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Killing SSR (relays) - Need to understand

Discussion in 'General Electronics Discussion' started by jwest7788, Mar 11, 2016.

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  1. jwest7788

    jwest7788

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    Mar 11, 2016
    Okay,

    Here's the setup:

    I have a project I am working on that uses heating coils in a kiln type setup.

    The whole thing is 240V 25-30amps

    I'm using a PID to control the elements, as I am hoping to hold precise temperatures inside the kiln.


    Something about my setup is killing SSR (relays) My relays are rated to 40A, 240V output, and the PID/input site is something like 3-32V.

    The switching and everything works fine, kiln heats up for awhile, but then within an hour, the SSR fails and gets stuck open.



    Though the SSR does have a heat sink with thermal paste, but heat could potentially be the cause.

    However, I feel like the issue is the sudden stopping and starting of such a large load on the SSR, as the PID flicks the SSR on and off several times a minute when it's trying to maintain the correct temperature.

    As a novice with electrical projects I want to confirm:
    Is the switching of the load a potential issue? --> Using the water analogy, I picture a "water hammer" type event, where the sudden switch off causes a voltage spike, or something similar?

    Any tips or solutions? I have gotten this far just reading various web pages, but now that I've killed 3 SSRs, and it's beginning to get expensive, I need help!

    Thank you! Seems like a nice forum you have here!

    JW
     
    Last edited: Mar 11, 2016
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi JW
    Do you have a schematic of your circuit we can see. Welcome to EP BTW.
    Adam
     
  3. jwest7788

    jwest7788

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    Mar 11, 2016
    [​IMG]
    Here's a mockup. Sorry, Not an actual schematic as learning the symbols is something still on my to do list.

    I'm sure this will generate some confusion, but I'll be here to answer questions, and I can grab pictures later if needed too.
     
    Last edited: Mar 11, 2016
  4. Minder

    Minder

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    Apr 24, 2015
    Your heating coils are drawing maximum current when cold, so if you are switching on/off this is causing maximum stress on the SSR.
    It sounds like your controller is using burst firing to control a PID loop or ?
    You may need to get a higher rated SSR or mock something up, circuit wise with a higher rated Triac.
    M.
     
  5. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Also, the heaters might be producing an inductive kick when turned off. A 240 V or 277 V transient suppressor across the output won't hurt. Many SSRs turn off at zero crossings and that should reduce this effect, but without more information this is a reasonable guess.

    ak
     
  6. Minder

    Minder

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    Apr 24, 2015
    I see you have added a layout, as suspected it appears to be a PID burst firing set up.
    Is this a commercially installed system or custom built?
    M.
     
  7. jwest7788

    jwest7788

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    Mar 11, 2016
    Inductive kick has been something on my mind, but had no idea what it was called. A transiet suppressor, like this thing?
    http://www.maplin.co.uk/p/transient-suppressor-metal-oxide-varistor-cp76h -- Any tips on findind one of these in Canada? Or somewhere that ships here I guess?

    This was once a residential Kiln for firing pottery, but it's since been modified. Portions of it were broken when I took ownership of it, and it's mostly amateur work trying to get it working with temperature control.

    Again, it works for awhile, but then later fails, not sure if super important info, but the coils are usually pretty hot before it fails. (This is a reason I think heat may still be an issue?)



    Right now my (probably misinformed) gut is telling me to focus on the transient supressor thing. Is the link above what you're referring to?
     
  8. Minder

    Minder

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    Apr 24, 2015
    Personally I don't think it is a inductive issue, I suspect it is more of a current issue and needs some empirical testing to find out the actual current that is occurring.
    But a heat sink on the SSR's should also help.
    M.
     
  9. dorke

    dorke

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    Jun 20, 2015
    1) Have you measured the heater current?
    2)have you measured the SSR case temperature?
    3) What type of SSR are you using (brand, model, datasheet)?
    4) You hoist a Canadian flag, isn't your line voltage at 120V?
    how come you have a 240V heater?
     
    Last edited by a moderator: Mar 12, 2016
  10. Minder

    Minder

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    Apr 24, 2015
    Canada/US is dual voltage 120/240 single phase, large demand systems and appliances are 240v fed.
    M.
     
  11. duke37

    duke37

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    Jan 9, 2011
    There was a thread some time back on trouble with a kiln. The matter was not fully sorted but it appears that the bare heating elements were mounted in the ceramic insulation and considrable leakage occurred both when cold due to damp and when hot due to ceramic conductivity.
    Some investigation is needed to detemine current consumption.
     
    hevans1944 and Arouse1973 like this.
  12. BobK

    BobK

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    Jan 5, 2010
    I hope the wiring of the SSR is not as shown. Note that the AC voltage is going to one of the control inputs.

    Bob
     
  13. jwest7788

    jwest7788

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    Mar 11, 2016
    hey Everyone, Thank you for the prompt replys!

    I'll take some measurements in the coming day or so.

    Can anyone point me to what exactly I am measuring?
    Voltage across heating coils?
    Voltage across SSR?

    I don't think my beginner multimeter can handle the amperage involved..

    Not sure how to proceed. Anyone have a tip or two for me?
     
  14. Minder

    Minder

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    Apr 24, 2015
    You really need a clamp on ammeter they come as attachments for DMM's.
    M.
     
    davenn likes this.
  15. dorke

    dorke

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    Jun 20, 2015
    You need to measure the current Through the heating element.
    You are correct about your DMM- they usually up to 10A.

    Like minder said,
    you need a high current(100A and above)clamp-on like here
    There are many to choose from.
    The professional,good and safe ones, are not cheap.

    There is a way to estimate the current without it :
    monitor your KW*H meter(electric company installed ) with only the heater connected,and calculate the current from it.

    Like so:
    Say you monitor the meter(start reading,end reading) for 6 minutes.

    KWH=(end reading-start reading)*10.
    Amperes=KWH/(line voltage)
     
  16. jwest7788

    jwest7788

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    Mar 11, 2016
    Thank you for the reply. Just to confirm, This should be sufficient, correct?
    http://screencast.com/t/Pj8C7C1lgZ7j
    http://screencast.com/t/DdUJfZqFW3zU

    --> This one because of free 2 day delivery, don't really want to wait weeks.


    Can I measure the current leading up to the heating coil, or does it need to be specifically the coil? Would hate to melt my ammeter, haha


    JW
     
  17. GPG

    GPG

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    Sep 18, 2015
    Inrush at switch on may be a problem. Make your own SSR using a zero crossing driver.
     
  18. Minder

    Minder

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    Apr 24, 2015
    Measure the heating element at the SSR input, some meters also have a peak store feature.
    M.
     
  19. dorke

    dorke

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    Jun 20, 2015
    Measure at the AC leads coming out of the SSR.
    You actually need to see all the current that the SSR is "carrying".
    The one you posted the link to has a Max reading which is a great thing in this case use it as well.

    Like so:
    1. Connect the clamp and turn one the meter in 200/400A
    MAX mode.
    2. Activate the heater with the SSR.
    the above will give you the max reading(inrush).

    3. Now put the meter in normal mode while the heater is on,
    this will give you the normal average current.
     
  20. jwest7788

    jwest7788

    15
    0
    Mar 11, 2016
    I've ordered the clamp on flow meter now. Expecting it on Tuesday, so will measure then and get back to everyone!

    Appreciate the assistance with this. :)

    JW
     
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