R.N.G.P.I.T, Bardoli Electrical Engineering Department

R.N.G.P.I.T, Bardoli Electrical Engineering Department

Subject: EMMI

Prepared By:

Dr. Shaikh Mohammed Suhel

Prepared By:

• Name: Dr. Shaikh Mohammed Suhel

(Assistant Professor RNGPIT, Surat)

• Former Assistant Professor in SCET, SNPIT&RC, VIT

• Qualification: PhD (Power- Electronics & Drives, NIT, Surat), M.Tech (Industrial Electronics, NIT-Surat), Gate, B.E. (Electrical Engineering., VNSGU-Surat).

• Experience: 13 Years.

CH: Electrical Engineering Materials

• This Lecture contain Dielectric Materials & It’s Properties

• Insulator & Dielectric Materials:

>Insulator are used to prevent the flow of electricity through them.

Insulator used for charge storage is called dielectric.

>A dielectric is a substance which does not allow the flow of charge through it but permit them to exert electrostatic field & Force

>Material such as glass, rubber, wood and porcelain provide electrical insulation between two conductors at certain potential difference; and also serve for storing electrical charge under certain other circumstances.

• Dielectric Materials:

>In an ideal dielectric material, all the

electrons are tightly bound to the

nucleus of the atom. As a result

there is no free electrons available for

conduction


In an atom positively charged nucleus at the center surrounded by orbiting electrons which are negatively charged. An isolated atom does not have any dipole moment.

When an external field is applied, it causes electron cloud to move away. Thus, the centroids of the positive and negative charges now no longer coincide and as a result electric dipole is induced in the atom

Dipole: A pair of equal and opposite charges separated by a small distance constitutes an electric dipole

Without Electric Field

Dipole: When Electric Field is applied


Polar Dielectric: A dielectric material in which a permanent dipole exists even in the absence of an external field is called Polar dielectric.

Non-Polar Dielectric: A dielectric material in which there is no permanent dipole existence in the absence of an external field is called Non-polar dielectric.

Polarization is defined as the process of creating or inducing dipoles in a dielectric materials by an external field

Non Polar H2, O2, CO2

Polar H2O, HCL,NH3


A dielectric material is an insulator

which contains electric dipole, that is

where positive and negative charge are

separated on an atomic or molecular

level.

Dipole moment

Polarization is defined as the process

of creating or inducing dipoles in a

dielectric materials by an external field

P Qd


Polarization process: polarization

is the process of separating

positive charge from the negative

charge.

A dielectric in the electric field

can be viewed as a free space

arrangement of microscopic

electric dipoles which are

composed of positive and

negative charges whose centers

do not quite coincide

These are not free charges and

they cannot contribute to the

conduction process. Rather they

are bound in place by atomic and

molecular force and can shift only

position in response to the

electric field.

• Properties of Good Dielectric Materials:

It should have high resistivity to reduce leakage current(Like sulphur)

It should have high dielectric strength (like mica)

It should have high mechanical strength (like steel)

It should have high fire resistance (Like silica)

It should have high chemical inertness (like platinum)

It should have low thermal expansion (like invar)

It should have high thermal conductivity (Like silver)

It should have low dielectric losses (Like vacuum)

It should have low water absorption quality

(like paraffin wax)

It should have high quality surface finish (Like ebonite)

THANK YOU


Subject: EMMI

Prepared By:


Prepared By:







• This Lecture contain Dielectric Materials & It’s Application, Dielectric losses, solid

dielectric

• Relative Permittivity or

Dielectric Constant:

>Dielectric characteristics are

determined by dielectric

constant, εr

>It is defined as the ratio

between the permittivity of

the medium ε and the permittivity of free space

εo.

r

o

> εr is dimension less quantity and varies widely

from material to material.

Its value for vacuum is 1,

and for all other dielectric

it is always greater than 1.

>Permittivity of a medium

indicates the polarizable

nature of a material.

• Dielectric Losses or Loss

tangent:

>loss of energy that goes into

heating a dielectric material in

a varying electric field.

>It can be understand by taking the case of charging and

discharging of a capacitor, if V

is the potential, then amount

of energy stored in the form

of electrostatic potential

energy in the dielectrics.

>during discharging the

same energy should be

released but it is an

observation only a part of

it is released while rest is

disappeared as a heat.

The amount of energy

dissipated in the form of

heat is known as dielectric

loss.

• Dielectric Losses or Loss

tangent:

>Let the plate of capacitor is

placed in an electric field ,due to

field dipoles orient in a particular

direction and opposes by internal

friction. The opposition by dipoles

is equivalent to resistance of

capacitor.

0 cos(90 )RDielectric loss VI VI

cos cI

I (2 )cI V fc

2 (2 )Dielectric loss V fc

• Types of Dielectric Materials:

>Dielectric materials can be

solid, liquid or gaseous.

>Solid dielectric are most

commonly used in electrical

equipment as these are very

good insulator.

>Example: Porcelain, glass,

plastic, rubber cotton, wood

and mica.


>Liquid Dielectric materials are basically three different types: (1)mineral insulating oils

(2) Synthetic insulating oils.

(3)Miscellaneous insulating oils.

>The function of insulating liquids is to provide electrical insulation and to dissipate heat (cooling agent).

>Example: Transformer oil, cable oil, capacitor oil, vegetable oil, vaseline, silicon liquids, Sovol, and Sovtol.


>Gaseous Dielectric materials

are used both as insulator and

as cooling agents

>Example: Air, hydrogen,

nitrogen, helium, sulphur-

dioxide, propane, methane,

sulphurhexafluoride.

• Solid Dielectric Materials:

>Mica: Mica is an inorganic

mineral material, made of the

silicate of aluminum with

silicates of soda, potash and

magnesia. It is crystalline in

nature and can be divided into

very thin flat sheets.

It is rigid, tough and strong.

It has high dielectric strength

and low dielectric losses. It is

not affected by moisture.

Application:

Electric Irons, hot plates

and toasters.

Insulation in armature and

field coils, high frequency

application.

• Solid Dielectric Materials:

>Glass: Glass is an inorganic material made by the fusion of different oxides like SiO2,ZnO and MgO.

Glass is brittle and hard. It has good mechanical strength and has low dielectric loss. It is insoluble in water and is highly chemical resistant to most corrosive agent.

Application:

As a dielectric in capacitor.

Radio and television tubes,

electric lamps and laminated

board.

THANK YOU


Subject: EMMI

Prepared By:


Prepared By:







• This Lecture contain Question based on Dielectric Materials, transformer oil

• How transformer oil is the best cooling as well as good insulator? justify it:

>Transformer oil (also known as insulating oil) is a special type of oil which has excellent electrical insulating properties and is stable at high temperatures. Transformer oil is used in oil-filled electrical power transformers to insulate, stop arcing and corona discharge, and to dissipate the heat of the transformer (i.e. act as a coolant)

>Transformer oil is also used to preserve the transformer’s core and windings – as these are fully immersed inside the oil. Another important property of the insulating oil is its ability to prevent oxidation of the cellulose-made paper insulation. The transformer oil acts as a barrier between the atmospheric oxygen and the cellulose – avoiding direct contact and hence minimizing oxidation.

>There are two main types

of transformer oil used in

transformers:

1)Paraffin based

transformer oil

2)Naphtha based

transformer oil

• Transformer Oil Properties:

>The properties (or parameters)

of transformer oil are:

Electrical properties: Dielectric

strength, specific resistance,

dielectric dissipation factor.

Chemical properties: Water

content, acidity, sludge content.

Physical properties: Interfacial

tension, viscosity, flash point,

pour point.

>Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer, is considered as 30 KV.

>Minimum standard specific resistance of transformer oil at 90oC is 35 × 1012 ohm–cm and at 27oC it is 1500 × 1012 ohm–cm.

• Transformer Oil Properties:

>Dielectric dissipation factor is

also known as loss factor or

tan delta of transformer oil.

When a insulating materials is

placed between live part and

grounded part of an electrical

equipment, leakage current will

flow.

>If the loss angle is small, then the resistive component of the current IR is small which indicates a high resistive property of the insulating material. High resistive insulation is a good insulator. Hence it is desirable to have loss angle as small as possible. So we should try to keep the value of tanδ as small as possible. The high value of this tanδ is an indication of the presence of contaminants in transformer oil

• why hydrogen is used in

rotating electrical

machine?

>Many power generators

over 150 MW in capacity

utilize hydrogen as a cooling

method to transfer heat

from the power generating

winding enclosure to the

heat exchanges known as H2

coolers.

>Hydrogen cooled power generators are more efficient and have less mass of materials of construction than their air-cooled cousins.

>Hydrogen gas is 7 times more effective as a heat transfer medium than air and 1/14th the density, resulting in less friction losses and more fuel converted to electricity.

• 1) Define dielectric

strength of an insulator.

Explain the factor affecting

dielectric strength?

>for a pure electrically

insulating material, the

maximum electric field that

the material can withstand

under ideal conditions without

undergoing electrical breakdown

and becoming electrically

conductive (i.e. without failure

of its insulating properties).

>For a specific piece of

dielectric material and

location of electrodes, the

minimum applied electric

field (i.e. the applied voltage

divided by electrode

separation distance) that

results in breakdown. This is

the concept of breakdown

voltage.

>It is the characteristic of insulator to withstand electric pressure. It is a measure of the voltage necessary to cause a puncture through a particular thickness of the material, and is measured in terms of kV/mm or volts/mm. The dielectric strength of the material should be such that it can withstand effectively the electric pressure of the electric system where it is used even at the event of temporary voltage rise and that too at an elevated temperature. The breakdown strength of material may have a value 20-30 kV/mm in most of the cases.

>Factors affecting apparent dielectric

strength

>It decreases with increased sample

thickness.

>It decreases with increased operating

temperature.

>It decreases with increased frequency.

>For gases (e.g. nitrogen, sulfur

hexafluoride) it normally decreases

with increased humidity as >ions in

water can provide conductive channels.

>For gases it increases with pressure

according to Paschen's law

>For air, dielectric strength increases

slightly as the absolute humidity

increases but decreases with an

increase in relative humidity

• Why air is replaced by nitrogen as an insulating material in certain applications?

>A dielectric gas, or insulating gas, is a dielectric material in gaseous state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric gases are used as electrical insulators in high voltage applications, e.g. transformers, circuit breakers (namely sulfur hexafluoride circuit breakers), switchgear (namely high voltage switchgear), radar waveguides, etc.

>A good dielectric gas should

have high dielectric strength,

high thermal stability and

chemical inertness against

the construction materials

used, non-flammability and

low toxicity, low boiling

point, good heat transfer

properties, and low cost.

>The most common dielectric gas is air,

due to its ubiquity and low cost.

Another commonly used gas is a dry

nitrogen.

>In special cases, e.g., high voltage

switches, gases with good dielectric

properties and very high breakdown

voltages are needed. Highly

electronegative elements, e.g., halogens,

are favored as they rapidly recombine

with the ions present in the discharge

channel. The halogen gases are highly

corrosive. Other compounds, which

dissociate only in the discharge pathway,

are therefore preferred; sulfur

hexafluoride, organofluorides (especially

perfluorocarbons) and chlorofluorocarbons

are the most common.

>Dielectric gases can also serve as

coolants.

>In special cases, e.g., high voltage

switches, gases with good dielectric

properties and very high breakdown

voltages are needed. Highly

electronegative elements, e.g., halogens,

are favored as they rapidly recombine

with the ions present in the discharge

channel. The halogen gases are highly

corrosive. Other compounds, which

dissociate only in the discharge pathway,

are therefore preferred; sulfur

hexafluoride, organofluorides (especially

perfluorocarbons) and chlorofluorocarbons

are the most common.

THANK YOU


Subject: EMMI

Prepared By:


Prepared By:

• Name: Dr. Shaikh Mohammed Suhel (Assistant Professor RNGPIT, Surat)


• Qualification: PhD (Power- Electronics & Drives, NIT, Surat), M.Tech (Industrial Electronics, NIT-Surat), Gate, B.E. (Electrical Engineering., VNSGU-Surat). • Experience: 13 Years.

CH: Electrical Engineering Material

• This Lecture contain

Material used in Electrical Machine

Magnetic Materials, types of different magnetic materials and its application

Electrical Materials used in machine

1) Conducting Material: Allow the current to flow

2) Magnetic material: To Produce magnetic field

3) Insulating Material: Separate or isolate conducting and non-conducting materials

Conducting Material:

1) Highly conducting material: Cu, Al, Au, Ag: Less opposition to current, less temp co-efficient, mech strength brittleness, rollability, drawability, weldability, solderability, adequate resistance to corrosion

2) Highly resistive material: Alloys, temp. co-efficient is low

R= 𝑅0 (1 + 𝛼𝑡)



1) Highly conducting material :

Copper: machine windings

Cost is low compare to gold and silver

Resistant to react to oxygen

Resistant to react to corrosion

Hard drawn copper wires are used in electrical machines because their mechanical strength is high

Temp co efficient (high)=0.00393/oC.

Aluminium:

Since Copper is getting depleted Aluminium is the next choice for conducting materials as it is abundantly available.

Aluminium can not be drawn into thin wire it can be used to form thin shit



1) Highly conducting material :

Aluminium:

For aluminium wire the size of slot require is higher as compared to copper

For induction motor above 100kW output aluminium can be used for cage rotor.

Aluminium can be used to form foil type low voltage windings in a transformer

Aluminium can be used to construct tank of a transformer to reduce stray losses

Aluminium gets easily oxidise to form Al2O3 layer which prevent further oxidation

Parameter Copper Aluminum

Cost 1 0.49

Cross section Area 1 1.62

diameter 1 1.27

Volume 1 2.04

Weight 1 0.49

strength 1 0.64

𝜎𝑐𝑢 > 𝜎𝐴𝑙

𝑅 = 𝜌𝑙𝐴 = 𝑙 𝜎𝐴

𝑅𝑐𝑢 = 𝑅𝐴𝑙

𝑙𝜎𝑐𝑢𝐴𝑐𝑢 = 𝑙 𝜎𝐴𝑙𝐴𝐴𝑙

𝐴𝐴𝐿 = (𝜎𝑐𝑢 𝜎𝐴𝑙 )𝐴𝑐𝑢



Electrical Carbon: brushes rotating to stationary

This material is made forms of graphite or other form of carbon.

Conductivity of carbon is less than the copper and aluminium but it surface is smoother so it is used to make brushes in an electrical machines

Metal to metal contact spark are more

Carbon brushes are graphited or heat treated to increase conductivity and reduce hardness.

Graphite has negative temperature coefficient

Current flow-> losses-> temp->R decrease Vbd= I R= constant


Magnetic Material:

The material that allow flow of

magnetic field through them are called

as magnetic materials

1) Diamagnetic material :

No dipoles in the absence of magnetic field

In diamagnetic material there is no magnetism in absence of external magnetic field

When magnetic field is applied dipole is induced and they orient in the direction opposite to external magnetic field.

Magnetization (M): Ability of dipole to align when magnetic field is applied

Xm is susceptibility

Susceptibility is a measure of the material to get magnetize when external magnetic field is applied.

1) Relative permeability 𝜇 = 𝜇𝑜𝜇𝑟

For diamagnetic material Xm<0 so µ< µ0

Permeability of a material is related to ability of the material to allow magnetic field line to pass through it

𝑀 = 𝑋𝑚𝐻

𝜇𝑟 = 1 + 𝑋𝑚


Magnetic Material:

Magnetic Flux density (B) :

Magnetic flux density represents the closeness of magnetic field lines to each others

In diamagnetic material μ< μo so Bair> Bdia

In a diamagnetic materials magnetic field line diverge away from each others

Copper, Silicon, Germanium, gold, diamond are diamagnetic.

𝐵 = μ𝐻


Magnetic Material:

1) Paramagnetic Magnetic material:

In paramagnetic materials magnetic dipole is already exist, due to spin of unpaired electrons in an atom

In paramagnetic material magnetic dipoles are randomly oriented but when magnetic field is applied such dipole is align parallel to external magnetic field.

So, Xm is positive;

μ>μo so Bair< Bpara

due to positive susceptibility the flux density in side paramagnetic material increases

Potassium, Oxygen, tungsten and rare earth metal.

Μr is close to 1 and not much differ in dia and paramagnetic material

𝑀 = 𝑋𝑚𝐻

𝜇𝑟 = 1 + 𝑋𝑚


Magnetic Material:

1) Ferromagnetic material:

This elements lie in D block or S Block in periodic table and they have large number of unpaired electrons which contribute to spin dipole moment. So they have large dipole moment and are strongly magnetized.

(Fe)Iron, (Co)Cobalt, (Ni) Nickel , Gd, dy

So, M is high. Xm>>0

μr >1so B> μoH So, Bair<< Bpara

Since Ferromagnetic material is strongly magnetized the magnetic field line is converged when they pass through ferromagnetic materials.

When magnetic field is removed the dipole remain align inside the material giving rise to residual flux density.

𝑀 = 𝑋𝑚𝐻

𝜇𝑟 = 1 + 𝑋𝑚


Magnetic Material:

Ferromagnetic material:

Coercive Force (Hc): It is the value of magnetic field that needs to be applied to make flux density zero.

Domain Theory: An iron piece normally does not exhibits magnetism unless it is magnetize which means net dipole moment in absence of magnetic field should be zero , But ferromagnetic material must have residual magnetism and to explain this discrepancy we study domain theory.

After removal of magnetic field the dipole inside each domain remain align but the domains are not align with each other.

Magnetostriction: It is the phenomenon in which dimension of a ferromagnetic material changes when magnetic field is applied.

If alternating field is applied than material expands and compresses in alternate half cycle by which mechanical vibration are produce which are observed as a humming sound in a transformer.

To damp out mechanical vibration, transformer should be mounted on mud/soil/rubber mat.


Magnetic Material:

Ferrimagnetic material:

Xm>0

In ferrimagnetic material when magnetic field is applied the successive dipole get align in opposite direction but their dipole moments do not completely cancel each others so there is net magnetization.

(μr)ferri< (μr)ferro

To derive ferrimagnetic material Fe is replaced by Mn in iron compound.

Ferrimagnetic materials have high electrical resistivity.

By operating at high flux density the size of machine can be reduced for constant flux.

B=ϕ/A.

The core of electrical machine is designed using ferromagnetic materials.

The core of high frequency low power transformer can be designed using ferrimagnetic materials

In Ferrimaterials-> μris less->so B is less->so ϕ is less-> So Volt is less-> So Power is less

𝑀 = 𝑋𝑚𝐻


Types of Magnetic Material:

Soft magnetic material:

This materials can be magnetized as well as demagnetized easily.

Such materials are Used to construct electromagnet where the current can be alternating by which magnetic fields needs to be reversed.

This materials are used in electrical machine

I



Hard magnetic material:

The materials which are hard to magnetize and hard to demagnetized are called hard magnetic materials.

Such materials are Used to make permanent magnet which can be used in low power electrical machines. PMDC motor.



Electrical sheet steel (non oriented steel)

The material used in early days of transformer was iron with small amount of carbon but major disadvantage is aging which increases the area of hysteresis loop and there by increasing hysteresis loss and reducing efficiency.

At present sheet steel is used in electrical where a small amount of silicon is added which is 0.3 to 4.5% by weight.

Si added 1> Hysteresis loop narrow 2> resistivity increases so eddy current loss decreases.

Electrical steel may be manufactured by hot rolling or cold rolling.

In large machine high permeability and high efficiency is desirable so 4.5% silicon is used

In small machine 0.3% silicon is used because efficiency is not critical

By adding silicon the material becomes brittle so we cannot add a large amount of silicon else we wont be able to shape the materials.

Steel with high silicon contain is called transformer grade steel.


Magnetic Material:

1) CRGO sheet steel :

The addition of silicon(Si) in iron(Fe) in right proportions with the help of certain manufacturing process significantly improves the magnetic and electrical properties of iron.

it was discovered that the addition of silicon to iron significantly improves the resistivity of iron and so silicon steel or what we know today as electrical steel was developed. It not only brought down the eddy current losses in steel, but significant improvement in magnetic permeability and reduction in magnetostriction was observed.

CRGO field should be used when magnetic field is only lies in one direction so that magnetic field can be oriented in the rolling direction.

In cylindrical or rotating machine CRGO steel does not offer much advantage because magnetic field direction changes at each point.

In transformer core CRGO steel can be used . Because:

1) High magnetic permeability leads to low excitation currents and lower inductions.

2)Low hysteresis and eddy current losses.

3)Excellent lamination factor leads to better and compact

designs and hence low material required.

4) High knee saturation characteristics.

5) Very low level of magnetostriction leads to noise reduction.

6) Enhances ease of winding and improves productivity.



Cold rolled Grain oriented steel (CRGO steel):

If cold rolling is performed on silicon steel then crystal structure gets distorted and grains are oriented in the direction of rolling so permeability is higher in the direction of rolling.

CRGO should be used when magnetic field only lies in one direction so that magnetic field can be oriented in the rolling direction.


Insulating Material:

Insulators are used in machine to separate conducting and non conducting part such as winding from the core or terminals from the tank.

Properties :

Discussed earlier

Conducting: Cu->winding ;Al-> Tank; Carbon-> Brushes

Magnetic: ferro/soft magnetic materials-> m/c core; ferri-> high freq. transformer; CRGO-> Transformer; si steel-> other m/c

Insulator:

Mica, Mica foium

Fibrous glass

Asbestos

Cotton wire, wood, silicon insulated conductor, Transformer oil

Electrical Materials used in Transmission line

Required Properties in Materials Used for Conductor in Transmission:

1)High conductivity

2)High tensile strength

3)Light weight

4)High resistance to corrosion in whether conditions

5)High thermal stability

6)Low coefficient of thermal expansion

7)Low cost

Materials use for transmission lines

are listed below-

Copper

Aluminum

Cadmium – Copper alloys

Phosphor bronze

Galvanized steel

Steel core copper

Steel core aluminum


Copper (Cu):

The extensively used, high conductivity material as conductor for electrical machines or equipment, is copper. Malleability, weldability and solder ability are most important properties of copper. Copper in pure form is having good conductivity. But the conductivity of standard grade copper is reduced due presence of impurities.

Properties of Copper:

Resistivity: 1.68 µΩ -cm.

Temperature coefficient of resistance at 20oC: 0.00386 /oC.

Melting point: 1085oC.

Specific gravity: 8.96gm /cm3

Copper is the most important and much suitable material for conductor of Transmission line as it having high conductivity and high tensile strength. More ever it is having good ductility. The only limitation is its cost.


Aluminum (Al):

Aluminum is having sufficient conductivity. More ever it is light in weight. Which results in low conductor weight and less sag. The only limitation is its low tensile strength. To overcome this limitation steel core is used for increasing the tensile strength of aluminum conductor such as in ACSR (Aluminum conductor steel reinforced) conductor. ACSR conductor is very much popular for high voltage overhead transmission lines.

Properties of Aluminium:

Resistivity: 2.65 µΩ -cm.

Temperature coefficient of resistance at 20oC: 0.00429 /oC.

Melting point: 660oC.

Specific gravity: 2.70 gm /cm3

The most extensively used material in transmission line is Aluminum.


Cadmium Copper Alloy:

The cadmium copper alloys contain cadmium from 0.6 to 1.2%. This small addition of cadmium increase the tensile strength and corrosion resistance of copper. The conductivity of cadmium copper alloys is 90 to 96 % of pure copper.

Use of Cadmium – Copper alloy:

For making conductors for High tensile

strength transmission line.

For making trolley wire.

Heating pads.

Electrical blanket elements.


Phosphor Bronze:

Phosphor bronze is an alloy of copper with 3.5 to 10% tin and upto 1% phosphorus. Sometimes, it is also is called as “Phos-Bronze”. The phosphorus is added as deoxidizing agent during melting. Phosphor bronze is having good strength, toughness, low coefficient of friction and fine grains. The addition of phosphorous increase the fluidity of molten which results in improved cast ability of alloy, and cleanup the grain boundaries which improves the mechanical properties of alloy.

Use of Phosphor Bronze

For making conductor for transmission line passing through marine atmosphere.

For making spring and bolts where high resistance to fatigue is required.

Ships propeller where high resistance to corrosion in required in marine environment.

For making electrical contacts.

In cryogenics, where fair electrical conductivity and low

thermal conductivity allows the making of electrical

connections to device at ultra-low temperature

without adding excessive heat.


Galvanized Steel:

Pure Iron and steel get rusted or corroded in open whether conditions. To avoid the corrosion, of sheet and wire etc. made of these metals are coated with Zinc. For Zinc coating Hot-dip galvanization is used. In this process the iron or steel in dipped in molten Zinc at a temperature around 449oC. When exposed to atmosphere, the zinc reacts with oxygen (O2) and forms the substance zinc oxide (Zno), which further reacts with carbon dioxide and form zinc carbonate (ZnCo3). This zinc carbonate is usually dull grey and fairly strong material, which protects the iron or steel underneath from corrosion in open whether conditions.

Use of Galvanized Steel

Galvanized steel wire is used for making conductors used in transmission line where resistance to corrosion to required.

Galvanized steel sheets and pipes are used for making poles of transmission.


Steel Core Copper:

Sometimes it is also called as copper clad steel conductor. For high tensile strength application to increase the strength of wire, the steel is used as a core of conductor and copper for increasing the conductivity of conductor. Here, copper not only provide the conductivity but also work as protective layer to stop the corrosion of steel by atmospheric weather conditions.

Use of Steel Core Copper

Steel core copper wire is used for earthling of electrical installations.

As inner conductor of coaxial cable.

Drop wire of telephone cables.


Steel Core Aluminum:

Aluminum is light in weight and is having good conductivity. But it is having very low tensile strength. To make it suitable to be used as conductor for transmission line, we have to increase its tensile strength. To increase the tensile strength, steel is used as a core of conductor. A good example of steel core aluminum is an ACSR (Aluminum Conductor Steel Reinforced) conductor. ACSR conductor widely used in transmission line. As it is have high tensile strength, good conductivity and economical.

Use of Steel Core Aluminum

Steel core Aluminum wire (ACSR) is used as conductor for transmission line.

As inner conductor of coaxial cable.


Subject: EMMI

Prepared By:


Prepared By:






Magnetic Materials continue…

Magnetic Material:

The material that allow flow of

magnetic field through them are called

as magnetic materials

1) Diamagnetic material :

No dipoles in the absence of magnetic field

In diamagnetic material there is no magnetism in absence of external magnetic field

When magnetic field is applied dipole is induced and they orient in the direction opposite to external magnetic field.

Magnetization (M): Ability of dipole to align when magnetic field is applied

Xm is susceptibility

Susceptibility is a measure of the material to get magnetize when external magnetic field is applied.

1) Relative permeability 𝜇 = 𝜇𝑜𝜇𝑟

For diamagnetic material Xm<0 so µ< µ0

Permeability of a material is related to ability of the material to allow magnetic field line to pass through it

𝑀 = 𝑋𝑚𝐻

𝜇𝑟 = 1 + 𝑋𝑚


Magnetic Material:

Magnetic Flux density (B) :

Magnetic flux density represents the closeness of magnetic field lines to each others

In diamagnetic material μ< μo so Bair> Bdia

In a diamagnetic materials magnetic field line diverge away from each others

Copper, Silicon, Germanium, gold, diamond are diamagnetic.

𝐵 = μ𝐻


Magnetic Material:

1) Paramagnetic Magnetic material:

In paramagnetic materials magnetic dipole is already exist, due to spin of unpaired electrons in an atom

In paramagnetic material magnetic dipoles are randomly oriented but when magnetic field is applied such dipole is align parallel to external magnetic field.

So, Xm is positive;

μ>μo so Bair< Bpara

due to positive susceptibility the flux density in side paramagnetic material increases

Potassium, Oxygen, tungsten and rare earth metal.

Μr is close to 1 and not much differ in dia and paramagnetic material

𝑀 = 𝑋𝑚𝐻

𝜇𝑟 = 1 + 𝑋𝑚


Magnetic Material:

1) Ferromagnetic material:

This elements lie in D block or S Block in periodic table and they have large number of unpaired electrons which contribute to spin dipole moment. So they have large dipole moment and are strongly magnetized.

(Fe)Iron, (Co)Cobalt, (Ni) Nickel , Gd, dy

So, M is high. Xm>>0

μr >1so B> μoH So, Bair<< Bpara

Since Ferromagnetic material is strongly magnetized the magnetic field line is converged when they pass through ferromagnetic materials.

When magnetic field is removed the dipole remain align inside the material giving rise to residual flux density.

𝑀 = 𝑋𝑚𝐻

𝜇𝑟 = 1 + 𝑋𝑚


Magnetic Material:

Ferromagnetic material:

Coercive Force (Hc): It is the value of magnetic field that needs to be applied to make flux density zero.

Domain Theory: An iron piece normally does not exhibits magnetism unless it is magnetize which means net dipole moment in absence of magnetic field should be zero , But ferromagnetic material must have residual magnetism and to explain this discrepancy we study domain theory.

After removal of magnetic field the dipole inside each domain remain align but the domains are not align with each other.

Magnetostriction: It is the phenomenon in which dimension of a ferromagnetic material changes when magnetic field is applied.

If alternating field is applied than material expands and compresses in alternate half cycle by which mechanical vibration are produce which are observed as a humming sound in a transformer.

To damp out mechanical vibration, transformer should be mounted on mud/soil/rubber mat.


Magnetic Material:

Ferrimagnetic material:

Xm>0

In ferrimagnetic material when magnetic field is applied the successive dipole get align in opposite direction but their dipole moments do not completely cancel each others so there is net magnetization.

(μr)ferri< (μr)ferro

To derive ferrimagnetic material Fe is replaced by Mn in iron compound.

Ferrimagnetic materials have high electrical resistivity.

By operating at high flux density the size of machine can be reduced for constant flux.

B=ϕ/A.

The core of electrical machine is designed using ferromagnetic materials.

The core of high frequency low power transformer can be designed using ferrimagnetic materials

In Ferrimaterials-> μris less->so B is less->so ϕ is less-> So Volt is less-> So Power is less

𝑀 = 𝑋𝑚𝐻



Soft magnetic material:

This materials can be magnetized as well as demagnetized easily.

Such materials are Used to construct electromagnet where the current can be alternating by which magnetic fields needs to be reversed.

This materials are used in electrical machine

I



Hard magnetic material:

The materials which are hard to magnetize and hard to demagnetized are called hard magnetic materials.

Such materials are Used to make permanent magnet which can be used in low power electrical machines. PMDC motor.



Electrical sheet steel (non oriented steel)

The material used in early days of transformer was iron with small amount of carbon but major disadvantage is aging which increases the area of hysteresis loop and there by increasing hysteresis loss and reducing efficiency.

At present sheet steel is used in electrical where a small amount of silicon is added which is 0.3 to 4.5% by weight.

Si added 1> Hysteresis loop narrow 2> resistivity increases so eddy current loss decreases.

Electrical steel may be manufactured by hot rolling or cold rolling.

In large machine high permeability and high efficiency is desirable so 4.5% silicon is used

In small machine 0.3% silicon is used because efficiency is not critical

By adding silicon the material becomes brittle so we cannot add a large amount of silicon else we wont be able to shape the materials.

Steel with high silicon contain is called transformer grade steel.


Magnetic Material:

1) CRGO sheet steel :

The addition of silicon(Si) in iron(Fe) in right proportions with the help of certain manufacturing process significantly improves the magnetic and electrical properties of iron.

it was discovered that the addition of silicon to iron significantly improves the resistivity of iron and so silicon steel or what we know today as electrical steel was developed. It not only brought down the eddy current losses in steel, but significant improvement in magnetic permeability and reduction in magnetostriction was observed.

CRGO field should be used when magnetic field is only lies in one direction so that magnetic field can be oriented in the rolling direction.

In cylindrical or rotating machine CRGO steel does not offer much advantage because magnetic field direction changes at each point.

In transformer core CRGO steel can be used . Because:

1) High magnetic permeability leads to low excitation currents and lower inductions.

2)Low hysteresis and eddy current losses.

3)Excellent lamination factor leads to better and compact

designs and hence low material required.

4) High knee saturation characteristics.

5) Very low level of magnetostriction leads to noise reduction.

6) Enhances ease of winding and improves productivity.



Cold rolled Grain oriented steel (CRGO steel):

If cold rolling is performed on silicon steel then crystal structure gets distorted and grains are oriented in the direction of rolling so permeability is higher in the direction of rolling.

CRGO should be used when magnetic field only lies in one direction so that magnetic field can be oriented in the rolling direction.


Insulating Material:

Insulators are used in machine to separate conducting and non conducting part such as winding from the core or terminals from the tank.

Properties :

Discussed earlier

Conducting: Cu->winding ;Al-> Tank; Carbon-> Brushes

Magnetic: ferro/soft magnetic materials-> m/c core; ferri-> high freq. transformer; CRGO-> Transformer; si steel-> other m/c

Insulator:

Mica, Mica foium

Fibrous glass

Asbestos

Cotton wire, wood, silicon insulated conductor, Transformer oil

• Losses in a magnetic materials

• Losses in a magnetic Core: It includes both hysteresis loss and eddy current loss. Because the core flux in a transformer remains constant for all loads. The core loss is practically the same at all loads.

• Hysteresis losses - power losses due to repeated change in magnetic polarity. It takes more mmf (NI) to demagnetize core in one direction than the other.

• Losses in a magnetic materials

• Eddy currents - ac currents induced in iron core due to changing magnetic field

• Control hysteresis losses - use alloy steels designed for magnetic circuits

• Control eddy current losses - laminate core, insulate laminates

1.6

max WatthW B fV

2 2 2

max WatteW PB f t


Subject: EMMI

Prepared By:


Prepared By:






Special purpose materials

Galvanization Materials :

Galvanizing is one of the most widely used to methods for protecting metal from corrosion. It involves applying a thin coating of zinc to a thicker base metal, helping to shield it from the surrounding environment. A number of methods can be employed for galvanizing of zinc to the metal substrate. Some of important techniques are described below.

1) Hot Dip Galvanizing :

The first step of this process is cleaning of the work piece that involves its degreasing by acid rinsing followed by water cleaning. The second step is its annealing and cooling in an oxide free atmosphere. During cooling when temperature of work piece reaches near to the temperature of molten zinc bath temperature. The work piece is dipped in to the bath. Very thin and uniform coating layer can be maintained by passing the sheets through rollers just after the coating.

This method is not recommended for galvanizing of very delicate and complex shaped parts having complex interior designs.

2)Flow Galvanizing :

It is also a type of crude way of galvanizing. In this process, hot zinc bath is made to flow over the surface of the sheet metal to be galvanized. Molten zinc is spread over the whole areas (surface) of the sheet metal. Excess zinc flowing down the surface is collected back for its recycling. This process is suitable for galvanization of flat sheet metals only. The thickness of coating by this process can be maintained to a uniform value. This process was later modified on the base of metal spraying process.

This modified process uses a metal spraying gun. The gun is equipped with a device to produce oxygen flame, through which a zinc wire is fed and melted. Air pressure is used to spray this molten zinc on to the surface of sheet metal. The limitation of dipping very large work piece in hot dip galvanizing is overcome. It also maintains a thin and uniform thickness layer of coating.

2)Sherardizing :

This process is used for galvanizing of those small parts having intricate shapes. In this process there is a box or container having filled with fine zinc powder. The parts are placed in this box, surrounded with the powder. The box is then heated in an oxygen. Zinc powder vaporizes. Zinc vapor comes in contact with the surface of work piece and zinc is deposited on the work piece. The work piece is then taken out of over and it is allowed to cool down to room temperature. In this way galvanizing of work piece can be completed.

3) Electroplating Galvanizing :

In case of electroplating galvanizing, zinc is deposited on to the work piece by making it cathode. It is just like a electroplating process. This process is time consuming so it is not recommended for mass production. Thickness of coating layer is very thin so it is not capable to provide corrosion resistant property to the work piece.

4) Cold Dip Galvanizing :

This process involves cleaning, buffing, degreasing of the work surface before galvanizing. Cold bath is used in this process. No heating of flow of current through electrolyte solution is required. The cold bath is prepared by dissolving shafts, like zinc chloride, tin chloride, ammonium chloride and potassium bitartrate, etc. in water and it is filled in a tank. The tank used in this process is a metallic tank, carrying a thick lining of rubber or PVC sheet on its internal surface. During the preparation proportion of tin chloride should always be less than half of the quantity of zinc chloride.

The parts to be galvanized are suspended immersed in the bath. How long these should be kept immersed depends on the thickness of the coating required. Dipping time varies from 3 to 12 hours. For thicker coating dipping time should be large

Refractory Materials :

A refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, inorganic, nonmetallic, porous, and heterogeneous.

High Alumina Bricks :

High alumina bricks refer to refractory materials with an AL3 content of more than 48%, mainly composed of corundum, mullite, and glass.

It is mainly used in the metallurgy industry to build the plug and nozzle of a blast furnace, hot air furnace, electric furnace roof, steel drum, and pouring system, etc.

Silicon Bricks :

The Si02 content of silicon brick is more than 93%, which is mainly composed of phosphor quartz, cristobalite, residual quartz, and glass.

Silicon bricks are mainly used to build the partition walls of the coking oven carbonization and combustion chambers, open-hearth heat storage chambers, high-temperature bearing parts of hot blast stoves, and vaults of other high-temperature kilns.

Magnesium Bricks :

Magnesium bricks are alkaline refractory materials made from sintered magnesia or fused magnesia as raw materials, which are press-molded and sintered.

Magnesium bricks are mainly used in open-hearth furnaces, electric furnaces, and mixed iron furnaces.

Corundum Bricks :

Corundum brick refers to refractory with alumina content ≥90% and corundum as the main phase.

Corundum bricks are mainly used in blast furnaces, hot blast stoves, refining outside the furnace, and sliding nozzles.

Ramming Material :

The ramming material refers to a bulk material formed by a strong ramming method, which is composed of a certain size of refractory material, a binder, and an additive.

The ramming material is mainly used for the overall lining of various industrial furnaces, such as open-hearth furnace bottom, electric furnace bottom, induction furnace lining, ladle lining, tapping trough, etc.

Plastic Refractory :

Plastic refractories are amorphous refractory materials that have good plasticity over a long period of time. It is composed of a certain grade of refractory, binder, plasticizer, water and admixture.

It can be used in various heating furnaces, soaking furnaces, annealing furnaces, and sintering furnaces.

Casting Material :

The casting material is a kind of refractory with good fluidity, suitable for pouring molding. It is a mixture of aggregate, powder, cement, admixture and so on.

The casting material is mostly used in various industrial furnaces. It is the most widely used monolithic refractory material.

Radioactive Material :

Radioactivity is a part of nature. Every thing is made of atoms. Radioactive atoms are unstable; that is, they have too much energy. When radioactive atoms spontaneously release their extra energy, they are said to decay. All radioactive atoms decay eventually, though they do not all decay at the same rate. After releasing all their excess energy, the atoms become stable and are no longer radioactive. The time required for decay depends upon the type of atom.

Most of the 92 naturally occurring elements on earth are unstable and can change into other forms. Radiation begins in the center of the nucleus of these elements, where energetic particles or waves of energy are released as the atom decay to stable forms.

More than 80% of the radiation we are exposed to comes from “background” radiation natural sources like sunlight, soil and rocks. Most remaining exposure come from manmade sources, such as x-rays and common household appliances like smoke detectors and color televisions.

Radioactive Material Uses:

Radioactive materials are used in producing many of the products we use every day: plastic wrap, radial tires, coffee filters, and smoke detectors.

Many medical facilities contain radioactive hazards (medical isotopes are use for diagnosis and treatment of many diseases).

Radioactive materials are used for diagnostic radiology, radiation medicine, and radiopharmaceuticals. Radiation hazards also exist wherever radioactive materials are stored or radioactive waste products are discarded.

Fires involving radioactive materials can result in widespread contamination. Radioactive particles can be carried easily by smoke plumes, ventilation systems, and contaminated water runoff. While radiation exposure outside of medical and research facilities is not common, you should be alert to its presence in labs, hospitals, and other treatment facilities

Structural Material:

Iron:

Wrought Iron: Wrought iron is the simplest form of iron, and is almost pure iron (typically less than 0.15% carbon). It usually contains some slag. Its uses are almost entirely obsolete, and it is no longer commercially produced.

Wrought iron is very poor in fires. It is ductile, malleable and tough. It does not corrode as easily as steel.

Cast Iron: Cast iron is a brittle form of iron which is weaker in tension than in compression. It has a relatively low melting point, good fluidity, castability, excellent machinability and wear resistance. Though almost entirely replaced by steel in building structures, cast irons have become an engineering material with a wide range of applications, including pipes, machine and car parts.

Cast iron retains high strength in fires, despite its low melting point. It is usually around 95% iron, with between 2.1% and 4% carbon and between 1% and 3% silicon. It does not corrode as easily as steel.


Iron:

Steel: Steel is an iron alloy with controlled level of carbon (between 0.0 and 1.7% carbon).

Steel is used extremely widely in all types of structures, due to its relatively low cost, high strength-to-weight ratio and speed of construction.

Steel is a ductile material, which will behave elastically until it reaches yield (point 2 on the stress–strain curve), when it becomes plastic and will fail in a ductile manner (large strains, or extensions, before fracture at point 3 on the curve). Steel is equally strong in tension and compression.

Steel is weak in fires, and must be protected in most buildings. Despite its high strength to weight ratio, steel buildings have as much thermal mass as similar concrete buildings.

The elastic modulus of steel is approximately 205 GPa.


Iron:

Stainless steel: Stainless steel is an iron-carbon alloy with a minimum of 10.5% chromium content. There are different types of stainless steel, containing different proportions of iron, carbon, molybdenum, nickel. It has similar structural properties to steel, although its strength varies significantly.

It is rarely used for primary structure, and more for architectural finishes and building cladding.

It is highly resistant to corrosion and staining.


Concrete:

Concrete is used extremely widely in building and civil engineering structures, due to its low cost, flexibility, durability, and high strength. It also has high resistance to fire.

Concrete is a non-linear, non-elastic and brittle material. It is strong in compression and very weak in tension. It behaves non-linearly at all times. Because it has essentially zero strength in tension, it is almost always used as reinforced concrete, a composite material. It is a mixture of sand, aggregate, cement and water. It is placed in a mould, or form, as a liquid, and then it sets (goes off), due to a chemical reaction between the water and cement. The hardening of the concrete is called hydration. The reaction is exothermic (gives off heat).

The elastic modulus of concrete can vary widely and depends on the concrete mix, age, and quality, as well as on the type and duration of loading applied to it. It is usually taken as approximately 25 GPa for long-term loads once it has attained its full strength (usually considered to be at 28 days after casting). It is taken as approximately 38 GPa for very short-term loading, such as footfalls.

Concrete has very favourable properties in fire – it is not adversely affected by fire until it reaches very high temperatures. It also has very high mass, so it is good for providing sound insulation and heat retention (leading to lower energy requirements for the heating of concrete buildings). This is offset by the fact that producing and transporting concrete is very energy intensive. To study the material behavior plenty of numerical models were developed, e.g. the microplane model for constitutive laws of materials.


Reinforced concrete:

Reinforced concrete is concrete in which steel reinforcement bars ("rebars"), plates or fibers have been incorporated to strengthen a material that would otherwise be brittle. In industrialised countries, nearly all concrete used in construction is reinforced concrete. Due to its weakness in tension capacity, concrete will fail suddenly and in brittle manner under flexural (bending) or tensile force unless adequately reinforced with steel.

Prestressed concrete:

Prestressed concrete is a method for overcoming the concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load.


Concrete:

Concrete is used extremely widely in building and civil engineering structures, due to its low cost, flexibility, durability, and high strength. It also has high resistance to fire.

Concrete is a non-linear, non-elastic and brittle material. It is strong in compression and very weak in tension. It behaves non-linearly at all times. Because it has essentially zero strength in tension, it is almost always used as reinforced concrete, a composite material. It is a mixture of sand, aggregate, cement and water. It is placed in a mould, or form, as a liquid, and then it sets (goes off), due to a chemical reaction between the water and cement. The hardening of the concrete is called hydration. The reaction is exothermic (gives off heat).

The elastic modulus of concrete can vary widely and depends on the concrete mix, age, and quality, as well as on the type and duration of loading applied to it. It is usually taken as approximately 25 GPa for long-term loads once it has attained its full strength (usually considered to be at 28 days after casting). It is taken as approximately 38 GPa for very short-term loading, such as footfalls.

Concrete has very favourable properties in fire – it is not adversely affected by fire until it reaches very high temperatures. It also has very high mass, so it is good for providing sound insulation and heat retention (leading to lower energy requirements for the heating of concrete buildings). This is offset by the fact that producing and transporting concrete is very energy intensive. To study the material behavior plenty of numerical models were developed, e.g. the microplane model for constitutive laws of materials.


Aluminium:

Aluminium is a soft, lightweight, malleable metal. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is ductile, and easily machined, cast, and extruded.

Corrosion resistance is excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.

Aluminium is used in some building structures (mainly in facades) and very widely in aircraft engineering because of its good strength to weight ratio. It is a relatively expensive material.

In aircraft it is gradually being replaced by carbon composite materials.


Composites:

Composite materials are used increasingly in vehicles and aircraft structures, and to some extent in other structures. They are increasingly used in bridges, especially for conservation of old structures such as Coalport cast iron bridge built in 1818. Composites are often anisotropic (they have different material properties in different directions) as they can be laminar materials. They most often behave non-linearly and will fail in a brittle manner when overloaded.

They provide extremely good strength to weight ratios, but are also very expensive. The manufacturing processes, which are often extrusion, do not currently provide the economical flexibility that concrete or steel provide. The most commonly used in structural applications are glass-reinforced plastics.


Masonry:

Masonry has been used in structures for thousands of years, and can take the form of stone, brick or blockwork. Masonry is very strong in compression but cannot carry tension (because the mortar between bricks or blocks is unable to carry tension). Because it cannot carry structural tension, it also cannot carry bending, so masonry walls become unstable at relatively small heights. High masonry structures require stabilisation against lateral loads from buttresses (as with the flying buttresses seen in many European medieval churches) or from windposts.

Since the widespread use of concrete, stone is rarely used as a primary structural material, often only appearing as a cladding, because of its cost and the high skills needed to produce it. Brick and concrete blockwork have taken its place.

Masonry, like concrete, has good sound insulation properties and high thermal mass, but is generally less energy intensive to produce. It is just as energy intensive as concrete to transport.


Timber:

Timber is the oldest of structural materials, and though mainly supplanted by steel, masonry and concrete, it is still used in a significant number of buildings. The properties of timber are non-linear and very variable, depending on the quality, treatment of wood, and type of wood supplied. The design of wooden structures is based strongly on empirical evidence.

Wood is strong in tension and compression but can be weak in bending due to its fibrous structure. Wood is relatively good in a fire as it chars, which provides the wood in the centre of the element with some protection and allows the structure to retain some strength for a reasonable length of time.

Documents

R.N.G.P.I.T, Bardoli Electrical Engineering Department