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Low frequency induction furnace:


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Vertical channel furnace (VCF) Low frequency induction furnace.










Maroon = molten iron.

Grey = refractory.

Red = inductor coil.

Black = inductor magnetic core.




This has to be the weirdest transformer I’ve ever worked on. The primary winding is just 30 turns of oval tube encased in epoxy resin, the secondary a single loop of molten iron. Working at full power up to 120,000A can be flowing in the molten iron loop.


Because of the proximity of the molten iron and the high current passing through the primary windings water is pumped through the cables and the coil tubing to cool them. The inductor box also has to be cooled. You can see some of the water pipes in the photograph. The first sign of trouble would usually be a rise in the return water temperature.





The coil is centered on and insulated from the magnetic inductor core by fiberglass rods.






Regulating the power to the induction coil is by series capacitors switched by SP contactors. Power factor correction capacitors in parallel were in the main used to fine tune the power level.

Normal running to hold the metal temperature would be roughly 700/800kVA depending on weight of the furnace burden. To heat the metal ready for pouring the power would be increased to 2MW. When I redesigned the control system I took the power to 2.6MW the inductor didn’t sound happy, that was the end of that experiment.








There's one slight problem when using induction to heat metal, it isn’t too fussy what it heats up. The inductor box is steel and the inductor is quite capable of melting itself. To get around this problem the box is fabricated in two halves insulated from each other to break the inductive loop, even the bolts have to be insulated.





I used to test the insulation between the two halves at 2.5kV DC. I always got readings in the TΩ range, this worried me for some reason. I did a bit more testing and found much to my surprise the zinc paint we sprayed the steel body with would actually withstand 5kV. The faces of the Paxoline insulated joint were also painted and varnished. No wonder I always got good results.


In the mid 60’s when the furnace was installed it was the largest vertical channel furnace in the world at 100 tons. Able to raise the temperature of a full burden by 1°C per minute.


My involvement came much later in 2004 when the control system was falling apart. I installed a Modicon TSX Micro PLC to control the power level and added safety shutdown systems. The furnace became my “baby” (more like an encumbrance), every time it cried or burped I had to attend to it.

On pressing the shutdown PB the 11kV OCB opens, this triggers an automatic system that opens the 660V ACB and then earths each capacitor in turn. The manual earth switch is interlocked with the 11kV OCB via a Castell key, the key can only be released with the OCB in the isolated position. Once the system is earthed another key is released to allow access to the power control panels and the inductor.


Finding an intermittent earth fault took me about three months, when I found the fault I wished I hadn’t. Iron was leaking through the refractory and dripping on to the coil causing the earth fault. It took a week to repair, during this time all the production workers were laid off.


A comical bit was the production manager asking me to restore power so the iron could be heated to make it easier to pour. I handed him the keys to my isolation locks and told him to phone me in twenty minutes. He asked “what are you doing?” “Getting in my car and getting as far away from you as I possibly can.” He asked one of the directors to stop me. “Why? I’m going with him and my cars faster.”




The beast in action:


First it has to be filled with molten iron.





It will then sits there grumbling to itself as it heats the iron to the casting temperature.





To get the iron out the entire furnace tilts toward the pouring launder.





The launder.






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