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

Subject: EMMI

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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.

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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
• Conducting Material: Allow the current to flow
• Magnetic material: To Produce magnetic field
• Insulating Material: Separate or isolate conducting and non-conducting materials

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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

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

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• Electrical Materials used in machine

Conducting Material:

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.

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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

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• Electrical Materials used in machine

Conducting Material:

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

• Electrical Materials used in machine

Conducting Material:

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 V_bd= I R= constant

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• Electrical Materials used in machine

Magnetic Material:

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Magnetic Flux density (B) :

• Magnetic flux density represents the closeness of magnetic field lines to each others
• In diamagnetic material μ< μ_o so B_air> B_dia
• In a diamagnetic materials magnetic field line diverge away from each others
• Copper, Silicon, Germanium, gold, diamond are diamagnetic.

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• Electrical Materials used in machine

Magnetic Material:

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1. Paramagnetic Magnetic material:
1. In paramagnetic materials magnetic dipole is already exist, due to spin of unpaired electrons in an atom
2. In paramagnetic material magnetic dipoles are randomly oriented but when magnetic field is applied such dipole is align parallel to external magnetic field.

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• So, Xm is positive;
• μ>μ_o so B_air< B_para
• due to positive susceptibility the flux density in side paramagnetic material increases
• Potassium, Oxygen, tungsten and rare earth metal.
• Μ_r is close to 1 and not much differ in dia and paramagnetic material

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• Electrical Materials used in machine

Magnetic Material:

1. Ferromagnetic material:
1. 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.
2. (Fe)Iron, (Co)Cobalt, (Ni) Nickel , Gd, dy

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• So, M is high. Xm>>0
• μ_r >1so B> μ_oH So, B_air<< B_para

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• 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.

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• Electrical Materials used in machine

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.

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• Electrical Materials used in machine

Magnetic Material:

Ferrimagnetic material:

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• 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.
• (μ_r)_ferri< (μ_r)_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-> μ_ris less->so B is less->so ϕ is less-> So Volt is less-> So Power is less

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• Electrical Materials used in machine

Types of Magnetic Material:

Soft magnetic material:

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• 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

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• Electrical Materials used in machine

Types of Magnetic Material:

Hard magnetic material:

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• 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.

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• Electrical Materials used in machine

Types of Magnetic Material:

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.

• Electrical Materials used in machine

Magnetic Material:

1. CRGO sheet steel :
1. 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.
2. 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.
3. 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.
4. In cylindrical or rotating machine CRGO steel does not offer much advantage because magnetic field direction changes at each point.
5. 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.

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• Electrical Materials used in machine

Types of Magnetic Material:

Cold rolled Grain oriented steel (CRGO steel):

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• 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.

• Electrical Materials used in machine

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

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• 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

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Materials use for transmission lines

are listed below-

• Copper
• Aluminum
• Cadmium – Copper alloys
• Phosphor bronze
• Galvanized steel
• Steel core copper
• Steel core aluminum

• Electrical Materials used in Transmission line

Copper (Cu):

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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.

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Properties of Copper:

Resistivity: 1.68 µΩ -cm.

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

Melting point: 1085^oC.

Specific gravity: 8.96gm /cm³

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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.

• Electrical Materials used in Transmission line

Aluminum (Al):

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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 20^oC: 0.00429 /^oC.

Melting point: 660^oC.

Specific gravity: 2.70 gm /cm³

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The most extensively used material in transmission line is Aluminum.

• Electrical Materials used in Transmission line

Cadmium Copper Alloy:

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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.

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Use of Cadmium – Copper alloy:

For making conductors for High tensile

strength transmission line.

For making trolley wire.

Heating pads.

Electrical blanket elements.

• Electrical Materials used in Transmission line

Phosphor Bronze:

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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.

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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.

• Electrical Materials used in Transmission line

Galvanized Steel:

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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.

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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.

• Electrical Materials used in Transmission line

Steel Core Copper:

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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.

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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.

• Electrical Materials used in Transmission line

Steel Core Aluminum:

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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.

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Use of Steel Core Aluminum

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

As inner conductor of coaxial cable.