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.

 

[marconi]

The computer inside the controller is making small changes to the P, I and D values and then seeing how the oven performs in terms of oven temperature PV and its error when compared with the desired or set temperature SV all over time. Iteratively it will be homing in on the P, I and D values which give the best oven temperature PV time performance. When it has arrived at these values any changes away from them will make the performance worse each time which is how the controller knows when to stop auto tune.

 

[marconi]

I should have said that in order for the auto tune to work you will need to cycle your oven from cold to hot to cold, a number of times for the AT function to work and the light to stop flashing, before you use the oven for business.

I also need you to confirm you have turned the derivative or d function on.

 

[marconi]

mdkmdk - Would you mind letting us know your progress please?

 

[mdkmdk]

mdkmdk - Would you mind letting us know your progress please?

Sorry for the delay in answering, I did get it to 800 degrees centigrade but the auto tuning light was still flashing. I saw the video at minute 4.18, should I set the AL1 value, if so why?

 

[marconi]

Good you have been looking at the controller's settings. Would you go through the steps as in the youtube video and report to me the settings you find? I can then tell you what to change or keep the same.

More immediately please me tell if you have set up 'InP' value as 'k' ( see at 5m57s) to tell the device you are using a thermocouple - and then tell me if the 'd' function is set to on or off (see at 6m57s).

 

[marconi]

AL1 and AL2 are over- and under- temperature limits to alert you if the oven goes to far above or below the set temperature. Would this be useful to you?

 

[mdkmdk]

Good you have been looking at the controller's settings. Would you go through the steps as in the youtube video and report to me the settings you find? I can then tell you what to change or keep the same.

More immediately please me tell if you have set up 'InP' value as 'k' ( see at 5m57s) to tell the device you are using a thermocouple - and then tell me if the 'd' function is set to on or off (see at 6m57s).

The input signal is set to the type K thermocouple, which is the one actually present and which has been replaced. The variable "d" corresponding to the derivation time is set with the default value "30", in the manual it is written that if it were set to "0" the derivation would be closed. The variable AL1, on the other hand, is set to the default value "10". The AL2 variable is not present. SLL (setting of minimum set value) and SLH (setting of maximum set value) are the variables that, if set, put a minimum and maximum limit on the temperature that can be manually set on the thermoregulator display, I wonder: what is the difference between these variables and AL1? I never understood what AL1 was really for, is it perhaps an audible alarm that sounds when the temperature is reached?

 

[marconi]

I am wondering now whether your controller is not exactly the same as the one shown in the youtube clip but similar in many respects.

How long does the oven take now to reach 800C?

Please set d to 200 and let me know how the oven performs.

I did ask for all the controller settings in my earlier post.

I will have another search for the manual for your controller. All I could find before was a leaflet with brief details.

Are you using the oven now to melt gold?

 

[mdkmdk]

I am wondering now whether your controller is not exactly the same as the one shown in the youtube clip but similar in many respects.

How long does the oven take now to reach 800C?

Please set d to 200 and let me know how the oven performs.

I did ask for all the controller settings in my earlier post.

I will have another search for the manual for your controller. All I could find before was a leaflet with brief details.

Are you using the oven now to melt gold?

I have not yet done the test of setting the variable 'd' to 200, however the auto tuning is still in operation, to get to 800 degrees the oven takes 2 hours and 27 minutes setting the temperature to 1000 degrees in order to make it rise continuously without being slowed down by the overshooting system. An anomalous thing I noticed is that if it reaches for example the temperature of 790 degrees and I set the temperature variable to be reached from 1000 to 790 to keep the temperature stable for ten minutes, I noticed a drastic drop, in 10 minutes it dropped to 736 degrees. The temperature maintenance system did not work, same thing happened to me when I dropped the temperature to 540 degrees and I set the temperature to be reached at 550 degrees, the temperature did not rise, I had to set 600 degrees to make it rise.

 

[mdkmdk]

Here is the manual of the MC-101 series temperature controller.

 

[marconi]

Thank you. I will study the manual and have a think. It really would help to know the settings which have been input since these determine the way the controller responds.

 

[marconi]

I have not yet done the test of setting the variable 'd' to 200, however the auto tuning is still in operation, to get to 800 degrees the oven takes 2 hours and 27 minutes setting the temperature to 1000 degrees in order to make it rise continuously without being slowed down by the overshooting system. An anomalous thing I noticed is that if it reaches for example the temperature of 790 degrees and I set the temperature variable to be reached from 1000 to 790 to keep the temperature stable for ten minutes, I noticed a drastic drop, in 10 minutes it dropped to 736 degrees. The temperature maintenance system did not work, same thing happened to me when I dropped the temperature to 540 degrees and I set the temperature to be reached at 550 degrees, the temperature did not rise, I had to set 600 degrees to make it rise.

I might be able to comment on what you have written if only I had the PID controller's current settings ...................

I have some ideas on how to proceed once I have the settings in front of me.

Are you writing and reading here in your first language?

 

[mdkmdk]

I might be able to comment on what you have written if only I had the PID controller's current settings ...................

I have some ideas on how to proceed once I have the settings in front of me.

Are you writing and reading here in your first language?

Some variables in the manual are not present in the menu of my temperature controller. I list below the variables related to the PID that are present in the menu:

At, Al1, SC, P, I, d, t, rSt, LCy are present.
The variable 'At' is still set to Yes since I have activated auto tuning.
The variable 'P' is set to 20.0 instead of 30.0 as it would be by default.
All other variables are set with factory default values. You can see these values in the central 'Factory Defaults' column of the manual.

Al2, oH, rE, bUFF are not present.

English is not my primary language. I really appreciate your interest and thank you for your help.

 

[marconi]

Thank you. Armed with this information I will be back in touch. Meanwhile could you do a trial for me please? Please set the Control Period 't' value to 2 seconds suitable for an SSR - at the moment it is set to 20s required for a relay. Then disable the Derivative D function by setting it to 0. Because the oven has a high thermal capacity and hence is slow to change in temperature the D function will not be that useful for temperature regulation. How does the oven perform now when the set Value SV is 800C? eg time to first reach 800C, overshoot and undershoot temperatures, time to 'stay at' 800C. Is it 'better' than when D is selected with a value of 20? Keep the AT selected.

The Control Period Value t being set to 20 is wrong because you are using an SSR which can switch more frequently than a relay without damage. I am pondering that your oven only needs PI control not PID. The reason the AT may be taking a long time or even not succeeding is that it assumes the controller has dual action ie it can raise the temperature and also lower the temperature by adding or removing energy. Of course the element can only add energy not remove it. For the oven to cool it needs heat energy to be removed and this must rely on it flowing out of the oven which I assume has reasonably well insulated walls so it is going to take more time to reduce the temperature by say 50C than to raise it by 50C - the controller does nothing but wait with a zero output and element off when the actual temperature PV has to decrease.

I am finding this problem interesting and a nice revision of the control engineering I learned/forgot decades ago.

 

[mdkmdk]

Thank you. Armed with this information I will be back in touch. Meanwhile could you do a trial for me please? Please set the Control Period 't' value to 2 seconds suitable for an SSR - at the moment it is set to 20s required for a relay. Then disable the Derivative D function by setting it to 0. Because the oven has a high thermal capacity and hence is slow to change in temperature the D function will not be that useful for temperature regulation. How does the oven perform now when the set Value SV is 800C? eg time to first reach 800C, overshoot and undershoot temperatures, time to 'stay at' 800C. Is it 'better' than when D is selected with a value of 20? Keep the AT selected.

The Control Period Value t being set to 20 is wrong because you are using an SSR which can switch more frequently than a relay without damage. I am pondering that your oven only needs PI control not PID. The reason the AT may be taking a long time or even not succeeding is that it assumes the controller has dual action ie it can raise the temperature and also lower the temperature by adding or removing energy. Of course the element can only add energy not remove it. For the oven to cool it needs heat energy to be removed and this must rely on it flowing out of the oven which I assume has reasonably well insulated walls so it is going to take more time to reduce the temperature by say 50C than to raise it by 50C - the controller does nothing but wait with a zero output and element off when the actual temperature PV has to decrease.

I am finding this problem interesting and a nice revision of the control engineering I learned/forgot decades ago.

I did the test with the configuration you suggested, what I noticed is that after 2 hours the temperature was at 750 degrees instead of 764 as in the other measurement with the setting at 1000 degrees and without the configuration you suggested. I don't understand why there is this 14 degrees difference. I don't think it could be the resistance that has degraded considering that it has been changed by a specialized technician a couple of months ago. I didn't always watch the oven continuously so I don't know if the heating stopped sometime, but when I took the measurement it was on.

The second thing I noticed was that at 760 degrees the heating turned off, after which it dropped to 753 degrees and stopped there; after a few minutes it began to rise. Then it reached 764 and switched off again, after that the temperature dropped and after a while it started again reaching 763 degrees, then it did the same thing stopping at 766. In 2 hours and 28 minutes it was at 766 degrees, instead in the measurement made days ago (with the temperature set at 1000 degrees) reached 800 degrees in 2 hours and 27 minutes. Since it seemed impossible that the temperature would rise more, I decided to finish the test.