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Department of Technical Education Andhra Pradesh Name : V. Narsimha Reddy Designation : Lecturer in EEE Branch : D.E.C.E. Institute : GMR. Polytechnic Gajwel Semester : VI Semester Subject : Electrical Utilization and Automation Subject Code : EE605A Topic : Electric Heating Duration : 100 Minutes Sub topic : Dielectric Heating Teaching Aids Revised By : : PPT, Diagrams, Animation K. Chandra Sekhar, L/EEE, GPT, HYD 9EE605A.19to20 1 Recap In the last class you have learnt about • Electric Arc Heating • Direct and Indirect Arc Furnaces 9EE605A.19to20 2 Objectives On completion of this topic you would be able to know • Principal of Dielectric Heating • Applications of Dielectric Heating 9EE605A.19to20 3 Dielectric Heating • When an insulating material is subjected to an alternating electric field, the atoms get stressed and due to interatomic friction caused by repeated deformation and rotation of atomic structure heat is produced • This loss is known as dielectric loss 9EE605A.19to20 4 Dielectric Heating •The dielectric loss is dependent upon the frequency and high voltage • High voltage at 20KV and above and frequencies in the range of 10 to 30 Mega cycles per second are employed 9EE605A.19to20 5 Dielectric Heating • The hysteresis loss is due to the reversal of magnetism or magnetic molecular friction which appears as heat • Electrically every atom is neutral, since the central positive charge equals the surrounding negative charge • The centers of positive charge and negative charge are coincident as long as there is no external electrical field shown in fig 1 9EE605A.19to20 6 Dielectric Heating Fig.1 Neutral Atom 9EE605A.19to20 7 Dielectric Heating • When atom is subjected to some external electric fields • The positive charge of nucleus is acted upon by some force in the direction of the field • And negative charge in the opposite direction • The effective centers of positive and negative charges are no longer coincident i.e., POLARIZED shown in fig 2 9EE605A.19to20 8 Dielectric Heating POLARIZED Fig.2 9EE605A.19to20 9 Dielectric Heating •This is known as electric dipole moment p = q d. Fig.3 where q = charge on the nucleus (coulomb) d = distance between the two centres (m) 9EE605A.19to20 10 Dielectric Heating •The atom in this state as said to be polarized • The electric field strength is increased, the degree of polarization also increases. After attaining a certain value of electric field, all the electric dipoles of a dielectric material will align themselves shown in fig 4 9EE605A.19to20 11 Dielectric Heating A.C. supply Fig.4 • The orientation of electric dipole will try to change according to the electric field applied. • Some of the energy applied direction of the will be waster towards the inter atomic friction and is called the dielectric loss. 9EE605A.19to20 12 Dielectric Heating • The loss increases with increase in frequency and strength of the electric field • Dielectric loss taking place in insulting material is analogous to hysteresis loss • This loss takes place in a ferro magnetic material • Hence it is also known as dielectric hysteresis 9EE605A.19to20 13 Dielectric Heating •As far as possible no air-gap should be left over between the electrode and material to be heated • The dielectric strength of air is smaller than any dielectric material • If voltage applied across the electrodes with air-gap and dielectric, air gets ionized first and result into the break down 9EE605A.19to20 14 Dielectric Heating • Therefore, it is desirable in dielectric heating not to apply high voltage but to use high frequencies. • All dielectric materials can be represented by a parallel combination of a leakage resistor ‘R’ and a capacitor ‘C’ shown in fig 5 9EE605A.19to20 Fig.5 15 Dielectric Heating • The total current I can be supposed to be made up of two components IR and IC. • The capacitive current IC leads V by 900. • The leakage current IR is in phase with applied voltage. Fig 6 9EE605A.19to20 16 Derivation of Dielectric loss Dielectric loss = V I CosΦ From fig 6 = VIR = V Ic Tan = V [ V / xc ] Tan = V 2 C Tan ( Tan ) = V2 2πf x o r A x watt. d 9EE605A.19to20 17 Dielectric loss Where v = Applied voltage (Volt.) f = Supply frequency (Hz). o = Absolute permittivity 8.854 x 10-12 F/m. r = Relative permittivity of the medium = one for free – space 9EE605A.19to20 18 Dielectric loss A = Area of the plate or electrode (m2) d = Thickness of dielectric medium or distance between electrode (m). = Loss angle (radian). r = Loss factor. From the above equation, the dielectric loss P α V2 and P α f 9EE605A.19to20 19 Dielectric loss • The use of high voltage is also limited due to break down voltage of the thin dielectric which is to be heated, safety conditions and corona • Under normal conditions, voltage gradient used is limited to 18 KV/cm • The choice of frequency depends on the loss factor of the dielectric. 9EE605A.19to20 20 Dielectric loss • Higher frequencies are used for low loss factor dielectric and vice-versa • Dielectrics having loss factor less than 0.05 are not economical to be heated by this method • Usual frequency used for dielectric heating is the range of 1 to 40 MHZ 9EE605A.19to20 21 Applications of Dielectric Heating 1. Drying tobacco, paper, wood and rayon 2. Welding of PVC 3. Stress annealing textile fibers 4. Heating of bones and tissues 5. Gluing and bonding of woods 6. Sterilization of cereals and medical equipment 7. Processing of rubber synthetic materials and chemicals during manufacture 9EE605A.19to20 22 Applications of Dielectric Heating Contd… 8. Heat-sealing of plastic resins 9. Preparation of thermo plastic resins 10.Sewing of rain coats, umbrellas made of plastic film materials 11.Diathermy treatment of certain body pains and diseases etc 9EE605A.19to20 23 Advantages of Dielectric Heating 1. Heat is produced in the whole mass of the material 2. Heating non-conducting materials is very speedy 3. Uniform heating 4. Materials heated by this method are combustible which cannot be heated by flame 9EE605A.19to20 24 Summary In this class we have discussed about • Principle of dielectric heating • Applications of dielectric heating • Advantages of dielectric heating 9EE605A.19to20 25 Quiz 1.Dielectric loss is ____ proportional to tan a. Directly b. Inversely c. Both a & b d. None 9EE605A.19to20 26 Quiz 2.Loss factor is ______ a. b. / c. / d. Zero 9EE605A.19to20 27 Frequently Asked Questions 1) What is dielectric heating ? 2) Explain the process of dielectric heating 3) List the applications of dielectric heating 4) State the advantages of dielectric heating 9EE605A.19to20 28 Thank You 9EE605A.19to20 29