INDUCTION HEATING
Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction. Induction heating is a non-contact heating process. It uses high frequency electricity to heat materials that are electrically conductive. In nonmagnetic material heat generated is due to eddy current losses, whereas ,if it is a magnetic material , there will be hysteresis losses in addition.
INDUCTION HEATING
If the current continues to flow in the disc ,the surface would attain extremely higher temperature which can t be obtained by any other method.
INDUCTION HEATING
VIDEO
INDUCTION HEATING
Since it is non-contact, the heating process does not contaminate the material being heated Heating process is much faster and efficient than any conventional method of heating metal because heat is actually generated inside the work piece rather than being transmitted through the surface of the metal. For these reasons Induction Heating lends itself to some unique applications in industry.
APPLICATIONS IN INDUSTRY
FACTORS CONTROLLING THE INDUCTED HEAT IN THE DISC
High
coil current ( ). (I Larger numbers of coil turns (N ). (N High frequency supply (f ). (f Close spacing between the coil and work (d ). (d The disc may be of magnetic material (). ( Higher electrical resistivity of the disc ( ). Heat induced all above factors
INDUCTION HEATING
Hysteresis
losses f Eddy current losses f2 The frequency of AC used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.
INDUCTION HEATING
The
depth of penetration of induced current into the disc:
where: =specific resistance in ohm-cm f = frequency in Hz =permeability of the charge
This shows higher the frequency of supply the lower the depth of penetration and more induced heat.
INDUCTION HEATING
Curie temperature: Temperature at which alignments of domain become random and material lose it s magnetism.
UN MAGNETIZED MATERIAL
MAGNETIZED MATERIAL
INDUCTION HEATING
At
higher frequency the heating due to hysteresis become very small as compared to eddy currents. This is due to higher temperature attained by the material at which it lose it s domain alignment thus become un magnetized. At higher frequency eddy current losses also do not follow f 2 law as frequency is increased higher and higher.
Types of induction furnaces
There are two types of induction furnaces: Core type or low frequency induction furnace. Core-less type or high frequency induction Corefurnace
CORE TYPE FURNACE
Furnace
having core with secondary short circuited in coil form around the furnace and primary connected to the supply.
INDUCTION HEATING
Furnace
operate at frequency of the order of
10 Hz. If current density exceeds about 500A/sq.cm ,the current around the melt, interacts with the alternating magnetic field and completely interrupt the secondary side. This is known as pinch effect . This furnace is inconvenient where different types of charges are to be melted.
PRACTICAL EXAMPLE OF CORE TYPE FURNACE
AJAX-WYATT AJAX-
FURNACE This is normally used for melting and refining brass and other non-ferrous metals. non-
DISADVANTAGE OF CORE TYPE FURNACE
The necessity of magnetic yoke, the short circuited secondary formed by the charge and the PINCH EFFECT , made the core type furnaces less popular.
CORECORE-LESS TYPE FURNACE
The
furnace consist of ceramic crucible cylindrical in the shape enclosed with in a coil which forms the primary of the transformer .
CORECORE-LESS TYPE FURNACE
The
flux produced in by the primary winding sets up eddy-current in the charge which flow eddyconcentrically with those in the primary winding .These currents heats up the charge to the melting point and provide stirring action to the charge.
CORECORE-LESS TYPE FURNACE
Since
the frequency of supply is very high, the skin effect in the primary coil increased the effective resistance of the coil and hence the copper losses tends to be high and artificial cooling is necessary. The coil is, therefore, made of hollow copper conductor through which cooling water can be circulated.
ADVANTAGES Following are some advantages of core less induction furnaces: They are fast in operation. If the frequency is high, the lower magnetic flux density due to the absence of iron core for a given primary current would be compensated for and the need for heavy iron core can be eliminated.
USES OF CORE-LESS INDUCTION COREFURNACE
They are used for steel production They are used for melting of non-ferrous metals like nonbrass, bronze, copper, aluminum, magnesium along with various alloys of these elements There is a large number of specialized applications e.g. vacuum melting in a controlled atmosphere and melting for precision casting where high frequency induction heating is used . It is very widely employed for various industrial activities like soldering, brazing, hardening and annealing drying paints, sterilizing surgical instruments etc.
