ElectromagnetismElectromagnetism

Three lessons of fun withThree lessons of fun with

motors and generatorsmotors and generators

1. Motors1. Motors

Motor EffectMotor Effect

► If a If a currentcurrent passes passes through a wire in a through a wire in a magnetic fieldmagnetic field, it feels , it feels a a forceforce

►The force is at The force is at right right anglesangles to both the to both the current and the field current and the field lineslines

Motor EffectMotor Effect►The The sizesize of force of force

increases if the increases if the current, field strength current, field strength or length of wire or length of wire increasesincreases

►The The directiondirection of force of force reverses if the current reverses if the current or field is reversedor field is reversed

►ElectricalElectrical energy is energy is converted to converted to kinetickinetic

Making a motorMaking a motor

►By By coilingcoiling the wire we can the wire we can Increase the wire Increase the wire lengthlength (and force) (and force) Get a force on Get a force on bothboth sides sides

Making a motorMaking a motor

►We need to solve a problem …We need to solve a problem …

Making a motorMaking a motor

►Solution:Solution: a a commutator commutator to reverse the currentto reverse the current and and brushes brushes to maintain contactto maintain contact

A real motorA real motor

Spring wire touchescontact for outer coil

Other end of springtouches one ‘brush’in hereOther end of brush

pokes out here

Commutator has 14Commutator has 14elements – each attacheselements – each attachesto a separate inner coilto a separate inner coil

Input mains leadconnects here

Outer coil makes anOuter coil makes anelectromagnet – wireelectromagnet – wirelooks bare but islooks bare but iscoated in plasticcoated in plastic

Electromagnet core isElectromagnet core islaminatedlaminated to reduceto reduceheat lossheat loss

2. Generators2. Generators

Starter QuestionStarter Question

►Which direction is the force felt?Which direction is the force felt?

Wire & current

Field lines

NONE!! They are not at right angles

Electromagnetic InductionElectromagnetic Induction

► If a conductor If a conductor cuts throughcuts through magnetic magnetic field lines a p.d. is generatedfield lines a p.d. is generated

►Either the wire or the magnet can Either the wire or the magnet can move (we need move (we need relativerelative motionmotion))

► If the wire is part of a complete If the wire is part of a complete circuitcircuit we get a we get a currentcurrent generated generated

►All our electricity is generated this wayAll our electricity is generated this way

Electromagnetic InductionElectromagnetic Induction

►The p.d. is The p.d. is reversedreversed if … if … the the movementmovement is reversed (in – out) is reversed (in – out) the the fieldfield is reversed (N-S) is reversed (N-S)

►For a For a coil coil of wire, the p.d. increases if …of wire, the p.d. increases if … the the fieldfield is stronger (field lines are closer) is stronger (field lines are closer) the the speedspeed of movement is more of movement is more the the numbernumber of coils is more of coils is more the the areaarea of the coils is bigger of the coils is bigger

Practical UsePractical Use

►Kinetic energy (turning the handle) Kinetic energy (turning the handle) becomes electrical energybecomes electrical energy

►Slip rings and brushes prevent the Slip rings and brushes prevent the external connections from tangling upexternal connections from tangling up

Practical UsePractical Use

►The generator produces The generator produces a.c.a.c.►As the coil turns the As the coil turns the directiondirection of of

current reverses as shown …current reverses as shown …

3. Transformers3. Transformers

StarterStarter

1.1. How do you increase the voltage How do you increase the voltage from an a.c. generator (four things)?from an a.c. generator (four things)?

2.2. Explain the purpose of the slip rings Explain the purpose of the slip rings and brushesand brushes

TransformersTransformers

► A transformer is just A transformer is just an a.c. electromagnet an a.c. electromagnet (primary coil) next to a (primary coil) next to a generator (secondary generator (secondary coil)coil)

► The a.c. electromagnet The a.c. electromagnet makes a makes a changing changing magnetic fieldmagnetic field

► This hits the generator This hits the generator coil making an coil making an a.c.a.c. currentcurrent flow in that coil flow in that coil

TransformersTransformers

►Why use electricity to make electricity?Why use electricity to make electricity?

►The The sizesize of the generated output voltage of the generated output voltage can be changed (more coils etc.)can be changed (more coils etc.)

►A transformer is used toA transformer is used to

change an change an a.c.a.c. voltage voltage

UsesUses

►Step-upStep-up Car HT coil – increases battery voltage for Car HT coil – increases battery voltage for

spark plugsspark plugs National grid – raise voltage for National grid – raise voltage for

transmissiontransmission

►Step-downStep-down Adaptors – reduce mains a.c. voltageAdaptors – reduce mains a.c. voltage National grid –reduce voltage for supply to National grid –reduce voltage for supply to

homeshomes

DemoDemo

► Note: the laminated Note: the laminated soft iron coresoft iron core CoreCore – helps – helps

‘concentrate’ the field ‘concentrate’ the field into the secondaryinto the secondary

LaminatedLaminated – reduces – reduces heat build-upheat build-up

Soft ironSoft iron – can – can magnetise and magnetise and demagnetise better demagnetise better than, say, steelthan, say, steel

Transformer FormulaTransformer Formula

Coils on primaryCoils on primary Voltage on primary Voltage on primary

-------------------------------------------- = = --------------------------------------------------

Coils on secondaryCoils on secondary Voltage on secondaryVoltage on secondary

V V

Primarycoils

Secondarycoils

HomeworkHomework

►Revise for testRevise for test