SSR relay problem solution

[mdkmdk]

Hello everyone,

I have a goldsmith oven to which I had to change an SSR relay because when it reached 475 degrees centigrade the temperature increase stopped. Since the oven must reach 800 degrees in about two hours, I have to solve the problem.

I changed all the main components to the oven: thermoregulator, thermocouple, resistance. The problem of increasing the temperature was present even before changing all these components.

I ordered an SSR relay of the same brand and model as the one mounted on the oven: Crydom D2425.

Since it was panel mounted I proceeded to modify the panel to put a heatsink in it.

Between the heatsink and the relay I put some Thermalright TFX thermal paste which has a high thermal conductivity power and is also used in computers and can withstand up to 300 degrees.

After having reassembled everything I turned on the oven, this time it reached 100 degrees centigrade after 10-15 minutes the increase in temperature stopped.

I wonder where he went wrong about the SSR relay changing as I think I've done everything possible. Could anyone help me? Thank you in advance for your reply.

I attach some photos to the message.

 

[James]

Part of the problem may well be that you Simpley are not giving the auto tune long enough to do its calculations.

it will never complete if you keep stopping it when you think it is not working correctly.
it may well take a number of hours to complete.
some large furnaces, the auto tune can take days to complete.

 

[mdkmdk]

Part of the problem may well be that you Simpley are not giving the auto tune long enough to do its calculations.

it will never complete if you keep stopping it when you think it is not working correctly.
it may well take a number of hours to complete.
some large furnaces, the auto tune can take days to complete.

I keep it on for a few hours, always with autotuning active, then at a certain point I turn it off.

 

[mdkmdk]

First it seems to me the controller is working and that 25C or so away from 500C seems to me quite good after the initial period of heat up. Would you please leave for longer and find out how close the oven temperature settles to 500C?

Jogged by Westward10's post I am thinking about the error in measuring the oven temperature PV introduced by the connection of the thermocouple's red and green leads to the metal terminals on the back of the controller. These two connections are the other half of the thermocouple circuit. A thermoelectric circuit has a hot sensing end thermocouple and a cold reference end thermocouple. The combined circuit produces a voltage/current proportional to the difference between the temperatures of each end. See:

Thermocouple Cold (Reference) Junction Compensation - https://blog.beamex.com/thermocouple-cold-junction-compensation

The reference thermocouple made by the terminal connections will reduce the voltage produced by the hot end. Thus temperature measurements by the hot end thermocouple will be seen by the controller as lower than they actually are. The controller has a an SC value for correcting sensor measurement errors so I will come back to you later about entering a value other than 0.

The problem is that the reference temperature 'cold junction' is actually getting warmer and warmer as the oven does because the terminals are enclosed in a space near the oven and ssr.

How accurate does the temperature need to be for your purpose?

One thing that is coming to my mind now is that when the oven was connected via the internal relay of the temperature controller and not directly with the SSR I did a test and saw that overshooting and undershooting near 800 degrees worked well. Could the problem be related to the fact that it is now connected directly with the SSR?

 

[Avo Mk8]

It is important for me that the temperature is accurate and real and that it reaches 800 degrees as soon as possible.

In your post #100 above, if the photo on the left is the new configuration, and on the right is the old, then can you tell us:
Do both the thermocouple wires transition to the same conductor material (eg copper) at the ceramic choc block on the side of the oven, or are the wires still maintained as K thermocouple lead materials until they get to the controller?
Hopefully the cold junction is not on the side of the oven?

Are you measuring, or able to measure, the actual temperature of the oven with an independent device?

 

[mdkmdk]

In your post #100 above, if the photo on the left is the new configuration, and on the right is the old, then can you tell us:
Do both the thermocouple wires transition to the same conductor material (eg copper) at the ceramic choc block on the side of the oven, or are the wires still maintained as K thermocouple lead materials until they get to the controller?
Hopefully the cold junction is not on the side of the oven?

Are you measuring, or able to measure, the actual temperature of the oven with an independent device?

The rigid conductors of the thermocouple terminate in the ceramic terminal from the top in the photo. In the lower part of the ceramic terminal two wires come out, one red and one green respectively made of the same materials as the thermocouple. Externally they are contained in a fiberglass sheath. I cannot understand what is meant by cold junction.

 

[James]

Thermocouple Cold Junction Compensation, Convert Thermocouple Voltage to Temperature, Convert Thermocouple Temperature to Voltage, ITS-90 Thermocouple Coefficients, N Type N, J Type J, R Type R, S Type S, T Type T, E Type E, K Type K

Thermocouple calibration and thermocouple cold junction compensation by measuring cold junction block temperature with a thermistor reference, RTDs or semiconductor IC temperature sensors and applying a calibration equation to convert voltage to temperature, using type N, type J, type R, type S...

www.mosaic-industries.com

 

[Avo Mk8]

The rigid conductors of the thermocouple terminate in the ceramic terminal from the top in the photo. In the lower part of the ceramic terminal two wires come out, one red and one green respectively made of the same materials as the thermocouple. Externally they are contained in a fiberglass sheath. I cannot understand what is meant by cold junction.

OK. you've done the right thing. No problem. I was just speculating in case the joint here was causing an issue.
I believe what you've done is fine as long as each wire coming out of the terminal block is made of exactly the same material as the corresponding thermocouple wire going in.

 

[marconi]

Good morning from slightly cooler London.

Let us see how the controller performs when we select digital mode which means the heater is simply on and off depending on whether the temperature is under or over the set temperature. The oven will be like a simple one in the home kitchen.

Select P = 0, I = 0 and D = 0.

Turn of the Auto tune AT.

Select 500C as the set temperature and leave it there and allow a few hours for the temperature to settle. Do not open the oven door. Use 500C so we can compare with the previous results.

Please tell us how quickly to first reach 500C and what are the over shoot and undershoot temperatures.

What does the temperature fluctuate between after a few hours?

 

[marconi]

mdkmdk - digital or on/off control is the most basic control method. I want to have some results using on/off control to compare with the use of PI control.

 

[marconi]

mdkmdk: Good afternoon. When you have done the test I mentioned in #108 this morning and the oven is cool, the next test is to slightly reduce the P proportional value and the I integral time. This will have the effect of making the controller more sensitive to changes in the temperature error (ie the heater will be on for longer for a smaller value of temperature error) and reduce the steady state error which is inevitable with just proportional control. The Integral action added to Proportional control will remove this steady state error and with a shorter integration time it will do so more quickly. Thus I hope the changes will show some reduction in time to settle at 500C.

So, set P to 15 ( - it was 20 before) and I to 160 ( - it was 180 before). Leave auto tune off for now.

Once you have observed the performance could you describe it to me? Allow at least a couple of hours to pass.

Then you can turn Auto Tune on and check the oven say hourly thereafter for the next three hours.

Just confirm the LCK Parameter Values is set to 0.

 

[mdkmdk]

Good morning from slightly cooler London.

Let us see how the controller performs when we select digital mode which means the heater is simply on and off depending on whether the temperature is under or over the set temperature. The oven will be like a simple one in the home kitchen.

Select P = 0, I = 0 and D = 0.

Turn of the Auto tune AT.

Select 500C as the set temperature and leave it there and allow a few hours for the temperature to settle. Do not open the oven door. Use 500C so we can compare with the previous results.

Please tell us how quickly to first reach 500C and what are the over shoot and undershoot temperatures.

What does the temperature fluctuate between after a few hours?

Good evening marconi,

I did the test of # 108, I state that by setting the values you suggested many items in the menu have disappeared and a new one has appeared. Now the only items present are: AL1 (value: 10), SC (0), ° H (2.0) and LCy (0). About the °H item I saw in the manual that it refers to "Two-digit Return Difference".

In 43 minutes the oven reached 500 degrees.

When this temperature is reached, it switches off.

Although the heating was off, the temperature rose to 509 degrees.

The temperature dropped to 499 degrees, at this temperature the heating started up again.

Although the heating had restarted, the temperature dropped to 489 degrees.

The temperature then rose up to 500 degrees and the cycle repeated itself.

I repeated this 4 times and each time the temperature range was the same, at most 1 or 2 degrees less or more.

Surely if instead I had set 800 degrees as the temperature to be reached when the heating was switched off, the temperature would have dropped much more than just 10 degrees.

I noticed something, the fourth time the red lights AT and AL1 came on, on the display of the temperature controller. After about 30 seconds, maximum 1 minute, the two lights that had switched on went out. The AT as you told me was disabled, usually when it is active the AT light is green and flashing.

Soon I will also do the other test you asked for.

 

[Darkwood]

I keep trying to catch up on the thread in my spare time and have seen a few things I cannot see anyone has picked up on, are we running into a dead end here in that the controller, thermocoupler may be working just fine and what you have is a failed element or two, this would account for it working to lower temp's but never getting high enough.
I work on and have designed and fitted many control systems for 50 ton and 80 ton heat presses, if temperatures are not been met then one of the first things to check is element failure, I have 16 on the presses I work on.
Again apologise if already mentioned and put out there but I missed it in my quick dash through the responses, it would fit the issues you are having at higher temp's but hide itself at lower settings, are we chasing our own tails here?

 

[marconi]

mdkmdk: Thank you for the information. I will think about what you have reported. But it seems quite good to me. In digit (on/off) mode, the heater turns off at the measured temperature PV = 500C and turns on again when lower than PV-H = 500-2 = 498C.

The further drop in temperature below 498 even though the heater turns on at PV-H is because it takes a while for the heating element to warm up to a temperature greater than the current oven temperature. There can only be heat transfer between the element and the oven if the oven is hotter than the oven. Until this happens the oven will fall in temperature. And you are right that at 800C the temperature drop will be greater because the heat loss is higher so the heater must be on longer to transfer enough heat energy to compensate. In digit or Proportional mode there will always be an error between PV and SV in order to compensate for heat loss from the oven. This is why we want to make the controller work in Proportional P and Integral I modes.

In the next trial, which uses P and I modes, the controller's Integral mode should eventually keep the element on at a power level less than full power which is sufficient to compensate for the heat losses from the oven to the surrounding environment and thus PV = SV or very close. But it takes time for the Auto Tune to calculate what this power level needs to be so please be patient.

For now the remarks about the AT light flashing confuse me because I thought auto-tune was not used in digit mode. I will have to check that.

 

[marconi]

mdkmdk: Good morning. When you have done the test in #110 please turn off the AT and then turn off all power to the oven. After a few minutes turn the power on again and set the temperature SV to 800C. Next turn on AT. This will test the performance of the oven at the temperature you want it to be 800C. During this test the AT will be attempting to tune the controller and you will probably see some overshoots and undershoots. The controller is changing controller values and trying them out to see which work the best. It will require a few hours to do this maybe longer. When AT is finished (if indeed it does) the AT light will turn off. See Section 5 of the controller manual for more on AT.

 

[mdkmdk]

mdkmdk: Good morning. When you have done the test in #110 please turn off the AT and then turn off all power to the oven. After a few minutes turn the power on again and set the temperature SV to 800C. Next turn on AT. This will test the performance of the oven at the temperature you want it to be 800C. During this test the AT will be attempting to tune the controller and you will probably see some overshoots and undershoots. The controller is changing controller values and trying them out to see which work the best. It will require a few hours to do this maybe longer. When AT is finished (if indeed it does) the AT light will turn off. See Section 5 of the controller manual for more on AT.

Good evening marconi,
I'm sorry if I haven't done the test yet but I've been very busy for my job, in the next few days I'll do the test and I'll let you know the results, thank you for your time.

 

[marconi]

mdkmdk. I thought I would contact you to ask if you need any more help. Do not worry about asking me because I enjoy these sorts of problems. They stop me being bored. At 61 I have liked thinking about PID controllers again.

 

[mdkmdk]

mdkmdk. I thought I would contact you to ask if you need any more help. Do not worry about asking me because I enjoy these sorts of problems. They stop me being bored. At 61 I have liked thinking about PID controllers again.

I started to do test #110 but I saw that the variable 'P' had a value of 0 and the variable 'I' and 'AT' were not present. The variable LCy instead has the value 0. I await your instructions on how to proceed in order to start test # 110. If I set P to '15' would the other missing variables reappear? Thank you.

 

[mdkmdk]

mdkmdk: Good afternoon. When you have done the test I mentioned in #108 this morning and the oven is cool, the next test is to slightly reduce the P proportional value and the I integral time. This will have the effect of making the controller more sensitive to changes in the temperature error (ie the heater will be on for longer for a smaller value of temperature error) and reduce the steady state error which is inevitable with just proportional control. The Integral action added to Proportional control will remove this steady state error and with a shorter integration time it will do so more quickly. Thus I hope the changes will show some reduction in time to settle at 500C.

So, set P to 15 ( - it was 20 before) and I to 160 ( - it was 180 before). Leave auto tune off for now.

Once you have observed the performance could you describe it to me? Allow at least a couple of hours to pass.

Then you can turn Auto Tune on and check the oven say hourly thereafter for the next three hours.

Just confirm the LCK Parameter Values is set to 0.

I set ‘P’ to the value 15 and the other variables were back, then I set the variable ‘I’ to 160.

In about 45-48 minutes it reached 500 degrees.

At around 472 degrees the heating turned off and on very very quickly, I could see it by the red light flashing. Around 491 the speed at which the heating light flashed decreased but still remained fast. The heating continued to switch on and off up to 500. With the heating off, the temperature reached a maximum value of 503 degrees. As soon as the temperature dropped to 500 degrees it started to turn off and on again quickly up to 493 degrees, at that temperature it turned on again steadily, it drops to 491 degrees and immediately after that it started to turn off and on again until the reaching 500 degrees.

I then did the test with the AT turned on, the temperature dropped to 472 degrees and the heating started up again, at 480 degrees the heating was off again. The cycle repeated itself constantly, I did not have the opportunity to keep it on for long, not having much time available and I could not do the other test by bringing the oven to 800 degrees.

 

[marconi]

thank you. I will read carefully later and reply.

 

[marconi]

Without AT for a setting of 500C the performance is good. Please now try a setting of 800C. Be careful to follow the sequence exactly as described before:

Turn off all power to the oven. After a few minutes turn the power on again and set the temperature SV to 800C. Next turn on AT. Observe the oven temperature and controller lights and be patient!

 

[marconi]

To what is the round orange panel lamp on the left of the controller connected?

what is the purpose of the round silver knob to the left of the round orange panel lamp? Is it a timer?

Is the clicking sound from the controller?

 

[mdkmdk]

To what is the round orange panel lamp on the left of the controller connected?

what is the purpose of the round silver knob to the left of the round orange panel lamp? Is it a timer?

Is the clicking sound from the controller?

The round red light that flashes is that of the heating, when it is on the heating is on, when it goes off it is because the controller does not send current to the SSR relay and consequently the heating light does not turn on either. The square light you see instead is a two-pole switch. The round knob is a switch-off timer which, if set, switches off the heating for a set amount of time. The noise you hear I believe is the controller, the light or the SSR relay I don't think they can make noise.

I read the previous message, if I put the AT on and set the temperature to 800 degrees, it could be that even after several hours the AT never turns off.

 

[marconi]

Yes. It could take several or many hours for the AT to tune successfully. But, even after a very long period, the AT as designed and implemented inside the controller might be unable to home in to the best values using its trial and error approach to tuning. In which case we leave AT off and use the figures I said to use and adjust them over the life of the oven until we are satisfied with the performance. Auto tuning algorithms are not perfect in all applications.

I must repeat that you have to allow AT at least 3 hours from when the oven first reaches 800C to attempt tuning and ideally longer say 6 hours. At the moment AT may not complete because you turn the oven off too early.

 

[mdkmdk]

Yes. It could take several or many hours for the AT to tune successfully. But, even after a very long period, the AT as designed and implemented inside the controller might be unable to home in to the best values using its trial and error approach to tuning. In which case we leave AT off and use the figures I said to use and adjust them over the life of the oven until we are satisfied with the performance. Auto tuning algorithms are not perfect in all applications.

I must repeat that you have to allow AT at least 3 hours from when the oven first reaches 800C to attempt tuning and ideally longer say 6 hours. At the moment AT may not complete because you turn the oven off too early.

I did the test by leaving the oven on and waiting for more than 6 hours after reaching 800 degrees. The lights of the AT were still flashing, the AT unfortunately did not work.

 

[marconi]

I conclude that AT will not work for your oven. We have tried to use it many times without success. Turn it off and just use the controller with a setting of 800C. Please tell me how the oven performs and allow an hour after reaching 800C.

AT makes some assumptions about the system (your oven) the controller drives. It may well be your oven does not satisfy one or a few of these assumptions which means trial and error does not home in on better and better values for P and I and the other variables.