HSC Physics K.I.S.S. Motors & Generators

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1

but first, an introduction...

HSC Physics Topic 2

MOTORS & GENERATORSWhat is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. ELECTROMAGNETIC FORCES & MOTORS

2. GENERATORS & POWER PRODUCTION3. TRANSFORMERS & THEIR USES

4. MORE ON MOTORS...all in the context of society’s use of electricity

What Has Gone Before...In the Preliminary Course you studied a topicabout electricity, including:

Electric Charges & Fields,Current , Voltage & Ohm’s Law,

Electric Circuits, Power & Energy,Magnetic Effects of Electricity.

In particular, it will be very useful to remindyourself about:

...and how todetermine

the direction and polarity of the

magnetic field,

AND...

What Happens Next...In this topic youwill learn how andwhy ElectricMotors work.

People often don’trealise how muchwe depend on thehumble electricmotor.

You will learn about how we generate theelectricity society needs,

...and how it is distributed and transformed to meet our needs.

It would be very wise to revise

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Electric Currents Produce Magnetic Fields

IRight Hand

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ELECTROMAGNETS

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2

ForcesBetween

WiresCarryingCurrent

Impacts onSociety &

Environment

EnergyTransformations

in Homes & Industry

DC ElectricMotors

Electric Meters&

Loudspeakers

Lenz’s Law & its

Effects

Features&

Advantages

Purpose&

Features

Step-Up&

Step-Down

MagneticFlux

&Flux

Density

MOTORS&

GENERATORS

CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorising the OUTLINE of a topic helps them learn and remember the concepts and important facts. As you proceed through the topic,

come back to this page regularly to see how each bit fits the whole. At the end of the notes you will find a blank version of this “Mind Map” to practise on.

Force on a Current-CarryingConductor in aMagnetic Field

The Motor Effect

ElectromagneticInduction

Generators

ElectromagneticForces

&DC Motors

Generators&

PowerProduction

TransformersAC InductionMotors



It was discovered in 1820 that a wire carrying anelectric current produces a magnetic field.Almost immediately, Andre-Marie Ampereinvestigated the way that TWO wires, bothcarrying current, would exert a force on eachother.

If the wires carry current in the SAME direction,the force ATTRACTS the wires.

If the currents flow in OPPOSITE directions, the force REPELS the wires.

Ampere found that the size of the force dependsupon a number of factors:

• the amount of current in the wires• the distance between the wires (separation)• the length over which the wires run parallel

For his contribution, we name the unit of currentafter Ampere.

You’ll notice that the force is very smallin the example at right. In fact, thistype of force is very weak and ingeneral electrical wiring is totally

insignificant.

However, the point is that it shows thatelectrical currents create magnetic

fields and forces, and that electricalcurrents can interact with magnetic

fields and with other currents.

This is the basis of ELECTRICMOTORS, GENERATORS andTRANSFORMERS... read on.

3


1. ELECTROMAGNETIC FORCES & MOTORS

I1Both wires carrying

a currentI2

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Currents in the Same Direction...


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...Attract

WIRESend-oon

Mathematically, F = k.I1.I2L d

F = Force in newtons (N) F/L refers to the “force L = Length in metres (m) per unit of length”

k = the “magnetic force constant” = 2.00 x10-7

I1 & I2 = the currents in the wires, in amps (A)d = the separation distance, in metres (m)

Example Problem:Two long, straight, parallel wires are carrying 5.60Aand 12.3A in the same direction. The wires are2.50cm apart.

a) Calculate the force per metre between them.b) If the parallel section of the wires runs for 4.75m,find the total force acting in this section.

Solution:a) F = k.I1.I2 = 2.00x10-7 x 5.60 x 12.3 / 0.025

L d= 5.51 x 10-4 N/m, attraction.

i.e. Each 1 metre of parallel wires has a force of0.000551N acting between the wires.

Note that the force is attracting the wires becausethe currents are in the same direction. If thecurrents flowed in the opposite directions, thesame force would be repelling the wires.

b) F = 5.51 x 10-4 newtons per metreL

So, F = 5.51x10-4 x 4.75= 2.62 x 10-3N attraction.

TRY THE WORKSHEET at the end of the section

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The Force Between Two Wires Carrying a CurrentThe explanation for the forces is quite simple...

If you look at the wires end-on, and use the“Right-Hand Grip Rule” to visualize theirmagnetic fields:


4

The Motor EffectAlthough the force between 2 wires carrying acurrent is rather weak, the effect can be muchmore powerful if more than one wire is involved,and if the magnetic fields involved are a lotstronger.

If, for example, a wire is carrying a currentthrough a reasonably powerful magnetic field,the wire will experience a significant, noticeableforce. This is called “The Motor Effect”.

Factors Affecting the Magnitude of the Force

• The Strength of the Magnetic Field (B)• The Current ( I ) carried in the wire• The Length (L) of wire within the magnetic field• The Sine ratio of the angle (Sinq) between thewire and the magnetic field lines

Since the force is directly proportional to eachof these factors, it follows that any increase inthe

• magnetic field strengthor • currentor • length of wire within the field

will increase the force, in proportion.

What about the angle?You know that Sin0o = 0

and Sin90o= 1Therefore, the maximum force on the wireoccurs when the wire and the field lines are atright angles. If the wire is parallel to the fieldlines, θθ = 0o and the force is zero.

Direction of the Force?In the diagram at left, notice that the magneticfield lines, and the current direction, and theresulting force are all at right angles to eachother.

The simplest way to determine the direction ofthe force is “THE RIGHT-HAND PALM RULE”



NorthPole

SouthPole Conventional

Current in wire

FORCEacting on wireFF

I

BBMagnetic

Fieldlines

Measurement of Magnetic Field:The “strength” of a magnetic field can be

thought of as the “density” of the magneticforce lines passing through an area of space.

The symbol used is “B”.The unit of measurement is the “tesla” (T),

named after an engineer/inventor who madegreat contributions to the practical

development of electricity generation.

You will learn more about the man Tesla, andthe tesla unit later in this topic.

F = B.I.L.SinθθF = Force, in newtons (N)B = Magnetic Field strength, in tesla (T)I = Current, in amps (A)L = Length of wire within the field, in metres (m)θθ = Angle between wire and field. See below

θ

B

wire

Direction ofConventional Current

Direction ofMagnetic Field

lines

Direction of Force.In this diagram, the force

is up out of the page.

I

F

B

Example Problem:A wire carrying 7.50A ofcurrent is within amagnetic field 0.450mwide, with a strength of18.6T. The field isdirected into the page asshown.

What force (including direction) acts on the wire?

Solution: F = B.I.L.Sinθθ= 18.6 x 7.50 x 0.450 x Sin90o

= 62.8N.The Right-Hand Palm Rule shows that the force isdirected down the page.

TRY THE WORKSHEET at the end of the section

B=18.6TI = 7.50A

L=0.450m


5

What Causes the Motor Effect?As you can probably figure out for yourself, theforce on the wire is due to the external magneticfield interacting with the field produced by thecurrent in the wire.

Verify the direction of the force using the RH Palm Rule.

The Concept of TorqueBefore we go any further, it is necessary for you tolearn about the way that one or more forces cancause things to rotate.

One particularly important situation is shown inthe following diagram:

Applying forces in this way will cause things torotate. The size of this “turning effect” is usuallymeasured by a quantity called “Torque”.

Torque and MotorsMotors and engines are usually used to turnwheels and axles to drive vehicles, or rotatemachinery, tools, etc. The key word is “rotate”.

Because they rotate things, it is appropriate tomeasure the effect of any engine or motor by itsTorque.

You might recall from a previous topic that

Work done = Force x distance = Energyby a force

Since Torque is also equal to Force x distance, itmeans that when you consider the torqueprovided by a motor, you are dealing with theenergy being converted by the motor.

In the case of an electric motor, the energyconversion is:

ELECTRICITY KINETIC ENERGY



Wire seen end-oon.Current into page

Force on wire due to interaction ofmagnetic fields

Magnetic Fielddue to current

Prac: Investigation of the Motor EffectYou will have done some experimental work inclass to see the Motor Effect in action.

There are many possible ways to do this, buta simple example is shown below.

The “external magnetic field” is provided by ahorse-shoe magnet (U-shaped). Thisproduces a magnetic field passing around thebottom wires of the rectangular coil.

Current passes through the rectangular coil ofwire. Only the bottom, horizontal strands ofwire are properly within the external field.

The wire is free to swing and deflects, asshown, when current flows.

Reversing either the current direction, or thepolarity of the magnetic field, will reverse theforce on the coil and the way it deflects.

Metal Bar, able to rotatearound centre

““dd”” == PPeerrppeennddiiccuullaarr ddiissttaannccee bbeettwweeeenn FFoorrcceess

FORCE = F

FORCE = F

Torque is a measure of the “turning moment” of aforce, or more commonly a pair of forces, causingrotation as shown above. Mathematically,

ττ = F.dττ = Torque, measured in newton-metres (N.m)

Note that the Greek letter “tau” is used for TorqueF = Force, in newtons (N)d = perpendicular distance between forces,

in metres (m)

Note: the syllabus requires you know the definitionof “torque”, but not to solve problems with thisparticular equation.

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on

Force pushes

coil

ExternalMagneticField

Pivot point


6

Torque on a Loop of Wire CarryingCurrent in a Magnetic Field

Previously, you learned how a straight wirecarrying current through magnetic fieldexperiences a force. What if the wire forms a loop?

If this loop of wire is able to rotate, the forces on eachside will provide a Torque and cause it to rotate about itscentral axis.

The force on each side is F = B.I.L (assume θθ =90o)

Remember that Torque = Force x distance betweenforces so the Torque on the loop is ττ = B.I.L.W

However, the factor (LxW) = the AREA of the loop, soττ = B.I.A

This is the torque provided by just one wire in theloop. If the loop is a coil made up of “n” strands ofwire, then

ττ = n.B.I.AFinally, it can be shown that as the coil rotates,there are positions where the forces on thewires do NOT cause rotation, so the torquevaries with the angle between the plane of thecoil, and the field.

Structure of a Simple DC MotorBasically, an electric motor is nothing more thana coil of wire, built onto an axle so that it canrotate within a magnetic field.

When current is switched on in the coil, the magneticforces create a torque which rotates the coil.

In small, simple motors (such as in a child’s toycar) the magnetic field is provided by apermanent magnet. In more powerful motors,the field is provided by an electromagnet, as inthe “demonstration” motor above.

The tricky bit is to supply electric current to arotating coil, and to maintain a steady,continuous torque... read on...



Magnetic Field

BB

This side of the loopexperiences a force

UP out of page

This side, NO FORCEbecause current flows

parallel to field

This side of the loopexperiences a force

DOWN into page

FF

FF

IFlow ofConventionalCurrent

WWIIDDTTHH OOFF CCOOIILL ““WW””

ττ = nBIACosθθττ = Torque on the coil, in newton-metres (N.m)n = number of loops of wire in the coilB = strength of the magnetic field, in tesla (T)I = current flowing in wires, in amps (A)A = Area of coil, in square metres (m2)θθ = angle between plane of coil and mag.field

Note that Cos 0o = 1Cos 90o = 0

so maximum Torque occurs when the plane ofthe coil lies “flat” in the field (θθ = 0o). Whenthe coil is “upright” in the field (θθ = 90o), theTorque is zero.

Example Problem:A rectangular coil (just like in the diagram at the left)made up of 50 loops of wire, is carrying a current of5.65A through a magnetic field of 20.0T strength.The dimensions of the coil are 4.50cm x 8.25cm.

What is the torque on the coil at the instant when itlies at 60o to the field lines?

Solution: ττ = nBIACosθθ∴∴ ττ = 50 x 20.0 x 5.65 x (0.0450x0.0825)x Cos60

= 10.5 N.m.

Notice that the dimensions of the coil were given incm, but must be converted to metres; S.I. unitsmust be used!

TRY THE WORKSHEET, at the end of this section

Powerleads

STATOR.

Coil for statorelectromagnet

ROTORCoils

Rotationaxle

Close-uup of back of motor

showingCommutator

& Brushes

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xxiiss


7

Main Features of a DC MotorRefer to the photo on the previous page.

The Rotor is the part that rotates. It is a coil of wire(or often several coils) mounted on an axle to allowrotation.

The Stator is the part that remains stationary. Itmay be a permanent magnet, or an electromagnet. Itspurpose is to provide the magnetic field.

Often, the magnetic poles are shaped in such a wayto create a “Radial Magnetic Field”... one in whichthe lines of force are directed like the spokes of abicycle wheel... radii of a circle. This means the planeof the coil is always “flat” in the field (θθ = 0o)throughout its rotation. Since Cos0o=1, the result ismaximum torque at (nearly) all positions.

The Brushes are fine, flexible metal wires, or (morecommonly) a spring-loaded stick of graphite. Thebrushes maintain electrical contact onto the rotatingmetal ring.

The Commutator is a metal cylinder, split into 2pieces. As it rotates, the direction of current in the coilis reversed every half-rotation. This way, the torque isalways in the same rotational direction, even thoughthe coil has turned over.

Other Applications of the Motor Effect

The GalvanometerAll electrical meters, ammeters and voltmeters,are based on a device called a “galvanometer”,named in honour of Luigi Galvani, one of thepioneers of Electrical Science.

The galvanometer works because of the MotorEffect; the more current that flows through itscoil, the greater the torque on the coil, and thegreater the deflection of the meter needle,working against a small spring . The needle thenpoints to a scale of measurements, which can becalibrated to read either current or voltage.

The Moving-Coil Loudspeakerwas explained in a Preliminary topic. Electricalcurrent (modulated with a signal from radio,microphone, etc) creates a fluctuating magneticfield around a coil. This field interacts with anearby magnet, and the coil vibrates rapidlyback-and-forth. The attached speaker conevibrates too, and sends compression waves(sound) into the air.



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Radial MagneticField

Measuring scale

Needle moveswhen coil rotates

SmallPermanentMagnetprovides radialfield for constanttorque

Coil (seen end-oon)experiences torquewhen current flows

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Two parallel wires, both carryinga)....................... will exert ab)............................ on each other. Thereason is because each wire willproduce a c).................................................... around itself, and these2 fields interact with each other. If thewires carry current in the samedirection, the force will d).........................the wires. If the current flowe)............................, the force willf)............................... the wires. Themagnitude of the force per unit ofg).................... is proportional toh).................................... in the wires, andi)................................ proportional to thedistance between them.

If a wire is carrying current through aj)............................. field, it willexperience a k)......................... Themagnitude of the force depends upon 4factors:• The strength of the Magnetic field,measured in l)..............• The m)...................... flowing in the wire• The n)............................ of the wire thatis within the o)................................., and• The p)............................ between wire &field.This force on a wire is the basis ofelectric motors and is called the“q)........................ Effect”. The directionsof current, field & force are all atr)................................. to each other, andcan be determined by the“s).................................. ................ Rule”.

“Torque” is a measure of thet)............................. effect of a pair offorces which cause something tou)............................ around a pivot pointor axle. A loop or coil of wire, carryingcurrent within a v)....................................................., will experience atorque, because the force acting on theopposite sides of the coil will bew)...........................................................

The size of the torque depends on 5factors:• The x).............................. which makethe coil• The strength of the y).........................................• The z)......................... flowing in the coil• The aa)............................... of the coil,and• The angle between the ab)...................of the coil, and the field. Maximumtorque occurs when the angle isac)...................... Zero torque occurs atan angle of ad)............... degrees.

A simple DC motor has just 4 mainparts:• The Rotor, made up of aae)................................. mounted on anaxle to allow it to af)..................................• The ag).................................. whichprovides the magnetic field, from eithera ah)............................... magnet, or anai)...........................................• The Brushes, which maintainaj)......................... contact between theelectricity supply and the rotating coil.• The ak)...................................., whichcauses the current toal)............................. every half-turn.

The Motor Effect is also involved in theoperation of a am).................................................., and a moving-coilloudspeaker. In an electric meter, theneedle moves along a calibrated scalebecause of the an)............................ on acoil inside a ao).................................(shape) magnetic field. In a loudspeaker,the sound is produced byap)............................ of a speaker cone.In turn, this is made to vibrate by a coil’smagnetic field aq).....................................with a permanent magnet.

8

Copying is permitted according to the Site Licence Conditions onlykeep it simple science®


Worksheet 1 Electromagnetic Forces & MotorsFill in the blank spaces. Student Name...........................................

COMPLETED WORKSHEETSBECOME SECTION SUMMARIES

1. Calculate the force per unit of length between 2long, parallel wires carrying 15.3A and 12.7Aand separated by 1.00cm. State the direction ofthe force, given that the currents are in oppositedirections.

2.Two long, parallel wires are carrying equalcurrents. The wires are 10.0cm apart. The forcebetween them is found to be 8.25x10-5 N permetre of length, attracting each other. Find themagitude, and relative direction, of the currentsin the wires.

3.The vertical wire runs for2.44m through a 105T fielddirected out of the page.The force on the wire is27.2N left.Find the magnitude anddirection of the current inthe wire.

4.A wire is carrying 8.00A of current over alength of 0.287m through a magnetic field of7.50T. A force of 3.72N acts on the wire.Find the angle between the wire and the fieldlines.

9



3.Two wires run parallel for a length of 1.48m. Thetotal force acting between them over this lengthis 6.44x10-4N when they are carrying currents of8.90A and 14.5A. How far apart are they?

4.Two power cables, both carrying 30.0A ofcurrent in the same direction, are separated by adistance of 8.00cm. The cables run parallel overa distance of 25.0m. What is the total force(including relative direction) acting betweenthem?

Worksheet 2 Practice ProblemsForce Between Wires Carrying Current Student Name...........................................

Worksheet 3 Practice ProblemsForce on a Wire Carrying Current in a Field Student Name ...............................

1.A wire is carrying 4.50Aof current through a11.0T field, directed asshown. The length ofwire in the field is 1.25m.Find the magnitude anddirection of the force onthe wire.

2.Find the magnitude &direction of the force which would act on thewire shown. The lengthof wire within the fieldis 0.385m.

B=11.0T

I = 4.50A

L=1.25m

BB == 2222..77TT

II == 55..9955AA 6600o

BB == 110055TT

FF == 2277..22NN LL==22..

4444mm

1. Calculate the amount of torque on a coil of200 turns of wire carrying 1.50A in a field ofstrength 5.25T. The area of the coil is 1.20x10-3m2. Assume that the field is radial, sothat the torque is always at a maximum.(i.e. θθ = 0o)

2.The coil shown is20cm square andcomposed of 35 turnsof wire.a) Find the torque onthe coil when it lies“flat” in the field (i.e. q = 0o).b) Will the coil rotateclockwise or anti-clockwise as viewedfrom the lower end?

3.The torque achieved by a small electric motoris found to be 3.86Nm, when a current of 3.20Aflows through the rotor coil which has an areaof 0.00262m2. The stator provides a radial fieldof 4.60T. (assume q = 0o)How many turns of wire in the coil?

4.The rectangular coil shown is 8.00cm x 5.00cm and is rotating anti-clockwisedue to the torque on it. The magnetic field of 22.3T gives maximum torque of 12.3Nmat the position shown. The coil consists of just12 turns of wire.a) What is the current?

b) Determine the direction of conventionalcurrent flow (clockwise or anti-clockwisearound the coil diagram?)

10



B=8.38T

I = 2.50A

NN SS

Worksheet 4 Practice ProblemsTorque on a Coil in a Field Student Name .........................................

Worksheet 5 Test Questions section 1 Student Name...........................................1. The diagram shows 2 wires carrying the samecurrent, but in opposite directions. The point“X” is mid-way between the wires.

What is the direction of the magnetic field atpoint X?A. down the pageB. into the pageC. up the pageD. out of the page

2. Two long parallel wires are carrying currents I1and I2 in the same direction. The wires are “d”metres apart. The wires exert a force per unit oflength on each other. If both currents weredoubled and the distance between the wireshalved, by what factor would the force per unitlength change?

A. increase, by a factor of 8.B. increase, by a factor of 2.C. remain the same.D. decrease, by a factor of 4.

3. ( 4 marks)Two parallel wires are carrying 12.0A and 7.50Aof current in opposite directions. The parallelsection of the wires is 1.85m long, and the wiresare 1.00cm apart.Calculate the total force (including direction)which will act between these wires.

4. (5 marks)a) In a simple DC motor, describe the role of:

i) the commutator

ii) the stator

b) Explain what is meant by a “radial magneticfield”, and describe the advantage it gives in arotating-coil motor.

X

Multiple Choice

The diagram is used for questions 1 and 2.

It shows 2 wires P & Q both carrying the same currentthrough the same magnetic field. The length of eachwire within the field is the same.

1. The force experienced by wire P would bedirected:A. to the rightB. to the leftC. out of the pageD. into the page

2. Compared to the force acting on P, the force onwire Q would be:A. exactly the same.B. about 87% as strong.C. exactly half as strong.D. zero.

3. A “torque” is produced when:A. a force causes circular motion.B. a force acts on a pivot point, causingacceleration.C. a pair of separated forces act in oppositedirections.D. a pair of forces act in the same direction.

4. Electric motors often have a curved statorstructure to give a “radial magnetic field”. Thebenefit of this field is that it:A. gives a more constant torque as the

coil rotates.B. reverses the current each half-revolution.C. intensifies the field in the centre of the

coil for increased torque.D. reverses the field so the coil will turn

the other way.

Longer Response QuestionsMark values shown are suggestions only, and are togive you an idea of how detailed an answer isappropriate. Answer on reverse if insufficient space.

5. (4 marks)A wire is carrying 9.00A of current in a directiondue north. 0.750m of the wire is within a verticalmagnetic field, which causes a force of 3.25N topush the wire due east.Find the magnitude and direction of themagnetic field.

6. (8 marks)

The rectangular coil PQRS is made of a singlestrand of wire. It is carrying current through afield as shown.a) Find the force acting on side RS, includingdirection.

b) What force acts on side QR?Explain your answer.

c) Find the torque on the coil at the momentwhen the plane of the coil is inclined at anangle of 10o to the field lines.

7. (8 marks)The diagram shows the simplified structure of a galvanometer, the basis of all electrical meters.

For each of the partslabelled (a), (b), (c) & (d)identify what that part is,and explain briefly itspurpose.

11



Worksheet 6 More Test Questions section 1 Student Name........................................

I

wire Qwire P

I

θθ=30o

B

Magnetic Field

BB= 8.00T

PP

QQ RR

SSI= 5.75A

Coil is 5.00cm square

Loop

can

rot

ate

on th

is a

xis

Flow ofConventionalCurrent

(a)

(b)

(c)

(d)

Electromagnetic InductionIf electrical currents produce magnetic fields, andinteract to produce forces and movement,shouldn’t the opposite occur too? So went theargument among scientists about 150 years ago.Many experiments were carried out before MichaelFaraday (1791-1867) proved the idea correct.

Faraday discovered that if there is relativemovement between a magnetic field and aconductor, then a current will be “induced” inthe conductor. This is called “ElectromagneticInduction”, and is the basis of the electricalgenerator and all of society’s large-scale powerproduction.

Magnetic Flux & Flux DensityTo explain his discovery of induction, Faradayintroduced the concept that a magnetic field ismade up of a series of “lines of force”. Heshowed that if a conductor moves so that it“cuts through” these field lines, then a currentis induced to flow in the conductor.

He invented the idea of “magnetic flux” as ameasure of how many field lines are cut by themoving conductor. From this concept arises theidea of Magnetic Flux Density

The “Magnetic Flux Density” is what we havebeen calling “Magnetic Field Strength”.

Faraday discovered that when there is relativemotion between a conductor and a magnetic field,a voltage (or “EMF” = ElectroMotive Force) iscreated.

The voltage (EMF) created then causes current toflow through the circuit according to Ohm’s Law.

Faraday developed the mathematical equationsrelating the induced voltage to rate of change offlux. The syllabus does NOT require you toknow these, but you DO need to know thedefinitions above.

Electromagnetic Induction is, of course, thebasis of the electric generator. Read on...

12


2. GENERATORS & POWER PRODUCTIONkeep it simple science

®


A Simple Induction ExperimentYou may have done a simple experiment tosee, first-hand, electromagnetic induction.

When the magnet is moved near, or into thecoil, the galvanometer needle registers a flowof current.

It doesn’t matter whether the magnet moves,or the coil moves... as long as there is relativemovement.

You may have investigated the factors which caneffect the nature of the induced current...

• Using a stronger magnet produces more current.• The closer the magnet is to the coil,

the more current.• The faster the movement, the more current.

You may also find that...

Reversing the direction of movement reversesthe current flow. Reversing the polarity of themagnet also reverses the current flow.

In practice, Induction usually produces ACelectricity... Alternating Current which flowsback-and-forth.

Large CoilMagnet

moved near, or in & out

of coil

Galvanometerdetects anycurrent flow

Same areas

Magnetic lines offorce passthrough thisarea

More field linesthrough the same area= more intense field

Field “P” Field “Q”

Magnetic Field Strength = Magnetic Flux Density

It is a measure of the intensity of a magnetic field,in terms of the number of force-field lines per unitof area.

Symbol used in equations = “B”Unit of measurement = tesla (T)

Rate of change of theFLUX

through the conductor

Size ofthe EMF

ααiiss pprrooppoorrttiioonnaall ttoo


13

Electrical Generators The main components of an electric generatorare basically the same as an electric motor. In amotor, electric current in a coil inside amagnetic field, causes the coil to rotate.

ELECTRICAL KINETIC ENERGY ENERGY

In a generator, rotating a coil inside a magneticfield induces a current to flow in the coil.

KINETIC ELECTRICALENERGY ENERGY

A simple motor can be a generator, and a simplegenerator can act as a motor...

Comparison: Motor & GeneratorStructureVery similar. Both consist of one or more coilsof wire which can rotate inside a magnetic field.The field can be provided by either permanentmagnets, or electromagnets.

Both require “brushes” to maintain electricalcontact with the rotating coil to pass currentinto the coil (motor) or carry induced current outof the coil (generator).

FunctionOpposites. Motors use electricity to produce movement inthe coil.Generators use movement of the coil to produceelectricity

PracticalitiesIn reality, motors and generators are built verydifferently for practical reasons.

For example, in power stations the generatorsare built to have the coils of wire stationarywhile the magnets do the rotating. This makes itsimpler and more efficient to transfer theelectricity to the power grid, without havingmassive currents sparking through the brushes.



Rotating CoilField Magnets

“Slip-rrings” on axle transfercurrent to the electric circuit

via “brushes”.

Connectionsto electric

circuitNN SS

The blades of theseturbines are turned by

the wind. This spins the generator

to make electricity.

What Makes the Generators Turn?

To get electricity from a generator you mustmake the coil (or the magnetic field) rotate.How?

In a “Hydroelectric” power station the generators are turned bythe flow of water falling through huge turbines.

In this NZ Geothermal power station (below)high pressure steam from volcanically heatedwater spins the turbines which drive thegenerators.

In a “conventional” power station theturbines are driven by steam made by heatingwater. The heat comes from burning a fuelsuch as coal.


14

AC and DC GeneratorsIn a simple generator as described previously, theinduced current will reverse direction every halfrevolution of the coil.

A graph of the EMF generated this way wouldlook like this:

It is also possible to make a generator which willproduce direct current (DC) which flows in thesame direction.

The electricity produced flows only in onedirection, (DC) but fluctuates according to theposition of the coil within the field.

Advantages & DisadvantagesThe fact that our “mains” electricity supply isAlternating Current (AC) tells you that there must beadvantages to generating electricity as AC, ratherthan DC.

One major advantage of AC has nothing to do with thegenerators, but relates to transmission of power andthe ease of altering the voltage in a transformer. Thiswill be studied a little later. For now, simply note thatthere ARE major reasons to do with distribution andusage, which make AC preferable to DC.

In terms of the generators themselves, anyadvantages & disadvantages relate to their structure:

Disadvantage of a DC GeneratorNo matter how well it is made, the commutator isthe weakness of a DC generator. Because it is asplit-ring structure, the brushes must spark andwear out as the commutator revolves. This isinefficient in terms of transferring electricity tothe external circuit, and causes maintenanceproblems as the brushes wear out and need tobe replaced.

Advantage of an AC GeneratorInstead of a commutator, the AC generator hascontinuous “slip-rings” so there is much lesssparking and less wear on the brushes.

Additionally, as already mentioned, an ACgenerator can be built with the massive, heavycoils stationary and the magnets doing therevolving on the inside. This simplifies theengineering and maintenance and eliminatesentirely the use of slip-rings and brushes tocarry the generated electricity. (However,smaller amounts of electricity still need to passthrough brushes and slip-rings to supply therotating electromagnets.)

Energy Losses in Power LinesOur modern electricity system consists of arelatively small number of large power stations,with the electricity needing to be distributed inpower lines over hundreds of kilometers.

Although the wires (usually aluminium) are madeto be of low resistance, over long distances therecould be major energy losses due simply to theresistance causing heating in the wires.

However, the energy loss due to resistance heatingis much greater at higher currents. So to minimiseenergy losses, electricity is carried at very highvoltage and very low current. Typically, long-distance power is distributed at 250,000 volts ormore, but only tiny currents, like 0.01 amp.

The fact that AC can be readily “stepped-up” to highvoltage for transmission, then “stepped-down” forconsumer use is a major advantage of AC electricity.The “step-up” & “step-down” is done bytransformers.



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VVoollttaa

ggee00

ttiimmee

DDCC ffrroomm aabbaatttteerryy

DDCC ffrroomm aa ggeenneerraattoorr

OOnnee ccoommpplleetteerroottaattiioonn ooff

ccooiill

NN SS

Induced current directionHalf arevolution

later, each sideof the coil cutsthe field in the

oppositedirection...

... so the currentreverses direction.

This is alternatingcurrent (AC)

Axlerotating clockwise

RotorRRoottaattiinnggCCooiill((ss))

NN SS

The current in the coilreverses every 11//22revolution, but thecommutator reverses it again to the externalcircuit.

External electric circuit

Brushes

Commutator




15

More on Power LinesPower lines are not usually covered with asheath of insulation. That’s why it is deadly totouch them with a ladder, or similar.

To prevent theelectricity beingconducted to theground, the powerlines are supportedon their poles ortowers by non-conductinginsulators.

The insulators areusually ceramic orglass and oftendisc-shaped tocreate a longerpath for a spark to jump.

Power lines and supporting structures need to beprotected from lightning strikes. Metal towers arewell “earthed” in that they can safely carry alightning strike into the ground. Wooden poles maybe equipped with a “lightning rod”... a thick metalcable running from the top down into the ground.

To protect the wires themselves, an extra wirecalled a “shield conductor” may be strung abovethe power cables. It is not supported by insulators,but electrically connected to the metal towers.Lightning will strike the “shield conductor” insteadof the power lines, and be safely conducted toground through the metal tower.

History... Edison v WestinghouseIn the early days of electricity generation and usagethe famous inventor, Thomas Edison, was pioneeringelectricity supply. He favoured the use of DCelectricity and had set up hundreds of DC powerstations around the New York area. His advantagewas that he had invented the light bulb, and nowstood to make a fortune selling both the bulbs and theelectricity to run them.

His main competitor was the Westinghouse company,which wanted to set up an AC electricity system.

In 1884, Nikola Tesla arrived in America fromSerbia. He was an engineer and inventor andhad developed new, improved versions of ACgenerators, motors and transformers. He got ajob with the Edison company, but soon left andwent to work for Westinghouse. Tesla sold hisinventions to Westinghouse, who built the firstlarge-scale AC power station at Niagara Falls.

Long distance transmission of AC electricitysoon proved more economical that the multiplepower stations and short-range Edison DCsystem. Also, Tesla’s new electric motor, whichran only on AC, proved very economical andreliable for factories, elevators and a host ofnew consumer machines like vacuum cleanersand washing machines. The modern electricalworld became established, and it was ACelectricity that became the standard.

Nikola Tesla’s contribution has been recognisedby the naming of the unit of magnetic fieldstrength (magnetic flux density) after him.

Impacts of the Development of AC GeneratorsThe syllabus asks you to “assess” these effects. This means to measure or “weigh-up” the

positives and the negatives, to both society and to the environment.It can be argued that the effects on human society are nearly all positive,

while environmental effects are all negative.

Effects on SocietyThe development of AC generators has led tothe wide-spread availability of low-cost energy.This has:-

• resulted in many improvements in life-style,with “labour -saving devices” such as washers,etc, and the comforts of air-conditioning andthe convenience of refrigerators and freezers,etc.

• promoted the development of electrical andelectronic inventions, leading to moderncommunications systems and computernetworks for finance, business andentertainment, to name just a few areas.

Like it or hate it, the fact is that the moderntechnological world, and the life-style mostpeople enjoy, with good health and manycomforts, is a direct result of electricity.

Effects on the EnvironmentAlthough much progress has been madetoward controlling pollution, the generation ofelectricity is linked to a number of enormousenvironmental problems:-

• Much of our electricity is still generated bycoal-burning power stations. The burning ofcoal is a major contributor to the “GreenhouseEffect” and “Global Warming”.

• Nuclear Power stations are “greenhouse-friendly”, but carry risks of disasters such asthe 1986 nuclear accident at Chernobyl.

• Even development of hydro-electricityinvolves massive disruption to ecosystems,when rivers are dammed and valleys flooded toprovide for power stations.


16

Lenz’s LawConsider a conducting wire being pushedacross a magnetic field. Because the wire iscutting the field lines, there will be an inducedEMF, and (if there’s a circuit available) currentwill flow..

BUT, when a current flows the Motor Effect willoccur and create a force on the wire. Which waywill it push the wire?

Heinrich Lenz figured it all out 150 years ago.The induced current will create a magnetic field(and Motor Effect force) which will oppose themotion that produced it in the first place.

Lenz’s Law arises as a consequence of theprinciple that energy cannot be created fromnothing... the “Law of Conservation of Energy”.

Look at the diagram above. If the inducedcurrent flowed the other way, then the motoreffect force would act to the right. This wouldaccelerate the motion of the wire. Since it wouldmove faster, it would cut more field lines(greater “flux change” Faraday would say) andthereby induce a greater EMF and greatercurrent. This would produce more force andaccelerate the wire even more... and so on. Thiswould mean energy being created from nothing!

In the diagram above, the induced current mustflow as shown, so that its own magnetic fieldopposes the motion of the wire, and soConservation of Energy is not violated.

This is why:

• when you push a magnet into a coil, you mayfeel an opposing force... the current induced inthe coil is creating a magnetic field which repelsthe one you’re pushing.

• When you wind the handle of a generator, theforce required is much greater than expected...Lenz’s Law opposes you!

Lenz’s Law has a number of consequences...

Back EMF in a MotorAs dealt with earlier, an electric motor rotatesdue to the torque on the coil due to the appliedcurrent interacting with the magnetic field.

However, as the coil rotates through themagnetic field, induction also occurs, creatingan induced EMF. Lenz’s Law guarantees that theinduced EMF will act against the supplied EMF.

Eddy CurrentsEven when there is no designed electrical circuitpresent, whenever there is relative motionbetween a conductor and a magnetic field, anEMF is induced and currents will flow. In a flatsheet or tubes of metal the induced currentsoften flow in circles... these are called “EddyCurrents”

Lenz’s Law guarantees that the eddy currentswill create magnetic fields to oppose the motionthat produced them.

Example: Get a small, but powerful “super-magnet” and drop it through a plastic tube. Thendrop it through a copper, or aluminium tube.



Magnetic Field up out of page.

Wire moving this way

Direction of induced current

Direction ofForce causedby current in

field

I

F

Lenz’s LawThe direction of an induced EMF

(and current) is such that it produces amagnetic field opposing the change

that produced the EMF

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CCOOPPPPEERR TTUUBBEE

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The final “net-EEMF”driving current

through the motoris the differencebetween the EMFsupplied, and the

“Back-EEMF”.

Current Flowdue to

external EMF

Current flow dueto induced EMF


17

Electromagnetic BrakingThis “braking effect” can be very useful.In some amusement rides, the passenger seat orcar is equipped with small, powerful magnets. Atthe end of the ride, there are sheets of copperwhich the magnets move past. (Or, vice-versa...the magnets are in the track and copper plate isonboard the car.)

Either way, eddy currents are induced in thecopper sheets. These currents produce magneticfields. These fields interact with the magnets toproduce a force opposing the motion. Thissmoothly slows the ride to a stop.

The beauty of this system is that:• it requires no power input to operate.• it involves no contact surfaces or moving partsthat can wear out.• it is “fail-safe”, so that in an emergency it willstill work and safely stop the moving ride.

Some trains use electromagnets (can be turnedon/off as needed) to induce eddy currents in therails below the train. As always, Lenz’s Lawensures that the induced currents create fields tooppose the motion, and acts as brakes.

Induction CookingAn “Induction Stove” has a flat ceramic top

with no visible heating elements.

Under the top are electromagnet coils. Whenswitched on, these produce oscillating

magnetic fields. If a metal saucepan is ontop, eddy currents are induced in the pan,which gets hot due to the resistance of themetal to the eddy currents. This heat cooks

the food in the pan.

AdvantagesHeat is produced directly in the saucepan,

rather than a heating “element”. This is muchmore efficient in energy terms, and thereby

cheaper to operate.

The flat ceramic top is easy to clean.



Rollercoaster TerminalPhoto by Stacy Braswell

Rails for cars to run on

Poles of largemagnets used forelectro-mmagnetic

braking ofrollercoaster cars

Electromagnetic Induction was discovered byMichael a).............................. It is the basis of theelectrical b).............................. and all large-scaleelectricity production.

A simple experiment to investigate inductioncan be done with a coil, a galvanometer and ac)........................... Any d).................................movement between e)................... and.......................... will induce a flow off)........................ in the coil. The magnitude of theinduced current can be increased by:• using a g)................................. magnet• moving the magnet h)........................ to the coil• making the motion i)...........................If the direction of movement is reversed, thenthe current j)........................ Reversing thepolarity of the magnet k).................... the current.

The strength of a magnetic field is technicallyknown as “Magnetic l)..........................................”This is a measure of the number ofm).................................. lines passing through agiven n).................. Faraday discovered that, inelectromagnetic induction, the size of theo)............................ induced is proportional tothe rate of change of p).......................... throughthe conductor.

In a simple generator, a coil is made toq)......................... inside a magnetic field, whichcan be provided by either r)..............................magnets, or by s).................................. Current ist).............................. in the coil and this is passedinto the external circuit by u)................................in contact with rotating “v)...................................”which are mounted on the axle. Overall, thestructure is very similar to an electricw)....................., but the energy transformation isexactly the opposite.

As the coil rotates, every half-revolution it cutsthe magnetic field in the opposite direction, sothe x)................................. is reversed. The resultis that it generates y)...........................................electricity. It is possible to generate DC bymounting a z)................................. on the rotatingaxle to aa)................................ the current againeach half-turn. However, DC generators havethe disadvantage that the commutator causes alot of ab)..............................., and the brusheswear out quickly. AC generators have theadvantage that the ac)...................-rings do notspark as much and do not wear out brushes asfast. In practice, AC generators are often builtwith the ad)................................... stationary, andthe ae)......................................... rotating. Thisdoesn’t need any slip-rings to connect the coilsto the af)............................................. at all.

Another major advantage of AC generation is todo with energy losses in transmission lines.Energy loss due to ag).............................. heatingis higher at higher ah)................................ levels.Therefore, to minimize energy loss, it is best totransmit electricity at ai)..................... voltageand very aj)....................... current. Since ACelectricity is easily stepped up or down by aak)................................., it is far better whenpower needs to be carried over long distances.

Transmission power lines use al).........................made from glass or am)......................, to preventelectricity running to earth through supportpoles and towers. They are protected from theeffects of an)................................ strikes by extrawires called ao)........................................... strungabove the power cables and connected to thesupporting towers. This allows lightning to beconducted harmlessly to the ap)..........................

In the early days of commercial electricityproduction, the aq)................................. companyfavoured the use of ar).............. electricity.Hundreds of local as)........................ stationswere built, since DC cannot be distributed overlong distances without massive loss ofat).............................. Nikola au)............................,a Serbian immigrant, invented improvedversions of AC av).........................., .......................and transformers. These were used by theWestinghouse company to build the first largepower station producing AC at aw)....................Falls. Tesla’s new AC motor was verysuccessful too, and this contributed to theacceptance of AC as the standard.

The effect of large-scale electricity generation:On society the impacts are mostlyax)......................., including many ay)..................-saving devices, and leading to the developmentof modern az).............................. systems forba)............................., .................................... and.....................................On the environment the impacts are mainlybb).................. For example, a lot of electricitygeneration involves burning of bc)......................which is a major contributor to thebd)..................................... and be)......................................................

Lenz’s Law states that the inducedbf)................................. is such that it’s magneticfield bg)....................... the change that producedit in the first place. This arises as aconsequence of the Law of bh)...................................................... It is because of this effect thatelectric motors are limited by“bi)......................................”. The effect can beuseful, such as in electromagneticbj)........................... of amusement rides andtrains, or in bk)............................ cooking. Theseeffects involve the induction ofbl).......................... currents in a conductor.

18



Worksheet 7 Induction & GeneratorsFill in the blank spaces. Student Name...........................................

Multiple Choice1. In an experiment, Sam noted that when the Npole of the magnet was pushed into the coil,the galvanometer needle moved left.

Which of the following would also cause theneedle to move left?A. pulling N pole out.B. pulling S pole out.C. reverse the wire leads, then push N pole in.D. push N pole in at other end of coil.

2. The magnetic field “strength” (B) is morecorrectly known as:A. Magnetic FluxB. Magnetic Flux DensityC. the rate of change of Magnetic FluxD. Electromagnetic Force, or EMF

3. In a simple generator with a rotating coil, thefunction of slip-rings and brushes is to:A. ensure a constant torque on the coil.B. pass current from the external circuit

into the coil.C. pass induced current from coil to the

external circuit. D. provide the external magnetic field.

4. Which of the following is NOT one of thereasons that we use AC electricity in preferenceto DC?A. AC generators do not need commutators.B. AC can be readily changed from one voltage

level to another.C. AC induction motors are efficient

and reliable.D. AC is more readily converted into heat, light, etc.


5. (3 marks)a) Give a definition of “magnetic flux density”and state the unit of measurement.

b) Complete this statement: “Michael Faradayfound that the size of the induced EMF isproportional to........”

6. (4 marks)During your studiesyou will have carriedout a first-handinvestigation intoinduction, usingequipment similar tothat shown in thisphoto.What is the effect onthe induced currentof:

a) using a stronger magnet?

b) placing the magnet inside the coil andleaving it stationary?

c) moving the magnet at a point further fromthe coil?

d) reversing which pole of the magnet isinserted into the coil?

7. (4 marks)Compare and contrast a simple moving-coilmotor and a simple moving-coil electricgenerator.

19



Worksheet 8 Test Questions section 2 Student Name...........................................

Multiple Choice1. A wire strung above the power lines andconnected to the towers without any insulation,is probably:

A. to carry the highest voltage electricity.B. a communication wire for electricity workers.C. to protect the wires from lightning strikes.D. a safety cable for maintenance workers.

2. During the early development of electricitysupply systems:A. Westinghouse favoured AC, Edison

favoured DC.B. Tesla sold his inventions to the

Edison company.C. Both Westinghouse and Edison favoured

AC supplies.D. The Edison company built the first AC

power station.

3. As you push a North pole of a magnet into a coila current will be induced in the coil. Thisinduced current will create its own magneticfield. You would expect the North pole of this“induced field” to be located:

A. at the opposite end of the coil from where you are.

B. at the near end of the coil.C. in the middle of the coil, pointing upwardsD. in the middle of the coil, pointing downwards.


4. (3 marks)Justify society’s choice of AC electricity supplyover DC.

5. (4 marks)Assess the effects of the development of ACgenerators on society and the environment.

6. (3 marks)A conducting wire is beingacted upon by a force asshown, so that it is movingthrough a magnetic field.A current is induced in thewire.

The induced current willresult in another force actingon the wire.a) In which direction the other force will act?

b) Deduce the direction of the current flow inthe wire.

c) State the scientific principle involved.

7. (4 marks)Outline the process of “electromagneticbraking”, giving an example of where it might beused, naming the scientific principleresponsible, and explaining how braking forcesare produced.

20



Worksheet 9 More Test Questions section 2 Student Name.....................................

B

F

TransformersIt has already been mentioned that the greatadvantage of using AC electricity is that it can be“stepped-up” to very high voltages for efficientdistribution, then “stepped-down” again forconvenient and safe usage by consumers.

It is this stepping-up & down of the voltages thatis the purpose and function of a transformer.

Step-Up, Step DownTransformers work by inducing a new EMF andcurrent in the “secondary coil”. Whether thesecondary voltage is higher or lower than theprimary voltage, is simply a matter of the ratiobetween the number of “turns” of wire in eachcoil.

Faraday discovered that the size of the inducedEMF is proportional to the number of turns ofwire in the coil. In a transformer, if the numberof turns in the secondary coil is greater than inthe primary, then the induced EMF is highertoo... the transformer “steps-up” the voltage.

In a “step-down” transformer, the opposite istrue... the secondary coil has less turns than theprimary, and the induced EMF is lower.

Current in Primary and Secondary Circuits

If you use a “step-up” transformer to get ahigher voltage, does this mean you just gotsomething for nothing?

No, of couse not! If the voltage goes up, thecurrent goes down in the same proportion. (Assuming perfect transformation of energy)

21


3. TRANSFORMERS & THEIR USESkeep it simple science

®


Schematic Diagram of “STEP-UUP” Transformer

PrimaryCircuit

(AC inputsupply)

Primary Coil(less turns)

SecondaryCoil

(more turns)Soft iron core

(Enhancesmagnetic field)

SecondaryCircuit

(AC output athigher voltage)

Schematic Diagram of “STEP-DDOWN” Transformer

PrimaryCircuit

(AC inputsupply)

Primary Coil(more turns)

SecondaryCoil

(less turns)

SecondaryCircuit

(AC output atlower voltage)

Practical InvestigationThere are many possible investigations you mayhave done in class to see the basic operation ofa transformer. One simple example is shown.

Light bulbconnected tocoil of wire =

“Secondary Coil”

AC power “Primary Coil”with steel core.(electromagnet)

Bulb lights up due toinduced current in

secondary coil

ExplanationThe AC supply to the primary coil (anelectromagnet) produces a fluctuating magneticfield. The field lines keep building, collapsing andreversing direction.

This moving field constitutes a “magnetic flux”through the wires of the secondary coil, and so EMFis induced.

This causes current to flow, which lights the bulb.

Note:This will NOT work with DC electricity from abattery. The key to the induction in the secondarycoil is the fluctuating field caused by the AC supply.(You WILL get induction with “DC” from a schoolpower pack because it fluctuates quite a lot)


22

The Transformer EquationThere is a simple relationship between thevoltages and the number of turns of wire in thecoils of a transformer.

Conservation of Energy in a Transformer

You will recall from a Preliminary topic that

Electrical Power = Voltage x Current P = V.I

and that Power is the the amount of Energybeing transformed per second.

In a step-up transformer, the voltage increases,and the current decreases by the same factor,so that:

Primary coil Power = Secondary coil PowerVpIp = VsIs

Energy per second = Energy per secondin Primary coil in Secondary coil

Therefore, the Law of Conservation of Energy isobeyed.

(Note: you are NOT required to solve problems using this relationship, but should be able to describe the situation)

Energy Losses in TransformersThe description above assumes that a transformerworks with 100% efficiency. This is often assumed forsolving simple problems, but you need to be awarethat, in the real world, nothing is perfect.

Real transformers are not perfect, and always losesome energy in the process of altering the voltage.The main loss of energy is by resistance heating, notonly in the coils, but due to “Eddy Currents” inducedin the iron core.

Once the transformer starts to heat up, the situationgets worse, because (as covered in Preliminary topic)resistance in a metal increases with temperature.

A number of methods are used to minimise theenergy losses:

• The iron core is not one large piece of iron, but ismade of thin sheets of iron, laminated together, butinsulated from each other. This way the eddy currentsinduced in the core are smaller, and cannot circulatevery far.

• The coil wires are thicker on the higher current sideof the transformer (depending whether step-up orstep-down). Thicker wires have less resistance, sothis minimises resistance heating in the coils.

• Transformers are designed to radiate heat away sothey stay as cool as possible, to reduce resistance.Large transformers may have cooling oil circulatingthrough a heat exchanger, rather like the radiatorsystem of a car engine.

In the photo on the left, the transformers are equippedwith small metal “radiators” to quickly lose heat tothe air.



Vp = npVs ns

Vp = Voltage in the primary coil.Vs = Voltage in the secondary coil.

np = No. of turns of wire in the primary coil.ns = No. of turns of wire in the seconary coil.(Assumes 100% efficiency in energy transfer)

Example Problem:A transformer has 750turns of wire in theprimary, and 5,000 turnsin the secondary coil.

Input voltage is 240V AC.a) Find the output voltage.b) Is this a “step-up” or “step-down”transformer?

Solution: a) Vp = np

Vs ns240 / Vs = 750 / 5,000

∴∴ Vs = 240 x 5,000 / 750= 1,600 V.

b) Step-up transformer, since it has moreturns in the secondary coil, and the outputvoltage is higher than input.

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23

From Power Station to Home...the Role of Transformers

Even though there is some loss ofenergy in a transformer, it is still worthit. The advantage is the way AC can bestepped-up to very high voltages andefficiently distributed over longdistances from large, economical powerstations.

The typical chain of transformations is:

Transformers inside the HomeEven after all this transforming going onbefore the electricity gets into yourhome, it’s still not finished.

Many appliances inside your home needa transformer because they need more,or less, voltage than the 240V supply.

For example:

Old-style TV picture-tubes need 1,500Vto operate. Much ofthe weight of a TVset is the heavystep-up transformerinside.

(These sets arerapidly disappearingas wide-screen LCDand Plasma TV’sreplace them)

Many electronic devices need only lowvoltages, such as 12V or less. Smallergadgets often have the necessarytransformer in a “box” combined withthe electric plug.



SStteepp-uupp

SStteepp-ddoowwnn

SStteepp-ddoowwnn

SStteepp-ddoowwnnSStteepp-ddoowwnn

Transformer on power poles

Typicaldomestic

Transformer- Rectifier

unit

A lot of smallerdevices not only runon low voltage, but

often need DC insteadof AC.

Their transformersare also “rectifiers” to

produce directcurrent.

Impacts of the Development of Transformers on SocietyThe syllabus requires you to be able to discuss the impact of transformers on

society. Development of transformers was, of course, an integral part of thedevelopment of our modern electrical supply system, based on large power

stations and long-distance transmission of high voltage AC.

The history and social impacts have already been outlined... see page 15.

Power StationGenerators

produce 20,000V AC

High Voltagedistribution250,000 V or more

District Areadistribution132,000 V

Town or Suburbdistribution

11,000V

Neighbourhooddistribution

2,000V

Home Supply

240V

The purpose and function of a transformer is toa).........................................................................This allows electricity to be stepped-up tob)................. voltages for efficient, long-distancec)............................, and then d)............................again for convenient safe use by consumers.

The basic structure of a transformer is simple: itconsists of e)....... coils, called thef)........................ and .............................. The coilsare arranged one inside the other, with a core ofg).......................... in the centre. If h)....................electricity flows in the i)................................ coil,it creates a j)......................... magnetic field. This,in turn, k)....................... an EMF in the secondarycoil, at a l)............................. voltage.

A “step-up” transformer has more turns of wirein its m)............................ coil, and its outputvoltage is n)............................ than the input. A“step-down” transformer has more turns in itso)............................ coil and its output voltage isp).............................

In a perfect transformer, the input and outputq)................... will be equal, because of the Lawof r)................................. of ....................................This means that if voltage is stepped up, thens).......................... will be lower.

In reality, there are t)............................................in any transformer, mainly due tou)......................................

This is partly due to resistance in the coils, butmainly because of resistance to v)......................currents induced in the iron core. Once someheat is produced, the resistancew)................................. at higher temperatures.To minimise these energy losses:• transformers are designed to x).........................heat.• thicker wires in coils reducey).............................• the iron core is made from z)...............................sheets of iron, aa).............................. together tominimise the ab)......................... currents.

The electricity generated at a power station isusually stepped ac)................ to at leastad)........................ volts for long-distancetransmission. It will then be stepped-ae)..................... in 4 or 5 separate transformersat district, suburb and af)..............................levels, before entering your home atag).................. volts.

Even inside the home there may be manytransformers working. For example, aah)......................... requires about 1,500V andhas a step-ai).......... transformer to do this. Smallelectronic equipment often runs onaj).................. voltage ak)........................ current.These devices often have a “transformer -al)....................................” unit combined withtheir electricity plug.

24



1. A transformer has 2,000 turns in its primaryand 200 turns in its secondary coil. Input is 240VAC.a) Find its output voltage.

b) Is this a step-up or step down transformer?

2.At an electricity “sub-station” the voltage isstepped-down from 66,000V to 11,000V. Themassive transformer has 52,000 turns of wire inits primary coil. How many turns in thesecondary coil, to the nearest thousand.

Worksheet 10 TransformersFill in the blank spaces. Student Name...........................................

Worksheet 11 Practice ProblemsTransformers Student Name...........................................

3.The 240V plug-in “recharger” for a mobile phonecontains a transformer with 50 turns in itssecondary coil. Its output is 6.0V DC.a) How many turns in the primary coil?

b) Apart from being a transformer, what else mustthis unit do?

c) Explain why this “other” function must bedone after the step-down transformer function isachieved.

The Induction Motor PrincipleOne hundred years ago, when Edison’s DC systemwas “fighting it out” with Westinghouse’s AC system,one of the factors that finally led to a victory for AC wasTesla’s Induction Motor.

The Induction Motor works on this same principle:

• The Stator is a series of coils, fed with AC current insuch a way that the magnetic fields “rotate” by arippling on-and-off in sequence around the outside.

• The Rotor is mounted on an axle for rotation. Itcontains a laminated iron core to intensify magneticfields. The main part, however, is a copper frameknown as the “squirrel cage” because it resemblesan exercise wheel for a caged pet.

The moving magnetic fields produced by the statorcoils induce “Eddy Currents” in the squirrel cage.These can circulate freely in the copper cage, andproduce their own magnetic fields.

The squirrel cage fields interact with the rotatingstator fields such that the rotor experiences torque,and rotates to “chase” the stator fields.

25


4. MORE ON MOTORSkeep it simple science

®


N SMagnet

Magnet rotated underneath metal disk. (e.g. by attaching it to a drill)

Copper or aluminium disksuspended on a thread Disk begins

to rotate,“chasing”

the magnet

Practical InvestigationYou may have carried out an experimentsimilar to this:

ExplanationThe moving magnet induces “Eddy Currents” inthe metal disk. These in turn create their ownmagnetic fields. The magnetic fields interactwith each other so that the disk experiences atorque, and begins to rotate, “chasing” therotating magnet.

EEddddyyCCuurrrreennttss

ccaanncciirrccuullaattee iinn

tthheessqquuiirrrreell

ccaaggee

Features of the Induction Motor• No external current needs to be fed into therotor, so there is no need for any slip-rings orcommutator. This simplifies the motor, reducesmaintenance, and makes it less likely thatanything can wear out or need replacing.

Therefore, the motor is reliable and low-maintenance.

• The motor works only on AC, and rotates at a constant speedaccording to the frequency of the AC supply.

This can be a limitation, and means that gearsor pulleys are needed to run machinery eitherfaster or slower than the motor speed.

Apparently the simplicity and reliabilityadvantages far outweigh the limitations,because it is estimated that about 95% of themillions of electric motors in the world are ACInduction types!

Electrical Energy ConversionsThe final point to be made in this topic is thesame as one of the first points made in therelated Preliminary topic...

Examples: In the home, electricity is convertedinto:

• Heat, by stoves, toasters, kettles, etc.• Light, by light bulbs and fluoro tubes.• Sound, by hi-fi speakers.• Microwaves, in a microwave oven.• Radio waves, by a cordless, or mobile phone.• Infra-red waves, by a radiant heater.• Kinetic & mechanical energy, by a blender ordrill.

In industry, electricity is converted into:

• Radio waves, for radio & TV transmissions.• Kinetic & mechanical energy, in industrial

machinery, conveyors and elevators.• X-rays, for medical imaging.• Light, in laser beams for communication.

... and many more examples.

Electricity is so useful because it is so easily converted into

so many other energy forms,quickly, efficiently & cleanly

Rotor from aninduction motor

showing the“squirrel cage”

Multiple Choice1. A transformer has 100 turns in its primary coiland 400 in the secondary coil. Its output voltageis 1,000V.Which statement is true?

A. This is a step-up transformer, and input voltage=4,000V

B. Input was 250V, and this is a step-downtransformer.

C. It is a step-down transformer, with input = 4,000V.

D. Input = 250V, and this is a step-up transformer.

2. Which statement is correct for a “perfect”transformer with 100% efficiency?

A. The voltages in each coil are equal.B. The product of (voltage x current) is equal

in each coil.C. The currents in each coil are equal.D. A heat exchanger is needed to cool

the transformer.

3. In an AC induction motor:

A. magnetic fields in the squirrel cage rotorchase the rotating stator fields.

B. the rotor fields in the squirrel cage arecreated by current fed to the coils via a commutator.

C. the stator consists of curved magnets toprovide a radial field to give constant torque to the squirrel cage.

D. the squirrel cage is made of soft iron pieces, laminated to minimize the eddy currents.


4. (4 marks)A small step-down transformer-rectifier unit hasan output of 8.00V from 240V mains input. Itssecondary coil contains 60 turns of wire.

a) How many turns in the primary coil?

b) What is the purpose of the unit being a“rectifier”?

5. (6 marks)a) Describe the basic structure and operation ofan AC induction motor.

b) Assess the features of the AC InductionMotor.

26



Worksheet 12 Test Questions sections 3 & 4 Student Name.....................................

Remember that for full marksin calculations, you need to show

FORMULA, NUMERICAL SUBSTITUTION,APPROPRIATE PRECISION and UNITS




27

CONCEPT DIAGRAM (“Mind Map”) OF TOPICSome students find that memorising the OUTLINE of a topic

helps them learn and remember the concepts and important facts. Practise on this blank version.

MOTORS&

GENERATORS


28

Answer SectionWorksheet 1a) electric current b) forcec) magnetic field d) attracte) is in opposite direction f) repel g) lengthh) the product of the currentsi) inversely j) magnetick) force l) teslas (T)m) current n) lengtho) magnetic field p) (sine ratio of) angleq) Motor r) right angless) Right Hand Palm t) turningu) rotate v) magnetic fieldw) equal but opposite x) currenty) magnetic field z) currentaa) area ab) planeac) 0o ad) 90o

ae) coil of wire af) rotateag) stator ah) permanentai) electromagnet aj) electricalak) commutator al) reverse directionam) galvanometer an) torque ao) radial ap) vibrationaq) inter-acting

Worksheet 21. F = k.I1.I2L d

= 2.00x10-7x15.3x12.7 / 0.0100 = 3.89x10-3 Nm-1.The force will repel the wires.2.

F = k.I1.I2 and I1 = I2 = IL d

8.25x10-5 = 2.00x10-7 x( I2)/0.100∴∴ I = Sq.root(8.25x10-5x0.100/2.00x10-7 )

= 6.42 A.Since force is attracting, the currents must be insame direction.3. F = k.I1.I2 so, F=k.I1.I2.L/d, L d

so d=k.I1.I2.L/F=2.00x107x8.90x14.5x1.48/6.44x10-4

= 0.0593 m (= 5.93 cm).4. F = k.I1.I2 so F = k.I1.I2.L/dL d

= 2.00x10-7x30.0x30.0x25.0/0.0800= 5.63x10-2 N, attraction.

Worksheet 31.F = BILsinq

= 11.0x4.50x1.25xsin90o

= 61.9N.RH palm rule shows force is directed up the page.

2.F = BILsinq

= 22.7x5.95x0.385xsin60o

= 45.0N, directed vertically into the page.

3.F = BILsinθθ, So I = F/B.L.Sinθθ

= 27.2/105x2.44xsin90o

= 0.106A. (1.06x10-1A)RH palm rule shows current flows down the page.4.F = BILsinθθ, So Sinθθ = F/B.I.L

= 3.72/7.50x8.00x0.287= 0.2160

∴∴ θθ = 12.5o

Worksheet 41.ττ = nBIA.Cosθθ

= 200x5.25x1.50x1.20x10-3xcos0o

= 1.89 N.m.2.a) ττ = nBIA.Cosθθ

= 35x8.38x2.50x(0.20x0.20)xcos0o

= 29.3N.m.b) RH Palm rule shows left side force is up, rightside force down, therefore coil will rotate clockwise.3.

ττ = nBIA.Cosθθso, n = ττ / BIA.Cosθθ

= 3.86 / 4.60x3.20x0.00262xcos0o

= 100 turns.4.a) ττ = nBIA.Cosθθso, I = ττ / nBA.Cosθθ

= 12.3 / 12x22.3x(0.0800x0.0500)cos0o

= 11.5A.b) To rotate as shown, the force on left side of coilmust be downward. RH Palm Rule indicates currentmust flow clockwise around coil.

Worksheet 51. B 2. A

3.F = k.I1.I2.L/d

= 2.00x10-7x12.0x7.50x1.85/0.0100= 3.33x10-3N.

Force will repel the wires, since currents opposite.

4.a) i) Commutator reverses the direction of current inthe coil every 1/2 revolution. This is necessary tokeep the torque in the same direction, and keep thecoil turning.

ii) The stator provides the magnetic field around thecoil. It can be one or more permanent magnets, orelectromagnet(s).

b) A “radial” field has its field lines in a pattern like thespokes of a wheel... radii of a circle. This means thatthe coil lies “flat” in the field (q=0o) through mostrotation positions. Since the torque on the coil isproportional to Cosq, this ensures maximum torquethroughout the rotation.




29

Worksheet 61. D 2. C 3. C 4. A

5.F = B.I.L.sinθθ, so, B = F / I.L.sinθθ

= 3.25 / 9.00x0.750xsin90= 0.481T.

RH Palm Rule shows field is vertically upwards.

6.a) F = B.I.L.sinθθ,

= 8.00x5.75x0.0500xSin90= 2.30N.

RH Palm Rule shows force is into page.b) Zero, because current flows parallel to field lines.c) ττ = n.B.I.A.cosθθ

= 1x8.00x5.75x(0.0500)2xcos10o

= 0.113N.m.7.a) Pointer needle. This rotates with the coil to give themeter reading on the calibrated scale.b) Permanent magnet. This provides the magneticfield for interaction with the coil. It is shaped to givea radial field for constant torque at all positions.c) Coil. When current flows in this coil it interacts withthe field, and experiences a torque to turn the needle.d) Spring. This works against the coil, so the needlemoves only so far for each level of torque produced.Also returns the needle to the zero mark when poweris off.

Worksheet 7a) Faraday b) generatorc) magnet d) relativee) coil & magnet f) currentg) stronger h) closeri) faster j) reversesk) reverses l) Flux Densitym) magnetic field n) areao) EMF p) magnetic fluxq) rotate r) permanents) electromagnets t) inducedu) brushes v) slip-ringsw) motor x) current directiony) alternating current (AC) z) commutatoraa) reverse ab) sparkingac) slip ad) coilae) magnets af) external circuitag) resistance ah) currentai) high aj) lowak) transformer al) insulatorsam) ceramic an) lightningao) shield conductors ap) groundaq) Edison ar) DCas) power at) energyau) Tesla av) generators, motorsaw) Niagara ax) positive/beneficialay) labour az) electronic/computerba) communication, entertainment, businessbb) negative bc) coal/fossil fuelsbd) Greenhouse Effect be) Global Warmingbf) EMF/current bg) opposesbh) Conservation of Energy bi) back-EMFbj) braking bk) inductionbl) eddy

Worksheet 81. B 2. B 3. C 4. D

5. a) Magnetic Flux Density is a measure of the intensityof a magnetic field. It can be thought of as the numberof “magnetic field lines” passing through a given areaof space. It is commonly known as “magnetic field strength”and measured in teslas(T).b) “... the rate of change of magnetic flux through theconductor”.

6.a) Using a stronger magnet induced more EMF, somore current flowed. (The galvanometer showed agreater deflection)b) While the magnet was stationary the galvanometershowed no current flow.c) If moved at a more distant point there was lessinduced current.d) Reversing the pole reversed the current flow. (Thegalvanometer needle deflected in the oppositedirection)

7.(“Compare & Contrast” means you must point outsimilarities AND differences.)A simple motor and a simple generator are verysimilar in structure: both are built from• one or more coils of wire on an axle. (The “Rotor”)• a magnet to provide a magnetic field. (The “Stator”)• a set of brushes to connect the coil to an externalcircuit.• a commutator or set of slip-rings on the coil axle, incontact with the brushes.The major difference between them is their function:• A motor uses electicity to produce the rotatingmovement of the coil. Electricity >> Kinetic energy.• A generator produces electricity from the rotation ofthe coil, usually caused by the rotation of a turbine.

Kinetic energy >> Electricity.

Worksheet 91. C 2. A 3. B

4.(“Justify” means to give reasons why this is corrector true)AC electricity has many advantages that make it moresuitable than DC for large-scale electricity supply:• AC can be easily stepped-up or down in voltage bytransformers. This allows for efficient distributionover long distances at very high voltages (minimumenergy loss) and then convenient and safe usage byconsumers at lower voltage.• AC generators can be constructed without acommutator to carry current to the external circuit.This is more efficient than a DC generator, andrequires less maintenance, too.• AC induction motors are cheap, reliable andefficient.




30

Worksheet 9 cont.5.(“Assess” means to measure or weigh up the prosand cons)The development of AC generators has had impactson society that have been mainly beneficial, such as:• development of many labour-saving andconvenience devices (e.g. washing machines,elevators, power tools) which have generallyimproved peoples’ life-style.• development of electronic devices and computerswhich are the basis of modern communications,entertainment, business and finance systems.

On the other hand, the impacts on the environmenthave been mostly negative ones:• burning of fossil fuels in power stations is a majorcontributor to the Greenhouse Effect and GlobalWarming.• nuclear accidents, such as Chernobyl 1986, carryserious risks to the environment and to people.• Building dams for hydro-electricity alterswatercourses and submerges ecosystemsOverall, the benefits to life-style have been seen bymost people as worth the environmental destruction.This attitude is changing as the environmentaldamage becomes critical, and becomes a globalthreat.

6.a) It will act left, to counteract the motion.b) RH Palm Rule shows that current must flow up thepage (in order to produce a Motor Effect force to theleft.)c) Lenz’s Law.

7.• Electomagnetic braking is used to stop someamusement rides (e.g. “Space Shot” rides involvingvertical free-fall) and used in some trains.• It works because of Lenz’s Law.• The moving car/train has strong magnets attached.To brake the car, the magnets move past coppersheets. This induces “eddy currents”, and Lenz’s Lawensures that these currents will produce fields thatoppose the induction magnets. Thus the car isslowed evenly and efficiently.

Worksheet 10a) change the voltage b) higherc) transmission/distributiond) stepped-down e) twof) primary & secondary g) ironh) AC i) primaryj) fluctuating k) inducesl) different m) secondaryn) higher o) primaryp) lower q) power/energyr) Conservation of Energy s) currentt) energy losses u) resistance heatingv) eddy w) increasesx) lose/radiate y) resistancez) (many) thin aa) laminatedab) eddy ac) upad) 250,000 ae) downaf) neighbourhood ag) 240ah) TV screen ai) upaj) low ak) DCal) rectifier

Worksheet 111.a) Vp = np

Vs nsso Vs = Vp.ns/np

= 240x200 / 2,000= 24.0V.

b) Step-down, because output voltage is lower.

2. Vp = npVs ns

so ns = np.Vs/Vp= 52,000x11,000 / 66,000= 8,666≅≅ 9,000 turns.

3.a) Vp = np

Vs nsso np = Vp.ns/Vs

= 240x50/6= 2,000 turns.

b) Rectify AC to DC outputc) Must be done after, because DC cannot easily betransformed, while AC can.

Worksheet 121. D 2. B 3. A4.a) Vp = np

Vs nsso np = Vp.ns/Vs

= 240x60/8= 1,800 turns.

b) It converts AC input to DC output.

5.a) The stator is a series of coils which producemagnetic fields that “rotate” when supplied with ACcurrent. The rotor contains a laminated iron core tointensify the fields, but also a copper cage-likestructure known as the “squirrel cage”. The statorfields induce eddy currents which flow freely in thesquirrel cage and produce their own magnetic fields.These interact with the stator fields and “chase” theirrotation. The result is a constant torque, so the rotorrotates.b) The AC induction motor has no need for anycommutator or slip-rings so it is efficient and low-maintenance.A disadvantage is that it runs only on AC, and canonly run at one speed, determined by the frequency ofthe AC cycle.Overall, its advantages outweigh this limitation, sinceover 90% of all electric motors are of this type.



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