heater 3kW on PWM control?

Discussion in 'The Projects Forum' started by bill nahd, May 23, 2015.

  1. bill nahd

    Thread Starter New Member

    May 15, 2015
    6
    1
    Hello technicians,

    I think i get a 3kWatt heater challenge to deal with.
    Someone asked me if i could make a simple fluid heater control (for a distiller). It should work like a gas stove; when the fluid is boiling too much, just turn the knob a bit down...

    I was thinking of a PWM control, but with 3kW?!?
    Maybe a very 'slow' PWM control with a solid state relay?
    An ssr switches at zero crossing, is that good for a fluid heater? (or: is it a resitive load?)
    Or is it anyhow better to use a real pid? -heater control? (example schematics plzz :oops:)

    i do not have the details yet about heater type and distiller size and so, but i'm sure you can point me to the right direction. ;)

    Many thanks in advance!
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,659
    632
    This is effectively a variation on PWM.

    I needed to reduce the power in our 5 kW on-demand water heater to keep it from boiling the water (because of low flow rate) in the hot season. I just built a high current version of a lamp dimmer.

    [​IMG]

    Note that the triac is an ST Semiconductor "snubberless" triac.
     
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  3. bill nahd

    Thread Starter New Member

    May 15, 2015
    6
    1
    Thanks for your quick answer DickCappels.

    Hmmm i for some reason a 'power dimmer' did not jump to my thoughts. I think (see) that can stay very simple then.

    I know enough for now, thanks again for pointing the right direction. :)
    *Thread closed.*
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,554
    2,375
    Typically phase angle control is not required for heating due to the hysteresis, Triac controllers are often used in a 'burst' full on/off fashion.
    Max.
     
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  5. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,238
    A solid state relay would be good since it provides isolation and has zero crossing to minimize EMI.
    You could control it with a variable duty-cycle, slow PMW signal ( ON and OFF times in seconds since a heater has a long thermal time-constant) using a 555 astable circuit such as this, for example.
    Increase the value of C1 to reduce the PWM frequency.
    Connect the SSR in place of Q1.
    R2 should be reduced to 1kΩ.
     
    Last edited: May 27, 2015
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  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,056
    It would be helpful to know the mass flow rate of the distillation, as this will tell you the wattage required over and above insulation losses. Most of the heat required from the heater - assuming the system is insulated - will be used to vaporize liquid and send it forward. This information will tell you, for instance, whether you need finer regulation in the top or bottom 10% of the capacity range, or right in the middle.
     
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  7. bill nahd

    Thread Starter New Member

    May 15, 2015
    6
    1
    Well thank you all for the information.
    I learned a lot.

    @wayneh: he bought a stainless steel beer barrel of 50 litres that will be modified with heater element and copper pipes and so.

    @crutschow: I like this solution. Will try this, to begin with a cycle time of 10 secs.

    Thanks.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,028
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    As I recall, the environmental ovens where I used to work had a cycle time of 2 or 3 seconds when controlling the oven air temperature, but a heater element in liquid could likely use a longer cycle time since its thermal time-constant would be longer.
     
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