HSC Physics – Motors and Generators – dot point notes

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HSC Physics – Motors and Generators syllabus dot point summary

This is a set of HSC Physics notes for each syllabus dot-point of Motors and Generators. HSC Physics tutoring at Dux College provides students with the right support to achieve a band 6 result in HSC Physics.

Contents

Motors use the Effect of Forces on Current-carrying Conductors in Magnetic Fields

Factors affecting Magnitude of the Force on a Conductor

  • Discuss the effect on the magnitude of the force on a current-carrying conductor of variations in:
    • the strength of the magnetic field in which it is located
    • the magnitude of the current in the conductor
    • the length of the conductor in the eternal magnetic field
    • the angle between the direction of the external magnetic field and the direction of the length of the conductor
  • Solve problems and analyse information about the force on current-carrying conductors in magnetic fields using: Latex formula
  1. A current carrying conductor placed within a magnetic field will experience a force
  2. The direction of this force may be determined using the RHP Rule.
  3. The magnitude of this force is determined using the equation:

Latex formula

Therefore:

  • Force ∝ Magnetic Field Strength
  • Force ∝ Magnitude of current
  • Force ∝ Length of conductor within field
  • Force is sine of angle that current makes with magnetic field
  • For a point charge moving through a B-fieldLatex formula

current-carrying conductors in magnetic fields

Forces Between Parallel Current-carrying Conductors

  • Describe qualitatively and quantitatively the force between long parallel current-carrying conductors: Latex formula
  • Solve problems using: Latex formula

Quantitatively

Latex formula

The force between long parallel current carrying conductors exists because the current in one conductor interacts with the magnetic field set-up in the other conductor.

magnetic field set-up in the other conductor

Qualitatively

Parallel conductorsparallel conductors produce forces toward one another

  • I1 sets up a magnetic field as shown
  • If I2 is placed to the right of I1, it will experience a B-field as shown.
  • This B-field will interact with the I2 to produce a force toward I1.

Antiparallel conductors – antiparallel conductors produce forces away from one another.

  • If Current 2 is reversed, so the conductors are antiparallel, F is reversed.

Torque

  • Define torque as the turning moment of a force using: Latex formula

Torque is the turning moment of a force (turning effect)

Latex formula

  • It is the product of the tangential component of the force and the distance the force is applied from the axis of rotation.
  • If force is not tangential, then we must find the tangential component

torque figure

The Motor Effect

  • Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field
The motor effect is due to the force acting on a current-carrying conductor in a magnetic field.
  • A current-carrying conductor produces a magnetic field.
  • When the conductor passes through an external magnetic field, the magnetic field of the conductor interacts with the external magnetic field and the conductor experiences a force.
  • The direction of the force on the conductor can be determined using the RHP rule

Force Experienced by a Current-carrying Loop in a Magnetic Field

  • Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces
  • Solve problems and analyse information about simple motors using: Latex formula

The forces experienced by a current-carrying loop in a magnetic field depend on the orientation of the loop relative to the magnetic field.

  • Assume that the axis of a rectangular coil is perpendicular to a magnetic field and the long sides of the coil are parallel to the axis and equidistant from it. Current is travelling ABCD
  • Each side of the coil will experience a Latex formula
  • AB and CD will always be perpendicular to the B-field and will experience a force.
  • Sides BC and AD experience none as the current is parallel to the field.
  • Since current in BC is travelling in the opposite direction to AD, the direction of the force acting on the 2 sides will always be opposing one another.
  • The net effect is that the coil experiences a torque causing it to rotate clockwise.

Latex formula

Proof:The force acting on sides AB and CD is given by Latex formulaWhere b is the length of sides AB and CDThe torque is given by Latex formulaSubstituting for F in the expression for Torque: Latex formulaIn general when the coil is inclined at angle Latex formula to the field Latex formula

current-carrying loop in a magnetic field

Operation of a DC Motor

operation of a dc motor

The Main Features of a DC motor

  • Describe the main features of a DC electric motor and the role of each feature
  • Identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets
Part Description Role/function
Magnets
  • Form the stator as either permanent magnets OR electromagnets
  • On opposing sides of rotor
  • Usually curved (radial)
  • Provides external B-Field which interacts with the current in the coils to produce motor effect.
Iron core
  • Cylinder of laminated soft iron around which rotor coils are wound
  • Mounted on the axle, in the rotor
  • Concentrates the external B-field to maximise Torque
  • Laminations limit the size of eddy currents.
Rotor (armature) Coils
  • Usually several coils of insulated wire wound around the iron core.
  • Either end is attached to one half of the commutator
  • Transmits current which interacts with the B-field to provide torque.
Split ringCommutator
  • 2 metal half rings connecting either end of the rotor coils to the brushes
  • Provides points of electrical contact between coils and external circuit.
  • Reverses current every 180o – ensures unidirectional torque
Brushes
  • Spring loaded graphite blocks mounted on either side of the commutator
  • Points of electrical contact between the rotor and circuit
  • Allows free rotation of the coils without tangling the circuit.
Axle
  • Cylindrical bar of hardened steel passing through the centre of the armature through iron core
  • Provides the axis of rotation for coils

main features of dc motors

Application of the Motor Effect

  • Identify data sources, gather and process information to qualitatively describe the application of the motor effect in:
    • the galvanometer
    • the loudspeaker

The Galvanometer

A galvanometer is a device which measures very small currents
  • It has a rotor (consisting of the iron core, about which coils are wound) surrounded by radial permanent magnets on the stator.
  • A spring and pointer needle are attached to the pivot point
  • The current which is to be measure is supplied to the coils via the ext. Circuit
  • The current interacts with the B-field to produce a force by the motor effect.
  • The forces on either side of the coils are in opposite directions, producing a torque which rotates the coils.
  • This causes the needle to move along the scale and spring to tighten.
  • When the restoring force of the spring exactly cancels the force from motor effect, the needle comes to rest along the scale and gives a reading.

galvanometer figure

Curved Magnets

  • The curved magnets provide a radial B-field which ensures angle between plane of coils and the B-field is constant. i.e. Latex formula is constant

Since Latex formula

  • n, B, l, Latex formula are all constant, so Latex formula– a linear scale can be used.

Loudspeaker

A loudspeaker is a device which transforms electrical energy into sound energy.

loudspeaker figure

  • A loudspeaker consists of a circular magnet (one pole) and a central bar (opposite pole)
  • A conducting coil is wrapped about the central bar and is supplied with sound which has been converted into electrical signals.
  • The current in the voice coil interacts with the B-field to produce a force by the motor effect.
  • The changing direction of the electrical signal causes the voice coil to move in and out along the central pole. (direction determined using RHP rule)
  • The movement of the coil vibrates an attached speaker cone which in turn, vibrates air particles to produce sound.
  • The frequency of the signal determines pitch while the amplitude determines volume.

The Relative Motion between a Conductor and Magnetic Field is used to Generate an Electric Voltage

Michael Faraday’s Discovery of the Generation of Current

  • Outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet
Electromagnetic induction is the generation of an EMF (electromotive force) or current due to the relative motion between a conductor and a magnetic field.

Experiment 1

  • Faraday discovered EM induction by taking a conducting coil connected in series with a galvanometer.
  • He moved a bar magnet in front of the coil and found the following:
Action Result
One Pole moved in Needle deflected one way
Same pole moved out Needle deflected other way
Magnet at rest No deflection
Slower movement Less deflection
Faster movement More movement

bar magnet in front of the coil

Experiment 2

  • HE also did an experiment with t insulated coils wrapped around a soft iron ring. One coil was attached to a battery (DC) with a switch and the other to a galvanometer
  • When the power was switched on, the needle deflected one way, returned to zero and deflected the other way when the power was switched off.

insulated coils wrapped around a soft iron ring

Conclusions

  1. A changing magnetic field can generate an electric current in a coil
  2. Magnitude of potential difference depends on the rate of change

This led to the development of electromagnetic induction

Magnetic Field Strength and Magnetic Flux Density

  • Define magnetic field strength B as magnetic flux density
  • Describe the concept of magnetic flux in terms of magnetic flux density and surface area

The strength of a magnetic field B is the magnetic flux density:Latex formula

The number of flux lines Latex formula per unit area (A)

In SI units, B is measured in:

  • Tesla / Weber’s per sq. m = Latex formula

 Magnetic Flux

Magnetic Flux is the amount of magnetic field passing through a given area
  • It is represented as Latex formula and denoted by the number of field lines in an area
  • B Flux is the product of magnetic flux density and the perpendicular surface area. Latex formula

Induced EMF and Faraday’s Law

  • Describe generated potential difference as the rate of change of magnetic flux through a circuit
Latex formula

Where n is number of coils

Faraday’s Law:

  • When a conductor experiences a net change in magnetic flux (due to relative motion), an EMF is induced
  • EMF is equal to rate of change of flux through the circuit OR the electromagnetic work done on a unit of charge in a circuit

Lenz’s Law in Relation to Back EMF

Lenz’s Law

  • The relative motion between a conductor and a B-field causes a net change in flux threading the conductor and induces an emf. (Faraday)
  • This EMF induces a current which produces a B-field opposing the original change. (Lenz)

For example:

  • As the coil is removed from the field, the permeating flux is reduced and an EMF is induced (Faraday’s law).
  • The induced current flows creates a magnetic field to oppose the original change (Lenz).
  • Therefore, a clockwise current is induced in the ring to produce magnetic field into the page by the right hand grip rule for coils.
  • The current stops flowing when the entire ring has been removed from the external magnetic field.

relative motion figure

Conservation of Energy

Suppose that Lenz’s Law was reversed so that the current is set up to produce a B-field that magnifies the original change in flux.

  • This would increase the net change in flux threading the conductor. (ie Latex formula increases)
  • Since Latex formula, the magnitude of induced EMF would therefore increase -> magnitude of induced current would increase
  • This will continuously result in the creation of electrical energy, which violates the C.O.E
  • Thus Lenz’s law must hold true.

Back EMF

  • Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors
  • Explain that, in electric motors, back emf opposes the supply emf

In motors, the rotation of the rotor coils within the external magnetic field produces relative motion, between the two, resulting in a net change in flux threading the coils

  • By Faraday’s law, emf is induced in the conducting rotor coils.Latex formula
  • By Lenz’s law, this emf is produced so as to oppose the supply emf. i.e. It travels backwards

As the applied emf increases, so too does the applied current and hence the motor speeds up. This increasing speed produces an increasing back emf.

Latex formula

The motor eventually reaches a steady speed when the supply emf equals the back emf.

This results in zero net torque

Starting Resistance

  • When an electric motor starts the back emf will be small and so the net emf and current in the coil will be large.
  • To prevent the large current burning out the motor, a variable starting resistance is placed in series with the coil to increase resistance which reduces current.Latex formula
  • As the coil speeds up (and the back emf increases) the starting resistance can be decreased and eventually removed

Eddy Currents in Terms of Lenz’s Law

  • Explain the production of eddy currents in terms of Lenz’s Law
Eddy currents are small circular currents induced in the surface of a flat conductor when it is moving relative to a magnetic field.

A flat sheet of conductor moving relative to a magnetic field experiences a net change in flux, which by Faraday’s Law, results in an induced current

  • These currents are circular and are induced in the surface of the conductor
  • By Lenz’s law, they are produced in a direction so as to set up a B-field that opposes the original change in flux

magnetic field figure

Factors affecting Generated Current

  • Perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet
  • Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when:
    • the distance between the coil and magnet is varied
    • the strength of the magnet is varied
    • the relative motion between the coil and the magnet is varied
  • Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

A change in EMF is induced (and a current is generated) when there is relative movement between a conducting coil and a magnet and there a change in flux threading the coil.

Distance between the coil and magnet

  • At smaller distances, the magnetic field is denser, meaning more flux lines will be cut and so a greater EMF will be induced

Strength of the Magnet

  • A stronger magnet field will mean a greater amount of flux is cut, inducing a greater EMF

Relative motion between the coil and the magnet

  • Greater relative motion means faster change in flux and a greater induced EMF

Induction Cooktops

  • Gather, analyse and present information to explain how induction is used in cooktops in electric ranges
An induction cooktop uses a rapidly changing magnetic field to produce eddy currents in the base of a pan, causing it to heat up.
  • The induction cook top uses a coil attached to an AC source under the stovetop to set up a rapidly oscillating magnetic fields.
  • This changing B-field threads the bottom of a sauce-pan, setting up a net change in flux across the base of the pan.
  • This prroduces eddy currents in the surface of the flat conducting base, so as to oppose the original change in flux.
  • These currents heat the pan by resistive heating.

NB: Resistive heating occurs due to the collision of moving electrons (in the eddy currents) with nuclei in the metallic lattice structure, causing them to vibrate.

induction cooktop

Advantages Disadvantages
  • Energy efficient as there is no heat loss to external environment as in gas
  • Safe- the stovetop itself does not heat up, only the pan.
  • Expensive
  • Only ferromagnetic pans may be used
  • Steel/iron jewellery may heat up

Electromagnetic Braking

  • Gather secondary information to identify how eddy currents have been utilised in electromagnetic braking

Consider the rotation of a disk/wheel within a localized external magnetic field.

  • The relative motion between the B-field and the wheel causes a change in flux threading the wheel section adjacent to the B-field’s area of influence
  • This gives rise to the induction of eddy currents on the surface of the wheel
  • By Lenz’s law, a magnetic field which opposes the original change is created.
  • The induced currents interact with the magnetic field to produce a force by the motor effect.
  • This force opposes the rotation of wheel causing it to brake.

electromagnetic braking

Advantages Disadvantages
  • No Friction – no wear
  • Very weak at low speeds – requires supplementary conventional brakes

Generators are used to provide Large Scale Power Production

The main components of a Generator

  • Describe the main components of a generator
A generator is a device which transforms mechanical energy into electrical energy

Features of simple Generator

Part Description Role/function
Magnets
  • Usually form the stator as either permanent magnets OR electromagnets
  • On opposing sides of rotor
  • Usually curved (radial)
  • Provides and external B-Field which interacts with the torque input to generate current in the rotor
Iron core
  • Cylinder of laminated soft iron about which the rotor coils are wound
  • Mounted on the axle, in the rotor (moving part of rotor)
  • Concentrates the external B-field to maximise output.
  • Laminations limit the size of eddy currents.
Rotor Coils
  • Usually several coils of insulated wire wound about the iron core.
  • Either end is attached to one half of the commutator
  • Rotates within the B-field and have current induced within them.
Split ringCommutator
  • Electrical contact between either end of the rotor coils and the brushes
  • 2 metal half rings
  • Reverses direction of output current every half rotation to provide uni directional output
Slip ring commutator
  • 2 metal full rings
  • Connected to coils and brushes
  • Do not reverse direction of output current so conduct an AC output
Brushes
  • Spring loaded graphite blocks
  • Allows free rotation of the coils without tangling the circuit.
  • Points of electrical contact between the rotor and the ext. Circuit
Axle
  • Cylindrical bar of hardened steel passing through the centre of the armature through iron core
  • Provides Axis of Rotation for coils

 

AC DC
ac generator dc generator

ac and dc generator output

Structure and Function of a Generator and Motor

  • Compare the structure and function of a generator to an electric motor

Structure

Similarities

  • Each has a stator which provides the magnetic field
  • A rotor consisting of coils, an axle and a laminated iron core
  • Brushes, which contact the commutator and external circuit

Differences

  • Motors use an input of current via a voltage source to provide torque as an output, while generators use an input of torque via a handle to provide an output of current
  • AC motors and generators use slip rings while DC M&G use a split ring commutator.

Function

  • The function of a motor is to convert electrical energy -> mechanical energy
  • The function of a generator is the opposite. (mechanical energy -> electrical energy)

Differences between AC and DC Generators

  • Describe the differences between AC and DC generators
DC AC
  • Simple generator uses a split ring
  • Can use a multi-part commutator with coils place at regular angles to one another
  • Produces a rippling current through one set of wires(when flux through one coil is weakening, commutator switches to other set of coils)
  • Smooth/ relatively constant output for applications requiring fairly steady voltage.
  • Simple generator uses a slip ring
  • Can use many angled coils with separate commutators and brushes.
  • Industrially, an AC generator has magnets as the rotor and coils on the stator.
  • This produces multi-phase output current through different wires (to avoid interference)
2 phase DC output 3 phase AC output
2 phase dec output 3 phase ac output
dc generator output ac generator output

Energy Losses in Transmission

  • Discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer
Latex formula
  • Power loss in transmission lines is due to resistance
  • Power loss can be best reduced by minimizing the size of the transmission current

Locations at which power loss occurs

Generator

  • Resistance in coils
  • Eddy currents induced in iron core cause resistive heating
  • Friction between moving parts produces heat.

During transmission

  • Resistive heating in transmission lines
  • Some EM induced.

Transformer

  • Resistive heating in coils
  • Eddy currents causing resistive heating in iron core

Impact of Electricity Generation on Society

  • Assess the effects of the development of AC generators on society and the environment

Advantages

Accessibility
  • Using transformers with AC means voltage can be stepped up/ down to minimise power loss.
  • It can be transmitted over long distances allowing remote areas to gain access to electricity.
  • This increased accessibility allows electricity to be used for cooking, lighting, refrigeration, improving the standard of living
Affordability
  • Minimising costly power losses makes AC more affordable for the consumer.
Applications
  • AC has allowed more applications for electricity to be developed as it can be stepped up or down to meet voltage requirements.
  • Microwaves, air conditioning, computers increase ease and convenience of everyday life.
  • increased reliability and affordability of products
  • Medical imaging techniques -> improved ability to diagnose disease, thus better prognosis.
Aesthetics
  • Power stations can be located further away from population centres,
  • Fewer transmission lines are required at the end point, as AC electricity can be transformed
  • This is more aesthetically appealing for society.

Disadvantages

Society

Widespread use and increased demand for AC electricity has led to:

  • Increase risk of electrocution
  • Automation of industry decreases demand for unskilled labour -> unemployment
  • Increased dependence on electricity
  • Invention of new leisure activities such as TV and computer games -> obesity
Environment
  • Increased demand for AC electricity has led to the increased burning of FF’s to generate electricity. This has led to:
    • Increased emission of air pollution – enhanced greenhouse effect, photochemical smog
    • Formation and effects of acid rain
  • There has also been the destruction of natural habitats and loss of wildlife in order to mine/deforest and construct dams for hydroelectricity.

Assessment

Despite the disadvantages of AC electricity (e.g. dependence, electrocution and environmental detriment) the benefits of AC on society outweigh the negative effects.

Advantages and Disadvantages of AC and DC Generators

  • Gather secondary information to discuss advantages/disadvantages of AC and DC generators and relate these to their uses

Transformers

AC generators can be used with transformers allowing:

  • More efficient and larger distance power transmission
  • The use of fewer wires (more aesthetically appealing)
  • The ability to use electricity to meet many different voltage requirements

DC generators cannot be used with transformers, therefore opposite is true

Structure (Split vs. Slip)

AC generators use slip rings (advantage)

  • They have smooth, continuous surfaces which the brushes are constantly in contact with.
  • This means they do not wear down quickly, require little maintenance and are quite reliable.

DC generators use split-rings (disadvantage)

  • This means the brushes are constantly striking the edges of the commutator causing them to wear quickly and require regular maintenance.
  • As they wear, they also do not maintain proper contact reducing its efficiency.
  • Conductive objects may also become lodged in the gaps between half-rings causing sparking and reducing the quality of the output signal.

Function of Parts

In an AC generator designed for high current applications, the current is produced in the stator windings, rather than the rotor. (advantage)

  • It is much easier to draw the current through a fixed connection on the stator and means that the output current can be increased by adding coils.
  • This does not increase the load on moving parts so maintains the efficiency of the generator.

A DC generator generates current in the rotor (disadvantage)

  • Increased output can only be achieved with more coils on rotor – higher demands on moving parts.
  • This causes more friction, wearing and heat loss, decreasing the efficiency of the generators.
  • Drawing large currents through the brush-commutator connection increases the risk of sparks forming adding “noise” to the output signal.
  • This limits DC to fairly low current uses.

Output Signal

AC generators can be designed using 6 stator poles in pairs and a single electromagnetic rotor to produce 3 phase output.

  • This makes AC generators ideal for large scale industrial applications.

DC generators can be designed with multiple coils placed at regular angles to one another around armature.

  • A multipart commutator has brushes which are only ever in contact with the commutator parts corresponding to the coil.
  • The current “ripples” around a mean value rather than fluctuating between 0 and max.
  • This current is suitable for application requiring a steady voltage which cannot be achieved with an AC generator without rectifiers.

Competition Between Westinghouse And Edison

  • Analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities
In the late 19th century, Edison favoured generating and supplying DC, while Westinghouse promoted the use of AC electricity.
  • Initially, Edison’s DC was favoured because technology using DC was already established and it worked well over short distances.
  • However, DC could only be generated and distributed at one voltage leading to expensive energy losses over relatively short distances.
  • Many generators and transmission lines were required to supply a small population.

Advantages

  • AC electricity avoided these issues because it could be used with transformers.
  • Transmission could occur over longer distances at less cost with fewer generators and power lines and the electricity could supply a range of voltage requirements.
  • Tesla’s invention of the induction motor which only operated on AC only gave it more support.

Foul play

  • However, competition was not always open and fair.
  • Edison tried to show that AC is dangerous by electrocuting animals.
  • He tried to have AC used in the 1st electric chair
  • Edison also tried to have the use of AC banned

Result

  • However, in 1891, when Westinghouse secured the Niagra Falls commission and was able to supply all of NYC with only 10 generators (1896), DC was well and truly phased out.

Transmission line insulation and protection

  • Gather and analyse information to identify how transmission lines are:
    • insulated from supporting structures
    • protected from lightning strikes

Insulation from supporting structures

Prevent discharge or sparking between transmission lines and metal supporting towers

Suspension and chain insulation

  • Linked ceramic disc segments linked designed to increase the distance the current has to pass over, decreasing the risk of current leaks.
  • Less chance that dirt and grime will collect on the underside of the sections – dirt becomes conductive when wet.
  • Usually quite long, higher voltage – longer insulator.

metal supporting towers

Protection from Lightning

When lightning strikes, it will usually pass between the bottom of a thundercloud and the highest point on the earth below such as metal transmission towers.

Chain Insulators

  • Prevent conduction of lightning current from metal towers to power lines, hence avoids dangerous power surges.

Continuous earth wires

  • Carries no current usually, but can if there is a power surge
  • Runs from the top of the tower to the bottom and into the ground
  • Therefore, if tower is struck by lightning, current can be conducted directly into the ground.

Shield Conductors

  • Uninsulated cable with no current positioned higher than other transmission lines
  • Lightning more likely to strike these cables, pass to the towers and into the ground.

Towers

  • They are conducting structures which are taller than cables and are well earthed.
  • Lightning is more likely to strike the tops of towers, so charge can be dissipated into ground.

Transformers allow Generated Voltage to be either Increased or Decreased before it is used

Purpose of Transformers

  • Describe the purpose of transformers in electrical circuits
A transformer is a device which can increase or decrease AC voltages

AC source is fed into primary coil, setting up an oscillating magnetic field.

  • This is conducted by the iron core through to the secondary coil setting up a net change in flux threading the 2nd coil.
  • Since by Faraday’s Law, Latex formula then an emf and output current is induced in the 2nd coil

Purpose in electrical circuits

The domestic supply voltage in Australia is 240V single phase AC. Industrial and commercial supply is usually 415V three phase AC.

Many appliances are designed to operate on different voltages.

  • A common reason is that they are imported devices which are made to run on their country’s voltages
  • In this case a transformer is place between the mains supply and the device to reduce the supply voltage

Many devices contain components which need very different voltages

  • g. a microwave can use main supply to produce microwaves or may even require a higher voltage, the display panel , however requires a much lower voltage.

electrical circuits

Step up and Step down Transformers

  • Compare step-up and step-down transformers
  • Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced
Step up transformer Step down transformer
  • Secondary coil has more windings than in the primary coil.
  • greater voltage output, lower current
  • Secondary coil had less windings than in the primary coil.
  • lower voltage output, higher current

Turns ratio

  • Identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage
  • Solve problems and analyse information about transformers using: Latex formula

 

  • AC current in the primary coil sets up an oscillating B-field conducted by the iron core into the secondary coil – net change in flux through secondary coil
  • This induces an emf and current – more coils means a greater total EMF is induced in the coil
Latex formula

Conservation of Energy

  • Explain why voltage transformations are related to conservation of energy

Conservation of energy: Power in the primary and secondary coils of an ideal transformer are conserved

  • Under ideal conditions, same change in magnetic flux will be experienced in both coils
  • Therefore, the induced voltage can be influenced by the number of coils in the secondary coil
Latex formula
Latex formula

Transformers in Electricity Substations and Homes

  • Explain the role of transformers in electricity sub-stations
  • Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer
  • Gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use

Electricity is typically consumed at 240V within households and 415V in factories.

  • If these voltages were supplied without the use of transformers would occur at high currents, leading to large, expensive power losses.

Transformers allow power to be generated and stepped up to high voltages for long distance transmission.

Latex formula
Therefore, a low I minimises Latex formula
  • The transmission distance can be increased and lowers costs.
  • Transformers also allow voltages to be stepped down as electricity enters population centres
  • Transformers can also be used to step up/down voltages to meet different demands.
  • Therefore, one mains supply can be used to power many devices.

transformers

Impact of Transformers on Society

  • Discuss the impact of the development of transformers on society

Advantages

  • The way transformers step up/down voltage reduces power loss in transmissions
    • This allows electricity to be affordably transmitted over longer distances (e.g. remote areas)
    • This significantly increases their quality of life lighting, heating, refrigeration, cooking)
  • Ability to step up/down voltages means electricity can meet demands of many appliances
    • This has led to widespread use of appliances which increase the ease of tasks at home.
  • Fewer transmission lines are required to transmit electricity and supply different voltage requirements, making the electricity network more aesthetically appealing for societies.

Disadvantages

  • It has led to increased dependence on electricity
  • It has led to the automation of industry, reducing demand for unskilled labour – loss of jobs
  • Increased risk of electrocution
  • Increased demand – increased burning of fossil fuels, causing decreased air quality and causing respiratory irritation.

Overcoming heating

  • The iron core in transformers concentrates the magnetic field to maximise efficiency
  • As changing flux interacts with the core, eddy currents are induced, leading to resistive heat loss.
  • The heat:
    • Represents energy loss from system
    • Increased resistance leading to power loss
    • Can damage wiring in transformers if heat become excessive

Energy Loss

  • Heating due to eddy currents being induced in the iron core of the transformer
  • Increased resistance in the coil wires due to heating causes greater power losses

Preventing/ limiting eddy currents

  • Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome

Iron cores are made of laminated iron.

  • This sheets of iron separated by insulative material
  • Laminations cut the plane at the induced magnetic field
  • Limit the size of eddy currents – reduce overall heating.

Iron cores can be made of materials called ferrites (capable of conducting B- fields but not electricity)

  • Oxides of iron and other metals
  • Prevents the induction of eddy currents.

Cooling techniques

Once a transformer gets hot, it must be cooled to prevent overheating.

  • Transformers have cases with heat sinks which increase surface are for heat dissipation.
  • Transformer cases are made out of black or dark material to encourage the radiation of heat produced internally to the external environment.
  • Raise transformers above ground inside ventilated cases to improve air circulation
  • Keeping the transformer out of direct sunlight
  • Immersing the transformer in non-conducting oil – pumped around to absorb heat produced at the core quickly and move it to the outside where it can be dissipated to the external environment.

Motors are used in Industries and Home to Convert Electrical Energy into more useful forms of Energy

AC Motors

  • Describe the main features of an AC electric motor

Universal Motor

The universal motor is very similar to a simple DC motor (split ring) but the stator is formed by electromagnets in series with the rotor coils

  • Using AC: When the direction of current switches, it is switched in both rotor and the stator simultaneously

ac motor

DC Function AC Function
  1. It starts the same way as a simple DC motor
  2. Rotation continues
  3. When the split-ring commutator parts crossover, current in the rotor changes direction and unidirectional torque is maintained

 

  1. It starts the same way (no starting torque required)
  2. When the AC switches polarity, the direction of current switches in BOTH the rotor and the stator. The result is that torque remains the same direction
  3. Rotation continues (with switches occurring at 50Hz)
  4. When the commutator parts switch over, the same as with DC occurs
 dc function  ac function
  • Note that in a simple AC motor where a starting torque is required to accelerate the rotor until it matches the rate of AC signal switching
  • With a universal motor (on AC), this is unnecessary as the uni-directional torque is maintained by the stator electromagnets simultaneously switching polarity

AC Induction Motor

  • Perform an investigation to demonstrate the principle of an AC induction motor

In an induction motor, torque is produced by the interaction of a rotating magnetic field produced by the stator with a conducting rotor.

The rotor consists of conducting bars made of copper or aluminium

  • These are attached to two end-rings to form the squirrel cage rotor.
  • The case is encased in a laminated armature with a soft iron core, laminations to limit eddy currents and an axle to provide an axis of rotation while minimising friction.

The stator consists of three pairs of coils that have iron cores.

  • They are mounted on a cylindrical frame
  • Each pair of coils is attached to one phase of three phase AC
  • This sets up a magnetic field which rotates around the cylindrical frame.
  • As each coil goes through one phase of AC, the B-field rotates such that the coil with max B-field strength changes

ac induction motor

Function

Method 1 (diagram a):

  1. As the B-field rotates clockwise, it moves up past bar A, increasing the B-flux going left between Bar 1 and 2
  2. This creates an EMF (Faraday) and induces a current + B-field to oppose original change (Lenz)
  3. This induces a current into the page on bar 1 (and out on bar 2)
  4. The current in bar 1 interacts with the external B-field and experiences an upwards force due to the motor effect.
  5. The squirrel cage thus “chases” the B-field

Method 2 (diagram b):

  1. Consider a positive point charge inside bar 1 as the B-field moves up past it.
  2. This is the same as the point charge moving down past a stationary B-field
  3. It would experience a force due to the motor effect which pushes it into the page (RHP rule)
  4. Consider many positive charges forming a conventional current into the page which interacts with the external B-field to experience an upward force due to the motor effect
  5. Each bar will move up on this side and the squirrel cage thus “chases” the B-field
Friction

There is always a load on the rotor, leading to friction which slows down the cage.

  • Therefore, the cage can never reach the exact same speed as the B-field.
  • The speed difference is known as the slip speed.
Latex formula
  • This constant speed difference means there is always relative motion between the conductor and the B-field so torque is constantly produced.
Advantages Disadvantages
  • Simplicity of design
  • High efficiency and low maintenance
  • Ability to self-start
  • Relatively low cost
  • They must have AC supply
  • Their max speed is limited by the supply frequency
  • They have low starting torque
  • Their speed drops as the load increases.
  • If cage and B-field lose sync, torque will momentarily disappear

Energy Transfers and Transformations

  • Gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry
Energy transfer – the same energy moves to a different location
Energy transformation – energy changes its from within an appliance

Energy Transfer

  • Energy from the heating element in a kettle boils water.
  • Heat from burning of FF’s used to heat water or melt metals.

Energy Transformation

  • In the home:
    • Electrical energy ->light energy in light bulbs.
    • Electrical energy -> heat energy for heaters and cooking devices such as toasters
    • Electrical energy -> kinetic energy in fans
  • Industry:
    • Electrical energy -> kinetic energy which then drives the machinery used in the production of goods.
    • Electrical Energy -> EM radiation for X-rays in imaging the interior of motors, foundations. Etc
    • Electrical energy -> chemical energy in electrolysis
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