Motors and Generators.

Motors and Generators

Magnetic Fields - Review
Magnetic fields are produced by magnetised magnetic materials such as iron electric currents I

A magnetic material contains magnetised regions called domains if the magnetic domains are randomly oriented, the material is not magnetised if the magnetic domains become aligned, for example due to an external magnetic field, the material becomes magnetised

The magnitude of the magnetic field produced by an electric current depends on the magnitude of the current - the greater the current, the stronger the magnetic field the distance from the conductor - the greater the distance from the wire, the weaker the magnetic field the shape into which the conductor is formed - e.g. a coil

Magnetic fields are represented by magnetic lines of force direction of the field - indicated by an arrow pointing in the direction in which the north pole of a magnet points in the field magnitude of the field - indicated by the spacing of the field lines - closer spacing represents a stronger field

Magnetic fields are represented by arrows - closer spacing  stronger B field X crosses – representing a field into the page dots – representing a field out the page

Bar magnet Solenoid Straight conductor

Remember the directions using the right hand grip rule - but NEVER quote this rule in the exam - it is just a memory aid! Straight conductor

Remember the directions using the right hand grip rule - but NEVER quote this rule in the exam - it is just a memory aid! Animations - see RHRule.avi MagneticFieldWire.avi

The magnetic field “close to” the end of a bar magnet is uniform X S N B

X B The magnetic field inside, and “close to” the ends of a solenoid is uniform

Background - the cathode ray tube
A cathode ray tube (CRT) is a highly evacuate glass tube containing a source of electrons (cathode), and in which there is a strong electric field created by a high voltage between the cathode and a positive electrode (anode) at the opposite end of the tube. E e Electrons travelling in straight lines through the vacuum, are accelerated from the cathode to the anode by an electric field, E

A beam of electrons is called a cathode ray. Cathode rays are not visible, since electrons neither reflect or emit light under these conditions. A demonstration cathode ray tube contains a sloping phosphorescent screen, which emits light when electrons strike it, making the path of the electrons visible.

Cathode ray tubes (CRTs) are a key component in devices including Television receivers Computer monitors Cathode ray oscilloscopes (CROs) Medical monitors and other scientific equipment based on the CRO

Moving charges experience a force
Electric charges moving in a magnetic field experience a force except when they move parallel to the magnetic field X The force is a maximum when the charge moves perpendicular to the magnetic field - in this case the magnetic field is into the page v B + F The force is perpendicular to both the velocity direction and the magnetic field direction

The magnitude of the force depends on the magnitude of the charge the velocity of the charge the magnitude of the field the angle between the direction of the field and velocity

Electric charges moving in a magnetic field experience a force except when they move parallel to the magnetic field This effect is called the motor effect It is the principle underlying the operation of cathode ray tubes (used in TVs and computers) electric motors and generators loudspeakers

First-hand investigation of the motor effect
The motor effect can be demonstrated by placing a magnet near a cathode ray so that the field of the magnet is perpendicular to the velocity of the cathode rays X The observed result, which demonstrates the motor effect is the deflection of the cathode ray, in a direction perpendicular to the magnetic field and to the direction of the cathode ray velocity

First-hand investigation of the motor effect
The motor effect can be demonstrated by placing a magnet near a current carrying wire so that the magnetic field is perpendicular to the direction of the current flow in the wire The observed result, which demonstrates the motor effect is the deflection of the wire, in a direction perpendicular to the magnetic field and to the direction of the current X deflection e

This equation is not in the syllabus
Moving charges experience a force The force on a charge moving in a magnetic field is at right angles to the velocity of the particle the magnetic field F = qvB This equation is not in the syllabus A constant magnitude force, which is always perpendicular to the velocity of a particle, results in the particle travelling in a circular path

Motion of charges in the Van Allen belts
Charged cosmic rays encountering the magnetic field of the Earth experience a magnetic force, trapping them in regions called the Van Allen radiation belts Van Allen radiation belts

The spiralling paths of the charged cosmic rays is a result of the particles having components of their motion parallel to the Earth’s magnetic field, which is unaffected by the field, and perpendicular to the field, which causes the particles to travel in circular paths. The combined effect is a spiralling path.

High energy charged particles interact with the Earth’s atmosphere at high latitudes (polar regions) to produce auroras.

A force is produced on a current carrying conductor in a magnetic field, except when the conductor is parallel to the field. The magnitude of the force depends on and is directly proportional to the magnitude of the current, I the magnitude of the magnetic field, B the length of the conductor, l in the field the sine of the angle between the field and the conductor F = BIl B If the wire is at an angle to the field, the force is reduced by a factor of sin(q) I q F = BIl sin(q)

Calculate the maximum force on a conductor of length 5 cm in a magnetic field with an intensity of 2 x 10–4 T when the current in the wire is 200 milliamperes. The maximum force is exerted when the conductor is perpendicular to the field, and is give by the expression Write the equation first! F = 2 x 10–4 x 200 x 10–3 x 5 x 10–2 F = 2 x 10–6 newtons F = BIl The force is perpendicular to the current and the field directions

What is the direction of the force on the wire? How could the force be doubled without altering the length of the wire? Note first The force is perpendicular to the current and the field directions Therefore it must be either into or out of the page Use whatever aid to memory you have decided to use… The force is into the page Since F = BIl, doubling either the current or the magnetic field strength would double the force on the wire.

A force is produced between two parallel current carrying conductors
Force between current carrying conductors A force is produced between two parallel current carrying conductors I I X Bin . Bout force force The force is a force of repulsion when the currents are in the opposite directions k = 2 x 10–7 NA–2

Force between current carrying conductors
A force is produced between two parallel current carrying conductors force force I I X Bin . Bout The force is a force of attraction when the currents are in the same direction Two parallel conductors carrying an electric current will experience a force between them because each one produces a magnetic field, which acts on the moving charges in the other wire. In the above diagram, two wires are carrying an electric current from top to bottom. The magnetic field shown is that produced by the wire on the left. This magnetic field is a cylindrical field around the wire and only the part of the field in the plane of the page has been shown on the diagram for simplicity. If we now consider the charges carrying the current in the other wire, since they are also moving down, the force on them, and hence on this wire will be to the left or towards the other wire. The same argument can now be applied to the current in the left hand wire as the charges move through the field of the right wire and it will be seen that this results in a force of attraction to the other wire. Having deduced the first result however, the second could also have been deduced from Newton's third law “when a force acts on a body, an equal an opposite force acts on the body producing that force”. k = 2 x 10–7 NA–2

A force is produced between two parallel current carrying conductors The magnitude of the force between the conductors is proportional to the magnitude of the currents in each wire inversely proportional to the distance between the wires dependent on the magnetic properties of the medium between the wires The medium between the wires determines the value of the constant k = 2 x 10–7 NA–2 in air or a vacuum

A force is produced between two parallel current carrying conductors The force is a force of repulsion when the currents are in the opposite directions (a) The force is a force of attraction when the currents are in the same direction (b)

Calculate the force between two straight conductors separated by a distance of 1.5 cm with a common length of 35 cm between them when the current in one wire is 200 milliamperes and the current in the other is in the opposite direction, with a magnitude of 5000 microamperes. Write the equation first! Then substitute values… F = 4.7 x 10–9 newtons The force is perpendicular to the current and the field directions

Calculate the force between the side of a square coil consisting of 20 turns carrying a current of 2 A and a straight conductor sharing a common length of 25 cm and carrying a current of 3 A if the distance between them is 2 cm. Write the equation first! Then substitute values… F = 3 x 10–4 newtons The force is perpendicular to the current and the field directions

Q1. What is the force between two parallel conductors carrying currents in opposite directions one centimetre apart if the current in one is 10 amperes, in the other is 5 amperes and the common length is 2 m? (Ans N, repulsion) Q2. If the distance between the wires was increased to 2 cm, what would be the new force between the wires? (use the fact that force and separation are inversely proportional) Write the equation first! Then substitute values… F = newtons The force is one of repulsion

Torque syllabus A torque is a force which acts to produce a rotational effect or a moment. The magnitude of a torque, t depends on the magnitude of the force, F distance of the force from the point of rotation, d

Consider a wire carrying a current across a magnetic field, B as shown
The motor effect - force on a current-carrying wire syllabus Consider a wire carrying a current across a magnetic field, B as shown + The force on the moving charges in the wire is into the page [don’t say “down”] This produces a resulting force on the wire that is also into the page A wire carrying a current in a magnetic field experiences a force due to the movement of charges in the wire. The net result is called the motor effect

Torque on a current-carrying loop
syllabus A rectangular loop of wire carrying a current can be placed in a magnetic field so that the force on opposite sides of the loop results in a turning force about an axis between the two sides. The force on side WZ is into the page [never say “down” - it is ambiguous] The force on side XY is out of the page [never say “up” - it is ambiguous] P Q The net result is a pair of moments creating a torque which, given a suitable mechanical arrangement, may result in the loop’s rotation about the axis PQ

Torque on a current-carrying loop
syllabus The force is measured in newtons The distance is measured in metres P Q The torque is therefore in … newton metres (N.m)

A DC electric motor converts electrical energy to mechanical energy
Features of a DC electric motor A coil on which a torque is produced by the interaction of a current and a magnetic field can be arranged, with other components, to produce an electric motor A current-carrying coil A magnetic field A commutator A brush A DC current source The current in the loop produces a torque on the loop, causing it to rotate A DC electric motor converts electrical energy to mechanical energy

The importance of the commutator
The coil in position (a) experiences a maximum torque. The torque causes rotation, clockwise viewed from above At the position (b), no torque is produced Inertia carries the coil past the position shown in (b) and the commutator reverses the current direction so that the torque direction remains the same and the coil continues to rotate.

The importance of the commutator
syllabus Figure (a) viewed from above B . x The torque produced causes the coil to rotate clockwise . B x B x . Inertia causes the coil to rotate past the position (b) and the commutator reverses the current in the coil In position (b) there is no torque because there is no current

Principle of a DC electric motor
syllabus The commutator in an electric motor is the moving component of a motor, which provides an electrical contact between the external circuit supplying the energy to the motor and the rotating armature of the motor. Contact is achieved through brushes made of carbon, which make contact with the commutator via a smooth contoured surface matching the brushes to the commutator.

This photograph shows the commutator of a motor made using many coils
Principle of a DC electric motor Commutator This photograph shows the commutator of a motor made using many coils The use of many coils, each in a different plane, results in a more uniform torque being produced. The invention of the commutator was important for the development of electric motors, since it is the device that allows the current in the coil to be reversed every 180° so that the torque is always in the same direction.

A DC electric motor converts electrical energy to mechanical energy
Principle of a DC electric motor A DC electric motor converts electrical energy to mechanical energy

A DC motor’s operation is based on the principle that a current carrying conductor placed in, and at right angles to, a magnetic field tends to move in a direction perpendicular to the magnetic lines of force A rectangular coil of wire placed in a magnetic field such that two sides of the coil always carry a current perpendicular to the field will experience a torque due to the forces produced on the sides - the torque causes the coil to rotate A DC motor is similar in construction to a DC generator A DC motor may be made to act as a DC generator by mechanically turning the coil in the field (a DC generator is a DC motor operating “in reverse” - the energy transformation is reversed)

syllabus DC Motor

Components of a DC electric motor
DC Motor

Practical Exercise - Constructing a DC Motor
Making a DC electric motor Practical Exercise - Constructing a DC Motor

Motor effect and loudspeakers
LOUDSPEAKERS operate in essentially the same manner as an electric motor. A varying voltage (called the signal) is applied to a coil in a magnetic field. The coil is attached to a diaphragm, which moves as the coil in the magnetic field. This vibration causes the surrounding air to vibrate thus producing sound. Research A loudspeaker converts electrical energy to mechanical energy. Alternating current in the coil produces a force that moves the speaker cone correspondingly. It uses the motor principle.

Motor effect and loudspeakers
Because of the inertia of the speaker cone, speakers have to be designed differently to reproduce different frequency sounds. The more rapidly the speaker cone must vibrate, the lower the mass must be so that the force produced by the motor effect on the coil can change the motion of the cone very rapidly. LOUDSPEAKERS operate in essentially the same manner as an electric motor. A varying voltage (called the signal) is applied to a coil in a magnetic field. The coil is attached to a diaphragm, which moves as the coil in the magnetic field. This vibration causes the surrounding air to vibrate thus producing sound. Research

I The centre-reading galvanometer
A centre-reading galvanometer is a very sensitive DC ammeter. The zero marker on the scale is in the centre, and current readings can be either positive or negative. I + Such meters are built using the convention that if the meter produces a positive reading (pointer deflects to the right), then the current is flowing into the positive terminal of the meter.

Motor effect and galvanometers
spiral spring ELECTRICAL METERS are essentially the same as electric motors. A current flows in a coil, which is in a magnetic field. This produces a torque on the coil. The rotation of the coil is limited by a spring. The coil has a pointer attached, which points to a scale, which has been calibrated appropriately. Ammeters have a low resistance (ideally zero). They are made from a galvanometer which has a low value resistor in parallel so that most of the current flows through the resistor and the presence of the meter in the circuit does not appreciably affect the resistance of the circuit in which it is placed. Voltmeters are galvanometers with a high value series resistance. Thus when the voltmeter is connected in parallel with the device across which the voltage is being measured, very little current flows through the meter. Research A galvanometer operates on the same principle as an electric motor A current in the coil in a magnetic field produces a torque on the coil, which rotates Equilibrium is achieved by having a spiral spring producing an opposing torque

spiral spring The torque on the current carrying coil is proportional to the current in the coil Research

The torque on the current carrying coil is proportional to the current in the coil The torque turns the coil to which the pointer is attached Equilibrium is reached, and the measurement can be recorded, when the opposing torque of the spiral spring equals that of the electromagnetically produced torque on the coil Research

Faraday’s discovery of the generation of electric current
Background In 1820, Oersted discovered that an electric current in a wire produced a force on a compass needle placed near the wire… thus establishing a connection between electricity and magnetism. Michael Faraday discovered that a force was exerted on a current flowing in a conductor in a magnetic field [1821]. This is the principle behind the operation of all electric motors.

The credit for generating electric current on a practical scale goes to the famous English scientist, Michael Faraday. Faraday was greatly interested in the invention of the electromagnet, but his brilliant mind took earlier experiments still further. If electricity could produce magnetism, why couldn't magnetism produce electricity? Michael Faraday

While carrying out investigations on how a current in one coil could produce a current in another coil, Faraday discovered that a permanent magnet moved in and out of a coil produced an electric current in the coil. The current flows in one direction as the magnet is pushed into the coil, and in the opposite direction when the magnet is pulled out of the coil. In 1831, Faraday found the solution. Electricity could be produced through magnetism by motion. He discovered that when a magnet was moved inside a coil of copper wire, a tiny electric current flows through the wire. Of course, by today's standards, Faraday's electric generator was crude (and provided only a small electric current), but he had discovered the first method of generating electricity by means of motion in a magnetic field. Nearly 40 years went by before a really practical DC (Direct Current) generator was built by Thomas Edison. In 1878 Joseph Swan, a British scientist, invented the incandescent filament lamp and within twelve months Edison made a similar discovery in America. Swan and Edison later set up a joint company to produce the first practical filament lamp. Prior to this, electric lighting had been crude arc lamps. Edison used his DC generator to provide electricity to light his laboratory and later to illuminate the first New York street to be lit by electric lamps, in September Edison's successes were not without controversy, however - although he was convinced of the merits of DC for generating electricity, other scientists in Europe and America recognized that DC brought major disadvantages. syllabus

Magnetic field strength and flux density
Seeing the pattern made visible by sprinkling iron filings around the magnet... Faraday developed the model that magnets were surrounded by lines of force. Cutting the lines of force with a coil produced a current. Faraday deduced that the current produced by a magnet inserted into a coil was greater if there was a greater number of lines of force were cut by the coil.

The total number of lines of force is proportional to a quantity called the magnetic flux. The amount of flux passing through the square between the magnetic poles depends on the area of the square the angle it makes to the field the strength of the field

The amount of flux, or number of lines of force per square metre is called the flux density. Flux density is another term for magnetic field strength syllabus

Magnetic flux, flux density and area
The flux density of this magnetic field… exceeds the flux density of… this magnetic field The number of flux lines through the square is less in the bottom diagram syllabus

Generation of potential difference by changing flux
Faraday established that the magnitude of the current produced was dependent upon the number of lines of force cut by the conductor in unit time. Investigate how the current can be changed. first What factors does the number of lines of flux cut by the coil depend upon?

Generation of electric current using a coil and magnet
Three factors determine the number of lines of flux cut by the coil in a given time, and hence the magnitude of the current produced when the magnet is moved the number of turns on the coil the strength of the magnet the speed at which the magnet is moved Increasing any of these variables increases the current produced. The generated potential difference is proportional to the rate of change of flux through a circuit. first

Generation of potential difference
The flux through a circuit can be changed in many ways including.. X conducing loop v B moving a conducting ring or loop such that the flux through the loop varies, depending on the position of the loop. At which point/s in the movement of the coil from left to right does the flux change? first

There is no flux change as the coil moves through positions W, Y or Z
Generation of potential difference There is no flux change as the coil moves through positions W, Y or Z The flux only changes as the loop passes through these positions. As the loop moves through these positions, a potential difference is generated in the loop first

The flux through a circuit can be changed by… A potential difference is generated across the ends of the coil through which the flux changes moving a magnet in and out of a solenoid ac Flux changes in this coil changing the current in one coil,the magnetic field of which passes through another coil [see transformers]

The flux through a circuit can be changed by rotating a conducting loop in a magnetic field loop north south coil As the angle the loop makes to the magnetic field, changes, the flux changes accordingly.

A conducting rod sliding along a conducting loop generates a potential difference in the circuit - the loop plus the conducting rod. B X While the rod is moving, the flux enclosed by the current loop changes, inducing a potential difference causing a current to flow clockwise around the loop

Review - the field produced by a solenoid
The magnetic field produced by a current in a solenoid (coil) is similar in shape to the magnetic field surrounding a bar magnet A current flowing in a completed circuit through the coil in the direction indicated results in the right hand end of the coil being a north pole. S N Use the right hand grip rule as a memory aid … but never quote this rule in the exam… it is not a physical law!

Lenz’s Law When a flux change induces a current in a conductor, the induced current produces a magnetic field that opposes the change in flux that caused the current If the magnet is moved in the direction indicated, v… v S then by Lenz’s law, the left hand end of the solenoid must become a south pole, opposing the motion of the magnet - the movement of the south end towards the solenoid. The direction of the current in the solenoid can be deduced from the magnetic polarity and the direction of the windings of the solenoid. syllabus

Lenz’s Law When a flux change induces a current in a conductor, the induced current produces a magnetic field that opposes the change in flux that caused the current Reversing the direction of movement of the magnet reverses the direction of the induced current in the coil. Note that the polarity of the solenoid opposes the change that is inducing the current - not simply the polarity of the moving magnet.

Lenz’s Law What is the direction of the current induced in the loop by the moving magnet? The current induced must produce a magnetic field that opposes the movement of the bar magnet. N S The current produces a north pole to the left of the coil and a south pole to the right. The current must flow anticlockwise, viewed from the side of the approaching magnet, to produce a field, which opposes the change of flux produced by the movement of the magnet.

Lenz’s Law As the magnet approaches the ring the magnetic flux increases Bind Changing flux  induces current  magnetic field B OPPOSES flux change that created it The system behaves such that it tries to keep the flux constant induced B field to the left offsets increased B due to magnet

Lenz’s Law Consider a metal conducting loop in a magnetic field
A conducting rod is then slid along the metal conducting loop

Lenz’s Law As the conducting rod moves to the right…
The flux increases through the area enclosed by the circuit

Lenz’s Law The increasing flux induces a current
The current must produce a field that opposes the changing flux

Lenz’s Law A current must flow anticlockwise around the loop to produce a field out of the page, opposing the increasing flux I I

Lenz’s Law A current flowing towards the top of the page in the conducing rod results in a force on the rod towards the left, opposing its motion I Force

Lenz’s Law While the rod is moving to the right, the flux enclosed by the current loop decreases. The induced current in the loop must create a current, the field produced by which opposes the change in the flux. Hence it is into the page. The induced current in the loop must be clockwise to create a magnetic field into the page. This is consistent with Lenz’s law, since the flux of the field B through the current loop is decreasing, the field created by the induced current must counteract this decrease. B X I

Using a coil, magnet and a centre-reading galvanometer (a very sensitive ammeter), an electric current can be generated. When the magnet is pushed into the coil, a current flows in one direction - indicated by the deflection of the meter pointer in one direction. When the magnet is moved in the opposite direction in the coil, a current flows in the opposite direction - indicated by the deflection of the meter pointer in the opposite direction. first

Quiz Predict the behaviour of the meter if the magnet is passed through the coil in the direction indicated. The polarity of the meter connections to the coil is shown. A + –

Answer When the magnet begins to move into the coil, the left hand end of the coil becomes a north pole. The current must flow away from the positive meter terminal for this to happen. The needle therefore deflects to the left (negative). As the magnet passes through the centre of the coil the current reverses direction. As it leaves… A + –

Lenz’s Law and the conservation of energy
When a flux change induces a current in a conductor, the induced current produces a magnetic field that opposes the change in flux that caused the current Imagine that the motion of the magnet as shown resulted in the end of the solenoid becoming a north pole. v N This would produce a force on the magnet, causing it to accelerate towards the solenoid. This would increase the rate of flux change through the solenoid, producing a larger current and solenoid field, causing the magnet to accelerate more. This feedback cycle would result in an increase in the kinetic energy of the magnet and the current in the solenoid that required no further input of energy. This violates the law of conservation of energy. It is therefore impossible. Lenz’s law is a special example of the law of energy conservation.

Lenz’s Law and back emf Review
Outline the principle of operation of an electric generator. when a flux change is produced through an electric circuit or conductor, an emf is produced, resulting in an electric current if there is a complete circuit this is the principle on which an electric generator operates GENERATORS operate on the principle that if a wire is made to move so that it cuts through lines of magnetic force then the charges in the wire will experience a force on them which results in an emf being produced. This may in turn cause a current to flow in a circuit. A generator converts kinetic energy to electrical energy. It is important to note that the principle of the motor and the generator are identical. i.e. A charge which moves across a magnetic field experiences a force on it. In the case of the motor the charges are made to move in the first place by the application of an electric field. In the case of the generator the charges are made to move initially through the field by the application of a mechanical force. The generator principle in producing an electrical signal (voltage or current) from magnetic tapes and computer disks. The information is stored in the form of magnetic variations on the tape or disk. When the magnetised tape or disk is moved near the detector a flux change results in the generation of an electrical voltage. Variations in this voltage correspond to stored sound or digital information. When the plane of the coil is perpendicular to the field, the flux is a maximum As the coil rotates and the flux changes, an emf is induced

Review question Explain the principle of the electric motor
Identify the energy transformations occurring in an electric motor Current flowing perpendicular to a magnetic field in a coil of wire produces a force on two sides of the coil which results in a torque on the coil. The torque produces rotation of the armature about the axle on which the coil is situated. The purpose of an electric motor is to convert electrical energy to mechanical energy. Motors are less than 100% efficient and some of the electrical energy is converted to heat energy.

Lenz’s law and back emf An emf is produced in any coil of wire rotated in a magnetic field because of the change of flux taking place through the coil. back emf Hence an emf is produced as the coil of an electric motor as the motor rotates. The faster the rotation, the greater the emf. Question Do you think the emf produced as the rotor coil spins in an electric motor opposes or aids the rotation of the motor? Explain your answer. The induced emf in an electric motor opposes the rotation of the motor. If it aided the rotation, it would cause the motor to spin ever faster with no further input of energy - a violation of the law of conservation of energy.

Lenz’s law and back emf DC motor
As an electric motor turns, an emf opposing the applied voltage is produced in the rotor coil of the motor. This induced emf opposing the rotation of the motor is called back emf. As the speed of a motor increases, the back emf increases until the effects opposing the rotation of the motor - the load, friction and back emf - result in an equilibrium being reached causing the motor to spin at a constant speed. Motors sometimes have resistor built in, which limits the current that can flow in the rotor coil until the motor is spinning fast enough for the back emf to prevent the motor from burning out due to excessive current in the rotor coil.

Back emf - its relation to the supply voltage
The back emf produced in an electric motor opposes the applied supply voltage used to operate the motor back emf – + As the speed of the motor increases, the back emf also increases, opposing the effect of the applied emf and hence limiting the speed of the motor. The motor reaches an equilibrium at its operating speed, with the torque generated by the motor being in equilibrium due to the combined effects of the torque produced by the current in the rotor coil, the load the motor is driving, the frictional effects within the motor and the effect of the back emf syllabus

Back emf - its relation to the supply voltage
Demonstration Connect a small DC electric motor to a battery with an ammeter in series When the motor is first turned on, there is a surge of current. The current quickly stabilises at a lower value. When the motor is held so that it cannot turn, the current increases dramatically - 5 to 10 times greater. Explanation: When the motor is prevented from turning, there is no back emf and so the effective potential is greater than when the motor is running, so the current is greater. syllabus

Lenz’s law and eddy currents
Eddy currents are produced in any conductor through which there is a changing flux Definition Eddy currents are circulating currents, or current loops, within a conducting material, produced as a result of potential differences induced by a changing magnetic flux through the conductor. Lenz’s Law Applies A flux change through a conductor induces a potential difference The induced potential difference causes a current to flow The current produces a magnetic field This magnetic field opposes the original change in flux EDDY CURRENTS are produced in any conductor in a region of changing magnetic flux. The changing flux induces an emf in the conductor according to Faraday's law. Because the conductor has a low resistance, even though the induced emf may be small, the eddy currents can be quite significant. This is a problem in transformers because the eddy currents cause heat to be produced representing a loss of energy. The heat may also damage equipment. To reduce the induced currents, transformer cores are usually laminated (layers of metal) with insulation between the laminations. Some large transformers such as those found in our electricity network are cooled with oil. syllabus

Induction cook tops Induction cook tops induce an electric current in a special metal cooking vessel using the principle of Faraday’s law of induction. The induced currents are referred to as “eddy currents” The current in the cooking vessel directly heats the cooking vessel, because of its low resistance. There is no direct electrical contact between the cooking vessel and the cook top. current  heat A changing magnetic field, produced by alternating current in coils below the cooking surface induces voltages in the cooking vessel causing eddy currents induced voltage Changing B field AC current in coils Reference:

Induction cook tops Components of a CookTek Induction-Efficient Pan* (1) 18/10 stainless steel is non-reactive to food and easy to clean. (2,3,4) 1145 aluminium with a layer of 3004 aluminium between for even heat distribution. (5) 18/10 stainless steel for superior bond. (6) Magnetic stainless steel for efficient induction. (7) 18/10 stainless steel resists pitting and rusting. A changing magnetic field produced by coils in the cook top induce eddy currents in the metal cooking vessel * from CookTek Online Reference:

Induction cook tops Summary of operation 4. Currents produces heat
3. Induces voltage in metal vessel 2. Produces changing B field 1. AC current in coils Reference: syllabus

Eddy currents and switching devices
A ground fault circuit breaker uses a sensing coil to detect magnetic flux changes produced by eddy currents in an iron ring to switch off the current if there is an earth related fault. A sudden change in the current to the earth (ground) connection induces changing eddy currents in the iron ring. A current is induced in the sensing coil. This current activates the circuit breaker, isolating the device from the electricity supply. xx syllabus

Eddy currents and magnetic braking
Induced eddy currents produce magnetic fields opposing the change in flux that causes the potential difference. These opposing magnetic effects can be used to produce a braking effect on the object, the movement of which resulted in the changing flux that induced the eddy currents. Applications include slowing of maglev trains slowing the “space probe 7” ride at Australia’s Wonderland syllabus

Investigating eddy currents and magnetic braking
1. Using a neodymium magnet, hold the magnet just clear (1 mm) of a long non-magnetic metal surface (such as aluminium) and quickly move the magnet quickly across the surface. Describe your observations Explanation: The relative movement between the magnet and the metal conductor causes a change in flux through the conductor. Thus a current is induced in the conductor, which produces a magnetic field that opposes the changing flux produced by the movement - hence opposing the movement. The induced current, and hence the magnetic braking stops when the movement stops.

Eddy currents When a metal disk moves in a magnetic field the induced emf results in currents known as “eddy currents”

Eddy currents Eddy currents result in heating of the metal plate
Eddy currents can be reduced by cutting slots in the metal plate. The slots act like open switches and prevent the flow of eddy currents and hence the loss of heat energy. This principle is important in reducing eddy current heat losses in motors and transformers.

1. Drop a neodymium magnet through a copper pipe with a diameter just a little larger than the magnet. Describe your observations Explanation: As the neodymium magnet falls through the copper tube (a non-magnetic material), the changing flux induces eddy currents in the copper. I The eddy currents produce magnetic fields that oppose the cause of the flux change - the falling magnet, thus opposing the movement and causing the magnet to fall slowly through the tube.

The south end of the falling magnet is pointing upward and north end is facing downwards. To oppose this motion (the cause of the flux change through the copper tube), by producing an upward force, the induced eddy currents must produce a north pole above the eddy current as shown. S I The eddy currents produce magnetic fields that oppose the movement, causing the magnet to fall slowly through the tube. Eddy currents above the falling magnet will flow in a direction that results in the south pole being attracted upwards - as per Lenz’s law. syllabus

Eddy currents and electricity meters
Eddy currents are induced in a metal disc in the electricity meter. These cause the disc to rotate. syllabus

Identifying the main generator components
syllabus Identifying the main generator components magnets In addition to the identified components of the generator, there must be some method of mechanically turning the coil in the magnetic field. This is may be done using a turbine driven by water or steam, or a belt, as in the case of the alternator in a car. coil slip rings magnetic field brushes

Generation of alternating current
Three methods can be used to generate AC in the lab Move a magnet in and out of a solenoid in a circuit Move a long wire back and forth across the Earth’s field lines Rotate a coil, connected to a pair of slip rings, in a B field Which method would be most appropriate in a commercial power station? Three things are essential to generate a potential difference A conductor A magnet Relative motion first

Generation of alternating current
When the magnet is pushed in and out of the coil, an alternating current is induced in the completed circuit through the microammeter. Note! The conductor - the coil connected to a completed circuit The magnet Relative motion - the magnet or coil must be moved first

The Earth’s Magnetic Field
The Earth’s field is similar in form to that of a bar magnet. The angle the Earth’s field makes to the the surface of the Earth depends on latitude. Near the poles the field is almost vertical, while at the equator, the field is parallel to the ground. first

The Earth’s Magnetic Field at Different Locations
Around Sydney, the angle of the field is about 30° to the ground, pointing in a northerly direction. At the South Pole Sydney At the Equator A conductor perpendicular to the field, moved back and forth perpendicular to the field will have an alternating current produced in it if it is connected to a circuit. first

Rotation of a coil in a magnetic field
syllabus Rotation of a coil in a magnetic field first

Comparison of generator and motor structure
Generators and motors have the same key components. A motor can be run as a generator, converting mechanical to electrical energy - although not very efficiently.

Comparison of generator and motor structure
Generator structure Key components Magnets Coil Commutator or slip rings Armature, stator and rotor Motor structure Key components Magnets Coil Commutator or slip rings Armature , stator and rotor Generators and motors have the same key components. A motor can be run as a generator, converting mechanical to electrical energy

Comparison of generator and motor function
syllabus Demonstrations A loudspeaker (in principle the same as an AC motor) can be operated as a microphone (in principle the same as an AC generator) Connect the inputs of two moving coil galvanometers to each other and pick up one of the meters and gently rock it so that the needle moves back and forth. The needle on the other meter moves correspondingly, because the meter being rocked is acting as an AC generator, and the other meter is acting as a motor. (moving coil meters and motors work on the same principle - called the motor principle)

Production of alternating current using an AC generator
Describing the operation of an AC generator An AC generator consists of one or more coils in a magnetic field. When the coil is turned, usually by a turbine driven by water or steam, the change in magnetic flux through the coil induces a potential difference across the ends of the coil. In a coal fired power station, the generators are turned turbines operated by steam produced from a boiler. Production of alternating current using an AC generator A model hand generator can be used to produce an AC current. As the handle on the large wheel is turned rapidly, a belt turns a coil attached to the axle of the small pulley. This coil spins in the magnetic field provided by two permanent magnets, generating one cycle of an AC voltage with each revolution of the coil. The generated voltage can be connected to an external circuit via the slip ring commutators. A number of observations can be made when operating this simple AC generator. The faster the rotor coil is turned, the larger the emf generated The frequency of the AC voltage is the same as the frequency of rotation of the rotor coil. The change of frequency of the emf produced as the speed of rotation is increased can be observed by connecting the output of the AC generator to a cathode ray oscilloscope. Results of this are shown in the following images, the image on the left being the result of turning the generator more slowly. Two features should be observed. Increasing the rate at which the generator is turned increases both the voltage and the frequency of the generated emf CD Reference: Basic Electrical Knowledge.pdf.

Describing the operation of an AC generator In a simple AC generator consisting of a single coil having many turns, the output of the coil is connected by brushes to an external circuit by a pair of slip rings. slip rings Production of alternating current using an AC generator A model hand generator can be used to produce an AC current. As the handle on the large wheel is turned rapidly, a belt turns a coil attached to the axle of the small pulley. This coil spins in the magnetic field provided by two permanent magnets, generating one cycle of an AC voltage with each revolution of the coil. The generated voltage can be connected to an external circuit via the slip ring commutators. A number of observations can be made when operating this simple AC generator. The faster the rotor coil is turned, the larger the emf generated The frequency of the AC voltage is the same as the frequency of rotation of the rotor coil. The change of frequency of the emf produced as the speed of rotation is increased can be observed by connecting the output of the AC generator to a cathode ray oscilloscope. Results of this are shown in the following images, the image on the left being the result of turning the generator more slowly. Two features should be observed. Increasing the rate at which the generator is turned increases both the voltage and the frequency of the generated emf CD Reference: Basic Electrical Knowledge.pdf. The AC potential difference produced has a frequency equal to the frequency of rotation of the coil.

Describing the operation of an AC generator In the AC generator below, in which positition/s of the armature is there a potential difference between the slip rings? Identify the positive terminal. [ B and D ] + Production of alternating current using an AC generator +

Describing the operation of an AC generator Two Types of AC Generators Revolving armature rotor is an armature which is rotating inside a stationary electromagnetic field seldom used since output power must be transmitted through slip-rings and brushes Revolving field dc current is supplied to the rotor which makes a rotating electromagnetic field inside the stator more practical since the current required to supply a field is much smaller than the output current of the armature Production of alternating current using an AC generator

Describing the operation of an AC generator Revolving Armature Production of alternating current using an AC generator

Describing the operation of an AC generator Revolving Field Production of alternating current using an AC generator

Describing the operation of an AC generator syllabus The AC generator converts mechanical energy to electrical energy in the form of an alternating current Mechanical energy is used to turn a coil in a magnetic field The magnetic flux change through the coil induces a potential difference across the ends of the coil The circuit with the rotating coil is completed by slip rings that connect the rotating coil to the stationary external circuit Alternating current flows with a frequency equal to the frequency of rotation of the generator Production of alternating current using an AC generator

Describing the operation of an AC generator syllabus Production of alternating current using an AC generator Generation of AC electricity for commercial distribution See excellent website -

Transmission line energy losses
Energy losses occur from transmission lines because they have a small resistance and they carry a current. E = P x t = I2R x t The energy loss is proportional to the square of the current, and so high voltages are used on the transmission line to minimise the current, requiring the use of step-up and step-down transformers. The energy lost per second is called the power loss in a transmission line. Power = VI A specific power requirement can be provided by any suitable combination of voltage and current. e.g. a low current - high voltage or low voltage - high current could be used to produce the same result. But V = RI (Ohm’s law) Hence Power = V2/R or I2R Since the resistance of the power line is fixed, it is evident that a high voltage - low current combination results in less energy losses in the transmission line.

Describing the operation of a DC generator
A DC generator converts mechanical to electrical energy. Mechanical energy is used to rotate the armature / coil assembly of a DC generator e.g. using a steam or water turbine. The rotation of the coil in a magnetic field produces a flux change through the coil, which induces a potential difference across the coil. The ends of the coil connect to the external circuit via a split ring commutator. The DC generator produces a varying DC voltage, as shown in the graph.

To convert the output of a DC generator to a constant voltage, a smoothing circuit must be used. This typically contains a capacitor placed across the varying DC output. This capacitor stores energy while the voltage is high. When the voltage drops, the stored energy in the capacitor is released, maintaining the voltage at a constant level. capacitor

syllabus Identify the generator in the diagram below. What component makes this identification possible? This is a DC generator since the coil is connected to the external circuit using a split ring commutator.

Describing the operation of an AC generator syllabus A model generator - this one has a split ring commutator What kind of generator is this? This is a DC generator In what way would an AC generator differ from this generator? An AC generator has slip rings instead of a split ring commutator. Production of alternating current using an AC generator

Producing an electric current with a voltaic cell
syllabus A voltaic cell produces an electrical potential by exploiting the different attraction which atoms have for their electrons. Chemists call this the reduction potential of the element. A simple voltaic cell uses two different metal electrodes in a conducting solution called an electrolyte. Two electrolyte solutions are use in the cell shown here. (zinc and copper sulfate) first Note

Carry out an investigation to measure the potential difference produced by two different pairs of metals used in a simple electrolytic cell as shown below. Record which metal electrode was the positive electrode in each case. Tabulate your results, and those of other groups in your class. Cu(+)/Zn __ volts Fe(_)/Zn __ volts Cu(_)/Fe __ volts Ni(_)/Zn __ volts The maximum potential difference that can be produced by a voltaic cell depends on the reduction potentials of the two metal electrodes used. The potential differences produced in this experiment are always less than the maximum, because in practice, the potential difference depends on factors including - the concentration of the electrolyte - the temperature - the area of the electrodes - the separation between the electrodes first

syllabus Carry out an investigation to measure the potential difference produced by two different pairs of metals used in a simple electrolytic cell as shown below. Record which metal electrode was the positive electrode in each case. Typical results Cu(+)/Zn 0.9 volts Fe(+)/Zn 0.5 volts Cu(+)/Fe 0.4 volts Ni(+)/Zn 0.8 volts The maximum potential difference that can be produced by a voltaic cell depends on the reduction potentials of the two metal electrodes used. The potential differences produced in this experiment are always less than the maximum, because in practice, the potential difference depends on factors including - the concentration of the electrolyte - the temperature - the area of the electrodes - the separation between the electrodes first

Quiz Three different metal electrodes, A, B and C were used in pairs to make three electrochemical cells. A B Two of the results obtained are as follows. A and B 0.5 volts A positive A and C 0.2 volts C positive Predict the voltage that was produced using electrodes B and C. A C B C AC vs DC generators Transformers only work on AC, which is one of the great advantages of using AC generators. Transformers allow 240V to be stepped down to convenient levels for digital electronics (only a few volts) or for other low power applications (typically 12V). Transformers step the voltage up for transmission and down for industrial and domestic use. Without transformers, the waste of electric power in distribution networks, already high, would be enormous. It is possible to convert voltages in DC, but more complicated than with AC. Further, such conversions are often inefficient and/or expensive. AC has the further advantage that it can be used on AC motors, which are usually preferable to DC motors for high power applications (despite what the syllabus says!) C is positive relative to A A is positive relative to B Therefore C must be positive relative to B The voltage produced will be 0.7 volts

Quiz Predict, at the instant shown in the diagram, the potential difference across the ends of the loop. Justify your prediction. Hint: Look carefully at the direction of motion of the conductor in the magnetic field. AC vs DC generators Transformers only work on AC, which is one of the great advantages of using AC generators. Transformers allow 240V to be stepped down to convenient levels for digital electronics (only a few volts) or for other low power applications (typically 12V). Transformers step the voltage up for transmission and down for industrial and domestic use. Without transformers, the waste of electric power in distribution networks, already high, would be enormous. It is possible to convert voltages in DC, but more complicated than with AC. Further, such conversions are often inefficient and/or expensive. AC has the further advantage that it can be used on AC motors, which are usually preferable to DC motors for high power applications (despite what the syllabus says!) The potential difference is zero. Since the conductor at this instant is moving parallel to the magnetic field lines, there is no force produced on the charges in it, so there is no PD.

Comparison of benefits of AC and DC generators
syllabus Comparison of benefits of AC and DC generators AC generators produce a voltage that can be readily transformed AC generators are simpler and more reliable (they have fewer parts) AC can be used for operating motors suitable for a range of applications AC vs DC generators Transformers only work on AC, which is one of the great advantages of using AC generators. Transformers allow 240V to be stepped down to convenient levels for digital electronics (only a few volts) or for other low power applications (typically 12V). Transformers step the voltage up for transmission and down for industrial and domestic use. Without transformers, the waste of electric power in distribution networks, already high, would be enormous. It is possible to convert voltages in DC, but more complicated than with AC. Further, such conversions are often inefficient and/or expensive. AC has the further advantage that it can be used on AC motors, which are usually preferable to DC motors for high power applications (despite what the syllabus says!) Reference:

Comparison of benefits of AC and DC generators
syllabus Comparison of benefits of AC and DC generators Thomas Edison pioneered DC and George Westinghouse and Nikola Tesla championed AC. AC is easily transformed permitting transmission over long distances with less energy loss, but AC is more deadly than DC. Reference: “The war of the currents, or let’s Westinghouse him” by Ira Flatow Edison’s electric chair, using AC, was used in the 1892 execution of Charles MacElvaine. It was part of Edison’s public relations campaign to portray AC power as a menace to public safety. AC vs DC generators Transformers only work on AC, which is one of the great advantages of using AC generators. Transformers allow 240V to be stepped down to convenient levels for digital electronics (only a few volts) or for other low power applications (typically 12V). Transformers step the voltage up for transmission and down for industrial and domestic use. Without transformers, the waste of electric power in distribution networks, already high, would be enormous. It is possible to convert voltages in DC, but more complicated than with AC. Further, such conversions are often inefficient and/or expensive. AC has the further advantage that it can be used on AC motors, which are usually preferable to DC motors for high power applications (despite what the syllabus says!) More

The effects of generators on the environment
Generators need an energy input to operate, usually either Water turbine Steam turbine

Water turbines Usually require the building of large dams, which may arguably have a very significant effect on the environment e.g. Tasmania, Snowy Mountains, China, Brazil Large areas covered by water when dams are built inevitably involve the destruction of habitat and species. Rotting vegetable matter in dams contributes further to carbon dioxide and methane production - both greenhouse gases. Dams may interfere with the movement of animals, affecting breeding and food supplies.

The Three Gorges Dam in China… When completed in 2009, this will be the largest dam in the world.

The Three Gorges Dam in China… Environmental issues Inundation Flood concerns Increased earthquakes Water pollution Sedimentation Species affected Human resettlement

Brazil … Tucurui Dam Hydroelectric generators produce over 90% of Brazil’s electrical energy

Generators at a large Canadian hydroelectric power station
hydroelectric energy production Generators at a large Canadian hydroelectric power station Hydroelectric power station

Steam turbines Steam turbines can use existing bodies of water, without the need for dams, however the water needs to be heated requiring either… The burning of fossil fuels, coal (in NSW), gas or oil, resulting in the production of large quantities of carbon dioxide, as well as other pollutants. The carbon dioxide may be linked to global warming - it is a greenhouse gas. The operation of nuclear power plants (which are used to heat water to steam to operate steam turbines) resulting in radioactive waste products, which must be safely stored for long periods.

Cooling towers at a nuclear power plant
The effects of generators on the environment Cooling towers at a nuclear power plant Thermal pollution and gaseous emissions

Location of nuclear power stations

syllabus The effects of generators on the environment Steam turbines Steam turbines produce thermal pollution. Not all the heat produced can be used efficiently. Hot waste water must be cooled before it can be recycled into the environment. Despite cooling, water enters Lake Macquarie in NSW from adjacent power stations at a higher temperature than the lake itself. This promotes growth of algae, especially when it is in combination with nitrogen and phosphorous based chemicals entering the system from homes and farms, acting as fertiliser for the algae.

The insulation of high voltage transmission lines
Glass or ceramic insulators are used on both high voltage and lower voltage transmission lines. These can be seen on most electricity poles. The higher the voltage, the more insulation is required. detail

High voltage transmission lines
High voltage transmission lines are the backbone of the national electricity grid in Australia. Voltages are stepped up at the power station (from 23 kV typically produced by the generators to 330 kV or 500 kV) for transmission across the electricity grid. The electricity grid is a network of interconnected transmission lines and power stations. Finally, the voltage is stepped down progressively at points in the grid, ultimately to 240 V for domestic use. Some historical background Reference: About 1880 attempts were made to establish power transmission over long distances up to some 50 kilometres. At that time telegraph lines were the only infrastructures bridging such lengths. To overcome the voltage drop over such distances the cross section of the wire had to be increased in proportion to the distance. So some observers remarked that at the end a wire of the size of a barrel would be demanded. The alternative interpretation of Ohm's Law provided the more practicable solution of increasing the voltage at the sending end and covering the line losses in that way. In spite of all this, direct current machines were of such a quality and capacity that moderately small distribution systems were operated with success

High voltage transmission lines
Some historical background Reference: About 1880 attempts were made to establish power transmission over long distances up to some 50 kilometres. At that time telegraph lines were the only infrastructures bridging such lengths. To overcome the voltage drop over such distances the cross section of the wire had to be increased in proportion to the distance. So some observers remarked that at the end a wire of the size of a barrel would be demanded. The alternative interpretation of Ohm's Law provided the more practicable solution of increasing the voltage at the sending end and covering the line losses in that way. In spite of all this, direct current machines were of such a quality and capacity that moderately small distribution systems were operated with success

The higher the voltage, the more insulation is required
The insulation of high voltage transmission lines The higher the voltage, the more insulation is required ceramic insulators Detail of the ceramic insulators on this high voltage transmission line tower are shown on the left power lines conducting bypass

Reference: ElectricitySaskpower.pdf

Ceramic insulators on transmission lines are very varied in their design Identify the supporting structures, insulators and conducting wires in these photographs Generally the greater the voltage, the greater the separation must be the separation between the current carrying wires and the supporting structures. Additional reference: ElectricitySaskpower.pdf

high voltage transmission lines
Lightning protection of high voltage transmission lines syllabus High voltage transmission lines are excellent lightning targets because earthed conductors they are metal conductors they are usually the tallest object in the vicinity Transmission lines must be protected from lightning strikes because high voltage transmission lines strikes produce voltage surges that damage both the supply system and connected appliances they are expensive to repair damage interrupts energy supply High voltage transmission lines are protected by earthed conductors connecting the highest points of the supporting towers to each other

Human health and high voltage power lines
Some studies appear to show a weak association between exposure to power-frequency magnetic fields and the incidence of cancer however… Epidemiological studies done in recent years show little evidence that power lines are associated with an increase in cancer A connection between power line fields and cancer remains biophysically implausible "The scientific evidence suggesting that [power-frequency electromagnetic field] exposures pose any health risk is weak.” (A 1999 review by the U.S. National Institutes of Health ) "Laboratory experiments have provided no good evidence that extremely low frequency electromagnetic fields are capable of producing cancer, nor do human epidemiological studies suggest that they cause cancer in general.” (A 2001 review by the U.K. National Radiation Protection Board (NRPB)) Most of the concern about power lines and cancer stems from studies of people living near power lines (Q12) and people working in "electrical" occupations (Q15). Some of these studies appear to show a weak association between exposure to power-frequency magnetic fields and the incidence of cancer. However, epidemiological studies done in recent years show little evidence that power lines are associated with an increase in cancer (Q19A and Q19B, Q19H thru Q19K), laboratory studies have shown little evidence of a link between power-frequency fields and cancer (Q16), and a connection between power line fields and cancer remains biophysically implausible (Q18). A 1996 review by a prominent group of scientists at the U.S. National Academy of Science concluded that: "No conclusive and consistent evidence shows that exposures to residential electric and magnetic fields produce cancer, adverse neurobehavioral effects, or reproductive and developmental effects."(Q27E). A 1999 review by the U.S. National Institutes of Health concluded that: "The scientific evidence suggesting that [power-frequency electromagnetic field] exposures pose any health risk is weak."(Q27G). A 2001 review by the U.K. National Radiation Protection Board (NRPB) concluded that: "Laboratory experiments have provided no good evidence that extremely low frequency electromagnetic fields are capable of producing cancer, nor do human epidemiological studies suggest that they cause cancer in general." (Q27H) Reference: Reference: Power Lines and Cancer.pdf taken from

Human health and high voltage power lines
syllabus Do power lines produce electromagnetic radiation? To be an effective radiation source an antenna must have a length comparable to its wavelength. Power-frequency sources are clearly too short compared to their wavelength (5,000 km) to be effective radiation sources. Calculations show that the typical maximum power radiated by a power line would be less than microwatts/cm^2, compared to the 0.2 microwatts/cm^2 that a full moon delivers to the Earth's surface on a clear night. How do radiofrequency radiation and microwaves cause biological effects? A principal mechanism by which radiofrequency radiation and microwaves cause biological effects is by heating (thermal effects). This heating can kill cells. If enough cells are killed, burns and other forms of long-term, and possibly permanent tissue damage can occur. Cells which are not killed by heating gradually return to normal after the heating ceases; permanent non-lethal cellular damage is not known to occur. At the whole-animal level, tissue injury and other thermally-induced effects can be expected when the amount of power absorbed by the animal is similar to or exceeds the amount of heat generated by normal body processes. Some of these thermal effects are very subtle, and do not represent biological hazards. * Use of electric blankets or electric mattress covers was not associated with an increase in female breast cancer [C60]. * An experimental study in humans reports that exposure to a 200 microT 16.7 Hz field had no effect on melatonin levels, body temperature or heart rate [E31]. * An experimental study in humans reports that day-time exposure to a 100 microT power-frequency field had no effect on night-time melatonin levels [E32]. * Exposure of mammalian cells to a 1000 microT 50-Hz field caused no increase in chromosome damage (micronucleus assay), but did increase the incidence of chromosome damage induced by a chemical carcinogen [G108]. * Two studies of whether power-frequency field cause DNA damage to brain cells of mice. One finds DNA damage [G107], but the other does not [G109]. * Exposure of normal human cells and human breast cancer cells to 20 or 500 microT power-frequency fields for 1 or 4 days had no effects on cell growth or morphology [H66]. * A review of epidemiological studies of the effects of electromagnetic fields on adverse human reproductive outcomes concludes that no adverse effects have been demonstrated, but that additional research is needed [J24]. Reference: elf_guidelines.pdf Reference: Power Lines and Cancer.pdf taken from

The purpose and principle of a transformer
A transformer is an electromagnetic device consisting of two conducting coils, isolated electrically from each other, but linked magnetically. The primary purpose of a transformer is to change a voltage from one value to another, using magnetic induction A secondary purpose of a transformer is to isolate one part of a circuit from another physically and electrically, while allowing the transfer of energy from one part to the other TRANSFORMERS consist of two coils, electrically insulated from each other. A changing voltage is applied to one coil called the primary coil. This produces a varying magnetic flux through the secondary coil, thus inducing a current in the secondary coil. The emf across the secondary coil is related to the input voltage and the number of turns on each coil by the relationship np /ns =Vp / Vs np = number of primary turns ns = number of secondary turns Vp = primary emf Vs = secondary emf It can be seen that a transformer may either increase or decrease the applied voltage, depending on the relative number of turns on each coil. It is important to note however that the energy available out of the secondary coil is always less than the energy applied to the primary coil. Remember that energy is the product (voltage x current x time). The coils are usually wound around an iron core to 1. Increase the flux of the primary coil. 2. Improve the flux linkage between the two coils.

A transformer is a device that transfers energy by electromagnetic induction Primary and secondary windings (insulated from each other electrically) are wound onto a ferromagnetic core Used to raise voltage (“step-up transformer”) or lower voltage (“step-down transformer”) Voltage is raised when the primary winding has fewer turns than the secondary winding, and voltage is lowered when the primary winding has more turns than the secondary winding TRANSFORMERS consist of two coils, electrically insulated from each other. A changing voltage is applied to one coil called the primary coil. This produces a varying magnetic flux through the secondary coil, thus inducing a current in the secondary coil. The emf across the secondary coil is related to the input voltage and the number of turns on each coil by the relationship np /ns =Vp / Vs np = number of primary turns ns = number of secondary turns Vp = primary emf Vs = secondary emf It can be seen that a transformer may either increase or decrease the applied voltage, depending on the relative number of turns on each coil. It is important to note however that the energy available out of the secondary coil is always less than the energy applied to the primary coil. Remember that energy is the product (voltage x current x time). The coils are usually wound around an iron core to 1. Increase the flux of the primary coil. 2. Improve the flux linkage between the two coils.

Transformers operate on the principle of Faraday’s law 1. A changing current in the primary coil, produces a changing magnetic flux 2. A The changing flux from the primary coil, induces a potential difference across the secondary coil

syllabus The purpose and principle of a transformer 1. The changing current in the primary coil, is usually achieved by applying an alternating voltage, resulting in an alternating current (AC) 3. The field from the primary coil is intensified and concentrated (also referred to as increasing the flux linkage) through the secondary coil by an iron core AC output AC input 4.The changing flux through the secondary coil, induces a potential difference across the secondary coil 2. As the alternating current changes magnitude and direction, a magnetic field is produced, which changes in a corresponding manner

Investigation - examining a transformer
Transformers frequently have more than two coil windings. first Examine a transformer and record the input and output information using a diagram. Quiz What type of primary input is required to produce a secondary output on a transformer? Ouch!! Predict Would connecting a battery to the primary coil of a transformer produce an output voltage?

Investigation - modelling a transformer
Construct a simple transformer by winding a primary coil of 20 turns and a secondary coil of 100 turns onto a soft iron bar Apply a low voltage (2-4 volts) AC to the 20 turn coil Measure and record the input and output voltages Apply a low voltage (2-4 volts) AC to the 100 turn coil

Investigation - modelling a transformer
Typical results 20 turn coil input voltage (primary) __ volts 100 turn coil output voltage (secondary) __ volts 100 turn coil input voltage (primary) __ volts 20 turn coil output voltage (secondary) __ volts Faraday’s original transformer (left)

Model of Faraday’s experiment demonstrating induction
Faraday’s Observations of Induction Quiz Predict what will happen when the switch is closed. Explain the processes underlying your prediction. When you close the switch, a current passes through the first coil and the iron ring becomes magnetised. Note that the compass in the second coil deflects momentarily and returns immediately to its original position. The deflection of the compass is an indication that an electromotive force was induced causing current to flow momentarily in the second coil. When you open the switch, notice that the compass again deflects momentarily, but in the opposite direction. The closing and opening of the switch cause the magnetic field in the ring to change: to expand and collapse respectively. Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit. The generation of an electromotive force and current by a changing magnetic field is called electromagnetic induction. Model of Faraday’s experiment demonstrating induction

Faraday’s Observations of Induction
Three levels of answers: Elementary: The compass needle will be deflected when the switch is closed. Better: The compass needle will deflect momentarily, tending to align parallel to the axis of the coil in which it is located. It will then return to the position shown in the diagram. Best: The compass needle will deflect momentarily, with the north end, tending to align parallel to the axis of the coil in which it is located, and pointing to the right. It will then return to the position shown in the diagram. When you close the switch, a current passes through the first coil and the iron ring becomes magnetised. Note that the compass in the second coil deflects momentarily and returns immediately to its original position. The deflection of the compass is an indication that an electromotive force was induced causing current to flow momentarily in the second coil. When you open the switch, notice that the compass again deflects momentarily, but in the opposite direction. The closing and opening of the switch cause the magnetic field in the ring to change: to expand and collapse respectively. Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit. The generation of an electromotive force and current by a changing magnetic field is called electromagnetic induction.

Faraday’s Observations of Induction
Explanation Closing the switch causes a current to flow in the coil connected to the battery. As the current changes from zero to a steady value, the magnetic field intensity increases. The iron torus links the changing flux from the primary coil connected to the battery to the secondary coil. The flux change through the secondary coil induces a current, because the circuit is closed, producing a magnetic field parallel to the axis of the secondary coil, causing the compass to deflect in that direction. When you close the switch, a current passes through the first coil and the iron ring becomes magnetised. Note that the compass in the second coil deflects momentarily and returns immediately to its original position. The deflection of the compass is an indication that an electromotive force was induced causing current to flow momentarily in the second coil. When you open the switch, notice that the compass again deflects momentarily, but in the opposite direction. The closing and opening of the switch cause the magnetic field in the ring to change: to expand and collapse respectively. Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit. The generation of an electromotive force and current by a changing magnetic field is called electromagnetic induction.

What happens when the switch is opened after being closed?
Faraday’s Observations of Induction Detailed Explanation The current flows clockwise around the primary circuit when the battery is connected. N N This produces the magnetic polarity shown, with the magnetic field intensity increasing. I I The current induced in the secondary, must flow in the direction shown, producing a magnetic field with a flux opposing the increasing flux of the primary. What happens when the switch is opened after being closed? The magnetic polarity and the current of the secondary is therefore as shown. When you close the switch, a current passes through the first coil and the iron ring becomes magnetised. Note that the compass in the second coil deflects momentarily and returns immediately to its original position. The deflection of the compass is an indication that an electromotive force was induced causing current to flow momentarily in the second coil. When you open the switch, notice that the compass again deflects momentarily, but in the opposite direction. The closing and opening of the switch cause the magnetic field in the ring to change: to expand and collapse respectively. Faraday discovered that changes in a magnetic field could induce an electromotive force and current in a nearby circuit. The generation of an electromotive force and current by a changing magnetic field is called electromagnetic induction. The magnetic field inside the secondary coil increases, and its direction is to the right, deflecting the north end of the compass in that direction. When the current in the primary reaches a constant value, the flux change is zero, so no current is induced in the secondary coil. The compass returns to its original position.

Comparing step-up and step-down transformers
A step-down transformer has less turns on the secondary coil than on the primary coil By comparison, a step-up transformer has more turns on the secondary coil than on the primary coil

Step-down transformers are found in all electronic devices that can be run from the domestic 240 V AC domestic supply, since all electronic devices require low voltages to operate the semiconductor components. Step-down transformers are used in the electricity supply grid to reduce the high voltages (up to 330 kV) used when transmitting energy to lower voltages (240 V) for domestic use

Step-up transformers are used at power stations to convert the lower voltage generator output (~600 V) to higher voltages (up to 330 kV) for transmission across the grid. Step-up transformers are needed to convert 240 V to several thousand volts needed to operate fluorescent lights. TVs and other devices containing a cathode ray tube (CRT) require high voltages (up to 50 kV) for their operation. X-ray machines require high voltages, requiring the use of step-up transformers for their operation.

Compare the number of turns on the secondary coil to the number of turns on the primary coil of this transformer This transformer has less turns on the secondary coil than the primary coil since it is a step-down transformer The ratio of the number of turns on the primary to the secondary coil is 240:6.3

syllabus Comparing step-up and step-down transformers The induction coil A low-voltage, pulsed DC (6 V) applied to the primary coil (typically having less than a hundred turns), produces a high voltage (~30 kV) across the secondary coil (having thousands of turns)

Induction coil Pulsed DC is used because the rate of change of flux is much greater than that produced by a 6 V alternating current. + –

The induction coil voltage supply*
the applied DC voltage causes a current to flow the current produces a magnetic field the field attracts the magnetic reed switch the circuit is broken, switching off the current the reed switch springs back, completing the circuit I reed switch iron cored coil + – + – DC supply I * not explicit in HSC syllabus

Induction coil This induction coil is producing a voltage of about 20 kilovolts spark A device similar to this is used to produce the spark to ignite the petrol in a car engine.

Transformer primary-secondary voltage relationship
The voltage input and output of a transformer are related to the number of turns on the primary and secondary coils by np = number of turns on the primary coil ns = number of turns on the secondary coil Vp = primary voltage (input) Vs = secondary voltage (output) syllabus

Solving primary-secondary voltage problems
A transformer was found to be transform 220 V AC to 5.5 V AC. If the number of turns on the secondary coil was 20, how many turns must there have been on the primary coil? 220 V 5.5 V np = 800 turns

There are 126 of turns on the secondary coil of this transformer. How many turns are there on the primary coil? There are 4800 turns on the primary coil.

Transformers Identify the component labelled A and outline its function. Describe one structural feature of this component and explain the reason for this feature. A Component A is a soft iron core. Its two main functions are (1) to increase the flux through the primary coil and (2) to provide a more effective flux linkage between the primary and the secondary coils. The iron core is usually laminated to reduce eddy currents in the core caused by the flux changes through it, and thus to minimise heat losses and to increase the efficiency of the transformer.

The law of conservation of energy
Conservation of energy and voltage transformations The law of conservation of energy Energy cannot be created or destroyed, but only transformed from one form to another. Step-up transformers increase the voltage applied to the primary coil, resulting in a greater voltage across the secondary, however the energy available at the output of the secondary coil is always less than the energy applied to the primary coil. Analysing the energy relationships in a transformer… Energy is the product of power and time. Power is the product of voltage and current.

Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but only transformed from one form to another Input 240 V Output 12 V transformer energy input energy output energy losses The energy output of a transformer is always less than the input Energy losses occur because eddy currents induced in the transformer core by the alternating current, result in resistive heat losses (the transformer core heats up) The ratio of the energy output to the energy input, expressed as a percentage is called the efficiency of the transformer.

Energy cannot be created or destroyed, but only transformed from one form to another. Input 240 V Output 12 V transformer energy input energy output energy losses Consider a simplified case in which… The transformer output remains at 12 V, regardless of the load The energy losses (mainly heat) are constant, say 30% i.e. the efficiency of the transformer is constant at 70%

Energy cannot be created or destroyed, but only transformed from one form to another. Input 240 V Output 12 V transformer energy input energy output energy losses If the lamp is a 36 watt lamp (12 V) then the output current is 3 A What is the input current? The input power is 51.4 watts The input current is therefore 0.21 A [ I = P / V ]

Energy cannot be created or destroyed, but only transformed from one form to another. Input 240 V Output 12 V transformer energy input energy output energy losses If a 48 W lamp (12 V) is connected instead of the 36 W lamp, then the output current is 4 A. What is the input current? The input power is 68.6 watts The input current is therefore 0.29 A [P=VI]

Energy cannot be created or destroyed, but only transformed from one form to another. Input 240 V Output 12 V transformer energy input energy output energy losses The output current of a step-down transformer is more than the input current The output power of a step-down transformer is less than the input power If the load (resistance) on the output is decreased, the output current increases accordingly and the input current increases

Energy cannot be created or destroyed, but only transformed from one form to another. Input 240 V Output 12 V transformer energy input energy output energy losses What would be the effect of increasing the number of turns on the secondary coil, with the same light globe attached?

syllabus Conservation of energy and voltage transformations Energy cannot be created or destroyed, but only transformed from one form to another. Input 240 V Output 12 V transformer energy input energy output energy losses Increasing the number of turns on the secondary coil would increase the transformer output voltage With the same lamp, the current would be more and hence more power would be produced and the lamp would be brighter A greater output current would result in a greater input (primary) current Energy would still be lost in the core as heat [How much more?]

Electricity substations and transformers
Electricity generated at a power station is usually produced at a voltage ranging from a few hundred volts to 10s of kV (Eraring power station at Lake Macquarie has four 660 MW generators with an output of 23 kV. It is transformed to 330 kV or 500 kV for transmission over the grid. Transformers allow 240V to be stepped down to convenient levels for digital electronics (only a few volts) or for other low power applications (typically 12V). Transformers step the voltage up for transmission, as mentioned above, and down for safe distribution. Without transformers, the waste of electric power in distribution networks, already high, would be enormous. It is possible to convert voltages in DC, but more complicated than with AC. Further, such conversions are often inefficient and/or expensive. AC has the further advantage that it can be used on AC motors, which are usually preferable to DC motors for high power applications THE ELECTRICAL POWER GRID Domestic electricity is for the most part produced in power stations where generators are driven by a turbine. The turbine itself may be turned by steam produced by burning coal or heat from a nuclear reactor. Some turbines are driven by water falling from storage dams as in the Snowy Mountains Scheme. (Hydro electricity) Most generators produce alternating current. Advantages of AC include the 1. ease with which alternating current can be generated. 2. ease with which alternating current can be transformed from one voltage to another. The latter is necessary because, in order to minimise energy loss while the electrical energy is being transmitted, the voltage has to be high and the current low. For consumption however, the voltage needs to be low and so the voltage is stepped down by a transformer. High transmission voltages are used to minimise heat losses in the transmission lines.

Electricity substations are used to transform voltages in the grid
Electricity substations and transformers The voltage must be transformed to lower voltages, usually a few thousand for local distribution, and then to 240 volts for domestic use. Electricity substations are used to transform voltages in the grid

Electricity substations are used to transform voltages in the grid
Electricity substations and transformers The distribution of electrical energy involves substations responsible for stepping up voltage for transmission and stepping it down for use Electricity substations are used to transform voltages in the grid

Domestic appliances and transformers
Many domestic appliances today have semiconductor (electronic) components requiring low voltage DC for their operation. Electronically operated domestic appliances require both a transformer to change 240 volts to about 5-20 volts rectifier to change the low voltage AC to DC Appliances with no transformer kettle, hot water heater, toaster, older room heaters, hair dryers, incandescent lights, old model refrigerators, some clothes dryers Appliances with a transformer TV, stereo, computer, CD player, clock radio, fluorescent lights, home security systems, microwave oven, answering machines, air conditioner, fax machines, washing machines, microwave oven

Domestic appliances and transformers
A 2002 transformer development…

The need for transformers in the electricity grid
syllabus The need for transformers in the electricity grid Transformers are required at the power station to step up the relatively low voltage from the generators (100 V) to high voltages (330 kV) for distribution over the grid. High voltage transmission Transformers are required at local substations to step down the very high voltages from transmission lines to lower voltages (11 kV) for suburban distribution. Finally, local transformers step the voltages down further for domestic use (240 V) Suburban step down transformer

Voltage produced by the power station generators ~ 23 kV

MANY transmission lines to the step-up transformers

Step-up transformers at the power station ~ 330 kV

Step-down transformers for industry ~ 415 volts

Local distribution ~ 110 kV - 33 kV

Domestic transformer ~ volts

All energy comes in via the home meter box

Below-ground distribution is becoming popular Above-ground is still common

syllabus The need for transformers in the electricity grid Step-up transformer at a power station. A suburban step-down transformer

Eddy currents and energy losses in transformers
syllabus Eddy currents and energy losses in transformers To increase the magnetic flux produced by the primary coil of a transformer, a soft iron core is used The changing flux induces eddy currents in the iron core, which results in resistive heat losses, and therefore inefficiency of the transformer. To reduce the eddy currents, the core of a transformer is usually laminated, that is, made up of many layers of soft iron, electrically insulated from each other

Transformers and their impact on society
Analyse the impact on society of the development of transformers The first practical transformer, using AC, was developed in 1883 Prior to this, direct current was seen as being the logical way to distribute energy using electricity AC triumphed, and by the early 1900s, its future impact on society was inevitable Transformers permitted the long-distance transfer of electrical energy with low resistive energy losses Without the high voltages possible through the use of transformers, the electrical wires required to transmit large amounts of electrical energy would have to have been too large to be practical Transformers and AC Systems Reference: A decisive advance in alternating current working came as a result of the experimental work of Gaulard and Gibbs, who, in 1883, completed their first successful transformer or, as it was called in those days: `Secondary Generator'. With the coming of the transformer a struggle began between engineers in favour of the transformer in alternating current systems and a more or less conservative group of adherents of the well established direct current practice. In history this episode has been called `The battle of the Systems'. From the start alternating current was applied to the operation of lighting equipment such as, most commonly, arc lamps but also to the Nernst lamp which used magnesium oxide and to the vacuum bulb with its carbon filament. So before long many AC systems were established by local authorities and by private enterprises. In 1908 the biggest transformer was of a power of 5 MW but by 1925 the figure was more than ten times the earlier one. In the same year Gisbert Kapp said: 'Most of the electrical energy now being generated passes twice, or even more often, through a transformer. Thus the total transformer power installed, over the whole world, exceeds at all times the power that is generated, however much this might be'

syllabus Transformers and their impact on society Animation! electricToaster.avi

syllabus Transformers and their impact on society Transformers were a key to establishing electrical energy as the driving force behind technological and industrial development in the 20th century. Electrical energy rapidly became the means of lighting homes and cities, with its distribution facilitated by the use of transformers Electrically operated machines thus replaced less efficient machines, resulting in the rapid growth of industry and commerce Communication networks grew rapidly as a result of electrical energy and its intimate association with radio, then television and ultimately the computer revolution of the late 20th century Every home has dozens of appliances that make use of transformers, permitting a host of electronic devices to be operated from the mains

Describing the main features of the AC electric motor
The main features of AC motors current direction is reversed by the alternating voltage used, rather than by a split ring commutator as in a DC motor the motor speed is determined by the AC frequency, rather than by the magnitude of the applied voltage, as with a simple DC motor There are two common types of AC motor synchronous motors induction motors

Synchronous AC motors An alternating voltage is applied to the rotor coils via a pair of slip rings. The stator field may be produced by either permanent magnets, or it may be produced by a DC electromagnet. This type of motor is called “synchronous” because the speed is synchronised to the frequency of the applied voltage. The frequencies may be different, but they bear a simple relationship to each other. Heavy loads cause synchronous motors to slow down too much and the frequencies will no longer be synchronised - hence the motor does not work efficiently. It may fail altogether. The synchronous motor is essentially an AC generator operated in reverse. In a synchronous AC motor, the field magnets coils are wound on the rotor, and are supplied with a DC voltage, and the armature winding is supplied with an AC voltage. The variation of the voltage in the armature causes a torque on the rotor because of the reaction with the poles of the field magnets, and the makes the rotor turn at a constant speed determined by the frequency of the AC voltage in the stator. The constant speed of a synchronous motor is advantageous in some applications. However, in applications where the mechanical load on the motor becomes very great, synchronous motors cannot be used, because if the motor slows down under load it will “fall out of step” with the frequency of the supply voltage and as a result, stop. Synchronous motors can be made to operate from a single-phase power source by using a design that produces a rotating magnetic field. In applications requiring a motor capable of running a heavier load, three phase AC is used and the rotor/stator combination consists of three pairs operating on the same principle discussed above.

AC induction motor This is simplest and most rugged type of electric motor Induction motors have current carrying coils wound on the stator, and a rotor assembly which has no electrical connections to the power supply. The AC induction motor is named because the electric current flowing in the rotor is induced by the alternating current flowing in the stator. The power supply is connected only to the stator. The combined electromagnetic effects of the applied alternating current and the induced rotor current produce the torque The simplest of all electric motors is the squirrel-cage induction motor operated by an AC voltage supply. Induction motors are the most common type of electric motor used in heavy industry. The key to the operation of the induction motor is the manner in which current is supplied to the rotor. There are no electrical connections between the rotor and the electricity supply. As the motor's name states, induction is used to produce a current in the rotor. The rotor consists of a core in which are embedded a series of heavy-duty straight conductors arranged in a circle around the shaft and parallel to it. Steel laminations inside the “squirrel cage” increase the flux through the rotor. If these laminations are removed, the rotor conductors resemble in form the cylindrical cages once used to exercise pet squirrels or mice.

The simplest of all electric motors is the squirrel-cage induction motor operated by an AC voltage supply. Induction motors are the most common type of electric motor used in heavy industry. The key to the operation of the induction motor is the manner in which current is supplied to the rotor. There are no electrical connections between the rotor and the electricity supply. As the motor's name states, induction is used to produce a current in the rotor. The rotor consists of a core in which are embedded a series of heavy-duty straight conductors arranged in a circle around the shaft and parallel to it. Steel laminations inside the “squirrel cage” increase the flux through the rotor. If these laminations are removed, the rotor conductors resemble in form the cylindrical cages once used to exercise pet squirrels or mice.

Main features of the AC electric induction motor
Reference: Squirrel cage motors get their name from the appearance of early rotors. They are the most common type of industrial AC electric motor, being rugged and requiring neither a separate DC power source nor slip- rings. They are essentially constant speed devices when energised by a constant frequency AC supply, however electronic speed control is available. The majority of industrial squirrel cage motors are foot mounted Totally Enclosed Fan Cooled (TEFC), in which the motor internals are isolated from the surrounding environment by shaft seals etc, thus minimising the ingress of dust and moisture. The inevitable heat generated by I2R losses in the internal windings is transferred to the surrounds by air circulation within the casing together with cooling air blown by an integral fan along fins on the casing's exterior. Squirrel cage motors are not without their drawbacks, notably during starting when current drain is high. It is essential to appreciate in broad terms the thermal -electrical behaviour when selecting a motor, for a particular task, since the winding temperature dictates the lifetime of the motor. It is the motor's heat dissipation capability dictated by heat transfer and the integral fan which to a large extent determines the motor's maximum continuous mechanical power rating for winding temperatures commensurate with an acceptable life.

Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations. Rotor: The rotor is the rotating component of an induction AC motor

Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations. Rotor The rotor is the rotating component of an induction AC motor. The rotor consists of conducting cast-aluminium rods. These rods are short-circuited by end plates completing the so called “squirrel cage”, which rotates when the moving magnetic field induces current in the conductors.

Rotor The squirrel cage may have a laminated, cylindrical soft iron core, electrically insulated from the squirrel cage. The iron core intensifies the magnetic field of the stator, inducing a larger current in the rotor, resulting in a larger torque. The purpose of the laminations is to reduce heat losses due to eddy currents induced in the core, thus making the motor more efficient

A squirrel cage rotor

These two photographs show two views of the rotor from an AC electric motor. Shaft squirrel cage rotor These conductors run parallel to the shaft, the full length of the squirrel cage syllabus

Principle of the induction motor
Investigating the principle of the AC induction motor The neodymium magnets rotating above the metal plate produce a changing flux through the plate. Principle of the induction motor The flux change induces eddy currents in the plate. The eddy currents produce a magnetic field which results in an interaction with that of the rotating magnets, producing a torque on the metal plate, which rotates.

Discuss why most motors are AC induction motors
AC induction motors have a simple design, requiring no brushes, commutator or slip rings for their operation, since there is no electrical contact between the rotor and the power supply. AC induction motors, because of their simplicity are cheaper to manufacture as well as being very reliable. They are well suited to applications requiring a constant torque and rotational speed - common criteria in applications such as fans, fridges, washing machines, clothes dryers, air conditioners. Three phase AC induction motors Single phase is used in domestic applications for low power applications but it has some drawbacks. One is that it turns off 100 times per second (you don't notice that the fluorescent lights flicker at this speed because your eyes are too slow: even 25 pictures per second on the TV is fast enough to give the illusion of continuous motion.) The second is that it makes it awkward to produce rotating magnetic fields. For this reason, some high power (several kW) domestic devices may require three phase installation. Industrial applications use three phase extensively, and the three phase induction motor is a standard workhorse for high power applications. See also How It Works Electric Motor.pdf syllabus

AC electric motors and power tools
Despite what the syllabus states… AC motors are used for high power applications. Three phase AC induction motors are widely used for high power applications, including heavy industry. However, such motors are unsuitable if multiphase is unavailable, or difficult to deliver, as in the case of electric trains. Many electric train systems run on DC, because it is easier to build power supply lines requiring just one active conductor for DC. Some notes about the syllabus The syllabus says in one of the dot points: "explain that AC motors usually produce low power and relate this to their use in power tools". AC motors are used for high power applications. Three phase AC induction motors are widely used for high power applications, including heavy industry. However, such motors are unsuitable if multiphase is unavailable, or difficult to deliver. For instance, many train systems run on DC, presumably because it is easier to build power lines and pantographs if one only needs one active conductor. Single phase induction motors have problems for applications combining high power and flexible load conditions. The problem lies in producing the rotating field. A capacitor could be used to put the current in one set of coils ahead, but high value, high voltage capacitors are expensive. Shaded poles are used instead, but the torque is small at some angles. If one cannot produce a smoothly rotating field, and if the load 'slips' well behind the field, then the torque falls or even reverses. Power tools and some appliances use brushed AC motors. Brushes introduce losses (plus arcing and ozone production). The stator polarities are reversed 100 times a second. Even if the core material is chosen to minimise hysteresis losses ('iron losses'), this contributes to inefficiency, and to the possibility of overheating. These motors may be called 'universal motors' because they can operate on DC. The obvious solution (if multiphase is unavailable) is therefore to rectify the AC and use a DC motor. High current rectifiers are expensive, but are becoming more widely used. Reference:

DC operated electric drill
syllabus AC electric motors and power tools Power tools and some appliances use synchronous AC motors with brushes. Brushes introduce energy losses (plus arcing and ozone production). Power tools using AC induction motors produce low torque at low speeds - this can be a problem. Eddy currents induced in the rotor core result in energy losses and the possibility of overheating. These motors are sometimes called ‘universal motors’ because they can operate on DC as well as AC. DC operated electric drill DC And Universal Motors Your electric drill uses a different kind of motor. Where an induction motor induces current and its accompanying magnetic field in the rotor, DC and universal motors deliver current to the rotor, usually called the armature, through direct physical contact. The current is sent to the armature through carbon blocks, or brushes, that bear on a component called the commutator. The commutator routes the current through the armature coils where its magnetic field interacts with the field of the stationary winding. In its basic DC form, a 2-segment commutator is simply a switch that reverses the polarity of the current flowing through the armature. As such, it converts direct current to a form of alternating current, and the armature is impelled to spin as described for AC motors. In practice, the commutator has many insulated segments each connected to a coil on the armature to produce the rotating magnetic field that spins the armature. Universal motors are similar to DC motors but have modifications so they can run under AC as well as DC current.

AC electric motors and power tools
syllabus AC electric motors and power tools Power tools can operate on DC as well as AC however the simplest type use a DC motor.

AC induction motors and their advantages
syllabus AC induction motors and their advantages Simple design Low cost Reliable operation Features and Advantages of AC electric motors The main features and advantages of an AC electric motor are discussed below. Simple Design The simple design of the AC motor makeand results in a generally highly reliable device.The changing field by the 50 hertz AC line voltage causes the rotor to rotate around the axis of the motor. The speed of AC motor depends on three variables 1. The fixed number of winding sets (known as poles) built into the motor, which determines the motor’s base speed. 2. The frequency of the AC line voltage. Variable speed motors incorporate electrical components that permit the frequency can be changed to alter the speed of the motor. 3. The load on the motor, that results in slip. Low Cost The AC motor has the advantage of being the lowest cost motor for applications requiring more than about 300 watts of power. This is due to the simple design of the motor. For this reason, AC motors are overwhelmingly preferred for fixed speed applications in industrial applications and for commercial and domestic applications where AC line power can be easily attached. Over 90% of all motors are AC induction motors. They are found in air conditioners, washers, dryers, industrial machinery, fans, blowers, vacuum cleaners, and many, many other applications. Reliable Operation The simple design of the AC induction motor results in extremely reliable, low maintenance operation. Unlike the DC motor, there are no brushes to replace. If run in the appropriate environment for its enclosure, the AC motor can expect to need new bearings after several years of operation. If the application is well designed, an AC motor may not need new bearings for more than a decade.

Conversion of electrical energy
syllabus Conversion of electrical energy Gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into other forms in the home and in industry Discussion in class! Coils from an AC fan motor (right)

syllabus AC vs DC motors* AC most common type for power applications, simple, cheap, constant speed operation DC easily controlled, variable speed operation, with or without brushes, also function as generator Stationary windings (stator) commutator Brushes Armature (rotating unit) Internal view of the “Universal” motor used in the SKIL electric hand drill - the so-called universal motor can be operated using either AC or DC voltage * Not specifically required by syllabus, however AC motor advantages must be understood

The end

Westinghouse and Tesla
George Westinghouse was a famous American inventor and industrialist who purchased and developed Nikola Tesla's patented motor for generating alternating current. The work of Westinghouse and Tesla gradually persuaded Americans that the future lay with AC rather than DC (Adoption of AC generation enabled the transmission of large blocks of electrical, power using higher voltages via transformers, which would have been impossible otherwise). Today the unit of measurement for magnetic fields commemorates Tesla's name. Return

What chemists do The electrochemical cell shown here separates the two electrodes into separate solutions so that the chemistry of what is being done can be more easily understood. It is not necessary to have two different electrolytes in two containers in order to produce a potential difference. Return

Share resources with your fellow teachers and students.
A word from the creator This Powerpoint presentation was prepared by Greg Pitt of Hurlstone Agricultural High School. Please feel free to use this material as you see fit, but if you use substantial parts of this presentation, leave this slide in the presentation. Share resources with your fellow teachers and students.

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