adccommunitymod (AutomationDirect) asked a question.
Created Date: August 11,2008
Created By: HughRob
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Hi, I need some help with a PID loop for a heater. I know that can control the heater by pulsing it on and off but this won't be adequate for my application, I need to vary the ppwer produced by the heater. My problem is that I am not really understanding how to do this, whether it is by a rheostat or a frequency controller, and also where to look to obtain an off the shelf item. In my mond I should be able to take an anlogue signal from the PLC which is a DL06 and send it to a gadget which will adjust the power required. Can someone please help me with this, I am quite sure its simple.
Created Date: August 11,2008
Created by: Steve Bailey
What type of heater do you have? If you have resiatance heaters, then about all you can do to vary the power is to switch individual heater elements into or out of the system.
You should also be aware that PID tuning values for of heating system of X watts may not be good when the heating system is Y watts.
Created Date: August 11,2008
Created by: KPrice
Steve Bailey, we did a project where we used SSRs (solid state relays) from Omega to control zones of electric resistance heaters. We used the PLC digital outputs to control the SSRs, and programmed the time-ptoportioning control in the PLC. Yes, as you said, there are also SSRs that include time-proportioning and will switch the load on and off based on a 4-20ma signal. However, we found for the large number of PID loops we had, it was less expensive and more flexible to program the time-proportioning in the PLC. The manual includes good instruction on programming the timme-proportioning digital output for the PID loop. Hope this helps.
Created Date: August 11,2008
Created by: 72hdflh
HughRob,
Take a look at these, we have several in our plant. I have not seen a failure in the 11 years that I have been here.
http://www.watlow.com/products/controllers/power/dinamite.cfm
Created Date: August 11,2008
Created by: KPrice
HughRob, sorry, I got your name wrong. I see you also asked for a possible supplier. Here is a link.
http://www.omega.com/ppt/pptsc.asp?ref=SCR19&Nav=heap07
I would suggest the zero-crossing switching, it does not generate noise like the phase-angle switching and your application may not need the tight control of the phase-angle switching, it's a balance between tight control and heater cycling. Even with 1 loop, it may be less expensive to program the time-proportioning in the PLC and use a SSR. Hope this helps.
Created Date: August 11,2008
Created by: Steve Bailey
I know that can control the heater by pulsing it on and off but this won't be adequate for my application, I need to vary the ppwer produced by the heater.
I interpreted that to mean that the original poster understood how to use a PID output to create a PWM digital output but that for some reason he wants to vary the power of the heater. I interpreted the word "power " to mean the rate of energy transfer.
If you are limited to a constant voltage, you can modify the overall heater power by adding or subtracting individual heater elements. If you have the ability to modify the voltage, you can reduce the power from a heater element by reducing the voltage. You should not raise the voltage beyond what the heater element is rated for.
Created Date: August 11,2008
Created by: HughRob
Heater Power Supplies
Thanks for answers
Created Date: August 11,2008
Created by: HughRob
Hi,
I need some help with a PID loop for a heater. I know that can control the heater by pulsing it on and off but this won't be adequate for my application, I need to vary the ppwer produced by the heater.
My problem is that I am not really understanding how to do this, whether it is by a rheostat or a frequency controller, and also where to look to obtain an off the shelf item.
In my mond I should be able to take an anlogue signal from the PLC which is a DL06 and send it to a gadget which will adjust the power required.
Can someone please help me with this, I am quite sure its simple.