testing thyristor module

[memberpeace]

Sorry guys

I think it did not take the pictures yesterday. I put them into word document instead today. Number 1 entrance point from fuse, 2-3 exit goes to contactor to energise elements.

Also I checked the graph yesterday again. Problem is still there. I need to check the things you told me. I will do them as soon as I have time. Thanks again.

 

[milldrone]

memberpeace,
I'm no expert at PID but it looks to me like you have a tuning issue with your PID loop. If you don't get any responses after my post “bumps” this thread. You might try re-posting with "PID" in the title of your new thread. Sometimes PID experts don't look at threads without "PID" in the title. If your real lucky Ron Beaufort will respond.

Be sure to list what type of PLC is doing the controlling, what type of sensors are being used, and a discription of the process. The chart you posted will help also. Was the chart from the bad oven? Maybee a chart from a good oven would be a good idea also. Lastly the lines on the chart need some identification as to what they represent ( the PID gurus may already know). The members of this forum dislike playing "20 questions", so be as precise as you can.

You said in post #14 "There are PID loops in it. But there is just on/off control coming from PLC which triggers the thyristor. In getting temperature setpoint all outputs for triggers are on. After that in soaking cycle according to the program it is switching the output on and off continously and keeping the temperature in same value for 3 hours if it is a good oven. In the other ovens the one which is making zigzag (in "graph.jpg" file) is same as the others. By doing zigzag it also raises the other zones' temperatures which is really bad." . Does this mean there are multiple zones with overlapping control areas? If so you need to state this also.




Alaric 's reply in post #9 asks some important questions that need answering

 

[TConnolly]

So if I understand you correctly, you are not phase angle firing the SCRs in the module with an analog ouput, but you are using a time proportional output method, that is you are truning on/off a discrete digital output to control your heating. Is that correct? If this is the case and you do not have a run away heating problem, you are just very unstable, then this is a PID tuning problem. Based on the information provided so far I would venture that your SCRs are just fine.

Was there a time recently that this furnace worked properly? Have you changed any of the PID gains or the TPO period since then?

 

[Clay B.]

So if I understand you correctly, you are not phase angle firing the SCRs in the module with an analog ouput, but you are using a time proportional output method, that is you are truning on/off a discrete digital output to control your heating. Is that correct? If this is the case and you do not have a run away heating problem, you are just very unstable, then this is a PID tuning problem. Based on the information provided so far I would venture that your SCRs are just fine.

Was there a time recently that this furnace worked properly? Have you changed any of the PID gains or the TPO period since then?

I agree with Alaric. Since you are just firing the SCR on and off then just look at them being a switch. IF you get no heat or it just runns away then SCR's would be a place to look.

From veiwing your chart it looks like you have an oscillating PID loop. It apears to be hunting for the tempature. I am not the best with loops so I generally just make small changes to the PID then look for the results. You may want to compare you PID to the other ovens that are working correctly and see if there are any big differences in the numbers. If the ovens are the same size etc. then they should have very similar setpoints.

 

[memberpeace]

good news

Hi guys

At last I found the reason. All the answers you send me were not the solutions may be but they were the possibilities. It gave me a great oppurtunity to check the rest of the things. Thank you very much

The fault was not because of the thyristors or PID tuning. The only way you can see was when the oven was in soaking cycle. Because it is switching on and off all the time it is very hard to check on PLC program on PC and thyristor in the electrical cabinets at the same time. What I have done today is I forced off PLC output and checked the power and saw that it switched off not at the same time but it did lately. It was because of a faulty PLC output. I replaced the output point. It is working in a great way like the others. The output card is standard 10-30 Vdc output at allen bradley PLC series. I think I should consider to replace with high speed output card in the future.

Thanks for your advices. It was really a brain storm for me. It give me a great ooportunity to learn and understand a lot of things really which I was not sure.