Download engineering physics ii dielectrics

ENGINEERING PHYSICS II
DIELECTRICS
4.8 DIELECTRIC LOSS
If a dielectric is subjected to an electric field, the electrical energy is absorbed by the dielectric
and certain quantity of electrical energy is dissipated in the form of heat energy. This is known as
dielectric loss.
The dielectric loss can occur both in direct and alternating voltages. The dielectric loss is less in
direct voltage than that of alternating voltages.
Loss in Purified gas
If an alternating voltage is applied across the capacitor having vaccum (or) purified gas then the
resulting current leads the applied voltage by 90 degree as shown in fig
If I lead V exactly by 90 degree we can say no electrical energy is lost.
Explanation
We know power loss PL = VI cos θ
When θ=90˚ ; PL=0
Loss in commercial dielectric
Now, when a practical dielectric is present the current leads the voltage by (90- δ ), then it
shows that there is some loss in electrical energy, and is called loss angle as shown
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ENGINEERING PHYSICS II
DIELECTRICS
Explanation
In this case the power loss PL = VI cos θ
Since θ=90-δ, we have
PL = VI cos(90-δ)
………..(1)
∴ PL = VI sinδ
We know V=IR, therefore I =
V
R
If the capacitive reactance is Xc then we can write,
I=
V
Xc
……….(2)
Substituting eqn (2) in (1) we get
PL = V 2 δ/Xc ………(3)
Power loss
We know frequency
1
2𝜋𝑅𝐶
1
=
2𝜋Xc 𝐶
𝑓=
∴ Xc =
1
2𝜋f𝐶
………(4)
Substituting eqn (4) in (3) we get
If δ is very small, then sinδ = tanδ
𝑃𝐿 = 2𝜋f𝐶𝑉 2 sinδ
∴ Power loss PL = 2πfCV 2 tanδ
Here tanδ is called the power factor of the dielectric. If f, C, V are constants then
𝑃𝐿 ∝ 𝑡𝑎𝑛δ
Naturally the power loss varies with frequency. The power loss at various frequency ranges is
shown in figure.
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ENGINEERING PHYSICS II
DIELECTRICS
In the electrical frequency regions the power loss is high, due to diffusion of ions from one
equilibrium position to another.
In the optical region the power loss is less because here the dielectric loss is associated with the
electrons.
4.9 DIELECTRIC BREAKDOWN
When a dielectric is placed in an electric field and if the electric field is increased, when the
field exceeds the critical field, the dielectric loses its insulating property and becomes conducting i.e.,
large amount of current flows through it. This phenomenon is called dielectric breakdown.
The electric field strength at which the dielectric breakdown occurs is known as dielectric
strength.
The dielectric strength =Dielectric voltage/Thickness of dielectric
There are different mechanisms by which the dielectric break down takes place. Some types of
dielectric breakdowns are

Intrinsic (or) avalanche breakdown

Thermal breakdown

Chemical and electrochemical breakdown

Discharge breakdown

Defect breakdown
Intrinsic Breakdown
When a dielectric is subjected to electric field then the electrons in the valence band acquire
sufficient energy and go to conduction band by crossing the energy gap and hence become conducting
electrons. Therefore large current flows and is called intrinsic breakdown (or) Zener breakdown.
Impurities in the dielectric material create additional energy levels in the energy gap and so
they help the intrinsic breakdown to occur at low applied voltages.
Avalanche Breakdown
This conduction electron on further application of field, collide with the valence electrons in
the co-valent bond and remove more electrons hence transferring them as conduction electrons.
These Secondary conduction electrons again dislodge some other bound electrons in the
valence band and this process continues as a chain reaction. Therefore very large current flows through
the dielectrics and hence called as avalanche breakdown.
Characteristics
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ENGINEERING PHYSICS II
DIELECTRICS

It can occur even at lower temperatures.

It requires relatively large electric fields.

This kind of breakdown occurs in thin sample.

It does not depend on the configuration of electrodes and shape of the material.

It occurs within a short span of time (milli seconds)
Thermal Breakdown
In general, when a dielectric is subjected to an electric field, heat is generated. This generated
heat is dissipated by the dielectric.
In some cases the heat generated will be very high compared to the heat dissipated. Under such
conditions the temperature inside the dielectric increases and heat may produce breakdown. This type
of breakdown is known as thermal breakdown.
Characteristics

It occurs at higher temperature.

It requires moderate electric fields.

It depends on the size and shape of the dielectric material.

Since, the dielectric loss is proportional to frequency, the breakdown occurs at relatively lower
field strength for a.c. fields than that of d.c. fields. It occurs in the order of milliseconds.
Chemical and Electrochemical Breakdown
This type of breakdown is almost similar to the thermal breakdown. If the temperature is
increased mobility of the ions will increase and hence the electrochemical reaction may be induced to
take place.
Therefore when mobility of ions are increased, insulation resistance decreases and hence
dielectrics become conducting. This type of breakdown is called as chemical and electro chemical
breakdown.
Characteristics

It occurs only at low temperatures.

It occurs even in the absence of electric field. In rubber, due to oxides produced in air, they
gradually lose their dielectric property.

It depends on concentration of ions, magnitude of leakage current.
Discharge Breakdown
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ENGINEERING PHYSICS II
DIELECTRICS
In some dielectric occluded gas bubbles may be present. When these type of dielectrics are
subjected to electric field, the gas present in the material will easily ionize and hence produce large
ionization current and is known as discharge breakdown.
Characteristics

It occurs at low voltages.

It occurs due to the presence of occluded gas bubbles.

It depends upon the frequency of the applied voltage.
Defect Breakdown
Some dielectrics have defects such as cracks, pores, blow holes etc. These vacant position may
have moisture (or) impurities which leads to breakdown called as defect breakdown.
4.10 REMEDIES FOR BREAKDOWN MECHANISMS

To avoid breakdown, the dielectric material should have the following properties.

It should have high resistivity.

It must posses high dielectric strength.

It should have sufficient mechanical strength.

Dielectric loss should be low.

Thermal expansion should be small.

It should be fire proof.

It should be resistive to oils, liquids and gases etc.

It must have less density.

There should not be any defects.

It must be in pure form.
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