1000 Watt induction heater boards

Discussion in 'Furnaces and their construction' started by Mark's castings, May 27, 2021.

  1. Ironsides

    Ironsides Silver

    I am not sure why you need to change the frequency for ferrous metals. At a college in Melbourne they had a hobby foundry course and they used a 100 Kw power supply at 3 Khz to run three different furnaces. One was 100 Kgs another was 70 Kgs, these two were only used for converting steel to cast iron. The smaller 50 Kgs was only used to melt brass and bronze. Only one furnace at a time could be used to melt any metal.

    I watched the furnace operator do hundreds of melts for cast iron and a few for bronze and the frequency was never changed to suit ferrous or non ferrous metals. I did ask him why that power supply used 3 Khz and he said that frequency was determined by furnace size, the larger the furnace the lower the frequency and the smaller the furnace high frequencies were needed. When ferrous metals are heated to past their curie point they become non magnetic and melt the same way as non ferrous metals do. He also said that if he filled the furnace with cast iron chips from a machine shop the chips would heat up but will never get to the melting point because 3 Khz would only melt larger chunks in that furnace. I did try to make a induction heater using 50 Hz and it worked heating up thick pieces of steel but when I tried to heat up thin pieces of sheet metal it would not heat them up. So it is possible to make an induction furnace using 50 Hz to melt metals but would be impossible to do in a backyard setting for many reasons to do with using a low frequency.
     
  2. The depth of the magnetic field penetration into metals is dependent on frequency, it's possible to case harden steels by using a high enough frequency to limit heating to the material surface. There is also dielectric losses in the capacitors that can be reduced along with the capacitor heating by using a lower frequency, on this board the capacitors need forced air cooling to survive more than a few minutes and some experimenter have cut the temps by 30% or so by going from 90 KHz to 43 KHz. It seems to be running at 90KHz as a cost cutting measure when 40 KHz or so would be less stressful on parts and give better heating penetration.

    I've listened to some descriptions of 50Hz induction furnaces used at Toowoomba Foundry and they needed a charge of molten metal to start them off and were a sort of flattened U tube that could be tilted to pour metal out of the front hole and charged with metal from a rear hole. They could not melt metal from cold unless they had some molten metal to begin with initially.
     
  3. Ironsides

    Ironsides Silver

    You would think that at 90 Khz or even at 43 Khz the skin effect would not melt melt metal but because metals are great conductors of heat it would melt the surface and the rest of the piece would get hot and melt. I had a look inside an induction furnace power supply at a foundry auction, every thing is water cooled including capacitors, thyristors and even bus bars. Passing so much current requires a lot of cooling. The 50 Hz furnaces I have seen are like a normal induction furnace with coils surrounding the crucible. To start this furnace a large solid cylinder that just fits in the furnace is melted but to avoid doing this again the furnace is never emptied to less than 50% and then small and large chunks are added to keep production going. This furnace is a coreless furnace and the furnace you mentioned is a cored furnace. It is a large transformer with the secondary part is the furnace.
     
  4. I have some motor start capacitors on order at the moment that are 1.5uF 425VAC which are cheap, three of those in series gives 0.5uF and I need around 4uF for 45KHz or thereabouts which would be a bank of 4 x 3= 12 capacitors. I'm hoping the higher current rating of such a type spread over 12 capacitors instead of the six much smaller ones will reduce the load on each capacitor enough for continuous use. Their construction usually has a large air gap inside which will impede cooling though so I may need to open them up.
     
  5. Ironsides

    Ironsides Silver

    I have pulled apart many microwave ovens and they have large capacitors and I was going to use them in parallel for electric motor starting. They .9 to 1.1 microfarad Maybe you could use them in your induction heater?
     
  6. They'd be ideal, around 2000 Volt rated too.
     

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