HSC HSC HSC HSC PhysicsPhysicsPhysicsPhysics Revision Revision Revision Revision Module Module Module Module 2222 David Pham

# Motors and Generators 1. identify that the motor

effect is due to the force acting on a current-carrying conductor in a magnetic field

A current-carrying conductor produces a magnetic field. When this current-carrying conductor passes through an external magnetic field, its magnetic field interacts with that field, as any conventional magnet would. The conductor thus experiences a force, known as the motor effect, discovered by Faraday. The right-hand palm rule gives the direction of the force.

* perform a first-hand investigation to demonstrate the motor effect

• Pracs • See Attachments

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

A galvanometer is a device used to measure the magnitude and direction of DC current passing through a particular point, using the motor effect to measure the magnitude. It consists of a rotor, attached to a spring and pointer, and a scale. Around this are stationary magnets. The rotor has external electrical input and is wound around and iron core to increase the strength of the magnetic field.

The coil consists of many loops of wire, connected in series with the rest of the circuit such that the current in the circuit flows through the wire. When the current flows, the coil experiences a force due to the presence of the external magnetic field (motor effect). The magnitude of the force (and torque) is dependant on the amount of current flowing through the coil. The iron core of the coil increases the magnitude of the force and allows more precise readings. The needle is rotated (as it is attached to the coil) until the magnetic force upon the coil is equaled by a counteracting force of the spring, as it winds up and increases the torque it applies on the coil. The magnets around the coil are curved (radial), resulting in a uniform magnetic field, such that the plane of the coil will always be parallel to the field and the torque is kept constant no matter the deflection. This also allows for the scale of the galvanometer to be linear, with amount of deflection being proportional to the current passing through the coil. The scale is previously calibrated such that it is accurate. Loudspeakers are devices that transform electrical energy into sound energy. A loudspeaker consists of a circular magnet with north and south radial poles, where, in the space, a movable coil is positioned. This coil is connected to an amplifier which produces an alternating current, which will eventually create the sound.

When the current flows through the coil, it experiences a force which causes it to vibrate back and forth depending on the direction of the current, via the motor effect. For example, as in the diagram, with a clockwise current the direction of force upon the coil is into the page and anticlockwise current causes the coil to move out of the page. As the input current is an AC source, the coil vibrates sinusoidally. It is attached to a paper speaker cone, which produces the longitudinal sound waves we hear from the vibration. The stronger the sound, the higher the magnitude of the current is, as the coil moves more.

discuss the effect on the magnitude of the force on a current-carrying conductor of variations in: • the strength of the

magnetic field in which it is located

• the magnitude of the current in the conductor

• the length of the conductor in the external magnetic field

• the angle between the direction of the external magnetic field and the direction of the length of the conductor

The equation for the force on a conductor placed within a magnetic field thus is: From this equation, we can see that force on a conductor depends on a number of factors: • The strength of the external magnetic field. The force on the conductor is

directly proportional to this. • The magnitude of the current in the conductor. The force is once more

directly proportional to this. The direction of the current does not affect the magnitude of the force.

• The length of the conductor in the field. Once more, the force is directly proportional to this.

• The angle between the direction of current in the conductor and the external magnetic field, depicted in the diagram as θ. The force is proportional to the sine of this angle. If the conductor is parallel to the field, sin θ is zero, and hence there is no force. Similarly, if the conductor is perpendicular to the field, the force is the maximum possible for B, I, and l. Note that the angle must be between the conductor and the field, not any other angle.

The direction of the force can be found using the right-hand-palm-rule. In this case, the fingers represent the direction of the field lines (north to south), the thumb the direction of the current, and the palm represents the direction of force. In the diagram, the force is out of the page.

* solve problems and analyse information about the force on current-carrying conductors in magnetic fields using:

• Skill Remember the direction of force and direction of current, as well as the correct angle between the conductor and the field.

describe qualitatively and quantitatively the force between long parallel current-carrying conductors:

The relationship between the magnitude of the force, currents and distance between the conductors is:

Where F is the force experienced by each conductor (with the directions determined as below), k being the constant 2x10-7 N A-2, I1 and I2 the magnitudes of the current, d the distance of separation, and l the common length. The force is proportional to the common length and currents in each wire and inversely proportional to the distance of separation, with constant of proportionality k. The direction can be determined: Around any straight, long current-carrying conductors there exists a magnetic field, whose direction can be determined with the right-hand grip rule (the right

thumb points in the direction of conventional current while the fingers, curling around, show the direction of the magnetic field). This can be depicted as such, around a current I2.

If we place another conductor in this magnetic field, and use the right-hand palm rule to find the direction of force upon that conductor, we find it is attracted to I2, if the directions of current are the same. Conversely, I2 is attracted to this other conductor. Thus two long parallel current-carrying conductors (with current in the same direction) will attract each other.

If we place I1 in the opposite direction, however, it becomes apparent that the conductors will repel each other.

Thus, two long parallel current-carrying conductors (with current in the opposite directions) will repel each other.

* solve problems using:

• Skill Remember to include directions of forces, and calculate the force based upon common, not total length.

define torque as the turning moment of a force using:

A torque can be thought of as a turning effect of a force acting upon an object. It is easier to rotate an objet if the force is applied at a greater distance from the pivot axis, and also if the force is at right angles to the line joining the pivot axis to its point of application, as can be noted in daily life (it is harder to open a door with the same force, near to the hinges rather than further away, for example). Thus this torque is defined as:

The torque is the product of the force and the distance from the pivot axis. However, this only applies in the case where the force is perpendicular.

A sinø term needs to be introduced so situation such as:

Notice at ø=90o, the torque reduces to the first equation.

describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces

There are several possible positions for the coil to be in, and the forces on the coil can be worked out for each situation by consideration the direction of the current and magnetic field. From this, the torque and hence net result of the force can be determined. In this series of rotations, the magnetic field is to the right and conventional current enters from the right graphite brush.

The initial position of the coil is (a), where use of the right hand palm rule shows the forces acting upon the coil, with KL experiencing an upwards force equal in magnitude to which MN experiences downwards. The forces are equidistant from the axle and perpendicular to the line joining the axle to the place of application of the force. This means the torque is at its maximum, such that the coil rotates clockwise, until it reaches position (b). During its travel, the sine of the angle between the force and the plane joining the pivot point to the point of application of the force, so the torque decreases over time until it reaches a minimum at (b). Here the resultant forces don’t create a torque. However, the momentum of the coil keeps it rotating and soon the commutator will change the direction of the current, at (c), which is the coil just afterwards. As the direction of the current has now changed the direction of the forces on side KL and MN are opposite to before, such that it keeps the coil rotating in the same direction. Here we can appreciate the necessity of the commutator, as otherwise the direction of the rotation would not stay the same.

The torque steadily increases as the coil rotates to position (d), essentially half a period of revolution away from the initial position. It is essentially the same except sides LK and MN are switched, and the torque is at its maximum once more again. Similarly, (e) is a reflection of (c). After this the coil moves back into the initial position again, to continue its rotation and powering.

The graph of torque versus angle is a cosine curve:

The forces acting on sides MN and KL are the same throughout the rotation, however. Note that sides KN and ML don’t experience appreciable force in the initial position as they are parallel to the magnetic field. However, this changes in rotation due to the gradually increasing angle. As the forces aren’t in the direction of motion, there is no effect on the movement of the rotor as the rotor is not free to move in any direction other than rotation. Also note that the torque remains in the same direction (the graph is an absolute value cosine curve) due to the commutator.

* solve problems and analyse information about simple motors using:

• Skill

Recall the equation for the force on a current-carrying conductor:

Real motors have many wires coiled around at once, so the force is given by:

However in this case the conductor is always perpendicular to the field so

Thus the torque, Fd sinø, is given by

Therefore, as

From this it can be seen that there are several ways to increase the speed (related to torque) of a motor: • Increasing the force on each side of the coil. This can be done by:

• Increasing the current in the coil (or increasing voltage of external circuit) • Increasing the number of loops of the coil • Producing a stronger magnetic field with the stator • Using a soft iron core as part of the armature, in the centre of the loop.

This concentrates the magnetic field and increases the torque. • Increasing the width of the coil • Using more than one coil mounted on the armature • Increasing the number of coils wound onto the armature (with extra split-ring

commutators as necessary) • Having stators with curved magnetic poles allows for a constant field and

smoother operation, as well as keeping the torque at its largest for the longest possible duration

describe the main features of a DC electric motor and the role of each feature

identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets

An electric motor is a device which transforms electrical potential energy into rotational kinetic energy. They produce this rotational kinetic energy by passing a current through a coil in a magnetic field. The DC electric motor can be simplified into several key components: • A source of emf and an external circuit with resistors if necessary. • A split-ring commutator to change the direction of current as necessary, with

carbon brushes connecting it to the conventional external circuit. • Stationary magnets. These can be either permanent conventional magnets or

current-carrying coils, which can be more convenient as they can run on the same electricity source. As these are stationary, they are called stators.

• A coil (many wires are used to increase torque) wounded on an armature (a frame). This is free to rotate on an axle, and is thus known as the rotor. The armature axle protrudes from the casing, enabling the movement of the rotor to do work. There is also an insulator to separate the coil from the axle.

The function of the emf source and the external circuit is to allow a current to flow in the internal circuit. This allows for a force to be produced inside the coils and hence a torque, which in turn adds rotational kinetic energy. However, note that in a DC motor the current that flows in the external circuit is in one direction; but the internal circuit would suffer problems due to this, as the direction of force upon the rotor would not remain the same and there is no net rotation produced, as once the coil moves through a half-rotation the direction of current would force it to move back.

A split-ring commutator is a mechanical switch that automatically changes the direction of current in the commutator when the torque falls to zero. It consists of a split metal ring, each part of which is connected to either end of the coil. As the coil rotates, first one ring then the other makes contact with the brush. This reverses the direction of current through the coil and keep the torque, and hence rotation, in one direction. Conducting contacts called brushes connect the commutator to the DC source of emf. Graphite is used in the brushes because it conducts electricity and acts as a lubricant. This also prevents the wires from being tangled. The magnetic field of a motor is provided by stationary magnets (permanent or temporary). This magnetic field allows for a force to be applied to the coils and thus creates the motor effect. They are fixed to the body of the motor, and electromagnets can be produced by coiling wire around a soft iron core. The same current can be used for the rotor and electromagnet, as shown:

Part Description Function

Stator (magnets) A pair of fixed magnets – possibly permanent or electromagnets

Provides magnetic field

Rotor (armature and coil) Many wires (the coil) wound onto a frame (the armature) to form the rotor

Creates rotational kinetic energy from torque

External Circuit Conventional DC circuit, ending in graphite brushes, with emf source

Provides emf, driving the current through the coil

Brushes/Commutator Split metal ring, with graphite brushes connecting it to the emf source

Keeps the rotation in one direction and prevents wire tangling

2. outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet

Michael Faraday carried out a series of experiments which showed that moving magnets can generate electric currents, following his 1821 discovery that a current-carrying conductor experiences a force within a magnetic field (motor effect). His discovery of electromagnetic induction eventually led to the means of electrical generation today. Faraday had carried out several experiments to show the nature of electromagnetic induction. In his first successful experiment Faraday set out to produce and detect a current in a coil of wire by the presence of a magnetic field set up by another coil. 70m of copper wire was wound around a block of wood, with a second length wound in between the gaps of the first wire, separated with twine. One coil was connected to a galvanometer while the other was connected to a battery.

When the battery circuit was closed, Faraday observed that there was a sudden and very slight deflection at the galvanometer, meaning a very slight current was detected in the secondary circuit. A similar (but in the opposite direction) deflection was observed if the battery circuit was stopped. He was careful to emphasise that the current was temporary, only when a change in current was experienced. Modifications of this experiment were followed up by Faraday. He showed that, by winding the secondary coil around a glass tube with a nail in it, that a current was produced in the secondary circuit as the needle was magnetised. However, it was only magnetised when the current was switched on or off, not when running or off. His second experiment involved a soft iron ring, with a primary coil wound around it on one side and a secondary coil, connected to a galvanometer, on the other. When the current was set up in the primary coil, the galvanometer needle reacted rapidly, greater than if no iron core was present. Similarly to his above experiment, however, the needle soon came to rest and only deflection when current was turned on or off.

He concluded when the magnetic field of the primary coil was changing, a current was induced in the secondary coil. He was also able to show that moving a magnet near a coil could generate an electric current in the coil, or taken away. When it is brought close or taken away, there is a current induced but when it is stationary no current is induced, similar to the above experiments in that electric currents are only induced when there is a change in the magnetic field density (magnetic flux). Moving a magnet away creates the opposite effect to moving it closer, as does moving the opposite pole. For example, moving a south pole away has the same effect as moving a north pole of equal strength towards it at the same speed.

Another observation in this experiment Faraday made was that the degree of deflection depended on the speed at which the magnet moved. The faster a magnet approaches the grater the induced current is.

Subsequently Faraday attached two wires through a sliding contact to touch a rotating copper disc located between the poles of a horseshoe magnet. This was the same as moving a magnetic field near an electric circuit. This induced a continuous direct current. Faraday had invented the first electric generator. Prior to this, continuous electricity could only be produced by batteries or galvanic cells.

define magnetic field strength B as magnetic flux density

describe the concept of magnetic flux in terms of magnetic flux density and surface area

Electromagnetic induction is the creation of an emf in a conductor when it is in relative motion to a magnetic field, or is situation in a changing magnetic field. This effect is caused by the change in magnetic field strength, or magnetic flux density, B. The magnetic field can be represented diagrammatically using field or flux lines, where the closeness or density represents the strength of the magnetic field. It can be imagined as a magnetic field ‘flowing’ from the north to south poles of a magnet. It is measured in T (Tesla) or weber per square meter (Wb m-2). The magnetic flux is the amount of magnetic field passing through a given area (can be thought of as field line density), denoted ΦB, and is measured in Weber (Wb). If a particular area A is perpendicular to the field lines B, then

ΦB = B⊥A

If they are not perpendicular, the flux is equal to the equivalent amount of flux passing through perpendicularly (the component which is perpendicular, calculated via trigonometry). The magnetic flux density, which, as above, is denoted as B, is the amount of such flux per unit area.

We can see that the close the magnet is to the coil, the more flux threads through it. It is because of this that Faraday’s experiment with relative movement between the magnet and coil produced an induced current, to counteract the change in magnetic flux. An increase in the strength of the magnetic field can also change the number of field lines threading through a coil.

describe generated potential difference as the rate of change of magnetic flux through a circuit

For a current to flow through the galvanometer in Faraday’s experiments there must be an electromotive force (emf, symbol E or ε). Faraday noted that there had to be change occurring in the apparatus for the emf to be created – the amount of magnetic flux threading through the coil. The rate at which the magnetic flux changes determines the magnitude of the induced emf. This was noted because the faster the magnet moved to the coil or away, the more current was induced and the greater the change in flux overall, the more current was detected. This leads to Faraday’s law, which states that the induced emf in a circuit is equal in magnitude to the rate at which the magnetic flux through the circuit is changing with time. That is,

The negative sign indicates the direction of induced emf, explained by Lenz’s law, and . Since ΦB = B⊥A, a change in ΦB can be caused by a change in the magnetic field strength, B or in the area of the coil that is perpendicular, or both. The n represents the number of turns of wire in the coil, as the total emf would be greater. As such, rotating coils in magnetic fields will experience changing levels of flux and thus can generate electricity.

account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors

explain that, in electric motors, back emf opposes the supply emf

Lenz’s law is a result of the law of conservation of energy. It states that an induced emf always gives rise to a current that creates a magnetic field that opposes the original change in flux through the circuit. This is also present (as the negative sign) in Faraday’s law. The case for Lenz’s law can be shown via contradiction. If the opposite of Lenz’s law, rather, were true instead, the principle of conservation of energy would be violated, as energy cannot be produced spontaneously, only transformed. This is such because if a change in flux created a similar change in flux (in the same direction, via the induced current), this in turn would induced further current and eventually create infinite ‘free energy,’ increasing in magnitude due to its own energy, without work. Thus to create electrical energy in a coil, work must be done, in this case coming from part of the energy used to move the magnet to (or from) the coil. Another method of proving Lenz’s law can be seen through consideration of total energy in a coil moving into a magnetic field.

The coil, moving with initial velocity v, has total energy equal to its kinetic energy. However, as it enters the magnetic field, it experiences a change in flux and thus a current is induced in the coil. This electromagnetic energy, in addition to the coil’s present kinetic energy, must equal to the energy before, the

initial kinetic energy. It is thus obvious that the kinetic energy of the coil must have decreased. This is because the induced current acts in such a way to oppose the motion, thus giving Lenz’s law. Lenz's law follows from the Law of Conservation of Energy. That law says energy cannot be created nor destroyed but can only change form. In the case of the magnetic field and coil, kinetic energy must be transferred to the coil to produce the induced current flow (electrical energy). That energy is the work done in inserting the coil or withdrawing it. For applications of Lenz’s law, simply create a magnetic field such that it ‘repels’ the change in magnetic field, and the direction of the current necessary to produce this field is the induced current. Otherwise, apply to the part which has just experienced the change in flux, such the resultant force opposes the motion which caused this change. For example, if a N magnet approaches a coil, the coil will produce and emf to counteract the incoming N by creating another N repelling that:

Thus the current flow in the indicated direction, creating a north pole which repels the incoming north pole slightly. The direction of the current can be found (for coils or rings) by the right-hand coil rule. Another example is when a ring moves out of a magnetic field, decreasing total field density. Hence an induced current is produced to retain the number of field lines passing through (deduced via the right-hand coil rule). Here, a field into the page is created by the induced clockwise current. The current stops flowing when the ring is totally removed from the field.

Lenz’s law explains the production of ‘back emf’ in electric motors. Electric motors use an input voltage to produce a current in a coil to make this coil rotate in an external magnetic field, so one would expect that as the amount of magnetic flux in the coil changes (due to rotation) so would an emf be induced. The emf induced in the coil is in the opposite direction to the input voltage or supply emf, by Lenz’s law. Is it known as back emf, as it is in the opposite direction to the supply emf. We can see that if this were not the case, the current would continue increasing and the motor would go faster forever (exponentially). The net voltage (resultant emf) across the coil equals the input voltage (or supply emf) minus the back emf. The magnitude of this back emf is proportional to the speed of the armature. When the motor is first switched on, the back emf is zero and the current passing through the coil is at its maximum. When the motor is not rotating, there is no back emf produced so the current will be greater. If there is nothing attached to the motor to slow it down (ignoring frictional effects) the speed of the armature coil increases until the back emf equals to the external emf. When this occurs, there is no voltage over the coil, meaning no net force acts upon the coil, and as such it rotates at a constant rate. When there is, however, a load on the motor, the coil rotates at a slower rate and the back emf is reduced. The force resultant from the small voltage over the coil is used to overcome this load and balance forces for constant motion. As the coil has a fixed resistance, the net voltage over the coil determines the amount of current flowing in the coil. The smaller the back emf is, the greater

the current flowing through the coil. If a motor is overloaded, it rotates too solely and does not produced sufficient back emf, making the current high. This causes the burning out of motors. When the motor is switched, as the coil turns slowly initially, there is a protective mechanism (series resistor, which is turned off at high speeds) used to protect the coil from the high currents. The production of back emf later reduces this current.

* plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when: • the distance between

the coil and magnet is varied

• the strength of the magnet is varied

• the relative motion between the coil and the magnet is varied

• See Attachments • Pracs

* perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet

• See Attachments • Pracs

explain the production of eddy currents in terms of Lenz’s Law

An eddy current is a closed loop current that flows in a conductor, such as the iron core of a coil of an electromagnetic brake plate, when there is relative motion between the object and a magnetic field. The eddy current, flowing in a closed loop, acts like the current in a coil or solenoid and produces its own magnetic field. The polarity of this magnetic field depends on the direction in which the eddy current circulates. Eddy currents are another by-product of Lenz’s law. The magnetic flux density change which induces this current can be from: • a magnetic field acts on part of the object and there is a relative movement

between the field and the object • a conductor moves in an external magnetic field • a metal object is subjected to a changing magnetic field The magnetic fields set up by the eddy currents oppose changes in the magnetic fields acting in the regions of the metal objects.

For example, if a rectangular sheet of metal is removed from a magnetic field. On the left side of the edge of the magnetic field charged particles in the metal sheet experience a force because they are moving relative to the field. By applying the right-hand palm rule it can be seen that positive charges will move upwards, as they ‘wish’ to push the metal sheet left. To the right particles are not moving relative to the field and thus experience no force, leaving accumulated positive charge free to move down and complete the loop. This forms a current loop known as an eddy current. Consider the north pole of a magnet moving over and close to the face of an aluminium plate. By Lenz's Law, the circulation of an eddy current ahead of the moving magnet should produce a north pole that will repel the moving magnet.

The direction of current flow to produce a north pole agrees with the direction of the induced emf in a conductor moving relative to a magnetic field, that is, down the plate within the region of the moving field. Similarly, Lenz's Law predicts that an eddy current induced behind the moving magnet will produce a south pole that will attract the moving magnet. Together these two induced poles oppose the motion of the magnet over the aluminium plate. Eddy currents find use in everyday applications, such as switching devices, induction braking and induction cooktops (latter two are explored in subsequent research dot points). Applications of eddy currents should apply to what is immediately about to enter the change, not what has already left, and the force should act to oppose the motion.

* gather, analyse and present information to explain how induction is used in cook tops in electric ranges

An effect of eddy currents is that they cause an increase in the temperature of the metals, due to the collisions between moving charges and the atoms of the metal, as well as the direct agitation of atoms by a magnetic field changing direction at high frequency. This is similar to a resistive conductor losing energy as heat to the environment due to the presence of a current. Induction heating works on this principle, as it is the heating of an electrically conducting material by the production of eddy currents in the material. It is undesirable in electrical components such as motors, generators and transformers, but has been put to use as induction cookers and induction furnaces. A gas stove cooks food by burning gas to produce hot gases, which flow across the bottom of a saucepan and transfer heat via conduction. However, a large amount of the thermal energy is carried away into the environment, and the leftover heat cooks the food. Thus, it is an inefficient and undesirable method of cooking. Some electric cooktops contain induction coils instead of heating coils. These set up rapidly changing magnetic fields, inducing eddy currents in the metal of the saucepan. The metal pot is not a perfect conductor, and as a result these eddy currents encounter some electrical resistance. This resistance converts the current into heat. The result is that the metal pot, and only the metal pot, heats up. Heat is transferred from the pot to the food inside the pot by conduction. The cooking surface is designed to be a good thermal insulator, so that a minimum of heat is transferred from the pot to the cooking surface (and thus wasted). In normal operation, the cooking surface stays cool enough to touch without injury. If the pot is made from an electrical insulator, then no current can flow through the pot. This means that no heat will be generated. Thus inductive cookers do not work with Pyrex glass or ceramic. Induction cookers are faster and more energy-efficient (about doubly increased) than traditional stoves. Additionally, the risk of accidental burning is diminished since the cooker itself only gets marginally hot (due to heat conduction down from cookware), allowing direct contact with a reduced chance of harm. Also, no heat is lost to the air directly from the cooker, keeping the kitchen containing the cooker cooler. Induction cooking gives more complete control over temperatures – without the time-lag experienced with conventional cooktops, even gas.

The induction coils are separated from the saucepan by a non-conduction ceramic plate. The pan must be made from a metal that has a high internal resistance to this induced AC current. A ferromagnetic or ferrimagnetic coated pot is placed above an induction coil for the heating process to take place. The

resistance to the rapidly oscillating currents within the pan results in heat being produced directly in the base of the pan. That heat is dissipated to the food in the pan and does the cooking. The ceramic cooktop itself is not heated other than by heat lost from the pan. The induction cook top works best when used with pans made of ferromagnetic metals such as stainless steel and cast iron. This type of cooktop does not work with non-ferromagnetic cookware, such as glass, aluminum, and most stainless steel, nor with ferromagnetic material covered with a conductive layer, such as a copper-bottomed pan.

* gather secondary information to identify how eddy currents have been utilised in electromagnetic braking

A metal disc, rotating relative to a magnetic field experiences changes in flux, thus producing eddy currents. Refer to diagram (a), where the application of the right-hand-palm rule and Lenz’s law shows the direction of the eddy current to be upwards, as shown in diagram (b). This current follows a downward, circular path after leaving the region of magnetic influence. The magnetic field exerts a force on the induced eddy current (essentially the wheel, as it is the conductor), and via more rule application it can be seen that the force opposes the direction of motion of the wheel, which is to be expected via Lenz’s law. In this way, eddy currents can be used in smooth braking devices in trams and trains. An electromagnet is switched on so that an external magnetic field affects part of the metal wheel or steel rail below the vehicle, creating eddy currents. These oppose the direction of motion of the vehicle and slow it down. In the case of the wheel, it is slowed down. For the rail, there is a force forwards upon the rail, and hence an equal and opposite (retarding) force is exerted on the vehicle. Because the strength of the induced eddy currents is proportional to the speed of the train, the braking force is reduced as the train slows, resulting in a smooth stop. Eddy currents are used for electromagnetic braking in many free-fall amusement park rides. A copper plate attached to the ride capsule passes between fixed strong magnets near the bottom of the ride, inducing eddy currents and associated magnetic poles in the copper plate. Each fixed magnet in turn induces a like pole as the plate approaches and an opposite pole as the plate leaves. The combined effect of interaction between the permanent and the induced fields slows the ride down smoothly because the strength of the eddy currents in the plate is directly proportional to the speed of the plate moving between the poles. As the ride slows the braking force is reduced.

3. describe the main

components of a generator compare the structure and

function of a generator to an electric motor

A generator is a device which transforms mechanical kinetic energy into electrical energy. In its simplest form, a generator consists of a coil of wire forced to rotate about an axis in a magnetic field. As the coil rotates, the magnitude of the magnetic flux threading through the area of the coil changes, and by a similar principle to the ‘back emf’ explored above, a current is induced in the coil. This follows Lenz’s law and Faraday’s law of induction. The magnetic field of a generator can be provided either with permanent magnets or using an electromagnet, though using electricity to produce it might seem counter-intuitive. As the production of an emf requires a magnetic field and coil, these are present. As similar to the motor, the stationary parts are called the stator (magnets) and the rotating parts the rotor (coil).

Component Description Purpose Stator (magnets) and magnetic field

The magnets are usually on either side of the coil, possible radially. Electromagnets are used for large-scale generators.

Provides the flux change great enough to induce a current in the coil when physically turned; radial field keeps current at maximum longer.

Rotor (coil and frame) The coil consists of many turns of wire (increasing amount of total flux change

Provides the internal circuit which allows the production of electricity.

as many areas are present). The armature consists of laminated iron wounded on an axle.

Torque is applied to make the rotor spin.

Commutator(DC) and Slip Rings (AC) and brushes

AC motors use slip-rings, made of two separate rings. DC motors use a split-ring commutator, similar to the motor. Commutators and rings are made of metal. Brushes are graphite ‘sticks’ which provide electrical contact and are not fixed.

Provides interface between the rotating coil and stationary external circuit, without wires tangling. The split ring changes the direction of induced current as necessary. Brushes connect the two circuits, made from lubricating graphite.

Energy Source There must be a source of mechanical energy turning the coil in a magnetic field. This can be done by driving a turbine with steam or water, or a belt (such as the alternator in a car). Industrial generators usually use propelled steam.

If the coil of a generator is forced to rotate at a constant rate, the flux threading the coil and the emf produced across the ends of the wire of the coil vary with time as shown:

The first graph depicts the amount of flux threading through the coil as a function of time. The second graph, being the negative of the derivative of the first graph, shows the amount of flux induced in the coil, as a result of Faraday’s (and Lenz’s) laws. The frequency and amplitude of the voltage produced by the generator depends on the rate of turning A generator’s structure is similar to that of an electric motor, possessing stator, commutator or slip rings, and rotor components. They differ in that the generator converts rotational kinetic energy into electrical energy while motors convert

electrical energy into rotational kinetic energy. Thus, in a generator, the terminals are not connected to a power source but to a circuit, and the axle is connected not to the load but the energy source. A motor can be used as a generator by physically rotating its coil (though not very efficient). An AC induction motor is different from an AC generator as its rotor coils are not connected to an external circuit and its field is always supplied by electromagnets.

describe the differences between AC and DC generators

AC and DC generators differ in some ways, though the principles they work on are the same, via induction of a current by Lenz’s law. The main difference between the two is how they are connected to the external circuit, with their commutator or slip-rings determining their output circuit. The net output of any generator is a sinusoidal wave – that is, inherently AC in nature. However, there exists still a tangling problem with the external circuit, and this is solved by the use of slip-rings (not split-ring commutators):

These slip-rings allow the direction to current to vary as it does in the coil, without tangling the wires. They are metallic rings connected to graphite brushes similar to those in the DC motor, and output the sinusoidal AC current.

The DC motor must have its output in the same direction all the time, and thus it employs a split-ring commutator, as in the motor, to convert the direction of output current. Note that the internal circuit still produces AC; the commutator changes the direction of output so the external circuit receives an absolute-value graph of current. (Magnets have been omitted in these diagrams.)

A DC motor can use more breaks in the commutator to deliver a more steady current, as the absolute sinusoidal graph varies greatly from zero to maximum, which can affect performance adversely, but a graph which incorporates out-of phase peaks leads to a more constant current, achievable from the use of a 4-split ring commutator (that is, two coils)

To find the direction of the current in the coil (and in the external circuit), there are several methods. The first method is to consider the force upon a positive test charge on one side of the coil (right-hand palm rule), and the direction of this force will correspond to the conventional current. Follow this through to the slip rings or the commutator to find the direction of current in the external circuit. Alternatively, one could apply Lenz’s law to the coil. Determine the change in flux of the coil at an instant, and the current is in the direction where it would produce a magnetic field that is opposite to the change in flux, via the right-hand grip rule.

For example, in this scenario consider the left side of the coil. By the first method, as the direction of the rotation on the side is *up* at that instant, it becomes the thumb (current). The magnetic field (fingers) point to the right, and hence the palm (force) points into the page, which is the direction of current (clockwise). Using the second method, we consider the left half of the coil. As the coil rotates, the amount of flux threading through will increase (north to south). To decrease this flux, we increase the magnetic field south-to-north, which would require (via right-hand coil rule) a clockwise current.

* gather secondary information to discuss advantages/ disadvantages of AC and DC generators and relate these to their use

Advantages Disadvantages Easy to transform EM radiation interferes

with other electrical equipment

Can be transmitted at high voltage/low current and be later transformed to low voltage/high current

Requires thicker insulation to minimise interference

Reduced energy losses in transmission due to low current Less moving parts; more reliable, longer life span, and less maintenance Rotating magnet: more practical and easier to obtain

AC Generator: used in large scale commercial production of electricity

Can produce three-phase voltage

More likely to kill or injure than DC due to fillibration

No EM radiation means less insulation is required

Cannot be transformed easily

DC Generator: Used in specialist fields such as No energy loss from More difficult to supply

induction in adjacent metal structures or lines

to homes by line distribution

Able to deliver a constant voltage under different loads

More moving parts meaning components wear from friction; less reliable, shorter life span, more cost

Does not require a rectifier to work; most appliances run on DC current Used for specialist chemical processes which require DC

aluminium refining, chlorine production, and locomotives and ships

Voltage output can be made smoother with more coils and more splits in commutator

Sparking causes interference

The commutator of a DC generator consists of a number of metal bars separated by narrow gaps. As the brushes remain in contact with the commutator under spring pressure, they are constantly striking the leading edge of each successive bar. This wears the brushes and they need to be replaced regularly. The commutator bars also wear down until the insulating material between them prevents the brushes from making proper contact with the bars, reducing the efficiency of the generator. Pieces of metal worn from the commutator bars can become lodged in the gaps, causing a short between bars and reducing the output of the generator. In contrast, the slip rings of an AC generator have continuous, smooth surfaces, allowing the brushes to remain continuously in contact with the slip ring surface. Thus the brushes in an AC generator do not wear as fast as in a DC generator. There is no possibility of creating an electrical short circuit between segments in an alternator because the slip rings are already continuous. An AC generator therefore requires less maintenance and is more reliable than a DC generator. Most commercial generators are AC generators. In a DC generator the current is generated in the rotor and is then drawn from the windings through the commutator and out via the brushes. The larger the current required, the heavier the rotor coils must be, placing high demands on bearings and supporting structures. In addition, drawing large currents through the commutator-brush connection increases the likelihood of electric arcs forming as the brush breaks contact with each bar in turn. This reduces the efficiency of the generator and creates radio “noise”. This limits the usefulness of DC generators to relatively low current applications.

discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer

Power stations are usually situated large distances from cities (due to environmental and pollution reasons) where most of the consumers are located. This presents problems with power losses in the transmission lines, as they are essentially long metallic conductors with (over such distances) significant resistance. This means that they have a significant voltage drop over them when they carry a large current, possibly resulting in greatly decreased voltages for the consumer. As V = IR And P = VI Thus Ploss = I2R, where I is the current flowing through the conductor and R is the resistance of the transmission line. There are some ways to reduce this power loss, which is important to both the consumer and produced. Transformers can change the current flowing through the coil, and if they decrease it the power loss is decreased twice as much. Using transformers allows for electricity to be provided over large distances without wasting too much power, and it is the reason why AC use is higher than DC use (transformers only work with AC currents). This has a significant impact on society, as it means that generators can be built at locations further from society than otherwise, creating less pollution and impact, and focusing development around sources of energy, such as coal mines.

Electricity from the NSW energy grid are transmitted at ranges from 500 to 66 kV (meaning current is very low, reducing power loss), which is stepped down for use in the city by transformers at substations. For energy losses to be minimised, the transmission voltage must be very high. This requires high poles or towers and large insulators. These are expensive to build and maintain and have an adverse effect on the visual environment. Trees must be kept well clear of high voltage transmission lines to avoid damage to the lines during storms and to reduce the possibility of a short to earth. This often requires a wide corridor to be cleared, sometimes through environmentally sensitive areas. Energy losses can also be minimised through careful choice of materials and design of conductors. Transmission lines are typically made of either copper or aluminium, as these metals have low resistivity, that is, they are good conductors. Resistance is inversely proportional to the area of cross-section of the conductor, so the thicker a conductor, the lower the heat losses. However, heavier conductors require more expensive support structures. Aluminium has higher resistivity than copper but it is much lighter than copper, and less susceptible to corrosion. The smaller weight and lower maintenance costs more than compensate for the larger diameter of aluminium needed to carry a certain current. Recent experiments with superconducting materials show some promise for reducing energy losses from high voltage transmission lines even further in the future. Energy is also lost through the induction of eddy currents in the iron core of transformers. This applies both to step-up transformers at the power station and to step-down transformers at the sub-station and on power poles on suburban streets. The circulation of eddy currents in the transformer core generates heat because of the resistance of the iron. Transformer cores are usually made of laminated iron, consisting of many thin layers of iron sandwiched together, with thin insulating layers separating them. This limits eddy currents to the thickness of one lamina and reduces the corresponding heat loss. Eddy currents may be further limited in transformer cores made of granular ferrites, as used in some recent experiments. The ferrites allow the magnetic flux to change freely but have high resistance to the eddy currents. Heat loss inevitably occurs in the core of a transformer. As overheating can damage the transformer, various cooling techniques are used to dissipate the heat. These include cooling fins on the outside of the transformer, radiator pipes to allow cooling oil to circulate by convection and transfer heat to the air, and electric fans to force cooling air to flow around the transformer. Ultimately, the price of energy losses is paid for by customers. In order to keep such costs reasonable, methods of reducing energy losses are applied. Further research directions include find low (or zero) resistivity conductors such as superconductors and other ferromagnetic materials to be used in transformers.

* analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities

In the late nineteenth century, Edison favoured generating and supplying direct current (DC) electricity while Westinghouse promoted the use of alternating current (AC) electricity. Edison had the initial advantage that the technology for generating DC was well established and DC worked well over short distances. However, DC could only be generated and distributed at the voltages at which it was used by consumers. This meant that currents in conductors were large, leading to huge and expensive energy losses over distances of more than one or two kilometres. To supply a large city required many power stations throughout the city and an unattractive proliferation of wires to carry the required current. The great advantage of AC was that, through the use of transformers the voltage could be stepped up or down as required. This meant that AC could be generated at moderately low voltages, stepped up to high voltages for transmission over great distances and stepped down again to lower voltages for consumers. The higher voltage meant that AC could be transmitted over greater distances than DC, with smaller energy losses. Power stations could be fewer and further apart and conductors could be lighter. The economic advantages of AC, including the smaller energy losses and the economy of scale in needing fewer power stations further apart, along with the unattractive web of wires required for DC, supported Westinghouse’s solution to

the supply of electricity over Edison’s. AC received a boost in popularity with Tesla’s invention of the induction motor which operates only on AC. Competition was not always open and fair. Edison had a vested interest in DC as he owned hundreds of DC power stations and all of his many electrical inventions to that time ran on DC. Edison attempted to prove that AC was very dangerous by electrocuting animals on stage and convincing authorities to use AC for the first electric chair. He resorted to legal tactics in an attempt to have AC banned and to prevent its use with his inventions. Edison seems to have unreasonably shunned AC electricity. Demonstrations (such as the Chicago World Fair and Frankfurt/Lauffen) eventually swayed observers to notice AC as superior transmission. Westinghouse eventually undercut Edison on two major the projects – the Chicago World Fair and production of a hydroelectric plant at Niagara Falls (to Buffalo, 30 km away), due to the superiority of AC and their lower-costed bids. AC was recognized as superior due to less attenuation and power loss, letting it become cheaper. AC eventually came to be the dominant form in which electricity is generated world-wide. But DC has the advantage of not causing losses through electromagnetic radiation or magnetic induction. With solid-state switching it is now relatively simple to change between DC and AC at high or low voltages. High voltage DC transmission is now practicable. Scientists are striving to develop super-conducting wires for power transmission. If they do, DC could become the preferred current for long distance transmission.

* gather and analyse information to identify how transmission lines are: • insulated from

supporting structures • protected from lightning

strikes

In dry air sparks can jump a distance of 1cm for ever 10kV of potential difference. A 330kV transmission line can thus spark up to 1/3 of a metre, and in high humidity the distance is larger as resistance is smaller. High voltage transmission lines are kept away from their supporting structures by large insulators to reduce the likelihood of a discharge between the conductor and the support structure. These suspension insulators are used on transmission lines with 33kV or higher.

a conductive path across the surface of the insulator. The disc-like shape of the segments, whether ceramic or rubber, ensures a long pathway for any spark discharge across the insulator. When lightning strikes, it will usually pass between the bottom of a thundercloud and the highest point on the Earth below, usually tall trees, tops of buildings and power towers. Transmission lines and supporting structures have a number of protective features associated with their design. In the event of a transmission tower being struck by lightning, the metal tower itself acts as a conductor to take the charge to the ground. The towers are well earthed, with a large surface area

Insulator chains can be up to around 2 m in length: generally, the higher the voltage, the longer the chain. Insulators are constructed either of ceramic segments joined together with metal links or of rubber discs with a fibre glass core. Their design reduces the possibility of charge leaking through the insulators themselves. The metal links in ceramic insulators are isolated from each other, and the fibreglass is a non-conductor, so there is no continuity of conduction. The insulator segments are designed to shed water and prevent dust from building up, as either moisture or dust can make

of metal buried in the ground, enabling the charge from any lightning strike to dissipate harmlessly in the earth. Towers are widely spaced to ensure that, should one tower be struck, the adjacent towers suffer no damage from the lightning strike. Not all the wires on a transmission tower carry the electric current. The uppermost wires are called shield conductors (continuous earth line), as they are designed to reduce the chance of a lightning strike to the transmission wires. Shield conductors are connected directly to the transmission towers without the use of insulators so that they can conduct charge between the clouds and the earth as it builds up, to neutralise the charge distribution. If the shield conductors are struck directly by lightning the current is conducted safely to earth. Also, as the cable normally carries no current, a current running through it (from a fault) can be detected and fixed at leisure.

assess the effects of the development of AC generators on society and the environment

The development of AC generators has had both positive and negative effects on society and the environment. The development of AC generators has led to the widespread application of some of the useful features of AC electricity. AC generators are simpler and cheaper to build and operate than DC generators. Because AC electricity can easily be transformed, it can be transmitted cheaply over great distances, allowing a wide range of primary energy sources to be exploited. This has allowed the development of extensive, reliable AC electricity networks for domestic and industrial use throughout much of the world. This in turn has had both positive and negative effects on society and the environment. From a social point of view, electricity generation has allowed the development of the highly mechanized and electronic lifestyles to which people in the developed world have become accustomed. Our lives are made easier every day by the use of vast numbers of electrical gadgets. The affordability of electricity has promoted the development of a wide range of machines, processes and appliances that depends on electricity. Many tasks that were once performed by hand are now accomplished with a purpose-built electrical appliance and most domestic and industrial work requires less labour. Electricity runs our lighting, our heaters & air conditioners, our computers and communications equipment, our refrigerators, toasters, electric fry pans & stoves, washing machines, vacuum cleaners, stereos, TVs, garden equipment, industrial equipment and so on – the list is almost endless. Other new tasks can now be achieved that were formerly impossible, such as electronic communication. However some predictions of the effects of electricity on society have not been realized. It was first predicted that electric machines would do all physical labour, and that workers would receive more leisure time. Housework would similarly be eliminated in favour of leisure. However, a reduction in unskilled jobs and increased unemployment is one factor arising from electrical use, different to predictions. Similarly, the prediction that people would go back to the countryside to live, due to decentralization, was shown wrong. Middle classes moved to outer suburbs while lower classes stayed in inner, poorer suburbs. The ready availability of electricity has led to increasing dependency on electricity. Essential services such as hospitals are forced to have a back-up electricity supply, “just in case”. Any disruption to supply compromises safety and causes widespread inconvenience and loss of production. A major electricity failure can precipitate an economic crisis. The global electricity industry lobby is very powerful but is not always just. Social values may give way to economic pressures, especially in developing countries where often the poorest people lose their livelihood to make way for new energy developments. AC power generating plants can be located well away from urban areas, shifting pollution away from homes and workplaces, thus improving the environment of cities. However, many environmental effects of the growth in the electricity industry are negative. Power transmission lines criss-cross the country with a marked visual impact on the environment, often cutting a swathe through environmentally sensitive areas. Remote wilderness areas can easily be tapped for energy resources such as their hydro-electric potential. Most power generation stations around the world still use fossil fuels as their energy source. Fossil fuel power stations produce thermal pollution, acid rain and air pollution due to the release of particulate matter and oxides of nitrogen

and sulfur. Fossil fuel power stations release huge amounts of carbon dioxide into the atmosphere, which adds to the greenhouse effect, which is believed to be raising Earth’s temperature. Fossil fuel power stations also indirectly cause the land desecration and pollution associated with the coal mining, necessary to maintain supply of fossil fuel. Nuclear power stations leave an environmental legacy of radioactive waste that will last many thousands of years. The effects of the development of AC generators on society and the environment have been far-reaching. Some effects have changed the way people live, but not always for the better. Many people now enjoy increased convenience and leisure, many new industries flourish on new technologies made possible by electricity, but the dislocation and unemployment experienced by some can be devastating. Many aspects of the development of electricity have led to environmental degradation, often in remote areas where the long-term effects are poorly understood. These effects seem likely to be ongoing, as the compromise between economic interests and social and environmental values often favours the economic. We have not yet learned to live with AC electricity in a sustainable way.

* plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

• See Attachments • Pracs

4. describe the purpose of transformers in electrical circuits

Transformers are devices that decrease or increase AC voltage, and work on a principle similar to Faraday’s experiments. They find use in CRTs and electronic appliances in order to provide the correct voltages, and are also used industrially in the electric power distribution system. They consist of two coils of insulated wire called the primary and secondary coils. These can either be wound onto the same soft iron core, or linked by a soft iron core.

They are designed so that as much as possible of the magnetic flux from the primary coil threads through the secondary coil. When an AC current flows through the primary coil, a constantly changing magnetic flux threads through the secondary coil as a result of the soft iron core. This flux in turn induces a current in the secondary coil with the same frequency (but not necessarily the same voltage or current; in fact this transformers are mainly used to modify these) in the secondary circuit. If a steady DC current were to flow through the primary coil, it would produce a constant flux threading the secondary coil. No voltage would be induced in the secondary coil, as a changing flux is needed. Thus, transformers work only with AC circuits. The domestic supply voltage in Australia is 240V single-phase AC. Many appliances, such as motors and lights, are designed to operate directly on these voltages. However, many domestic and industrial appliances contain components that require voltages well below the supply voltage, such as display panels, printed circuit boards or semi-conductor devices, which typically require between 3V and 24V. Some components, such as television picture tubes, require voltages well above the supply, around 1500V. Transformers are placed in the circuit between the AC supply and the component to reduce or increase the supply voltage to that required for the component. It is common for the step-

up or step-down transformer to be built into the appliance as part of its power supply.

compare step-up and step-down transformers,

identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage

The difference between the magnitudes of the primary (Vp) and secondary voltages (Vs) is dependant on the number of coils of the two coils (their ratios) – that is, ns and np. If the transformer is ideal, it is 100% efficient and the energy output at the primary coil is equal to the energy output of the secondary coil – the rate of change of flux through both coils is the same. Faraday’s law is used to derive the relationship between Vs, Vp, ns, and np: For the secondary coil:

For the primary coil:

By manipulation, we get:

If ns is greater than np, Vs will be greater than Vp. Such as transformer is known as a step-up transformer (increases voltage). A transformer with smaller ns than np will have a correspondingly smaller Vs than the Vp. This is known as a step-down transformer, as the output voltage is smaller than the input. Type Step-up transformer Step-down transformer Structure Consists of two inductively coupled coils wound on a

laminated iron core Coil Ratios More turns in the secondary

coil than the primary coil Fewer turns in the secondary coil than the primary coil

Voltage Outputs

Higher output voltage than input voltage

Lower output voltage than input voltage

Current Outputs

Lower output current than input current

Higher output current than input current

Industrial Use Used at power stations to increase voltage and reduce current for long-distance transmission

Used at substations and in towns to reduce transmission line voltage for domestic and industrial use

Domestic Use Used in television sets to increase voltage to operate the picture tube

Used in computers, radios, and CD players to reduce household electricity to very low voltages for electronic components

* solve problems and analyse information about transformers using:

• Skill

explain why voltage transformations are related to conservation of energy

The principle of conservation of energy states that energy cannot be created nor destroyed, only transformed from one form to another. All physical systems obey this law. The amount of electrical energy entering a transformer in a certain time must equal the total amount of energy in all forms leaving the transformer in the same period of time. That is, power in equals power out. Thus, we cannot get more energy out of a transformer than you put into it (some energy is in fact lost, due to eddy currents’ dissipation of thermal energy and loss of flux). The rate of supply of energy is known as power and is found via P = VI. If there is no power loss (that is, ideally Ps = Pp) we can substitute P = VI to get VpIp = VsIs. Rearranging this as a ratio gives:

Note that this is an inverse relationship, not a proportional relationship like the one linking voltage and coil turns. This means that the ratio of secondary current to primary current is the inverse of the ratio of secondary voltage to primary

voltage. The secondary current is less than the primary current in a step-up transformer, and greater in a step-down transformer. Thus, overall:

However, real transformers produce heat because of the resistance of the iron core to induced eddy currents. This represents an energy loss to the system as heat is a form of energy. The power output of a transformer cannot exceed the power input, and the useful electrical power output is less than the input by the amount of the power loss through heating within the transformer.

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

A transformer has an iron core to concentrate the magnetic field to achieve the maximum possible inductive coupling between the primary and secondary coils. As the changing flux intersects the core, eddy currents are induced in the iron. Heating occurs because of the rather high resistance of the iron to the eddy currents. This heat represents a power loss to the electrical system and excessive heating can damage or destroy the transformer. One of the best ways to overcome difficulties of heating in transformers is to reduce the size of the eddy currents. Transformer cores are made of laminated iron, that is, many thin sheets of iron pressed together but separated by thin insulating layers. This limits the circulation of any eddy currents to the thickness of one lamina, rather than the whole core, thus reducing the overall heating effect.

In addition, construction of transformers from ferromagnetic materials will reduce overheating. Ferrites are complex oxides of iron and other metals, and are magnetically permeable but poor electrical conductors. The magnitudes of the eddy currents are thus reduced. Once the transformer does get hot it must be cooled to prevent overheating. Several strategies have been developed to keep transformers cool: • Heat-sink fins are added to the metal transformer case so that heat dissipation

to the environment can occur more quickly over a larger surface area. • The transformer case may be made of a black material so that the heat

produced internally is efficiently radiated to the environment. Most small transformer rectifier units found around the home are coloured black.

• Pad-mounted transformers at ground level have ventilated cases to allow air to remove heat by convection. They may also have an internal fan to assist air circulation to remove excess heat faster.

• The transformer case may be filled with a non-conducting oil that transports the heat produced in the core to the outside where the heat can be dissipated to the environment. The oil may circulate from hotter to cooler regions by convection alone, or circulation may be assisted by a pump. The case may have design features such as cooling tubes and radiator slats to increase the rate of heat dissipation.

• Large transformers such as at substations are always located in the open or in well-ventilated areas to maximise airflow around them for cooling. These are fitted with a combination of cooling mechanisms including pumps to circulate cooling oil through large radiators, and fans to increase the airflow over the radiators. The fans are often thermostatically controlled and cut in at a specified temperature, usually around 50°C.

discuss why some electrical appliances in the

Australian homes are provided with AC electricity of 240VRMS. Many domestic appliances are designed to run most efficiently at this voltage. Such appliances

home that are connected to the mains domestic power supply use a transformer

are connected directly to the mains supply without the need for a transformer. Some appliances contain components that require a transformer because they operate best at lower voltages than the mains supply. In a microwave oven, for example, large, energy consuming parts such as the turntable motor and the microwave transducer may be connected directly to the mains, while the control and display panel is supplied with low voltages from a step-down transformer in a built-in power supply unit. Most electronic circuits are designed to operate at low DC voltages between 3 and 24V. Thus, household appliances that have electronic circuits in them will have step-down transformers; either as a ‘power-cube’ or built into them. For example, power-cube transformers can be founds in rechargeable appliances such as portable vacuum cleaners, cordless phones, portable music players, and laptop computers. These transformers have a rectifier circuit to convert AC to DC. Other components such as television receivers and computer monitors contain cathode ray tubes that require voltages well above the mains supply, up to around 25 kV, to accelerate electrons toward the screen. These use a built-in step-up transformer to provide the necessary voltage. The power supply unit may contain both a step-up and a step-down transformer.

explain the role of transformers in electricity sub-stations

* gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use

Electricity is typically consumed in homes and industry at 240 V or 415 V. If there were no transformers, electricity would have to be generated and distributed at these same voltages. To supply the power demands of even a small town, the current at these voltages would be very large, leading to large and costly transmission losses and possible overheating of conductors. For a large city there would need to be many power stations spaced every few kilometers. If different voltages were needed, these would require separate power stations and separate distribution systems, adding to the problems. The use of transformers with AC electricity overcomes many of these problems. It is more efficient to generate electricity at high voltages, such as 23 kV, than at low voltages. Power stations can run efficiently at their design voltage and different transformers can be used to simply step the voltage up for transmission or down for local use as required. It is much more efficient to use very high voltages, up to 500 kV, for transmission lines, because at these voltages the currents are relatively small and transmission line losses are less of a problem. The higher the voltage, the smaller the line losses, and the greater the distance of transmission, the more important this saving is. Because the current is smaller at high voltages, fewer, smaller conductors are necessary for any particular power load than at lower voltages. High voltages are easily achieved for economical transmission by the use of step-up transformers. Electrical energy is usually consumed at low voltages, but at widely scattered locations. Transformers are used to progressively step the voltage down from the transmission lines to the consumer. At the generator, levels of 23kV are generated, and stepped up to transmission levels (330kV in this case). At regional sub-stations, step-down transformers reduce this to 110 kV or 66kV for regional distribution (zone power substations). Local sub-station transformers step this down further to 33 kV or 11 kV for distribution along suburban streets. Finally, pole transformers step this down to 240V (domestic) or 415V (industry). At each stage, the output voltage is chosen to match the power demand and the distances over which supply is needed. The stepped-down voltage used at each stage of distribution is chosen to balance the power, and hence the current, requirements, and therefore also the transmission losses, against the area over which distribution is required. Substations can perform several different tasks: step down voltages with transformers, split the distribution voltage to several directions, or control the availability (with circuit breakers and switches) of electricity.

discuss the impact of the development of transformers on society

The development of transformers made it possible to transmit electrical energy efficiently over great distances. This has had a range of impacts on society. If electrical energy could not be transmitted efficiently over large distances many more power stations with their associated pollution would be necessary to supply power to their local areas. Even very remote communities now have access to grid-supplied high-voltage

electricity which is stepped down locally by transformers. This has raised living standards in rural communities through provision of, for instance, electric lighting, refrigeration and air conditioning, and increased the scope of rural industries. Large cities have been allowed to spread, because electricity is readily available as an energy source, thanks to transformers. This has led to social dislocation in urban “deserts”, as people have moved further from family and friends and workplaces. Industry is no longer clustered around power stations or other sources of energy. Power stations can be in remote locations and high-voltage electrical energy can be distributed almost anywhere, to be stepped down near the point of use. This has allowed industries to be decentralised and has facilitated the development of industrial areas away from residential areas. This has relocated pollution away from homes, but it means that many people now spend significant time travelling between home and work. Transformers enable power from the one power station to be used in many different applications. Sub-stations can step the power down in stages to the various levels required by households, industry, public transport and so on. Without transformers, different industries requiring different voltages would have to build generators to produce those specific voltages. The existence of transformers has enabled the construction of many of the electronic labour-saving and entertainment devices we take for granted. TVs, computers, mobile phones, stereos, radios, electronic clocks, many kitchen appliances and countless other electronic devices require transformers for their operation. With the development of the transformer, people have changed the way they live, as electricity to every home has become an affordable necessity rather than a luxury.

* perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced

• See Attachments • Pracs

5. describe the main features of an AC electric motor

As with DC motors, AC motors have two main parts – the stator and rotor. The stator is the stationary part of the motor and it is usually connected to the frame of the machine. The stator of an AC motor produces the external magnetic field in which the rotor rotates, as the magnetic field produces a force (and torque) on the rotor. Most AC motors have a cylindrical rotor that rotates about the axis of the motor’s shaft. This type of motor rotates at a high speed, with the rotor completing about one revolution for each cycle of the electricity supply (50Hz or 3000 revs/min). To change speeds, gearboxes with the correct ratio are used. They are found in electric clocks, electric drills, fans, pumps, compressors, conveyor belts, and other factory machines. The rotor is mounted on bearings which are attached to the frame of the motor. In most AC motors the rotor is mounted horizontally and the axle is connected to a gearbox and fan, which cools the motor. Both the rotor and stator have a ferromagnetic core, usually steel, which strengthens the magnetic field. The parts of the coil experiencing alternating magnetic flux are laminated to prevent excess loss of energy via eddy currents. There are two main types of AC motors. Single-phase motors operate on one of the three phases produced at power generation plants. They can operate on domestic electricity. Polyphase motors operate on two or three of the phases produced at power generation plants. Universal Motor One type of single-phase AC electric motor is the universal motor. They are designed to both operate on AC and DC electricity. They are constructed along similar lines to the DC motors studied previously. The rotor has several coils wound onto the rotor armature. The ends of these coils connect to opposite segments of a commutator. The external magnetic field is supplied by the stator electromagnets which are connected in series with the

coils of the armature via brushes. The interaction between the current in a coil of the armature and the external magnetic field produces the torque which makes the rotor rotate. Note that regardless of the current or its frequency the direction of rotation remains constant as the magnetic fields and the coil current directions change at the same time. A variable resistor controls the speed of the universal motor by varying the current through the coils of the armature and the field coils of the stator. It is commonly used for small machines such as portable drills and food mixers.

AC Induction Motor Introduction: Squirrel-cage induction motors are by far the most common type of AC motor used domestically and in industry. They are found in some power drills, beater mixes, vacuum cleaners, electric saws, hair dryers, food processors, and fan heaters, and more. Induction motors are named such as a changing magnetic field set up in the stator induces a current in the rotor. The rotor, being free to move, creates a rotation effect. The simplest form of AC induction motor is known as the squirrel-cage motor, as the rotor resembles a rodent exercise wheel. The current in the rotor is induced in the conductors that make up the cage of the rotor by a changing magnetic field. It is an induction motor as no current passes through the rotor directly from the mains supply. This motor is best understood as a three-phase induction motor, using each of the three phases of electricity generated in power stations and supplied to factories.

Stator: The stator sets up a rotating magnetic field with constant magnitude. The stator usually consists of three sets of coils that have iron cores. The coils that make a pair are located on opposite sides of the stator, and they are linked electrically. It is connected to the frame of the motor and surrounds a cylindrical space in which it sets up the rotating magnetic field. In this type of three-phase induction motor, this is achieved by connecting each of the three pairs of field coils to a different phase of the mains electrical supply. The magnetic field inside the stator rotates at the same frequency as the mains supply.

Squirrel-cage Rotor: The rotor of the AC induction motor consists of a number of conducting bars made of either aluminium or copper. These are attached to two rings, known as end rings, at either ends of the bars. They short-circuit the rings and allows the current from one side to flow to the other side.

These bars and end rings are encased in a laminated iron armature. This intensifies the magnetic field passing through the conductors of the rotor cage and the laminations decrease the heating losses due to eddy currents. The armature is mounted on a shaft that passes out through the end of motor. Bearings reduce friction and allow the armature to rotate freely.

The induction motor includes no brushes or commutator meaning easier manufacture, no wear, no sparks, no ozone production and none of the energy loss associated with them.

Operation: As the magnetic field rotates in the cylindrical space within the stator, it passes over the bars of the cage. This has the same effect as the bars moving in the opposite direction through a stationary magnetic field. The relative movement of the bars through the magnetic field induces a current in the bars, which experience a force as they are in a magnetic field. The force is always in the same direction as the movement of the magnetic field, and the cage is forced to ‘chase’ the movement of the field in the stator.

The magnetic field moving to the right has the same effect as the conductor moving to the left. Application of the reverse of the right-hand rule is used to determine the direction of induced current – in this case, into the page.

The current in the bar produces a magnetic field as shown. Due to magnetic pressure in the interaction of the external magnetic field and the field inside the bar, the bar moves in the same direction as the field’s movement. Also, the right hand rule can be used to determine the force on the conductor bar. Slip: If the bars of the squirrel cage were to rotate at exactly the same rate as the magnetic field, there would be no relative movement between the bars and the magnetic field – thus no induced current and no force. If the coil is to experience a force there must be a relative movement, such that the cage ‘lags’ or constantly slipping behind the magnetic field. This slip is caused by the motor operating under a load, with the retarding force slowing the cage down such that it moves slower than a field. The difference between the rotational speeds of the coil and the magnetic field is known as the ‘slip speed.’ It is unitless and is a ratio of the relative speed of the magnetic field as seen by the rotor to the speed of the rotating field. This means that the motor is always slower than the magnetic field of the stator when it does work.

When any induction motor does work, the rotor slows down. When this occurs, the amount of slip is increasing. This means that the relative movement between the magnetic field and the conductor bars is greater and that the induced current and magnetic force due to the current are increased.

* perform an investigation to demonstrate the principle of an AC induction motor

• See Attachments • Pracs

* gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

The use of electrical energy is widespread in domestic and industrial uses, as it is easy to produce and distribute, economically competitive, and easily converted into usable energy. Energy transfer is the movement of energy from one object or location to another as the same form of energy. For example, a transformer moves electrical energy from the primary coil to the secondary coil as the same type of energy (though transforming it along the way into magnetic energy and back again – but note magnetism and electricity are the same type of energy, electromagnetic). Other examples of energy transfers are the conduction of electricity in wires and radiation/convection/conduction of heat from electrical resistance to the surroundings. An energy transformation is when energy is converted from one form into another. Electrical energy is transformed into other types of more useful energy in domestic and industrial applications.

Energy transformations in the: Home

Electrical Energy to: Industry:

Electrical Energy to: Heat: cooking, home heating, hot water

X-rays: medical imaging

Microwaves: microwave oven Light: laser printing Radio waves: cordless phones Radio waves: communication Light: light globes Kinetic: industrial motors, transport Kinetic: blender, fan Chemical: electroplating Sound: speakers Heat: induction ovens Chemical: recharging batteries

The energy transformations and transfers that occur when an electrical appliance operates depends on its purpose and current use. Take for example a hair dryer-blower. The electric motor transforms electrical energy (the rotor spins). Some energy is lost via the eddy currents in the laminated iron core. The mechanical energy of the rotor is transformed into sound and heat energy inside the motor, and transferred into the kinetic energy of air particles. The air passes through a heating element where electrical energy is transformed into heat and light energy. The heat and kinetic energy is then transferred out of the dryer by conduction to moving air particles and convection as the particles carry the energy away from the dryer. In electric kettles and toasters, current from the mains causes heating in a high-resistance element. In an induction oven, changing magnetic fields cause eddy currents to flow in the metal parts which become hot because of their electrical resistance. Thus electrical energy is transformed into heat energy, both in the home and in industry, when large electric currents flow through metals with high resistance.