Fast Resistance/Joule Heating

Discussion in 'The Projects Forum' started by amspurge, Jan 15, 2016.

  1. amspurge

    Thread Starter Member

    Jan 15, 2016
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    Hello everyone,

    I'm trying to find different methods of resistance heating (metal) and how AC vs DC changes the outcome. My goal is to heat metal as fast as possible and I'd like to know what techniques I can use to achieve this (AC waveform, frequency, voltage, material, etc.).

    I'm also curious to know if there is a special type of transformer that could help me with this rapid heating of metal.

    Please let me know if you have any thoughts. Any help would be great!
     
  2. WBahn

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    Mar 31, 2012
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    The first thing you might do is look at the power requirements you are talking about. How much metal, how much heat, how fast. Heating a small amount of material a few degrees in a few seconds is vastly different from heating a big hunk of metal several hundred degrees in a fraction of a second.

    There are lots of other issues as well that make it impossible to give good suggestions unless they are laid out.
     
  3. amspurge

    Thread Starter Member

    Jan 15, 2016
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    What I'd like to use is a small strip of material (0.25" wide x 0.015" thick) and heat it up. I'm thinking Nichrome will work best and I'd like to get it up to 400-500 degrees in about a second.
     
  4. WBahn

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    Mar 31, 2012
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    400 to 500 degrees what? Celsius? Farhenheit?

    What is the starting temperature?

    How long is this strip of material?

    What is its thermal capacity?

    How are you planning to get the heat from the heating element into the material?

    How well is the strip of material insulated from its surroundings?

    How is the strip of material held in place -- or more specifically, how well does it's mount conduct heat?
     
  5. amspurge

    Thread Starter Member

    Jan 15, 2016
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    I think you're missing the point. Regardless of the metal and its properties, I want to know how DC or AC would change the speed at which it is heated. I would also like to know how a waveform, RMS, etc. affect the speed of heating.

    The goal is purely to heat the fastest.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    The answers to those questions depend on how you are doing the heating. If you are doing it inductively, then DC isn't going to work worth a darn. But if you are doing it conductively, then it won't make much of a difference and the primary driver will be the effective power delivered to the element. But if the heating path involves reactive elements (such as a heating coil's inductance) then using an circuit that has a resonance and operating it at an AC waveform tuned to that resonance can have a very significant impact.

    There are also techniques you can use to get very high power for short durations, such as charging a coil with current and then dumping it into the heating element or doing something similar with a capacitor. Which would work best would largely depend on the resistance of the element.
     
  7. #12

    Expert

    Nov 30, 2010
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    The RMS voltage of a sine wave is the same as the DC voltage which would cause exactly the same amount of heat in the same amount of time. There is no particular wave form which enhances this function. There is no particular frequency which enhances this function unless you are trying to do induction heating. How complicated do you want to make this?
     
  8. amspurge

    Thread Starter Member

    Jan 15, 2016
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    Thanks for the responses, fellas. I think you both might be on to something.

    WBahn: It seems like AC would be the way to go for sure, and I have some other reasons why. Can you elaborate more on the circuit resonance and AC waveform tuning? We have been using different materials, but our output power has remained the same (no tuning).

    Here's a company that is doing this, I would like to be able to replicate it: http://www.mecasonic.com/en/gen/hot-air-welding/hot-air-impulsion-welding/impulse-welding_62.html

    If you look at their interchangeable tips, you can see they also have slots cut out, and I assume these are for directing the current.

    Your last comment about charging a capacitor is very interesting, because the company says on their website that the tips are "heated by a welding head that discharges up to nine low-voltage impulses of electricity" Maybe they're doing this?
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The method is going to depend alot on the material and extent you want the heating. They are wide and varied.
    Many applications that use resistance heating is for very localized applications, these are machines that are basically built along the lines of a resistance welder but the electrodes heat and weld, applications such as tank seams and fasteners such as threaded nuts with projections.
    Then there is induction heating and this uses AC of varying frequency dependent on the density of the objects heated.
    IOW, Material heating can also run the gamut of Resistive, Inductive and Capacitive.
    Max.
     
  10. Dodgydave

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    Jun 22, 2012
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  11. amspurge

    Thread Starter Member

    Jan 15, 2016
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    Thanks Dodgydave. We're trying to heat a metal tip so we can melt plastic quickly. I don't know if induction would work for that.

    Is there a way to do this without the water cooling?
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What about resistance welding technique Ala the weller soldering iron principle, the simple hand held device has over 100 amps at the tip.
    Simple fast and can be controlled simply by triac on the primary.
    Any process that cycles very frequently usually requires assisted cooling of some kind.
    Max.
     
  13. amspurge

    Thread Starter Member

    Jan 15, 2016
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    Hey Max! That's interesting you say that. We have two of them that we were looking at trying to understand. Do you know how these work? We have one that heats up very rapidly, but I wonder if it could heat up a strange geometry rapidly.

    Below is some information about what we're trying to do. It's called "Impulse Welding" (pictures are of the tips that are to be heated).



    [​IMG] [​IMG] [​IMG]
     
  14. MaxHeadRoom

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    Jul 18, 2013
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    The same principle as spot welding is used for seam welding and projection welding, In a seam welder, the electrodes are actually large rotating copper discs and 'roll' or draw the seam through as they rotate, the energy is pulsed by controlling the primary of the transformer.
    Just a glorified version of a spot welder, is how the Weller works, only the tip just heats up the solder instead of welding.
    It is a simple 1 turn secondary of a transformer that steps down to low voltage very high current.
    The same principle could be what is being used there.
    Max.
     
    Last edited: Jan 15, 2016
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  15. amspurge

    Thread Starter Member

    Jan 15, 2016
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    We've tried to do something similar (different amount of turns), but have not been able to reproduce the same results. Our transformers have a lot of secondary turns though. I'm wondering where our problem is. Could it be the geometry of the tip (electrode), material, or some electrical property that we're missing? Do you know how Weller is able to focus the heat at the tip? That's what we're after We want to only heat the tip and do so very quickly.

    Also, can you elaborate on why the energy is pulsed rather than constant? Is this just because of the waveform of AC power?
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The Weller has a large 1 turn secondary, the tip consists the smallest area of the whole secondary, hence this area is where the heat occurs.
    With the Weller it is a constant AC wave form, to pulse it or regulate it, you would need a Triac control of the primary.
    They do have a double pull trigger which switches in a different primary winding.
    Max.
     
  17. amspurge

    Thread Starter Member

    Jan 15, 2016
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    Forgive me, I'm a newb when it comes to electronics. When you say it has a "large" one turn secondary, what advantage does that provide? I assume that it would halve the voltage and double the amperage, but I'm curious as to whether the size factors in as well (or how it may factor in).

    What do you think of materials? Any thoughts on what may heat up the fastest while maintaining strength?
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    By large I meant large diameter, to carry the current involved. The tip dia is smaller area so this is where the heating will occur.
    The normal resistance welding copper electrode is high grade copper, not sure of the grade #.
    Max.
     
  19. amspurge

    Thread Starter Member

    Jan 15, 2016
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    So the diameter of my transformer winding wires will have a large impact on the power delivered to the tip? I take it if the windings are too small that I'll lose a lot of that heat in the transformer?
     
  20. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    No you don't loose heat, you loose current, bigger wire diameter means bigger currents, hence more heat generated at the tip.
     
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