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What stainless alloy for heating element

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  • What stainless alloy for heating element

    Just wondering what stainless alloy would be best if it was to be used as a very low ohms, high power resistor. Does one have a higher resistance, or survive better when it goes through hundreds of cycles of heating to near red and cooling ? I have an application in mind, and for that the change in resistance with heating won't be an issue. Nichrome wire would be the obvious choice, but I'm talking about a fraction of an ohm and currents in the several tens of amps. I'm leaning towards a flat strip cut out of sheet material.

    Another question concerns inductance- would a magnetic alloy be more inductive than a non-magnetic alloy?
    Last edited by darryl; 05-20-2007, 06:14 AM.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Look up the thermal conductivity of various alloys. The thermal conductivity and the resistivity are governed by the same principles. It can be inferred by the individual alloying elements and their proportions although that is a bit riskier as the conductivity doesn't always follow a strict proportional relationship to the percentage of each element in an alloy. If you calculate it that way you must normalize the values and then take a weighted average.

    Inductance is an effect caused by changing magnetic fields cause by changing current flow. It doesn't depend on the magnetic property of the conductor.

    [added]

    For obvious reasons I would suspect the best bet would be the SS alloy with the most nickel and chromium.
    Last edited by Evan; 05-20-2007, 08:12 AM.
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    • #3
      BTW, in a strip resistor you can eliminate all inductance by making a mobius resistor. Layer two strips of conductor with an insulator and connect the ends with a 180 twist to make a bifilar mobius strip. Connect the leads to the strip directly opposite one another at any point on the strip. The mobius resistor does not exhibit inductance as the current flow takes two paths that produce equal and opposite magnetic fields that cancel.

      Last edited by Evan; 05-20-2007, 08:32 AM.
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      • #4
        Evan, 'avin' us on, right? Moibus? As long as the current flows around in continuous turns inductance will sum with the turn count regardless of moibus fillips. Even with careful non-inductive technique there is always some inductance present in every conductor if only from forward inductance due to the length. Ask any RF or commercial power transmission line designer

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        • #5
          No, no joke. Bifilar wound resistors exhibit nearly zero self inductance as the fields cancel.

          From US patent Office:

          62Helical or wound element This subclass is indented under subclass 61. Subject matter wherein the resistor includes an elongated resistance element formed as two or more continuous closed coils or loops as a spiral, helix, or toroidal winding as defined in subclass 296 below, together with additional structure or arrangement of the coils or loops for reducing or eliminating the inductance or the effect thereof of the coils or loops.
          http://www.uspto.gov/go/classificati...tm#C338S062000
          Last edited by Evan; 05-20-2007, 08:59 AM.
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          • #6
            Originally posted by Evan
            No, no joke. Bifilar wound resistors exhibit nearly zero self inductance as the fields cancel.
            If I recall correctly they do have higher capacitance, but this is an unlikely problem at 60 Hz. In digital circuits it could be a factor as anything that alters the square waves has impact.

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            • #7
              Hmm, interesting. I'm familiar with non-inductive winding, but I've never seen the mobius strip used like that.

              I have made stainless steel resistors before and they worked fine, though it seems with a lot of expansion issues. No concern where it's air cooled and not encapsulated.

              Yes, choosing on the basis of high nickle and chromium content would make sense.

              I'm reminded of the old electric ore cars that used a large resistor for speed control. I think the wire was just steel rod, but I'm not sure. Apparently they would glow red after awhile, obviously during the ascent out of the mine shaft. Eight hours on one charge, hauling ore all day long. Hmm, seems pretty decent, especially considering the primitive speed control method (like a slot car control, only on a much larger scale).
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                OP, what's your application, if you don't mind sharing? I'm just curious now
                You never learn anything by doing it right.

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                • #9
                  While cleaning up the other day, I found my electric car project. This is the one man, three wheeled, laid back deathtrap that I'm hoping to be able to get to work and back in. It's still in test mode, so I don't want to rig up a full bore elecronic speed control just yet. Because it will be running full bore almost all the time I'm out on it, a simple resistor with a couple taps will do fine for the time being to give a soft start and somewhere to dump braking current. That's the idea anyway.

                  Ironic- due to the threat of flooding here, I have more important things to attend to, so I won't have time to worik on this again for awhile. But if I hadn't become motivated to do a major shop preparedness, I may not have gotten the urge to work on this again. As it stands now, it's in my kitchen- the only place in the house where there isn't any carpet. Usually you would put the project (bike or whatever) in either the dining room or the bedroom. Well, it's sort of in the kitchen/dining room. The bedroom is going to be filled with tools and such, and materials are going to reside in the hallway. I'll be living in the office for awhile.

                  At any rate, the electric is back on the stove, if only on the back burner. I'll be riding it soon.
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                  • #10
                    FWIW, I've been using 316 SS 0.6mm mig wire for heater elements for years now, with nary a prob
                    Just got my head together
                    now my body's falling apart

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                    • #11
                      Hi There,

                      How about Nichrome wire?

                      Good Luck!
                      -Blue Chips-
                      Webb

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                      • #12
                        Here is the "Resistivity" in Microhm-cm*
                        (for resistivity in ohms per circular mil ft.,multiply by 6.017 )
                        spec----at 68 deg------at 1200 F

                        301--------72----------116
                        302 -------72----------116
                        303--------72----------116
                        304--------70----------116
                        308--------72
                        309--------78-----------114.8
                        316--------74-----------116

                        403--------57-----------108.7
                        410--------57-----------108.7
                        414--------72
                        416--------57.0
                        420--------55.0
                        431--------65.0
                        440 (A,B,C)-60.0


                        Rich

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                        • #13
                          Thanks for the numbers, Rich. That's interesting. I don't think I've ever tested to find the difference in resistance with heating. From those numbers I can calculate to maybe cut only once and come up with the right range fairly quickly. Thanks again.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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