Correless Induction Furnace G2+

Coreless Induction Furnaces /Crucible furnaces Are made from robust steel shell that is mounted on trunnions /pivot and fitted with mechanism for tilting. The furnace has a cylindrical refractory unit with a top open for charging. Around the cylinder it is used a spiral watercooled electrical coil. A refractory lid is used to reduce the heat losses from the surface of the liquid metal and some facility are used to extract the fume and toxic gases. The furnace can operates at 50Hz or at hundreds of Hz. The 50Hz furnaces have a better stirring process, but a lower high power density (the necessity for a higher volume for the furnace). The stirring process is very useful to assure the mixture of the composition in order to be homogenous.

Operation The induction coil act as a primary of a transformer, with many turns, and the charge acts as a secondary with only a single turn. When the a.c. current is applied to the cooper coil of the furnace, a large current is induced in the metallic charge. A joule-Lenz effect will heat up the charge until it melts. Once the metal is molten the magnetic fields generate a stirring motion in the crucible, resulting a homogeneity of the chemical composition and assimilation of the substances in addition (for alloy materials). Water cooling system It is a very important component because the flow of current through the induction coil generates heat and heat is also generated through the refractory walls from the molten metal inside the crucible. Different sensing systems are used to provide warning if liquid metal is penetrating the crucible refractory. Any leakage could bring to an explosion. Also, if the cooling system fails the refractory could be rapidly brake down.

In some situations are used two furnaces as a multi-installation system. In this case, one will be in melting mode and the second will be holding for pouring. This will reduce the production time and will increase the efficiency of the process. The coil turns are electrically insulated to reduce the risk of shortcircuits. A refractory screen (in many layers) is used between the coil turns and the crucible to protect it from the heat and the molten damage. The crucible is a highly refractory material, but also a very resistant to the important forces inside the molten metal. The material depends on the melted metals and alloys. An important drawback is the low power factor, which has a negative impact over the supply system. So, a capacity group is used to increase the power factor. The limits of the crucible furnaces could be: - the stirring of the metal; -The meniscus of the molten metal (in some situation it needs about 1/3 of the total volume); - the electro-dynamic forces could be to strong; -- an overheat of the installation.

Iron core Induction furnaces Two main types of iron-core furnaces exist: (i) furnace with closed horizontal channel and (ii) furnace with closed vertical channel. The Inductor 1 is realized from copper as a cylindrical coil placed on a ferromagnetic core. The magnetic circuit 2 is from plates /tole and it is removable. The channel 4 is a refractory one and it contains the molten metal 3. Usually, the furnace has a tank/reservoir to keep the metal.

1 2 ϕ

G

Fe

4

F

3

Operation: For starting, the channel is filled with molten metal, and the furnace is connected to the supply. Because of the stirring effect the metal will flow between the channel and the tank. Solid metal pieces can be added. The furnace is not entirely depleted, a part of the molten metal being kept inside it.

Electrodynamic effects/Forces into the channel

1. Shrinkage effect/ Fc: Fc1 – the repulsion force results as an interaction between the magnetic field H1 and the induced current I2. This is orientated from the inductor to the exterior of the channel and is minimum at the interior, so it results a movement of the molten metal along the channel walls. Fc2 – the attractive force is due to the interraction between the elementary currents I2*, with a maximum value in the center of the channel. The transversal section of the channel will shrink with the danger of the cut off for the molten metal (and for the currents), which could stop the molten process.

2. Eddy effect Ft: Is due to the interaction between the currents with different densities J2 and the own magnetic field H2. The resulted force Ft acts to the channel’ axle. 3. Centrifugal effect Fm: Is due to the interaction between the currents in the channel and the leakage flux Φs . The forces Fm will press the metal to the exterior of the channel. Combined with the force G, due to the mass, the surface of the metal will be oriented after the combined force F. 4. Thermal convective effect: Is due to the different densities of the molten metal between the channel and the tank. This thermal convection is helping the mixture of the metal (the “colder” metal) fall down because it has a higher density.

A basic block diagram of one induction furnace is illustrated. Normally the input voltage from the secondary side of the power transformers which feed the induction furnaces are 230V or 400V, the first block. In the second block, incoming voltage will be converted to the fixed DC voltage. By means of power electronic switching devices, incoming DC current to the third block will be inverted to the one phase AC current. Adoption of the required load and inverters’ output will be done in the fourth block. Frequency or phase of inverter or both of them, output of the system and the DC level of converters’ output will be adjusted in the control section.