Electric Furnace Controller

Discussion in 'Furnaces and their construction' started by Al2O3, Sep 21, 2018.

  1. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I decided to start a new thread on my new furnace controller build since the topic of electric furnace and kiln control comes up here occasionally.

    The original thread I started on this subject was just intended for the lost foam casting for the SSR heat sink. That is here:


    As a quick refresher, my electric furnace has two 14ga Kanthal A1 coils that run on 240vac single phase power and are ~4.2kw each. This controller operates those resistive heating elements and also has 12v DC control circuit for the furnace lift actuator. Details on the coils and original controller build can be found here:


    I’ve been bargain shopping and collecting all the electrical components for the new controller over time. I made the cabinets from 18ga sheet stock to match the other cabinets on my rig. With the heat sink cast for the SSRs and the cabinets built,

    12 On Cabinet.JPG

    I went about the task of laying everything out, cutting cabinet openings, and painting the enclosure. From here it’s just mounting the components and making the wiring harnesses. Here it is compared to my original controller and with components mounted.

    14 Gen I Vs Gen II Controller.jpg

    It’s pretty much the same layout as my original controller except for a couple tweaks to the primary power circuit. That is explained in more detail below. For convenience I also added an additional Thermocouple (TC) display for my metal contact pyrometer. I was previously using a hand-held unit for that. This will keep the TC cables routed better and eliminates any need to make/break TC connections during the melt. The DC controller for actuator on the furnace lift was also incorporated into the new enclosure whereas it was previously hung on the side of my old controller. The new controller still has some available real estate for future additions. The old controller will be moved over to my big furnace and used with the electric insert for heat treating and potentially as a holding furnace.

    That thread is here and I’ll update that thread when I move the old controller to that furnace:


    The power panel is pretty simple. A two pole 25 amp breaker with one pole for each resistive heating element, furnace power contactor, a couple of SSRs, and terminal strip. The rest is a 12vdc power supply and wireless controller for the lift actuator. Ordinarily I’d use low voltage for the control circuit but for both this unit and the previous furnace controller, all controls are a separately fused, low power, 240 vac circuit.

    15 Power Panel Components.jpg 16 Cabinet Top Components.jpg

    The controls and instruments are a little more involved but allow independent operation and performance monitoring of each resistive coil.

    The PiD uses a K Type thermocouple in the lid of the furnace and solid-state relays to actively control temperature to set point. The PiD displays temperature set point and furnace temperature status. There is a panel meter that displays voltage, current, power, and energy. I originally thought this was just frill but it turns out to be a great status and diagnostic tool. It also indicates the total energy used in kw-hrs thus an estimate of cost of each heating session. There is an LED status lamp, hour meter, and selector switch for each resistive element. The selector switches just open the control circuit from the PiD for each respective SSR so either one or both coils can be selected for use. The contactor opens both legs of the 240vac to electrically isolate the furnace heating elements. My previous furnace just relied on the SSR selector switches to remove power from the coils but this still left one leg of the of the 240vac connected so you could still be exposed to 120vac to ground if you contacted the coils. I’ll post up a schematic in a future post. I’m going to rewire my old controller to accomplish same.

    17 Panel Components.jpg

    I’ve got some wiring to do!

    Rtsquirrel likes this.
  2. PatJ

    PatJ Silver Banner Member

    I have standardized on 120VAC for all control wiring circuits since it has enough power to control many items and no power supply is need.
    240 V would work as well.
    Its really a matter of finding coils and other components that have the right voltage rating.

    Nice panel and layout.
    And that cast heat sink is way too cool.

  3. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Got in a little shop time today and made some progress. If anyone was wondering what that second smaller cabinet was in my earlier post, it’s a three-drawer tool box that mounts under the controller. I got tired of having stuff lying around loose on the furnace cabinets and needed a writing surface so I made this in the style of the other cabinets to fill the space under the new controller. Finished the tool box up today and started the controller wiring.

    18 Tool Box.jpg
    19 Tool Box and Controller Paste.jpg

    Here’s a little cut and paste of what it’ll look like when completed and installed.

    20 Cut and Paste.jpg

  4. PatJ

    PatJ Silver Banner Member

    That is some exceptionally fine fabrication work.
    First class to say the least.
    The tool box is a great idea too.

    Nice small floor footprint on the furnace and associated gear.
    Space is always a premium, especially in my shop/garage.

  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Many thanks Pat. Much appreciated.

    I hear ya on shop space. Never have enough of that. Minimal foot print was one of the original build goals. It rolls easily on hard surfaces but with 4" wheels is a bit of a chunk to get over the transition back into the shop....but I manage.

  6. It looks great, my brain refused to register the scale and that it was hand made: I thought it was a repurposed photocopier stand at first, it looks that good.
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Rainy day....good for shop time! Here’s the new Gen II controller with the wiring harnesses completed, installed, and fired up for a few minutes.

    21 Wiring Harnesses.JPG 22 Installed.JPG 23 Fired Up.JPG

    Wow, it certainly refreshes my memory why wiring isn’t my favorite task. There’s a lot going on there to get all the instrumentation and controls for both the furnace heating elements and lift actuator connected from front panel to the power panel. Not sure how many feet of 18 ga instrument and control wire I used, but a lot. Moral of story, every bell and whistle requires multiple wires……but they are useful bells and whistles. Only disappointment, the blue latching power switches were supposed to be illuminated and they are not....what do you expect for $2 I guess.

    Here’s a little trick I picked up along the way. I keep 8 colors of control wire on hand, but I supplement that by applying tracer lines on the wire with different color Sharpy felt tip markers. That way I have a large number of identifiable combinations from the 8 basic colors and it allows me to trace the lines between components more easily in the future and color code AC and DC lines.

    24 Wire tracer.JPG

    I still have some additional real estate on the front and power panel in case any future additions may be needed…..perhaps a blower control for a forced air natural gas burner. This Gen II controller is more or refinement than additional capability but as I mentioned at the start of the thread, I have need for a second controller on my large furnace sitting in the background.

    Even at 8kw the furnace operating cost is very low. I’m paying 8.7cents/kw-Hr. Coming to temperature takes about 30 minutes so that costs 36 cents to initially heat things up but from there on the duty cycle of the resistive coils is only about 33% or about 24 cents an hour to operate. It’s clean, automated, no fuel to store or run out of, and temperature control is very precise which makes it useful for all kinds of heat treating and melting tasks up to about 2200F.……Suits me just fine. I like it!

    Tomorrow I’ll see if I can get the old controller mounted and fired up on the big furnace.

    oldironfarmer likes this.
  8. I was thinking long and hard about your furnace and going electric when I found this kiln at the scrapyard rated for 1300 deg C or 2372 deg F, for scale it's about 5'6" tall and 415 V.

  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    What a great find. Do you own it?

  10. No, I was sorely tempted, but my workshop is packed tight and I have to finish the current projects. I tipped off a friend who can fix it up and use it in his studio. I'm sure your made for the task design would be a lot more efficient than using a pottery kiln.
  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    It may not be that great for a melting furnace but for burn out kiln for larger investments.....nice. Most kilns don't like to be opened hot though and though easy to load, but with the large door I'm sure it would dump heat quickly. I'm with you though....not enough space in my shop for it.
  12. joe yard

    joe yard Silver


    I am scrapping the electronics of this 50s public service radio? The radio only had 1 crystal that was cut for 56 MHZ. At this time I have no use for the cabinet. Still it is a 70 year old classic electric cabinet that just looks cool with the meters on top!
    I can tell from your work that you also would find room for this cabinet Kelly.

    Attached Files:

  13. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Nice looking cabinet Joe and an interesting piece of history in it's own right. I'm in the process of completing projects I've already started.....before starting more. That can be difficult for me to resist. I have a lost foam rig and several sheet metal forming machines on the drawing board and also several automotive induction systems on the go but must tidy up and make space before adding to the clutter!

    On another note, glad to see you back in action.

  14. PatJ

    PatJ Silver Banner Member

    That is a good idea on color coding the control wiring.
    Makes hookup and troubleshooting much easier.
  15. joe yard

    joe yard Silver

    Thanks Kelley I don’t need the cabinet but could not stand to see it scrapped. Luckily for me room is not a problem. When I run out of room. It is due to poor organization, not lack of structure. The very small town my shop is in is about as close to Hootervill as any thing. The towns population hovers around 60 and ½ is related. The other half don’t care, 3 ½ miles in every direction is corn. We brought in another small building around a month ago. It is just a very nice 10 x 12 shed. This makes the 4 th shed. The only bad thing is they are all wood so I would not do any hot work in them.
    If it was not for the distance. I would gladly store then give the cabinet to you Kelley. The shipping cost would probably pay for a small house. It will go into the shed of someday. My god I should have a yard sale from the shed of someday.
    The wire marking is a great idea!
    I will be starting a thread on the lay out of a new structure for the foundry building soon.

  16. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I spent $6.50 and ordered two of the blue maintained push button power switches.....this time confirming they are indeed illuminated. The purpose of the furnace switch was to isolate the electrical power from the furnace for safety. I like good visual indicator of status, that way before I handle tools inside the furnace, I have three indicators of whether power has been removed from the coils; the status light on the furnace switch, the position of the selector switches, and the current display on the panel meter. It was bugging me. They'll take a month before they arrive. Me and the furnace can wait.

  17. Jason

    Jason Silver Banner Member

    How's that wiring diagram coming along??? I'd like to see the method to your madness. :D
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I did sketch that up but never posted it. I'll see if I can dig it up Jason.

  19. Gippeto

    Gippeto Copper

    @joe yard That cabinet would make a pretty darn cool beer fridge....:cool:

  20. Jason

    Jason Silver Banner Member

    Yeah, the BRITS would love it! They don't mind piss warm beer fresh out of a Lucas refrigerator. :p I'm a big fan of bitburger beer... I asked a german guy one time what was the correct temperature to drink it at. He said, put it outside on your doorstep. Whatever temp it is outside, that's it. Funny how he was right... Until it was about 70degrees. YUCK!

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