Basic Principle of Electromagnetism

BASIC PRINCIPLES OF ELECTROMAGNETISM

& TRANSFORMER

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CONTENTS Page OBJECTIVE 3 A . BASIC PRINCPLES OF ELECTROMAGNETISM 4 1. Magnet

2. Principles of magnet 3. Magnet law 4. Type of magnet 5. Electromagnet 6. Electromagnet strength 7. Magnetic quantities characteristic

i. Magnetomotive force ii. Magnetic field strength iii. Magnetic flux and flux density

4 4 4 5 5 6 6 7 7 7

B. TRANSFORMER 8 1. Principle of operation

2. Type of transformer and construction 3. Transformer ratio 4. Auto-transformer

9 10 12 14

C. CONCLUSION 16

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OBJECTIVE

1. To learn & understand the basic principles of electromagnetism and its application

2. To learn & understand function of transformer, its operation principles, type, construction and its application.

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A. BASIC PRINCIPLE OF ELECTROMAGNETISM

1. Magnet

i. Ancient people learned about magnetism from lodestones, naturally magnetized pieces of iron

ore. They are naturally created magnets, which attract pieces of iron. The word magnet in Greek meant "stone from Magnesia", a part of ancient Greece where lodestones were found.

ii. It can be defined as a substance that can attract a metal or iron. This ability is known as magnetism.

iii. Magnet has 2 poles which is north and south

2. Principles of magnet

Magnet has a magnetic field around the magnet itself. Magnetic field is the force around the magnet which can attract any magnetic material around it. The line form around the magnet bar is magnetic field which is known as flux magnet.

3. Magnet law

The flux line of magnetic have a direction and pole. The direction of movement outside of the magnetic field line is from north to south. The magnetic poles (north & south) have the strongest magnetic field. It basic law, is different poles will attract each other while the same magnetic poles will reject each other. In can be called as magnetic attraction and repulsion law. The flux will form a complete loop and will never intersect with each other and will be in smallest form possible.

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4. Type of magnet

There is two type of magnet: i. Pure magnet ii. Manufacture magnet Pure magnet: Pure magnet is a magic stone. The stone originally have the natural magnetic and normally in a form of iron ore. Manufacture magnet: Two type of manufacture magnet which is permanent magnet and temporary magnet.

a. The permanent magnet is manufacture so it can kept its magnetism. It can be obtained naturally or magnetic induction and placing a magnet into a coil then supplied with high electrical current. Normally it is used in speakers and metering devices.

b. Temporary magnet is created by mean of using electric current. It is known as electromagnet. It will have it magnetic properties when there is electric current and will lost the magnetic properties when the current is cut off. Example of such application is relays.

5. Electromagnet

Electromagnet produced when there is electric current flowing through a coil of wire (in circular path) and through a conductor. The direction of magnetic field produced by the current in the solenoid can be determined using two methods:

i. Right hand grip Right hand grip is a principle applied to electric current passing to a solenoid coil resulting in a magnetic field. By gripping the right hand around the solenoid, thumb is pointing in the direction of the magnetic north pole and remaining fingers is pointing of direction of current flow.

ii. Maxwell’s screw law

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A right handed screw is turn clock wise so that it moves forward in the same direction as the current. The direction of screw rotation (clockwise) indicate the direction of magnetic field from south to north.

6. Electromagnet strength

There are 4 factors that affect electromagnetic strength: i. Number of turns

The strength of the electromagnet is directly proportional to the number of turn in the coil. By varying the number of turns in its coil can produce very strong magnetic fields and its strength.

ii. Current strength

The strength of the electromagnet is directly proportional to the current flowing in the coil. Greater the current flow through the coil, stronger will be the magnetic fields produced.

iii. Length of coil The strength of the electromagnet is directly proportional to the length of the coil. By coil up the wire can increasing the length and increase the force of magnetic field.

iv. Types of conductor Depend on the nature of the core material. The use of soft of core can produces the strongest magnetism.

7. Magnetic quantities characteristic

There are many magnetic quantities in the System International (SI) unit. There are magnetomotive force, magnetic field strength, magnetic flux, flux density.

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i. Magnetomotive Force, Fm

Magnetomotive force is a cause of the existence of magnetic flux in a magnetic circuit. The total flux produced is depends on the number of turn (N) made in the circuit. It is also proportional to the current (I) passing through the coil. Then, the magnetomotive force is the product of current and the number of turns. Fm = N / ampere turn

ii. Magnetic Field Strength, H

Magnetic field strength or magnetizing force is defined as magnetomotive force, Fm per meter length of measurement being ampere-turn per meter.

iii. Magnetic Flux and Flux Density

Magnetic flux is the amount of magnetic filed produced by a magnetic source. The symbol for magnetic flux is phi (ф). The unit for magnetic flux is the weber, Wb. Magnetic flux density is the amount of flux passing through a defined area that is perpendicular to the direction of flux.

The symbol for magnetic flux density is B. The unit of magnetic flux density is the tesla, T, and the unit for area A is m2 where 1 T = 1 Wb/m.

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B. TRANSFORMER

A transformer is an electrical device that transfer energy from one electrical circuit to another by magnetic coupling but without any moving part. It is often used to convert between high and low voltage and between low and high impedances

A transformer is a device which uses the phenomenon of mutual induction to change the values of alternating voltages and currents. In fact, one of the main advantages of alternating current transmission and distribution is the ease with which an alternating voltage can be increased or decreased by transformers.

Transformers range in size from the miniature units used in electronic applications to the large power transformers used in power stations. The principle of operation is the same for each.

1. Principle of Operation

In simple form, a transformer consists of two windings connected by a magnetic core. One winding is connected to a power supply and the other to a load. A circuit diagram symbol for a transformer is show below.

N1 N2

AC supply

Load

Flux

Secondary winding Primary winding

Core

A transformer construction

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When the secondary is an open-circuit and an alternating voltage V1 is applied to a primary winding, a small current flows which set up a magnetic flux in the core. This alternating flux links with both primary and secondary coils and induces in them e.m.f.’s of E1 and E2.

The induced e.m.f., E in a coil of N turns is given by

E = -Ndt

dΦ volts

where dt

dΦ is the rate of change of flux.

In the ideal transformer, the rate of change of flux is the same for both primary and secondary thus

1

1

ΝΕ =

2

2

ΝΕ .

The induced e.m.f. per turn is constant. Assuming no losses, E1 = V1 and E2 = V2

1

1V

Ν =

2

2V

Ν or

2

1

V

V = 2

1N

Ν

When the load is connected across the secondary winding, a current I2 flows. In an ideal transformer losses are neglected and a transformer is considered to be 100% efficient.

Hence input powers = output power or V1 I1 = V2 I2 , the primary and secondary volt-amperes are equal.

Thus 2

1

V

V = 1

2

I

I

Combining equations gives

2

1

V

V = 2

1

N

N = 1

2

I

I

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2. Types of transformer and construction

i. There are broadly two types of single-phase double-wound transformer construction – the core type and the shell type as show below:

ii. Transformer is used to convert AC power at one voltage/current level into AC power

of the same frequency at another voltage/current level.

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iii. It can be used for voltage sampling, current sampling and impedance transformation.

iv. As shown on above picture, power transformer are constructed on one of two type of cores. • Rectangular laminated piece of steel with transformer windings wrapped

around two sides of the rectangle as shown below. This is core-form transformer construction.

• The other consists of a three-legged laminated core with the windings wrapped around the center leg. This is known as shell-form transformer construction. See below.

v. The core is constructed of thin lamination electrically isolated from each other in order to minimize eddy current.

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vi. The primary and secondary windings are wrapped on the top of the other. It serves two purpose which is to simplifies the problem of insulating the high voltage winding from the core and the other purposes is to lessen leakage flux.

vii. Transformers with high primary voltage and produce low secondary voltage is known as potential transformer.

viii. Transformer provides a secondary much smaller than but directly proportional to its primary current is known as current transformer

3. Transformer ratio

Transformer ratio is the comparison between primary and secondary elements.

The primary and secondary elements

Flux which is produced by primary winding completely inducing to secondary winding. Therefore, e.m.f induced in all the winding are similar for primary and secondary winding.

Np NsVp Vs

Flux produced by primary winding

Secondary winding

Primary winding

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== / windinge.m.f x Np

/ windinge.m.f x Ns

ndingprimary wiin induced e.m.f of Total

windingsecondary in induced e.m.f of Total sN

Np

With expected transformer ratio as K :

K = p

s

p

s

p

s

V

V

E

E

N

N==

Where S = secondary e.m.f induced P = primary e.m.f induced Vs = secondary terminal voltage

Vp = primary terminal voltage Notes If K < 1 i.e. Ns < Np : this transformer is called step-down transformer If K > 1 i.e. Ns > Np : this transformer is called step-up transformer If K = 1 i.e. Ns = Np : this transformer is called coupling transformer

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4. Auto-transformer

An auto-transformer is a transformer having a part of its winding common to the primary and secondary circuits

An auto-transformer.

I1 and I2 = primary and secondary currents respectively N1 = no. of turns between A and B N2 = no. of turns between B and C n = ratio of the smaller voltage to the larger voltage Neglecting the losses, the leakage reactance and the magnetizing current, we have for Fig 8.9.

n = 1

2

V

V=

2

1

I

I=

1

2

N

N

The nearer the ratio of transformation is to unity, the greater is the economy of conductor material. Also, for the same current density in the windings and the same peak values of the

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flux and of the flux density, the I2R loss in the auto transformer is lower and the efficiency higher than in the two winding transformer.

Auto transformer are mainly used for interconnecting systems that are operating at roughly the same voltage and starting cage-type induction motors. Should an auto transformer be used to supply a low voltage system from a high voltage system, it is essential to earth the common connection, for example, B in Fig 8.9 otherwise there is a risk of serious shock. In general, however, an auto transformer should not be used for interconnecting high voltage and low voltage systems.

Advantages and disadvantages of auto transformers

The advantages of auto transformers over double wound transformers included:

a. It is cheaper than a conventional two winding transformer of a similar rating. b. It delivers more power than two winding transformer of similar physical dimension c. For a similar power rating, an auto transformer is more efficient than two winding

transformer. d. An auto transformer requires lower excitation current than a two winding transformer

to establish the same flux in the core.

.

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Conclusion

1. Electromagnetism produced when there is a electrical current flow.

2. Electromagnetism has been applied to most of applications around us and have important roles. Such devices use this application is speaker, mobile phone charge, electric fan, blender and so on.

3. Transformer operating principles itself use magnetic principles.