Before understanding the Dielectric Heating you should need to know about Dielectric loss .Dielectric Heating is below Dielectric loss.
The structure of an atom of any material / substance is in two parts, one is internal nucleus and the other is external nucleus.
Whole atom consists of nucleus having protons and neutrons.
Protons have positive charge, so net positive charge is at the centre and electrons carry equal negative charge surrounding the nucleus.
The molecular structure of an atom is as shown in Fig. below.
The molecular structure of an atom when external force or electric force is applied is shown in Fig. below.
When an atom is subjected to the electric field, the positive plate repels the positively charged nucleus and the symmetry is now disturbed.
Such atom in stretched form known as dipole or atom in this state is said to be polarized as shown in Fig. below.
Now for a dielectric material, there are number of such atoms which will be polarized.
If the electric field strength is increased, the degree of polarization will go on increasing i.e. more and more number of atoms will be polarized.
There will be certain value of electric field for which all the electric dipoles will align themselves as shown in Fig. below.
Dipole means tiny particle of dielectric material having two poles.
The dielectric material in this state is said to be saturated.
If the alternating voltage is applied across the capacitor plate, electric dipoles will also try to change their orientation according to the direction of impressed electric field.
In doing so, some energy will be wasted as inter atomic friction which is called as dielectric loss.
In dielectric heating, the use of the dielectric losses is made. The material to be heated is placed as a slab between metallic plates or electrodes connected to a high-frequency A.C. supply.
For producing sufficient heat, frequencies between 10 MHz to 30 MHz and voltage of about 20 k V must be used.
The required high frequency supply is obtained from electronic oscillator circuit. Fig. shows dielectric heating circuit arrangement.
The heat is developed by dielectric loss within the substance which is indeed utilized for heating the substance itself.
The electric field applied should not be too large otherwise, it will dislodge the electrons from the orbit, thereby ionizing the dielectric i.e. breakdown occurs.
Thus, the applied electric field is always less than the breakdown voltage. Dielectric heating is also known as high-frequency capacitance heating.
The dielectric loss taking place in insulating material is analogous to hysteresis loss taking place in magnetic materials hence it is called as dielectric hysteresis.
The material to be heated may be considered as the dielectric or a condenser and maybe, therefore, represented as capacitance placed in parallel With resistance as shown in fig. given below.
Vector diagram of the circuit is also shown in
Fig. given below.
Dissipation factor (tan 𝛿)
The dielectric loss expressed as the tangent of the angle between reactance and impedance vectors of a capacitor is called as dissipation factor.
Advantages Of Dielectric Heating
- If the material to be heated is homogeneous, and the alternating (or varying) electric field is uniform, heat is developed uniformly and simultaneously throughout the entire mass of the charge.
- As materials heated by this process are non-conducting, so by other methods heat can not be conducted to inside so easily.
Applications Of Dielectric Heating
- The cost of the equipment required for dielectric heating is so high that it is employed only where other methods are impracticable or too slow.
- It is mainly employed for heating certain materials to a moderate temperature that has low thermal conductivity e.g. rubber for vulcanizing, wood for drying and setting glue in plywood manufacturing, textiles for drying, plastic for softening, de-hydration of food and tobacco, heating of bones, etc.
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