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Principle of Dielectric Heating , Applications of Dielectric Heating

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Principle of Dielectric Heating , Applications of Dielectric Heating
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
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