new to this so i'm not sure if this range has been set in the program and where. Secondly to try and bypass the problem of the output range i bought an externally powered signal convertor (6-9mA input with 4-20mA output), which worked perfectly for one conveyor but drained the power from the other 2 active output channels rendering them useless. So can anyone tell me if at some point someone realised that the full 4-20mA signal from each channel was unobtainable for some reason and changed it to allow all 3 to operate all be it in a smaller range..and also why is the output voltage for 1 card 1.6v and the other 2.3v?

[Mike Dyble 0]

Could you post the wiring diagrams, first thought is that it's a wiring problem.

[Hartman 0]

Hi mike, I dont really have an e-drawing i can give you but i can tell what we checked out so far. We obviously checked the cable and resisistances and they were fine-we also hooked up a process calibrator from the plc panel and injected the 4-20mA signal and at the other end (60m away at the power plug) we got the full range as required. We also tried a spare power plug wired direct at the plc terminations-still only the 6-9mA range. Also we have tried floating earths and grounding both ends but nothing seems to change the output range. I'm almost certain that the signal from the output card is just not powerful enough for some reason, maybe

[Moggie 17]

Have you checked the power supply to the analog card? (24V DC) Is the analog output wired correctly? As Mike say's it sounds like a wiring/connection problem!! Here is the link to the Siemens web site for "S7 300 module data". http://support.automation.siemens.com/WW/view/en/8859629

[BobLfoot 301]

You mention a "Process Calibrator" would you be using something like a Fluke 787 Processmeter? If so I'd suggest the following tests. 1. Hook only the 787 direct to the PLC output and record the current range attained. 2. Hook the 787 direct to the Vickers unit 4-20ma input and cycle it thru the range of 4-20 and observe operation. 3. Repeat Step 1 but place the 787 where the Vickers presently is. 4. Repeat Step 2 but place the 787 where the PLC Presently is. Your results should give you some idea were the problem lies.

[Hartman 0]

Hey, thanks for the reply. We are using a fluke 744 process meter. And yes we believe it is wired correctly. We have tested each part extensively and whatever we put in at one end we get at the other. I should point out that the way it is at the moment (with the output range 6.2mA to 8.7mA) the conveyors all run,and have done for years, but setting the power plugs up with this range is quite difficult. The output from the plc does range perfectly via the scada user control screen 0-100%-just in the smaller range. All 3 conveyors recieve the smaller range from their respective channels. Is it possible to limit the output range of a 4-20mA signal from an analogue module from the program itself? The program is designed to control the speed range of all three conveyor feeds over 8 stages according to the absorbed power of the shredder (hammer mill), could it be stuck in one speed stage? Maybe someone could look at the program-i dont follow the siemens codes very well! Thanks

[Moggie 17]

Please post a copy of the program, also a copy of the schematic is possible

[ramaslabas 1]

Grounding only one side of cable shield. Sometimes it really works.

[Hartman 0]

Hi again! Thanks for all the replies, the situation at the moment has been resolved by ignoring the outputs from the plc. In other words we now control the 3 conveyors via a PID controller in our dcs which outputs three 4-20mA on seperate cards to drive the proportional valves. The plc still has the active outputs we just disconnected them and re-routed from our dcs. The result has been very satisfactory with throughput from our mill increased dramatically. i'm going on a siemens course to help with future problems!

[ianbuckley 0]

The analog input cards I have used have a plastic jumper on the side of the card that sets the analog type. If the analog output cards have this also, make sure that it is set to the correct position. Also make sure the program is updating the analog outputs.