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Thread: PID Question

  1. #1
    Boolit Mold
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    PID Question

    I have collected most of the components to put together a control for an electric ladel casting pot. 50 - 60 lbs. of alloy.
    My intent is to use 2-240 volt elements for quicker heat up. At a set temp. 1 element would drop off & the second 1 would maintain casting temp."k" thermocouple & ssr's will be used.
    This is the control I am looking at to get the job done https://www.omega.ca/pptst_eng/CNPT_SERIES.html. 3 dc pulsed outputs.
    The third out put would be used as a fail safe in case of a runaway.
    My question is:will this controller do what I am want or is there something else out there?

    I this is a little confusing to you,please forgive me,I have no experience with pid's.
    Any suggestions welcome.
    Thnx Lots
    Bob

  2. #2
    Boolit Master
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    I think you are over thinking this.

  3. #3
    Boolit Master
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    My brief review of that controller does not show the capability to manage a two stage heating element. It does appear to have multiple stage profiles such as might be used in a heat treat furnace for tool steels, i.e. ramp up, soak, and ramp back down over separate time intervals.

    I briefly entertained the same two stage heater concept to shorten heat up time but abandoned it due to controller complexity and heating element cost. I found that any significant reduction in heatup time increased heater element watt density above manufacturers design limits for commonly available heater bands. Better band materials are available but cost is not reasonable for even a first class melting pot in a hobby. The pot, heater, and controller I built heat up in less than 20 minutes. I just use that time to arrange and prepare other aspects of the casting session.

  4. #4
    Boolit Mold
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    Quote Originally Posted by Retumbo View Post
    I think you are over thinking this.
    I know this is far more complicated than I need for simple casting purposes, but I am looking to learn more about pid's and what can be done with them.
    IF this works out,I would try incorporating it into a process we are working on at the shop.
    Hopefully a call to the Omega tech. staff will help.
    Thnx Lots
    Bob
    "Ridin' For The Brand"

  5. #5
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    JonB_in_Glencoe's Avatar
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    I'm not sure how much the Omega PID is? but it seems two cheap PID systems would work...just set one at a lower temp and be sure the second one "learns" while both units are on...and including, until the first one cuts out.

  6. #6
    Boolit Mold
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    BeeMan
    I thought the above control might work because it has dual set points & three outputs.
    I downloaded the manual for it but at this point on my learning curve,most of it went over my head.
    What control configuration would be needed to do what I want to do?
    Thnx Lots
    Bob
    "Ridin' For The Brand"

  7. #7
    Boolit Master
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    I used the low temp alarm circuit for high power heating. You can set the alarm to whatever you want completely independent of the regular temp control. Also I do not trust the SSRs for heavy loads. You can use a 24volt transformer and 30 or 40 amp contactors.
    Sent from my PC with a keyboard and camera on it with internet too.
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  8. #8
    Boolit Master
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    Bob,

    Yes, there are two set points. The first is for the process parameter and the second is the alarm. See page 48 in the manual. Similarly, it looks like the outputs are intended to drive different loads, not two copies of the same type load in a staged fashion.

    It may be possible to spoof the primary functions and achieve a dual heater staged control function, but doing so may require an external relay, latching logic, or inverting the alarm vs process parameter for the heat mode. As configurable as these controllers usually are, just about anything can be done.

    The multi segment ramp soak mode is not needed for a simple process like controlling a casting pot. I would look for a simpler (read less expensive) controller, even if you want to try spoofing the process and alarm set points to have a 2 stage heater element.

    Too bad we can't easily re-purpose a thermostat for a heat pump with electric heat backup. It has exactly the functionality we want in terms of a higher power load (electric resistance backup heat) further from setpoint and a lower power load (heat pump in heat mode) for when you are within a few degrees of set point.

    Keep us posted if you continue down this path and make a staged heater solution work. I still think your next challenge is going to be watt density of the heater elements causing excess temperature gradients and burning out the heaters. The challenge is pumping a lot of power in fast over a limited surface area of the pot, using off the shelf components to keep cost manageable on a one off project.

    BeeMan

  9. #9
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    you are over complicating the entire process.

    I would use a inexpensive PID and just a single SSR
    With a SSR your not gonna get a runaway situation as 99% of the time they fail to the off position.
    Get a 40 amp SSR and your golden. With that amperage, I would use a heatsink for sure.

  10. #10
    Boolit Master
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    LWKnight,

    Nicely done on sorting out a way to use the alarm. I have not used that Omega controller, so was not sure if it could do this. As far as SSRs go, just up size them so they are overrated and run cool even without a heat sink. The zero crossing feature vs a contactor makes them more reliable in the long haul and you don't need to limit the relevant PID parameter to conserve contact life. Without this constraint SSRs allow the PID setup to shorten on on off times, which in turn should help heater life because the element swings less between max (on) and min (off) temperatures.

    BeeMan

  11. #11
    Boolit Master
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    Hatch,

    I agree, simpler is better. It is where I ended up with my shop fabbed bottom pour pot and it works just fine. But if the OP wants to explore it, more power to him.

    BeeMan

  12. #12
    Boolit Master popper's Avatar
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    For your purpose a pid is used to get the temp to the set point as fast as the available heat will allow. A single controller is fine with as many elements as you want. SSR can fail open or close so if unattended, some alarm is needed.
    Last edited by popper; 10-10-2017 at 09:40 AM.
    Whatever!

  13. #13
    Boolit Mold
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    Thank You for your help.
    I will try to talk with Omega today & see what they have to say.
    I have 65amp ssr's.
    Element size is also a concern.
    I realize this is overly complicated for casting purposes,but looking at adapting this to other process's as well.
    Again Thnx Lots
    Bob
    "Ridin' For The Brand"

  14. #14
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    Makes really no sense to put two bands in and try to control separately. I understand what you are trying to do, but for this application it most probably won't work. Run two off of the same controller if you want, sizing properly the SSR and then once up to temp, both will cycle. If it learns properly, there really won't be any savings with both coming on, they'll just cycle shorter times.

  15. #15
    Boolit Master
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    Quote Originally Posted by BeeMan View Post
    LWKnight,

    Nicely done on sorting out a way to use the alarm. I have not used that Omega controller, so was not sure if it could do this. As far as SSRs go, just up size them so they are overrated and run cool even without a heat sink. The zero crossing feature vs a contactor makes them more reliable in the long haul and you don't need to limit the relevant PID parameter to conserve contact life. Without this constraint SSRs allow the PID setup to shorten on on off times, which in turn should help heater life because the element swings less between max (on) and min (off) temperatures.

    BeeMan
    I'm not sure about the Omega controller but anything that has a switch can be connected to a relay circuit. I plan to set up dual soloniod valve for the smelting pot. On low temp alarm , it will fire full power then drop to low till temperature setting reached then shut off to pilot mode.
    If I ever actually get it into 3D I simply must post pictures so I won't need the 1000 words.

    I opted for actual contactors for the electric pots because ( maybe its just me) I never really had much faith in SSRs You can get 40 amp contacts for about $7.00 per each and around the same for a 24volt transformer. This way the controller switch only carries low voltage and ultra low amperage. Contacts can last for years in an A/C unit so I think you would never wear one out casting.
    Sent from my PC with a keyboard and camera on it with internet too.
    Melting Stuff is FUN!
    Shooting stuff is even funner

    L W Knight

  16. #16
    Boolit Mold
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    I finally got a chance to talk with the tech. people at Omega. Great people.
    After a long(and some what confusing for him)chat,he figures the above control will work by configuring the alarms and output setting.He did say that this will take some time trying various combinations.
    I placed the order,along with a few other items.
    If it doesn't work out,at least I will end up with a(albeit,very expensive)PID for the new pot.
    THNX LOTS
    Bob
    "Ridin' For The Brand"

  17. #17
    Casting bullets since '66

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    You are overthinking.
    In fact a PID set to use the PID function is using a sledgehammer to kill a fly.
    I have a cheap ($12) PID set as a on/off thermostat. The Temp drops 1/2of a degree, the pot element heats.
    The temp goes 1/2 of a degree over, the electricity shuts off to the pot element.
    Under target temperature the pot element gets full 110 .
    It works just as well as with the unit set to the PID function.
    Go now and pour yourself a hot one...

  18. #18
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    HATCH's Avatar
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    It all depends on how critical you want your temp to be.
    A quality made PID will compensate for temperature overshooting.
    Basically when you kill the power to the heater yet the temp still increases.

    But for the most part of you just want to know the exact temp and have it operate just like the factory mechanical thermostat did, then setting it for just on/off operation will accomplish that goal.


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