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Electromagnetism

Soedibyo

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References

1. Theraja, B. L., ‘Electrical Technology’, S. Chand & Company Ltd., 1978.

2. en.wikipedia.org

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RE1334 KONVERSI TENAGA LISTRIK I 1: Energi dan permasalahannya 2: Konsep konversi energi 3: Pembangkitan energi listrik konvensional &

non konvensional 4: Dasar elektromagnetik, pengenalan bahan

magnetik dan elektromekanik 5: Mesin arus searah (DC) 6: Generator DC: cara kerja, klasifikasi dan

persamaan tegangan 7: Generator DC: rugi-rugi daya dan efisiensi,

efisiensi maksimum serta karakteristik 8: 9-10: Ujian Tengah Semester

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Last Assignment: Electricity Generation

Each group’s assignments (ppt format) are to be put into one folder and to be submitted next week (& to be copied to each other groups).

See the flash animations at http://www.pln.co.id/InfoUmum/PembangkitanListrik/tabid/77/Default.aspx

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Electric machines:

Mechanical energy

electrical energy

Magnetic field

Generator; motor; transformer

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Basic Principles

1. Ampere’s Law:An electric current produces a magnetic field.

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Review of the Basics

i1 in

a. b.

Ampere’s Law

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1. Ampere’s Law

netIdlH

H = magnetic field intensity produced by the current Inet.

dl = differential element of length along the path of integration.

If the core is composed of iron/other similar metals, essentially all the magnetic field produced by the current will remain inside the core -> path of integration is path length of the core lc.

c

c

l

NiH

NilH

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1. Ampere’s Law (2)

.The strength of the magnetic field flux produced in the core also depends on the material of the core.

HB H = magnetic field intensity (At/m) = magnetic permeability of material (H/m)B = resulting magnetic flux density produced (Wb/m2 or T)

Permeability: the degree of magnetization of a material in response to a magnetic field

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1. Ampere’s Law (2)

mH /104 70

0 = permeability of free space

Relative permeability =Permeability of any other material compared to 0

To compare the magnetizability of materials.

0 r

For example:The steels used in modern machines have r of 2000 to 6000 or more.

This means that, for a given amount of current, 2000 to 6000 times more flux is established in a piece of steel than in a corresponding area of air.

Obviously, the metals in machines core is important in increasing and concentrating magnetic flux in the device.

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1. Ampere’s Law (2)

mH /104 70

0 = permeability of free space

Relative permeability =Permeability of any other material compared to 0

To compare the magnetizability of materials.

0 r

For example:The steels used in modern machines have r of 2000 to 6000 or more.

This means that, for a given amount of current, 2000 to 6000 times more flux is established in a piece of steel than in a corresponding area of air.

Obviously, the metals in machines core is important in increasing and concentrating magnetic flux in the device.

The advantage of using a ferromagnetic material for cores in electric machines is that one gets many times more flux for a given mmf with iron than with air.

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Magnetomotive force (ampere-turn (At))

Magnetomotive force is any physical cause that produces magnetic flux.In other words it is a field of magnetism (tesla) that has area (meters square), so that (Tesla)(Area)= Flux.

It is analogous to electromotive force or voltage in electricity.

The standard definition of magnetomotive force involves current passing through an electrical conductor, which accounts for the magnetic fields of electromagnets.

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Magnetic Circuit

Ni

(a) A simple electric circuit

(b) The magnetic circuit analog to a transformer core

N is the number of turns of the coil, I is the current in the coil, Φ is the magnetic fluxis the reluctance of the magnetic circuit. The latter equation is sometimes known as Hopkinson's law.

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Magnetization curve

(a) Sketch of a dc magnetization curve for a ferromagnetic core.

(b) The magnetization curve expressed in terms of flux density & magnetizing intensity

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Magnetic Circuit (2)The advantage of using a ferromagnetic material for cores in electric machines is that one gets many times more flux for a given mmf with iron than with air.

Since real generators & motors depend on magnetic flux to produce voltage & torque, they are designed to produce as much flux as possible.-> most real machines operates near the knee of the magnetization curve.

(a) Sketch of a dc magnetization curve for a ferromagnetic core.

(b) The magnetization curve expressed in terms of flux density & magnetizing intensity

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Magnetic Circuit (2)

(c) A detailed magnetization curve for a typical piece of steel

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Energy Losses in a Ferromagnetic Core

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Hysteresis Loss (1)

x

x

x..

.

Magnetic domains oriented randomly Magnetic domains lined up in the presence of an external magnetic field

.When the external magnetic field is removed, the domains don’t completely randomize again.

Because turning them back requires energy.

.The fact that turning domains in the iron requires energy leads to a common energy loss in all machines and transformers -> hysteresis loss.

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Energy Losses in a Ferromagnetic Core

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Hysteresis Loss (2). P1:both field strength and flux density are zero.

.The field strength is increased in the positive direction and the flux begins to grow along the dotted path until we reach P2. This is called the initial magnetization curve. .When the applied field is returned to zero there will still be a remaining (remnant or remanent) flux density at P3.

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Hysteresis Loss (3)

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Hysteresis Loss (3)

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Area of Hysteriesis Loop

.Hysteresis loop measures the energy dissipated due to hysteresis which appears in the form of heat and so raises the temperature of that portion of the magnetic reversals.

.The shape of the hysteresis loop depends on the nature of magnetic substance.

.Loop 1: for hard steel.

Due to its high retentivity & coercivity*, it is well suited for making permanent magnets.

But due to large hysteresis loss it’s not suitable for rapid reversals of magnetisation.

Certain alloys of alumunium, nickel, & steel called Alnico alloys is found suitable for making permanet magnets.

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Area of Hysteriesis Loop

.Loop 2: is for wrought iron & cast steel.

It shows that these materials have high permeability & fairly good coercivity, hence making them suitable for cores of electromagnets.

.Loop 3: is for alloyed sheet steel & it shows high permeability & low hysteresis loss.

Hence such materials are suitable for making armature & transformer cores which are subjected to rapid reversals of magnetization.

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A family of B-H loops for grain-oriented electrical steel (BR denotes remanence and HC is the coercivity).

When an external magnetic field is applied to a ferromagnet, the atomic dipoles align themselves with the external field.

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Electromagnetic Induction

Faraday’s Law of induction

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Review: Fleming’s Right-hand Rule

Fleming's right hand rule (for generators) shows the direction of induced current flow when a conductor moves in a magnetic field.

The Thumb represents the direction of Motion of the conductor.

The First finger represents the direction of the Field.

The Second finger represents the direction of the induced or generated Current

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Coercivity Coercivity: the coercivity, also called the

coercive field, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC.

When the coercive field of a ferromagnet is large, the material is said to be a hard or permanent magnet.

Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation.

A ferromagnet with a low coercive field is said to be soft and may be used in microwave devices, magnetic shielding, transformers or recording heads.

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Retentivity The retentivity of a material is its capacity to

remain magnetized after the external magnetizing field has ceased to exist.

A material with high retentivity (i.e. iron) will keep some magnetic properties, it will become a permanent magnet, whereas a material with low or no retentivity will not keep the magnetic properties—it will lose its magnetization.

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Thank You

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(d) A plot of relative permeability r as a function of magnetizing intensity H for a typical piece of steel.

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Review of the Basics

Faraday’s Law of induction

i1 in

a. b.

Ampere’s Law

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.

Lenz’s Law*

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Electromagnetic Induction

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it.

Lenz’s Law