No core is provided in the coreless induction furnace. A crucible of more convenient shape can be employed. In this case, also the charge to be melted is made the secondary of the transformer. The primary is wound over the crucible itself.
The eddy currents produced in the charge not only heat it up but also account for the stirring action. Iron laminations are provided outside the primary winding to create a low reluctance path for flux and also contain the stray field which may otherwise induce the heavy current in supporting steel structure.
The frequency employed depends upon the size of the coreless induction furnace. For lower capacities of furnaces higher frequency of the order of 3000 Hz are employed whereas for higher capacities frequencies are down to 600 Hz. To reduce copper losses, hollow copper tubes are used in which cold water is circulated.
The operating power factor of such furnaces is very low (between 0.1 and 0.3). Therefore, static capacitors are used to improve the power factor of the installation.
Since the power factor does not remain constant during the operation of the furnace, the capacitance in the circuit during heat cycle is varied to maintain power factor approximately unity.
The coreless induction furnace is chiefly used for melting of steel and other ferrous metals. The capacities available vary from 50 Kg to about 20 tones. The initial cost is more as compared to arc furnace.
Advantages of Coreless Induction Furnace
The advantages of coreless induction furnaces over other types are given below:
- Low operating cost.
- The automatic stirring action produced by eddy currents.
- Low erection lost.
- The absence of dirt, smoke noise etc.
- Less melting time.
- Simple charging and pouring, precise control of power.
- Most suitable for the production of high-grade alloy steels.
Power Supply for Coreless Induction Furnaces
For coreless induction furnaces power supply is obtained from ordinary supply system and its frequency is converted to the higher value by means of oscillators. The oscillators can generate a very high frequency in the order of megahertz.
The AC supply is stepped up by a transformer and then rectified by using a bridge rectifier circuit. The rectified voltage is applied to the oscillator and high-frequency output is fed to the charge to be heated through an output transformer.
Eddy Current Heating
This is also known as the induction heating. The material to be heated is placed inside the coil. The heat in the material to be heated is produced by eddy currents. Power loss due to eddy currents is eddy current loss and appears in the form of heat.
The metal to be heated is placed within a high-frequency current carrying coil. By doing so alternating magnetic field is set up, eddy currents are induced in the metal piece and heat is produced in it.
The eddy current loss
We α (B2maxf2)/ρ watts/m3,
where, Bmax = maximum flux density,
f = supply frequency,
ρ = resistivity of the metal piece.
Since the eddy current loss is proportional to the product of the square of supply frequency and square of flux density, therefore by controlling the flux density and supply frequency the amount of heat can be controlled.
The frequency may vary from 50 Hz to 8 MHz depending upon the type of work done. High-frequency eddy current heating is frequently used for forging and annealing. The process is economical for continuous heating.
Advantages of Eddy Current Heating
- It is quick and clear.
- There is little wastage of heat in eddy current heating as heat is produced in the body to be heated up directly.
- Temperature control is easy i.e. by controlling the supply frequency.
- The heat can be made to penetrate into the metal surface to any desired depth.
Disadvantage of Eddy Current Heating
- It is the costly method for the production of heat.
- Low efficiency.
- The initial cost of eddy current heating apparatus is high.
Eddy current heating is used for the heat treatment of metals i.e. annealing, tempering, surface hardening etc.