noxx - 13-1-2008 at 17:10
Hello guys,
I have a PID controller that controls my electric furnace. The heating element is 1600W. And the volume of my furnace is about 700ml.
I would like to know if it's possible to tune my PID to make heating more slow. Because the heating is so fast that it cracks my refractory.
Here is what I can set:
-Proportional Band
-Integration Time
-Diffenciation Time
-Overshoot Suppression Coefficient
-Control Period
-Digital Filtering Strength
Also AT skew and Temp sensor error correction coefficient (useless?)
So is it possible to tune the heating time ?
Thanks a lot.
Twospoons - 13-1-2008 at 19:40
You should be able to slow it down by dropping the proportional gain, and probably the integral gain too. What you are aiming for is gross
undershoot on the control.
Its rather a crude way to control your heating time, and may impact on the thermal stability once up to temperature. A proper furnace controller
usually has a setting for ramp time - look for a pottery kiln controller.
The only other way is to slowly raise the temperature setpoint manually, at the rate you want.
I assume its an AC heater? If so a really nasty (because your electicity supplier will hate you), but simple way would be to switch a diode in series
with the heater while heating up (making it only 800W), then switch back to full power later.
noxx - 13-1-2008 at 20:22
Thank you Twospoons.
I'll give it a try.
Thanks
noxx - 14-1-2008 at 13:03
It works. Thanks
Now if I want the PID to control the cooling (not too fast) because it falls fast (2°C per second) and I ear my refractory cracking. Is there a way
to make it slowly cool ?
Thanks
Twospoons - 14-1-2008 at 20:39
The same slowdown should work in both directions.
Better still, insulate your furnace!
The controller can't take heat out so insulation will definitely slow down cooling, even if the controller was completely turned off. And
you will save power as well 
[Edited on 15-1-2008 by Twospoons]
noxx - 14-1-2008 at 21:55
Yes I will use 3 layers of Kaowool. Do you think the outer layer of my furnace will get hot with this insulation ? (I'll reach 1200°C)
Thanks
garage chemist - 14-1-2008 at 22:36
Another good solution to slow down the heating would be to use a simple thyristor circuit in series to your PID controller.
Set the thyristor circuit to a value that still gives a reasonable heating time but avoids the excessive temperature gradients that have caused
problems.
This will also prolong the lifetime of your heating elements since it reduces surface loading of the wire, contrary to the on-off switching that the
PID controller does.
bio2 - 15-1-2008 at 11:57
......I assume its an AC heater? If so a really nasty (because your electicity supplier will hate you), but simple way would be to switch a diode in
series.........
Your motors and transformers will hate you also.
I tried this with about a 15 amp load and all the motors and transformers in the house started "groaning" from the noise induced on the line. Some of
the cheaper household
transformers sondd so bad I decided to terminate the
setup. The microwave oven also sounded very unhappy!
What I was doing was using the half wave generated
to throttle an electric resistance heater embedded into a large cooking pot.
I have read using this method to induce noise on the line can slow kwh meters but filters would be needed to
protect the inductive devices in the area and it seems the
noise would travel through the utility transformer to the
neighbors appliances as well.
garage chemist - 15-1-2008 at 12:19
A 2600VA thyristor circuit costs about 20$ from ebay, does not give any such problems, and can be adjusted to any required power while in operation.
I use one to control my tube furnace.
[Edited on 15-1-2008 by garage chemist]
noxx - 15-1-2008 at 20:37
Thanks for the infos !
I didn't know how it was called.
noxx - 15-1-2008 at 20:42
I just heated my electric furnace to 640°C and two big cracks formed...
I was pretty sure it was going to happen since I did it outside at -15°C and the refractory wasn't insulated at all to wind and such. My electric was
very thin, about 5mm walls. Any idea what I could add to my refractory to make is less sensitive to thermal shock ? Or maybe if I make it thicker ?
The only thing I know about my refractory is that it contains silicates and can go up 1600°C . And I don't need to add water, it's already wet.
Thanks
[Edited on 15-1-2008 by noxx]
garage chemist - 15-1-2008 at 20:47
Did you dry it properly before heating it?
The first heating usually has to be done very slow, like 50°C per hour, to slowly drive out all water without cracking it due to the shrinkage that
results when the water is being driven off.
noxx - 15-1-2008 at 23:20
It wasn't the first firing and I was very very dry.