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triac phase control

Discussion in 'Microcontrollers' started by anandpatel27, Jan 13, 2008.

  1. ronald_jansen

    ronald_jansen New Member

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    Ok, it was indeed the zero crossing detection which failed.

    I attached a schematic. The problem is, that I get some sort of drop-outs in the resulting signal.

    I added C27 today. This should have been there all along (shame on me), but it does not seem to completely solve the problem.

    I set the software to supply a single phase (= one positive+ one negative) each 200ms. appr. 19 out of 20 times this works and then either a pulse is skipped, or a double pulse is provided.

    I am also having troubles to get the oscilloscope to trigger on the mains current measurement.
    I ordered for a current sense transformer so I can trigger on the zero crossing signal and view the mains current at the same time.
    Hopefully I will be able to find out more soon.
     

    Attached Files:

  2. Hero999

    Hero999 Banned

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    I think the problem might be caused by phase shift in the transformer aupplying the circuit and the current not being zero when the voltage is zero due to the fact that the trasformer is inductive.
     
  3. ronald_jansen

    ronald_jansen New Member

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    I don't think so. I also tried the controller on a purely resistive heater. The same effect is seen here.

    Also I found that with the load attached the transformer actually behaves a lot like a normal resistive load. I even do not see the rather large 6A startup peak when the load is attached. In that case the current amplitude is 3A in all cases.
     
  4. dave

    Dave New Member

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  5. nemo50

    nemo50 New Member

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    heat control ssr pid with mitsubishi alpha2 controller

    I'm reading with excitement all about the heat controll. Since I'm designing a big 16kw heated platen for vacuum laminator I would like to confirm if i understand it well.
    The heart of my system wil be mitsubishi alpha 2 controller. It has a pid controll function block. From it I go (using function bloc) to PWM and set period of 2 seconds. On the output i get pwm that varies accordingly to MV value of the PID function. From the output of the alpha I go to crydom 50A zero switched 3 phase SSR. Finaly the multiple incoloy heating elements will be conected to delta configuration and heat the aluminium platen.
    Thermocouple finishes the close controll setup with signal converted by ad595cq to useable 0-10V range for alpha2 controller. The platen will be about 1.5m2 area and working temperature about 155C.
    Is this a possible setup?
     
  6. ronald_jansen

    ronald_jansen New Member

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    I am not familiar with all of the hardware you are using.

    Basically it should be possible using a PID. The system will be rather large, so I am not sure about the regulation speed you can achieve.
    It might be useful to make the platen rather thick. This added thermal capacity will allow for slower regulation.

    Also be cautious that if you use many interfaces (thermocouple to 0-10V to controller to switch) there might be a significant lag. A slower control loop might be more stable.
    Also be sure that the measured temperature is accurate. Put the thermocouple at a location where the temperature measurement is representative for the whole platen. Ie. not to far away from the heater, and not too close to it.
    A thicker platen will probably have a better equality.

    Hope it helps.
     
  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there,


    Yes sometimes switching a transformer on and off causes continuous line surges
    which isnt always good either. It's a nice idea though.

    Another idea is to use DC to power the heater element. Using a rectifier and
    PWM power supply you can control the heat to a very specific level and very
    smoothly. If you dont mind the extra complexity this works out pretty good.

    A little less complex is to try switching AFTER the transformer instead of BEFORE it.

    Another idea is to build an AC chopper circuit, where you actually chop up the
    AC line with one or more transistors. This is a bit more complex too but also
    offers excellent power control.

    Perhaps you can tell us what your load (heater) actually is, and how many watts.
     
  8. ronald_jansen

    ronald_jansen New Member

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    After I got my problems with the zero crossing fixed, I was rather satisfied with switching the transformer. Not funny spikes anymore.

    I would recommend against the triac after the transformer, because all TRIACS have an appr. 1,5V voltage drop.
    1,5V at 1A (before transformer) = 1,5W
    1,5V at 16A (after transformer) = 24W = very hot Triac + relatively high losses.
    In that case I would recommend rectifying and using a FET instead.
    Or you could use the TRIAC to do the actual heating ;-)

    About the 16kW heater. I am not sure about phase cutting a 16kW device. This will cause such a large phase cut load to the net that it might even be illegal.
    You might have to use some kind of power factor correction.
    It might work with something like frequency controller motor driver. This can probably also be controlled witha 0-10V control voltage.
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    You mean the OP is using a 16kwatt heater?
     
  10. ronald_jansen

    ronald_jansen New Member

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    For the 16kW heater I was referring to nemo50 who is working on something like this.
     
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    Oh ok, i was hoping the OP would come back with the power of his heater
    so we would have a better idea what we are dealing with.
    Yeah, 16kw is a lot of power.
     
  12. ronald_jansen

    ronald_jansen New Member

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    No, I am affraid that he is longer here.

    I myself am working in the range 200-900Watts. This works quite well with a toroid transformer and phase cutting.
     
  13. nemo50

    nemo50 New Member

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    16kw heated platen, pid regulation

    Thank you all for your ideas. I'll use 12mm alu plate plus reinforcement because of the all pressure from vacuum. So there will be some mass to heat for sure. The slower control loop i will try for sure. For the first test I make a smaller model with 1/10 of the power/mass and test the stability first. This is a 3 phase system, So each phase is about 5.3kw only. The SSR crydom says switching at zero crossing in the datasheet. So basicly I have K type thermocouple (thinking about 2-4! to average the reading in alpha PLC software/hardware and make it somewhat bulet proof, sensors will be burried in the platen not too close to the incoloy elements,and from various sources i will add a second overheat protective system. In case of the SSR manfunction they stay conducting i was told. So I guess it is really a must to add some kind of temperature limit switch in case something fails. Will keep you informed but this is a projet for more than 1 or 2 months so don't expect many posts any time soon, I'm also a happy father now.
     

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