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Department of Technical Education

Andhra PradeshName : 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 : PPT, Diagrams, Animation Revised By : K. Chandra Sekhar, L/EEE, GPT, HYD

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Recap

In the last class you have learnt about

• Electric Arc Heating

• Direct and Indirect Arc Furnaces

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Objectives

On completion of this topic you would be able to know

• Principal of Dielectric Heating

• Applications of Dielectric Heating

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Dielectric Heating

• When an insulating material is subjected to an alternating

electric field, the atoms get stressed and due to inter-

atomic friction caused by repeated deformation and

rotation of atomic structure heat is produced

• This loss is known as dielectric loss

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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

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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 co-

incident as long as there is no external electrical field shown

in fig 1

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Dielectric Heating

Fig.1 Neutral Atom

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• 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

Dielectric Heating

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POLARIZED

Fig.2

Dielectric Heating

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Dielectric Heating

•This is known as electric dipole moment

p = q d.

where q = charge on the nucleus (coulomb)

d = distance between the two centres (m)

Fig.3

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•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

Dielectric Heating

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• 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.

A.C. supply

Fig.4

Dielectric Heating

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• 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

Dielectric Heating

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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

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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

Fig.5

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• 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

Dielectric Heating

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Dielectric loss = V I CosΦ

From fig 6 = V I R

= V Ic Tan

= V [ V / xc ] Tan

= V 2 C Tan ( Tan )

= V2 2πf x o r A x watt.

Derivation of Dielectric loss

d

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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

Dielectric loss

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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

Dielectric loss

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• 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.

Dielectric loss

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• 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

Dielectric loss

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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

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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

Contd…Applications of Dielectric Heating

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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

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Summary

In this class we have discussed about

• Principle of dielectric heating

• Applications of dielectric heating

• Advantages of dielectric heating

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Quiz

1.Dielectric loss is ____ proportional to tan

a. Directly

b. Inversely

c. Both a & b

d. None

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2.Loss factor is ______

a. b. / c. / d. Zero

Quiz

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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

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Thank You