Kanthal wire connection to mains cables overheating ?

Discussion in 'Furnaces and their construction' started by metallab, Nov 16, 2021.

  1. metallab

    metallab Silver

    Last year I made my own electric furnace using 1mm Kanthal wire with a total resistance of 20 Ohms, running on 230V AC with a dimmer, controlled by an on/off thermostat switch. Here the build of this furnace (https://www.metallab.net/kanthal.php), see section "New furnace December 2020" or the top two videos if you are interested.
    I have used it since many times and today I did a bronze melt but at the end the furnace stopped. Luckily the metal was already molten, so I could pour it and the casting was succeeded.
    But I thought is was burnt out :-(
    After cooling down I inspected it and it was not burned out, but the connection between the power cord and the Kanthal wire was broken due to overheating. I have the coils double braided at the ends to prevent this, but despite this it became overheated. I have replaced the connection (photo) and tested shortly and it ran again, but I don't consider it as safe, because the plastic insulation of the wires is already partly blackened.

    Is there a way to protect the cord wires (1.5 mm2 wire rated for 16 A) against overheating ? Modern cords and wires always have PVC insulation.

    IMG_2211.JPG
     
  2. I used a ceramic connector block of a suitable size for my modest 1.8kw foundry.
     
  3. metallab

    metallab Silver

    Thanks, I just ordered a few ceramic connector blocks.
     
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    The failure is because the junction temperature between conductor and resistive wire is so hot, the copper conductor alloys with the resistive wire, lowers the melting temp in the alloyed area, then quickly fails. Alloying is more appropriately done in your crucible! :)

    Tripling the resistive wire will further help reduce the resistance and temperature of wire at the junction point with the conductor. It looks like you have the area of the junction packed with insulation. It is customary to ventilate, usually just naturally and not forced air and some distance outside the furnace to further reduce heat.
    The ceramic junction block mentioned is one way.......also....
    The other thing that is customary, the initial connection to the resistive wire is usually a high temperature service wire. This can be connected with ceramic junction block or a ceramic wire nut. I prefer MG wire because it has both Mica and fiberglass for insulation (not PVC) and has nickel plated copper multi-strand conductor which is very conductive and usually much higher ampacity for the same gauge wire. It is of course more expensive, but not terrible, and you only need a short lead of it to transition to an ordinary conductor/insulator.

    You are doing bronze with Kanthal so are pushing the upper end of the operating range. Some or all of the aforementioned may be required for reliable operation.

    I don't know about EU sources but here are a couple I've used for reference. If you select the high temp wire category you will see other high temp wire but MG (Mica/Glass) is the best. Select the gauge for specs.

    https://www.prowireandcable.com/hi-temp-125c-538c/mg-mica-glass/ and also here: https://www.mcmaster.com/high-temperature-wire/

    https://www.mcmaster.com/ceramic-twist-on-wire-connectors/

    Best,
    Kelly
     
    Last edited: Nov 16, 2021
    Melterskelter likes this.
  5. Smoking Shoe

    Smoking Shoe Silver

    Just happened to be thinking about this and there may be another reason for the Kanthal to fail at the junction with the copper.
    Kanthal is an aluminum alloy that depends on forming an oxide skin to protect the wire. This same kind oxide skin is what caused a lot of problems with aluminum house wiring. Even though the junction was in a cooler location aluminum spontaneously oxidized in the presence of oxygen. It is one of the reasons we don't glue bare aluminum for structural parts.

    Maybe the use of aluminum wire paste, as used where aluminum wire is used in buildings would help prevent this kind of failure?
     
  6. Thermal cycling of a screwed connection will cause it to fail over time, also aluminium wires in crimped lugs and screwed connections have a tendency to lose contact force due to the aluminium cold flowing over time. I had a stick welder crimped connection to the aluminium transformer secondary wire overheat and fail and had to solder it to get a reliable connection. Aluminium can be soldered with plain old rosin core tin-lead solder for non heater connections by pooling solder on the aluminium and poking a scraper through the molten solder to remove the oxide layer. This would be no good for heater wires though with such a low melting point for the solder.
     
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    It's certainly true about the protective skin but at 5.8% Al, I don't think I would call FeCrAl an aluminum alloy.......iron alloy that has some Al in it.;)

    It could be a contributing factor and nothing wrong with trying to insure as low as junction resistance as possible, but it is worth noting that once the junction temperature is reduced there aren't failures and there are a zillion KA1 coils operating in Kilns. Even though the junction temps are lower they are still hot. What temp will the paste survive?

    Best,
    Kelly
     
  8. Smoking Shoe

    Smoking Shoe Silver

    I don't know. Will have to do some more research. Finding fully informative data sheets looks like it will take some digging. From my first little bit of looking these compounds have Zinc and hydrophilic fumed silica in carriers that appear to be considered trade secrets....and they aren't any friends of the environment.
     

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