wiring pid controller circuit

Discussion in 'Circuit Help' started by dve256, Jun 25, 2011.

  1. dve256

    dve256

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    I am building a heat treat furnace and need help with the wiring.
    I have this very nice diagram of how the curcuit is wired but I still have a few questions.
    Please forgive my stupidity (learning)

    the picture of the wiring is at the bottom of the pdf
    www.e3b6a5y.com/365/WiringDiagrams/new2006power.pdf

    On the diagram, where the heating element is wired to PID and the power cord. How does one make this 3 way connection? My power cord to plug is 3 core flex.
    Again on the diagram there is another 3 way connection between SSR output, PID and power cord.

    Another loose end (har har) I have is my earth in the power cord? Does not seem to be any where to connect this wire. I need an earth is the circuit don't I?

    Another problem I have also got a non contact (magnetic) interlock which I am unsure how it would fit into the circuit.

    Any advise much appreciated :)
    Look forward to a reply
     
    Last edited: Jun 25, 2011
    dve256, Jun 25, 2011
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  2. dve256

    Resqueline Moderator

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    It's quite simple really, see attached picture for one practical wiring example.
     

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    Resqueline, Jun 25, 2011
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  3. dve256

    dve256

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    Thanks Resqueline you are a genius, just to be sure you have used the 2 wires coming from the interlock to bridge -ssr to -PID? I like it!

    I was going to do something different, and probably incorrect so I am really glad to have come on here. Now I can see how it should go yeah it does seem easy and I can see how the current will flow around and connect the parts.
     
    dve256, Jun 26, 2011
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  4. dve256

    Resqueline Moderator

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    Yes. I expected it to be an ordinary closed-when-closed door switch. It could sit in either wire going to the SSR.
     
    Resqueline, Jun 26, 2011
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  5. dve256

    dve256

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    Just finished wiring it all up and tested, pluged it in and the PID came on. Once I figured how to set the temp and all the output light came on for a couple of seconds and then house power went off.
    In those couple seconds the inside of the kiln had become warm so it sure was heating fast.
    All I can think is maybe its wired wrongly or more likely drawing too much power.
    The element is rated 3220W. Is there anything I can do to test this or something? Any ideas.
     
    dve256, Jun 28, 2011
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  6. dve256

    duke37 VIP Member

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    There could be two reasons for the power going off.
    1. Too high a current. At 230V you will need 14A, at 110V abot 30A.
    2. Leakage to earth due to insulation that has absorbed moisture since the furnace was last used.

    Check what had tripped the house system, whether it was an overcurrent trip or an earth leakage trip.
     
    duke37, Jun 28, 2011
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  7. dve256

    dve256

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    thanks for the reply, the breaker for the garage did not go down it was the middle power breaker switch that was down in the fuse box. Does that help?
    So how do I find out if its an over current trip or earth leakage trip?
    Will try testing it again, this time from points in the house to see if it does the same. (when everyone is out..)
    This is a newly built kiln so I don't think moisture is an issue?
     
    dve256, Jun 28, 2011
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  8. dve256

    dve256

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    Oh ok so it will work at 30A to sort of make up for the lower volt supply?
    So maybe if I get a 40A SSR instead of the 25A SSR that might sort it on a 110V supply?
     
    dve256, Jun 28, 2011
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  9. dve256

    duke37 VIP Member

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    I have never worked with 110V which from your reply I assume you have. You will need some massive cables and they must not be overloaded. Do not change to a 40A breaker unless the cables are up to the job.

    You do not say how the furnace is constructed. If the heating element is laid in grooves in the ceramic bricks, then earth leakage is possible. If the ceramics are new, they will have picked up some moisture which should evaporate when the furnace is first heated. Earth leakage will increase again when the ceramics get really hot.
     
    duke37, Jun 29, 2011
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  10. dve256

    dve256

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    yes you are correct the element is layed in groved in insulating fire bricks but the bricks feel very dry so I don't think that could be the problem.

    If I streched out the element would that make the resistance higher and current lower?
    I am not entirely sure of how it works, ie what makes it rated 3220W? How else can one change the ammount of amps?
     
    dve256, Jun 29, 2011
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  11. dve256

    TBennettcc VIP Member

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    TBennettcc, Jun 29, 2011
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  12. dve256

    dve256

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    I have just tryed pluging in, in the house, 240V.
    This time the output came on for about 20 seconds then power went off, didn't expect that :eek:

    Here is a pic of the inside of the kiln, maybe my spirals are too tightly packed, if the spiral touched itself :p would that cause this because there are a few bits that get awfully tight together around corners ect.
    [​IMG]

    Thanks for the links, it is quite a complicated issue, especially as this is my first project in electrics. I will keep trying to understand it.
     
    Last edited: Jun 29, 2011
    dve256, Jun 29, 2011
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  13. dve256

    duke37 VIP Member

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    What country are you in?
    What power voltages do you have?
    The power is determined by the resistance of the element and the voltage of the supply. If you have 110V and 230V, then run it on 110V which will give about a quarter of the power and half the current which the higher voltage will give.

    You seem to have made a neat job of the construction. Quite ambitious for a first project.

    In order to reduce the power, the resistance of the element needs to be increased. This can be done by lengthening it or making it of thinner wire. The odd touching turn should not make too much difference although an even spread of turns means that the temperature is uniforn and you do not get hot spots which could burn out. You should try to spread the turns a little in the corners since the heat will not be able to escape so easily from there, At low temperatures, the element may have significant temperature differences along its length but at high temperatures these tend to reduce since radiation is proportional to absolute temperature to the power of four.

    You cannot tell if there is moisture in the ceramic just by looking at it. How much moisture is there in a peanut, 10% or 20% ?
     
    duke37, Jun 29, 2011
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  14. dve256

    dve256

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    I am in UK, voltage is 240V in house, I guess as the power went off on both attempts (in garage socket and house socket) the power in the garage is maybe 240V as well. Which would be good as its what I need. Not sure why it stayed on for longer the second time around though (house socket).

    The bricks I bought second hand so I guess its hard to say if they contain moisture.

    Well I think I am going to try making new chanels and streching the element, as it does seem quite tightly packed to look at it. It came unstreched at 50cm I streched it to 140cm as it said to strech it out to 2-3 times. However I have seen other peoples heat treat ovens online and some seem to have their coils much more spaced out.
    Will post back once I have modified frankinsteins coil and retested.
     
    dve256, Jun 29, 2011
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  15. dve256

    duke37 VIP Member

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    Stretching the coil will not add to the length of the wire so will not affect the resistance, I would not mess up your bricks by modifying the element.

    You have approx 230V in all parts of your estate, the difference between the on times could be due to the fact that you are probably going through two different breakers.

    Note that to get the power that is specified, you need 14A and this is above the 13A allowed from a power plug. When I worked for an electrical power company we were only allowed to pull 8A through a normal socket, this was considered the safe limit.

    You have two options:-
    1. Add about 50% more wire to drop the current to more like 10A.
    2. Connect it up to a dedicated circuit like a cooker supply. This will run up to 30A.
     
    duke37, Jun 29, 2011
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  16. dve256

    dve256

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    Oh ok, only seen your message now and have already stretched the coil into a longer shape, nevermind its done now and I think it will improve the heat dispersion hopefully.
    Will post a pic later.
    So the element was 50ft (15m) so I need another 8m(50% extra wire) of heat resistant wire? I have no more room in the kiln for more elements so I assume you are saying add the wire into the circuit on the outside, as in normal wiring.

    Please clarify, even after lengthening the circuit then your saying the plug is under rated? Many thanks for your help!
     
    dve256, Jun 29, 2011
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  17. dve256

    dve256

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    I have just tested it again and its running atm and the power is still on wtf yes!
    Stretching it seems to have done it?
    Is the output always at 1 level (full power) or should I expect increases in amps?

    Heres a pic of the kiln ON! :D

    [​IMG]

    KEWL!
     
    Last edited: Jun 29, 2011
    dve256, Jun 29, 2011
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  18. dve256

    duke37 VIP Member

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    To get a current of 10A you need a resistance of about 25 ohms. Measure the resistance of the element you have and then work out how much extra length you need.
    You must not have the extra element outside the furnace as it will get very hot and you will need to put it in an earthed box with a fan to keep it cool.
    It worries me that you appear to have one end of your element outside the furnace. This looks very dodgy to me.

    Use a digital meter to measure the resistance of the element and let us know what it is.
     
    duke37, Jun 29, 2011
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  19. dve256

    daddles VIP Member

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    It appears this is a home-made furnace, right? If so, it looks like you wound your own heating element. You said that stretching the element fixed the problem, so it's possible what happened is that you had some of the element shorted out, reducing the resistance of the load and thus drawing too much current.

    Ideally, the first thing to do before winding the heating element is to accurately measure the room temperature resistance. This lets you calculate the resistance per unit length, a useful number to know. Then, after winding the element with a known amount of wire, you'd again measure the resistance to make sure there were no shorts in it. You can make these measurements pretty accurately with two $5 Harbor Freight digital multimeters (i.e., make a Kelvin measurement with a battery).

    Personally, I don't like exposed elements like that in a furnace, but sometimes it can't be helped. If you're using it for heat treating metals, then you'll probably be keeping the door closed most of the time. I would make sure there's a safety interlock switch that shuts the power off if the door is opened (use e.g. a double pole relay to shut off both legs of the power). The reason is that it's also tempting to use such a furnace as a forge, meaning you might reach in there with metal tongs to grab a piece of metal. If you touch the heating element in the right place, you might get a fatal shock. I'd also make sure it was on a circuit with a ground fault interrupter breaker or outlet. If you're unsure of some of this stuff, it would pay to have a licensed electrician come in and check things out for you. Sure, this stuff will cost extra money, but the total cost would be much less than a hospital visit or funeral costs.

    I don't see a gas connector on it -- are you going to make it into an inert gas oven? That would be nice for helping to keep the scale off things.
     
    daddles, Jun 29, 2011
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  20. dve256

    duke37 VIP Member

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    You ask
    Is the output always at 1 level (full power) or should I expect increases in amps?

    The output will drop somewhat as the temperature rises since the resistance will rise with temperature. The variation will depend on the material of the wire, you can probably find this on the web or get the information from your wire supplier.

    Looks like the loading will drop by 6 to 10% on heat up. (Nichrome)
     
    Last edited: Jun 29, 2011
    duke37, Jun 29, 2011
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