Induction heating advantages; induction furnace

Induction heating heats a conducting charge by eddy currents (and hysteresis loss in magnetic materials) induced in it by an alternating magnetic field; the charge acts as the short-circuited secondary of a transformer.

Advantages of induction heating

1. Heat is produced inside the charge itself, so heating is fast and efficient.
2. No contact between coil and charge, so no contamination; pure metals and alloys can be melted.
3. Accurate temperature control and uniform heating; electromagnetic stirring mixes the melt well.
4. Selective heating of a chosen depth or area by choosing frequency (skin effect), e.g. surface hardening.
5. Clean, no smoke, no fuel storage, quiet and safe working conditions.
6. Quick start, low heat loss to surroundings and low running cost for large outputs.

Core-type (low-frequency) induction furnace

        +------------------+
        |   laminated core |
   +----+----+        +----+----+
   | primary |        | molten  |
   |  coil   |  core  | metal   |
   | (50 Hz) |        | ring =  |
   +----+----+        | secondary
        |             +----+----+
        +------------------+

• A primary coil on a laminated iron core is fed at supply (or low) frequency.
• The charge, in an annular or V-shaped channel around the core, forms a single-turn short-circuited secondary.
• Large current flows in the charge, heating it by $I^2R$.
• Problems: poor pf (high leakage reactance), pinch effect at high current density, and the furnace must be started with molten metal in the ring. The Ajax-Wyatt (vertical core) furnace overcomes many of these.

Coreless (high-frequency) induction furnace

        water-cooled copper coil
        o  +--------------+  o
        o  |   crucible   |  o
        o  |  [ charge ]  |  o   <- HF supply
        o  |              |  o      (0.5-10 kHz)
        o  +--------------+  o

• A crucible containing the charge is surrounded by a water-cooled copper coil fed from a high-frequency source (500 Hz–10 kHz).
• There is no iron core; the high frequency produces strong eddy currents in the charge.
• Capacitors are connected across the coil to correct the very low power factor.
• Melting is fast, with good stirring and accurate control.

Fields of application

• Melting of steel, copper, brass, aluminium and special alloys in foundries.
• Surface hardening of gears, shafts and crankshafts.
• Brazing, soldering, annealing and forging billets.
• Zone refining of semiconductors, sterilising, and induction cooktops.