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Friday, 4 October 2019

Dielectric Insulator Material

Dielectric material basically is an insulating material, but it has no free electrons. When a source of e.m.f is connected across it, no current flows. The behavior of dielectric is primary electrostatic electrodynamic effects that also takes place due to their non-zero conductivity are quite insignificant in normal operating conditions. Since no dielectric is perfect it contains a small number of free electrons and a very small current called leakage current flows through it when electric field is applied. While the function of insulating material is to obstruct the flow of current, the function of dielectric material is to store electrical.


REQUIREMENTS FOR DIELECTRIC:

The dielectric material may be solid, liquid or gases. Whatever be the material, the basic function is the same i.e., to store electric charges. Since most of the dielectric materials are not perfect insulators in that they contain, few free electrons which result in leaking current. When a dielectric is placed between two parallel plates and a.c voltage is applied to the plates, if the dielectric is a perfect insulator with zero electrical conductivity an alternating electric field is set up in the dielectric. Thus in turn, sets up a flow of wattless capacitive current in the dielectric and may be found from the equation.


Ic = 2 π f CV amps

Where,
lc = capacitive current (Amps)
f = frequency (Hz)
C = Capacitance of the capacitor (Farads)
V = Applied voltage (Volts)

On putting the capacitor across AC voltage the capacitive current flows continuously till the voltage is applied, but if D.0 voltage is applied across the capacitor, the capacitive current will flow only until the electric field is set up in the dielectric. Besides the dielectric strength depends on the following factors.

(a) Thickness
(b) Temperature
(c) Applied frequency
(d) Wave form
(e) Electrode form
(f) Area
(g) Heat conductivity
(h) Surrounding medium
(i) Rate and time of application of voltage.

To increase the breakdown voltage, generally the thickness is increased. Therefore the essential requirement for a dielectric is

(a) Its dielectric strength for a given thickness should be high.
(b) The material should be able to withstand one and half times the operating temperature.
(c) It should not appreciably vary its performance for varied frequency.
(d) It should not get affected by surrounding atmosphere like moisture, acids, alkalies etc.
(e) The leakage frequency should be as least as possible.
(f) It should not breakdown due to mechanical vibrations.
(g) The material should have a uniform thickness such that spark or puncturing does not takes place.

Table gives the dielectric strength and permittivities of some of the medium.

Material
Dielectric strength
Permittivity
tan δ
1
Bakelite
22 kV / mm
6
0.02
2
Bitumen
14 kV / mm
4.5
--
3
Cotton
4 kV / mm
5
0.2
4
Ebonite
25 kV / mm
2.8
0.01
5
Empire cloth
15 kV / mm
2

6
Fibre
5 kV / mm
6

7
Glass
8 kV / mm
6.5
0.004
8
Guttapercha
15 kV / mm
4

9
Rubber
17 kV / mm
4
0.016
10
Marble
6 kV / mm
8

11
Muscovite
95 kV / mm
6
0.0003
12
Mycalex

7
0.003
13
Dry paper
7 kV / mm
3
0.005
14
Paraffin wax
8 kV / mm
2.2
0.0003
15
Polystyrene

2.6
0.0002
16
Polythene

2.3
0.0001
17
Porcelain
15 kV / mm
6
0.002
18
Shellac
13 kV / mm
3
0.008
19
Slate
3 kV / mm
6

20
Steatite

5
0.002
21
Mineral oils
30 kV / mm
2
0.0002

Table shows the relative dielectric constant (ε) of dielectric material at different frequencies.


Frequency in Hz
102
104
106
108
3 x 109
Pyranol
4.42
4.40
4.40
4.08
2.84
Cable oil
2.25
2.25
2.25
2.25
2.22
Teflon
2.1
2.1
2.1
2.1
2.1
Polystyrene
2.59
2.56
2.56
2.55
2.55
Polythylene
2.25
2.25
2.25
2.25
2.25
Bakelite
4.87
4.62
4.36
3.95
3.70
Porcelain
8.98
8.95
8.95
8.95
8.90
Nylon
3.88
3.60
3.33
3.16
3.03
Glass
6.68
6.57
6.43
6.33
6.10

Below Table shows the value of loss tangent of various dielectrics at different frequencies.


Frequency in Hz
102
104
106
108
3 x 109
Pyranol x 10-4
36
4
25
1300
1200
Cable oil x 10-4
3
0.4
--
--
18
Teflon x 10-4
5
3
2
2
1.5
Polystyrene x 10-4
0.5
0.5
0.7
1
3.3
Polythylene x 10-4
5
3
4
--
3
Bakelite x 10-4
300
200
280
3
438
Porcelain x 10-4
22
6
2
4
11
Nylon x 10-4
144
233
257
210
128
Glass x 10-4
77
35
16
23
60


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