A coreless induction furnace is a high-frequency induction furnace used for melting steel and other ferrous metals. Its working principle is based on the principle of ac transformers i.e., when the primary is connected to a high-frequency ac supply, eddy currents are induced in the charge to be heated.
Construction of Coreless Induction Furnace :
The construction of a coreless induction furnace is very simple without any core as shown in the below figure. The furnace in coreless induction heating is made up of ceramic crucible cylindrical in shape surrounded by coils that act as the primary winding of the transformer as shown below. The charge to be heated is kept in the crucible and it acts as the secondary of the transformer.
Working of Coreless Induction Furnace :
When the ac power supply is given to the coils of the induction furnace, the coils setups alternating flux. The flux set up by the primary winding will induce the eddy currents in the charge. These eddy currents heat up the charge to its melting point and also set up electromagnetic forces producing stirring action which is essential for obtaining uniform quality of metal. The eddy current developed in any magnetic circuit is given by,
Where Bm is the maximum flux density, f is the frequency and We is the eddy current loss.
Due to the absence of the core in this type of induction furnace, the flux density will be low. In order to compensate for this low flux density, the frequency of the current supplied to the primary winding should be increased which is necessary to induce the required voltage in the secondary.
But when the furnace is operated at a high-frequency current, due to the skin effect, the effective resistance of the coil increases thereby increasing the copper loss. The increase in losses increases the temperature of the primary winding which in turn necessitates artificial cooling of the winding. To achieve sufficient cooling primary winding, coils made up of hollow copper tubes are used through which cooling water is circulated.
This type of furnace employs insulated supporting structures so that the current flowing in the coil may not induce eddy currents in the supporting structure which results in the reduction of efficiency.
The design consideration of a coreless induction furnace is such that a minimum stray magnetic field is maintained. The frequency of primary coil current suitable for the operation of the furnace can be known from the penetration formula.
Where,- ρ = Specific resistance of molten charge,
- f = frequency in Hz,
- μ = Permeability of the material,
- t = Thickness up to which the current in metal has penetrated.
Advantages of Coreless Induction Furnace :
The advantages of coreless type induction furnace are,- They are fast in operation.
- Precise control of power into the charge can be employed and thus uniform quality of the product obtained is unattainable by any other method.
- Absence of dirt, smoke, noise, etc.
- Crucible of any shape can be used.
- The eddy current in the charge results in automatic stirring.
- The erection cost of a coreless furnace is less.
- Operation cost is also low.
- It is possible to operate a coreless induction furnace intermittently as no time is lost in warming up.
Applications of Coreless Induction Furnace :
- This type of furnace is used for the production of steel and are also used for melting nonferrous metals like brass, bronze, copper, aluminum, magnesium, etc.
- They are also used for specialized applications such as vacuum melting, duplexing steel, and heating of charges of nonconducting materials by the use of conducting crucibles.
In case the charge to be heated is of nonconducting material, then crucibles made up of conducting materials are used which act as the secondary. Whereas if the charge to be heated is of conducting material, then the crucible can be of either conducting or nonconducting material (mostly of nonconducting material).