magnetism - hong kong polytechnic...

EE1D01 Electrical Science for Everyone

1

Magnetism

Magnesia, province of Greece

Unusual property of lodestone noted over 2000 years ago


2

Magnetism

Press SPACE to find out the correct answer.

What substance do the What substance do the paperclip and weight paperclip and weight

contain that make contain that make them attract to them attract to

magnets? magnets? ______________. IRONIRON


3

Magnetic Field in a Bar Magnet

• A magnet has two poles, North (N) and South (S)

• Like poles repel, opposite poles attract

• The lines of magnetic field from a bar magnet form closed lines.

• By convention, the field direction is taken to be outward from the North pole and in to the South pole of the magnet.

compass


4

The Earth’s Magnetic Field

The spinning iron core of the earth produces a magnetic field.

The magnetic north pole corresponds to the geographic south pole.

The earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the earth.


5

Magnetic Field of a Straight Wire

• The magnetic field lines around a long wire which carries an electric current form concentric circles around the wire.

• If a wire is grasped with the thumb of your right hand pointing in the current direction, the fingers encircle the wire in the direction of the magnetic field.


6

Magnetic Field of Current Loop

Electric current in a circular loop creates a magnetic field which is more concentrated in the center of the loop than outside the loop.

Magnetic flux Φ measuredin Webers (Wb)


7

Magnetic Field in a Solenoid

Stacking multiple loops concentrates the field even more into what is called a solenoid.

Flux density measured in Teslas (T) = (Wb/m2)


8

Electromagnets

• Electromagnets are usually in the form of iron core solenoids.

• The ferromagnetic property of the iron core causes the internal magnetic domains of the iron to line up with the smaller driving magnetic field produced by the current in the solenoids.

• The effect is the multiplication of the magnetic field by factors of tens to even thousands.


9

Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.

What are magnetic domains?


10

An electromagnet can be used to move large quantities of iron. When the current is on the magnet will pick up the iron. When you want to drop it you turn off the power and the electromagnet is disabled and the iron drops.

Electromagnets


11


12

Magnetic Levitation Train (maglev)

Japanese MLX01 can travel at speeds over 500 km/h (310 mile/h)


13

Maglev Train “Tracks”

Train moving on Guideway

The Yamanashi Guidewayfor the MLX01


14

Maglev Train in Shanghai


15

Magnetic Levitation

Some Benefits:

Relatively quiet operating noise

High speed capability: (“268 mph reached by maglev train linking Shanghai’s financial district with airport” in 2004)

Relatively pollution free

Some Drawbacks:

Electrical energy requirement is very great

“Superconducting materials offer the only realistic potential for this means of transportation”


16

Magnetic Force on Current-Carrying Wire


17

Coil in a Magnetic Field


18

DC Motor


19

Commutator and Brushes

Commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet. Commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment.


20

DC Motor


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22

DC Motor


23

DC Motor

The rotating part of the motor, including the electromagnets, is called the armature.

This diagram shows a small battery-powered electric motor and what it looks like inside with one end of the motor case removed.


24

As the rotor spins, the three plates come into contact with the positive and negative brushes.

Electric current flows through the brushes into the coils.

DC Motor


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DC Motor


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DC Motor


27

Electromagnetic Induction


28

Factors Affecting InductionElectricity is induced when a magnet moves inside a coil of wire. When the magnet stops moving, so does the electrical current.What factors affect the size of the electrical current induced?

The speed of the magnet – the faster it moves, the larger the current.

The strength of the magnet – the stronger the magnet, the larger the current.

The number of turns in the coil – the more turns in the coil, the larger the current.


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30

NS



31

NS



32

NS

STRONGER FIELD (B)



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NS

STRONGER FIELD (B)



34

NS

FASTER



35

NS

FASTER



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37

AC Generator A coil of wire turns in a magnetic field. The flux in the coil is constantly changing, generating an emfin the coil.


38

AC generator

ENG327 Basic Electricity & Electronics I 39


40

It is possible to predict the direction of the induced current produced by a generator if the direction of the force (or motion) or the magnetic field are known. Fleming’s right-hand rule is used to do this.

Fleming’s Right-hand Rule

First finger = magnetic Field

seCond finger = Current

thuMb = Motion


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42

Resistor Current & Power

-350

-250

-150

-50

50

150

250

350

0 0.01 0.02 0.03 0.04

Time /s

Vs

/V

-0.5

-0.3

-0.1

0.1

0.3

0.5

Ir

/A

Vs /V Pr /W Ir /A

Vs

50Hz220Vrms

r

1kIrrms = 0.22APave = 48.4W

Peak = 311V

Peak = 0.311A


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-350

-250

-150

-50

50

150

250

350

0 0.01 0.02 0.03 0.04

Time /s

Vs

/V

-1.5

-1

-0.5

0

0.5

1

1.5

IL /

A

Vs /V PL /W IL /A

ILrms = 0.7APave = 0

Inductor Current & Power

Vs50Hz220Vrms

L

1H

Peak = 311V Peak = 0.99A


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-350

-250

-150

-50

50

150

250

350

0 0.01 0.02 0.03 0.04

Time /s

Vs

/V

-1.5

-1

-0.5

0

0.5

1

1.5

Ic /A

Vs /V Pc /W Ic /A

Capacitor Current & Power

Vs

50Hz220Vrms

c

10uF

Irrms = 0.69APave = 0

Peak = 311V Peak = 0.976A


45

Three-Phase AC Generator

Larger generators tend to operate in thee-phases: there are three separate, evenly spaced coils embedded in the stator producing three separate voltage and current waveforms, separated by 120o


46

Three-Phase AC Generator


47

Steam Turbine and Electric Generator


48

Steam Turbine Blades


49

Cylindrical-Rotor Synchronous Generator

Stator

Cylindrical rotor


50Four-pole salient pole generator concept.

B-

B+

A+

C+

C-

A-

-- +

+

--+ +--

+ +

--+

+

N

SS

N

A+

C-

B+A-

B-

C+

Salient Pole Synchronous Generator


51

Hydro Generator


52

How is electrical energy produced?

Most methods of producing electricity involve converting movement (i.e. kinetic energy) into electrical energy.

Coal, oil, gas and nuclear power stations convert kinetic energy from high-pressure steam into electrical energy.

Wind turbines convert kinetic energy from wind into electrical energy.


53

Black Point and Castle Peak Power Station

Natural Gas Coal


54

Nordjyllandsværket, unit 3

The world’s most efficient coal-fired unit with 47% efficiency

Nordjyllandsværket, unit 3Net output : 385 MWSteam parameters : 300 bar/585 °C/585 °CMaximum district heating : 460 MJ/sCommissioning year : 1998


55

Power Plant / Power StationFossil Fuel Power Plant – Coal

Waste Heat

Flue

1

23

4 5

6

7


56

Natural Gas Power Station

Waste heat:

a. Cooling Towers:Released to atmosphere

b. Combined Cycle:Recycled in secondary boiler


57

Gas Turbine Components


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Nuclear Power Plant


59

Pressurized Water Reactor (PWR)

Thermal Turbine

G

Generator

CoolingTower

Condenser

Pump

Pump

Body of waterReactorVessel

SteamGenerator

Pump

Stea

m


60

Hydroelectric Power Plant


61



62

Propeller turbine for low head plant.



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64

Types of Hydropower Turbines


65

Francis turbine in Hydro power station


66

The next part of the story is that a changingmagnetic field produces an electric current in a loop surrounding the field

Transformer


67

Transformer

A current in the primary coil produces a magnetic field, like a solenoid.

The magnetic field couples around through the secondary coil.

A voltage is induced in the secondary coil.


68

Small transformer construction

a) Lamination, b) Iron core with winding

Iron core

Terminals

Secondarywinding

Insulation

Transformer


69

Typical Transformers

Primary-Secondary Terminology

90 V

Secondary

Load

Transformers are bidirectional devices.

However, either winding can be used as the primary.

120 V

Secondary

Load

This transformer was designed to step 120 V down to 90 V.

120 V

Primary

Source120 V

Source90 V

Primary

90 V


71

Transmission of Electric Power


72

Three-phase Transformer


73

Electricity Supply Network

GenerationGeneration 400kV Overhead Line

EHV Substation

Area

Primary

Substation

132kV132kV

Transformer

Room

Transformer

Room

Industrial Commercial Residential

Village Overhead lines

Underground Cable

Customer

Substation

11kV Ring Network11kV Ring Network

Transformer Room

11kV/380V


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Power System

74


75

500 kV TransmissionPower Plant

Generation

TransmissionSystem

DistributionSystem(12kV)

UndergroundDistribution Transfomer

ResidentialCustomer

Commercial/IndustrialCustomer

ResidentialCustomer

OverheadDistributionTransformer

UrbanCustomers

69 kV Sub-transmission

230 kVTransmission

Distribution Substation(69/12 kV)

High-Voltage Substation(230/69 kV)

Extra-High-Voltage Substation(500/230 kV)

Distribution Line

Underground Cable

To OtherHigh-VoltageSubstations

Electrical Power System


76

Transmission structures

three-phase “live” wires

500,000 230,000 138,000 69,000 7–13,000long-distance neighborhood

to house


77

Control Centre


78

Tariff

The rate of electrical energy at which it is sold to the consumers is called tariff .The supply companies invest money to generate, transmit and distribution of electrical energy, a tariff is fixed .The cost of generation depends upon the magnitude of energy consumed by the consumers and his load conditions. Therefore, due consideration is given to different types of consumers (e.g. domestic, commercial and industrial) while fixing a tariff .


79

Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in?

1 unit of electricity = 1 unit of electrical energy

Example:

How many units of electricity is 17.6 kWh? 17.6 units

The amount of electrical energy (i.e. the amount of electricity)used by an appliance depends on its power and how long the electricity is used for.

electrical energy = power x time

Calculating the Units of Electricity

= 1 kilowatt hour (kWh)


80

Electronic Meter

AC Energy Meters

The meters record how much electricity is used in a house in units of electrical energy.


81

The cost of electricity is the number of units of electrical energy multiplied by the cost per unit.

Example:

How much would 10 units of electricity cost at a price of 9p per unit?

cost = 10units x 9p/unit

cost = number of units x cost per unit

How much does electricity cost?

= 90p


82

An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time?

Number of units:

Cost of electricity:

Buying Electricity

= 12units

= 12kWh

= 3kW x 4h

number of units of electricity = number of kilowatt hours

= 96p

= 12 units x 8p/unit

cost = number of units x cost per unit


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84

Tariff Structure – Hong Kong Electric


85

(a) Basic Charge

(b) Fuel ClauseThe fuel clause charge is 22.4 cents per unit.

Domestic Tariff – CLP Power


86

Comparison based on monthly domestic customer consumption of 275 units (3,300 units p.a.), tariffs and exchange rates are as at Nov 2013

Tariff Comparison

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