PHYS2: Motors and Generators

1. Motors use the effect of forces on current-carrying conductors in magnetic fields

IDENTIFY THAT THE MOTOR EFFECT IS DUE TO THE FORCE ACTING ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

Size of force on an electric charge moving through a magnetic field: F = q v B

o q: size of the chargeo v: velocity of the chargeo B: magnetic field strength

Moving electric charges create magnetic fields which interact with the external magnetic field, causing a force to be applied on the electric charges

A current-carrying loop in a magnetic field has electric charges moving through a magnetic field. Since all these electric charges experience a force, the entire conductor will experience a force; this is the motor effect

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

Force is directly proportional to magnetic field strength Bo Because force on a charged particle in a magnetic field is proportional to magnetic field strength

Force is directly proportional to current in the conductor, Io Because higher current means more moving charged particles per unit time, and each moving charged

particle experiences a force, therefore higher current increases force experienced by the current-carrying conductor

Force is directly proportional to length of conductor in the field, Lo Because a longer conductor length has more moving charged particles which experience a force due to

the magnetic field Force is directly proportional to sine of angle between magnetic field and the conductor, or component of the

field that is at right angles to the conductoro Force is maximum when conductor is at right angles to field because charged particles move along the

length of the conductor, and force on the charged particles is maximum when they are moving at right angles to the magnetic field

o Force is zero when conductor is parallel to fieldo F=BILsin θ

SOLVE PROBLEMS AND ANALYSE INFORMATION ABOUT THE FORCE ON CURRENT-CARRYING CONDUCTORS IN

MAGNETIC FIELDS USING: F=BI l sin θ

PERFORM A FIRST-HAND INVESTIGATION TO DEMONSTRATE THE MOTOR EFFECT

DESCRIBE QUALITATIVELY AND QUANTITATIVELY THE FORCE BETWEEN LONG

PARALLEL CURRENT-CARRYING CONDUCTORS: Fl=k

I1 I 2

d

Qualitatively

Each of two parallel current-carrying conductors will experience a force due to the interaction of the magnetic fields that exist around each

If the two conductors carry currents in the same direction, then they will experience a force towards each other (applying right-hand push rule to force induced on one conductor by magnetic field of the other)

If the two conductors carry currents in opposite directions, then they will experience forces away from each other.

o Like currents attract; unlike currents repel The force is equal on both conductors, even if they carry different currents Force per unit length is proportional to current in each conductor, and

inversely proportional to separation distanceQuantitatively

k = 2.0 x 10-7 N A-2

Since force F is a vector quantity, direction of force must be specified

SOLVE PROBLEMS USING: Fl=k

I1 I 2

d

DEFINE TORQUE AS THE TURNING MOMENT OF A FORCE USING: t=Fd

Torque is the turning moment of a force. It is the product of the tangential component of the force and the distance the force is applied from the axis of rotation.

Torque increases when force is applied at greater distance Torque is greatest when the force is applied at right angles to a line joining the point of application of the force

and the pivot axis Torque has SI unit newton metre (Nm) If force is not perpendicular to the line joining point of force application and pivot point, then component of

force perpendicular to the line (sin θ) is used instead.

DESCRIBE THE FORCES EXPERIENCED BY A CURRENT-CARRYING LOOP IN A MAGNETIC FIELD AND DESCRIBE THE NET RESULT OF THE FORCES

Forces are applied to a current-carrying loop in a magnetic field, which create torque, enabling the loop to rotate.

Force acting on the sides of the coil that are perpendicular to the magnetic field can be

calculated using: F=nBI L sin θo Coil has n turns of wire on ito Force is proportional to # of turns

of wire Each end of the coil will experience a force which varies from zero, when the plane of the coil is parallel to the

field, to a maximum when the plane of the coil is perpendicular to the field. The forces on the two ends can be shown, by the right-hand palm rule, to be opposite in direction, always parallel to the axis, and alternating in direction through a full rotation of the coil.

As the forces on the two ends are always opposite in direction, and always parallel to the axis, their net effect is zero.

Each long side of a current-carrying loop in a magnetic field experiences a force up or down in accordance with the right-hand push rule. If one side is experiencing a force up, the other will experience a force down simultaneously

o The net result of these forces is that a torque is created, and the current-carrying loop rotates on its axis

o The magnitude of force acting on long sides of the loop is constant since current in loop is constant, magnetic field is near constant, and sides of coil are always perpendicular to magnetic field

However, torque is greatest when the plane of the loop is parallel to the magnetic field, i.e. force acting on sides of the coil is perpendicular to line joining axle (pivot line) to point of application

Torque decreases as angle between plane of coil and magnetic field increases Torque is zero when plane of coil is perpendicular to the magnetic field, i.e. force acting on sides of the coil is

parallel to the line joining axle to point of applicationo Momentum of coil keeps it rotating even though torque is very smallo Using more coils prevents the occurrence of a ‘dead spot’ with no

torque After every half rotation, when plane of coil is perpendicular to magnetic

field, the commutator switches direction of electric current, so that side of coil that is on top experiences a force downwards, and vice versa

o This keeps coil rotating. If no commutator was present, the coil would oscillate about the plane perpendicular to the magnetic field, and eventually stop moving

DESCRIBE THE MAIN FEATURES OF A DC ELECTRIC MOTOR AND THE ROLE OF EACH FEATURE

Feature Description RoleStator Non-rotating magnetic part of the motor Provides the external magnetic field, either via

permanent magnets or electromagnets, to cause the

coils to rotate when current is passed through themPermanent Magnets

Two permanent magnets on opposite sides of the motor, with opposite poles facing each other. The pole faces are curved to fit around the armature.

The magnets supply the magnetic field which interacts with the current in the armature to produce the motor effect.

Electromagnets Each stator coil (or “field” coil) is wound on a soft iron core attached to the casing of the motor. The coils are shaped to fit around the armature.

Each opposed pair of stator coils produces a magnetic field which interacts with the current in the armature to produce the motor effect. The iron core concentrates the field.

Rotor Rotating part of the motor; consists of armature and coil

Armature Consists of a cylinder of laminated iron mounted on an axle. Axle protrudes from the casing, enabling movement of coil to be used to do work

The armature carries the rotor coils. The iron core greatly concentrates the external magnetic field, increasing the torque on the armature. The laminations reduce eddy currents which might otherwise overheat the armature.

Rotor coil(s) One or several coils, usually several turns of insulated wire, wound onto the armature. Ends of the coils are connected to commutator

The coils provide torque, as the current passing through the coils interacts with the magnetic field. As the coils are mounted firmly on the rotor, any torque acting on the coils is transferred to the rotor and thence to the axle.

Split-ring commutator

Consists of a broad ring of metal mounted on the axle at one end of the armature, and cut into an even number of separate bars (two in a simple motor) separated by insulating material. Each opposite pair of bars is connected to one coil.

Makes electrical contact between the rotor coil, and the external circuit via brushes. Mechanical switch for reversing the direction of current flowing the coil every half-turn so that coil continues rotating in same direction

Brushes Conducting contacts made of compressed carbon connected to external circuit; spring-loaded to contact split metal ring of commutator as it turns

Connect commutator to the source of emf. Sliding contact necessary to stop connecting wires from becoming tangled

Axle A cylindrical bar of hardened steel passing through the centre of the armature and the commutator.

The axle provides a centre of rotation for the moving parts of the motor. Axle protrudes from casing, enabling movement of coil to be used to do work

IDENTIFY THAT THE REQUIRED MAGNETIC FIELDS IN DC MOTORS CAN BE PRODUCED EITHER BY CURRENT-CARRYING COILS OR PERMANENT MAGNETS

Magnetic field of a DC motor can be provided either by permanent magnets or by electromagnets Current that flows through the armature coil can be used in the electromagnetic coils wound around iron cores

SOLVE PROBLEMS AND ANALYSE INFORMATION ABOUT SIMPLE MOTORS USING: t=nBIAcos q

A is area of coil θ is angle between plane of coil and magnetic field

IDENTIFY DATA SOURCES, GATHER AND PROCESS INFORMATION TO QUALITATIVELY DESCRIBE THE APPLICATION OF THE MOTOR EFFECT IN:

- THE GALVANOMETER

- THE LOUDSPEAKER

Galvanometero Galvanometer is a device used to measure magnitude/direction of small DC currentso Coil consists of many loops of wire wrapped around an iron core, connected in series with circuit to be

tested. The iron core strengthens the magnetic field, and also provides electromagnetic damping so that the pointer stops swinging quickly

o When current flows, coil experiences a force due to presence of external magnetic field Iron core of coil increases magnitude of this force Radial magnets ensure that torque will be constant regardless of deflection, i.e. linear scale

o Needle is rotated until magnetic force acting on the coil is equalled by restoring torque of the spring, indicating the magnitude of the current on a suitable scale

Loudspeakero Consists of circular magnet that has one pole on outside and other on insideo Voice coil sits in space between the poleso Amplifier provides a current that changes direction at same frequency as sound to be produced, and has

magnitude in proportion to amplitude of the sound By motor effect, voice coil is caused to move in and out of the magnet Voice coil is connected to speaker cone that creates sound waves in the air as it vibrates

2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage

OUTLINE MICHAEL FARADAY’S DISCOVERY OF THE GENERATION OF AN ELECTRIC CURRENT BY A MOVING MAGNET

Electromagnetic induction: the generation of an emf (and electric current if in a closed conducting circuit) through the use of a magnetic field

Wood block: Faraday set out to detect current in a coil of wire by presence of magnetic field set up by another coil

o Coiled copper wire around a block of wood, and a second length of copper wire was coiled around the block in the spaces between the first coil

o Coils separated with twine; the primary coil was connected to a battery, the secondary to a galvanometer

o When primary circuit was closed, Faraday observed a momentary deflection at the galvanometer, indicating that a current was temporarily created in secondary circuit

o When current in primary circuit was stopped, deflection of galvanometer was in opposite direction Iron ring: Wound primary coil to one side of a soft iron ring, and secondary coil to other side

o When current was set up in primary coil, galvanometer needle responded ‘to a degree far beyond….without an iron core were used’

Concluded that when magnetic field of primary coil was changing, a current was induced in secondary coil

Moving magneto Showed that moving a magnet near a coil could generate an electric current in the coil

When a pole of a magnet is brought near one end of the coil, galvanometer needle momentarily deflected. When pole of magnet is not moving, galvanometer needle is at central point. When pole of magnet is removed from one end of the coil, galvanometer needle momentarily deflected in opposite direction

o Magnitude of induced current depends on the speed at which magnet is moving

Rotating copper disc: (first continuous current-generating device)o Faraday attached two wires 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's explanation was that the electric current was induced in the moving disc as it cut a number of lines of magnetic flux emanating from the magnet (the magnetic field).

Faraday’s conclusions:1) To generate a current, there must be relative motion between a conductor and a

magnetic field, and the conductor must be a close conducting circuit. 2) Electric current is induced because the conductor cuts the number of lines of

magnetic flux3) The magnitude of an induced emf, and hence current, depends on the rate at which flux lines are cut, or the

speed of relative motion between conductor and magnet.

DEFINE MAGNETIC FIELD STRENGTH B AS MAGNETIC FLUX DENSITY

Magnetic flux (φB): the amount of magnetic field passing through the given area o Measured in weber (Wb)

Magnetic field strength B, also known as magnetic flux density, is the amount of magnetic flux passing through per unit area

o Measured in tesla (T), or weber per square metre (Wb m-2) The stronger the magnetic field at a point, the higher the magnetic flux density B is at that point and the more

magnetic flux lines there are cutting or threading a given area.

DESCRIBE THE CONCEPT OF MAGNETIC FLUX IN TERMS OF MAGNETIC FLUX DENSITY AND SURFACE AREA

If a particular area, A, is perpendicular to magnetic field of strength B, then magnetic flux φB is product of B and A

o φB = B Ao i.e. magnetic flux is the magnetic field strength or amount of magnetic flux per unit area, multiplied by

total surface area, giving total magnetic flux Magnetic flux, φB, passing through an area is reduced if magnetic field is not perpendicular to the area

o φB = B+ A , where B+ is component of magnetic flux density, that is perpendicular to area, A.

DESCRIBE GENERATED POTENTIAL DIFFERENCE AS THE RATE OF CHANGE OF MAGNETIC FLUX THROUGH A CIRCUIT

When there is relative movement between a conductor and a magnetic field, a potential difference is generated. If the conductor is part of an electric circuit, a current is induced in the circuit.

Faraday’s Law of Induction: The magnitude of the induced emf in a circuit is equal to the rate at which the magnetic flux through the circuit is changing with time

o i.e. the change in amount of magnetic flux threading the coil generates a potential difference, hence creating a current

A change in φB can be caused by a change in magnetic field strength, B, or in area of the coil that is

perpendicular to the magnetic field. If a coil has n turns of wire on it, the emf induced would be n times greater that that produced if the coil had

only one turn of wire.

The rate at which magnetic flux changes, and hence emf produced, is varied by: o decreasing the distance between the conductor and the magnetic field, as the flux lines are closer

together nearer the magnet; o increasing the strength of the magnet, as there are more flux lines in the same space in a stronger field o increasing the speed of the relative motion between the conductor and the magnetic field, as the

conductor cuts more flux lines per unit time o increasing the angle between the direction of motion of the conductor and the direction of the magnetic

field from near zero towards 90 degrees, as the conductor cuts the maximum number of flux lines per second when its motion is at right angles to the field.

o Using a coil with more turns Cause of the induced emf and induced current

o As conductor moves relative to magnetic field, the direction of force on electrons within the conductor can be found using right-hand palm rule

Movement of electrons leaves a deficiency of electrons (a positive charge) at one end of the conductor, i.e. there is an emf between the ends of the conductor

If this conductor is part of an external circuit, a current will flow through the circuit

PERFORM AN INVESTIGATION TO MODEL THE GENERATION OF AN ELECTRIC CURRENT BY MOVING A MAGNET IN A COIL OR A COIL NEAR A MAGNET

PLAN, CHOOSE EQUIPMENT OR RESOURCES FOR, AND PERFORM A FIRST-HAND INVESTIGATION TO PREDICT AND VERIFY THE EFFECT ON A GENERATED ELECTRIC CURRENT WHEN:- THE DISTANCE BETWEEN THE COIL AND MAGNET IS VARIED- THE STRENGTH OF THE MAGNET IS VARIED- THE RELATIVE MOTION BETWEEN THE COIL AND THE MAGNET IS VARIED

ACCOUNT FOR LENZ’S LAW IN TERMS OF CONSERVATION OF ENERGY AND RELATE IT TO THE PRODUCTION OF BACK EMF IN MOTORS

Lenz’s Law: The direction of an induced emf (and current) is such that it produces a magnetic field which opposes the original change in flux that produced the emf.

Using Lenz’s Law: Use right-hand rule for coils. Point thumb in direction opposing the change in external magnetic field. Curl of fingers indicates the direction of induced current in the coil

Principle of Conservation of Energy: “Energy cannot be created nor destroyed, but it can be transformed from one form to another”

o If the opposite of Lenz’s Law were true, a changing flux in a coil would induce an emf creating a current that produced a magnetic flux in the same direction as the original change of flux. This would lead to a greater change in flux threading the coil indefinitely, and the induced current would continue to increase in magnitude, fed by its own changing flux

Since Principle of Conservation of Energy clearly states that energy cannot be created without doing any work, this clearly cannot occur.

o When a magnet is moved towards or away from a coil, the resultant change in magnetic flux as a result of the induced current opposes the original change in magnetic field, so that work must be done to move the magnet relative to the coil. This ensures that the electrical energy induced in the coil, has come from work done in moving the magnet (Principle of Conservation of Energy)

If the south pole of a bar magnet is inserted into the coil the current induced in the coil will flow in a direction such that it produces a south pole opposing the insertion of the bar magnet. Pushing the bar magnet against this field means that work must be done.

Back emf in motors: Electric motors use input voltage to produce a current in the coil, to make coil rotate in external magnetic field. However, an emf is induced in a coil that is rotating in an external magnetic field. If this emf was in same direction as the supply emf, the current would increase, and motor coil would rotate faster and faster indefinitely.

o Due to the Principle of Conservation of Energy, this cannot occur. The induced emf is in opposite direction to supply emf

EXPLAIN THAT, IN ELECTRIC MOTORS, BACK EMF OPPOSES THE SUPPLY EMF

Back emf: electromagnetic force that opposes the main current flow in a circuit. When the coil of a motor rotates, a back emf is induced in the coil due to its motion in the external magnetic field.

Net voltage across the coil equals supply emf minus the back emfo Back emf opposes the supply emfo Supply emf is constant, back emf is proportional to speed of motor

With no load, speed of armature coil increases until back emf is equal to external emfo When this occurs, there is no voltage and current flowing through the coil. There is no net force acting

on the coil and the armature rotates at constant rate When there is a load, coil rotates at a slower rate, causing a decrease in back emf, until the net voltage across

the coil and current flowing through it is higher to provide the torque to match the extra load If the motor is overloaded, it rotates too slowly, back emf is much reduced, and the voltage across coil may be

too high, resulting in a high current that could burn out the motor. o When motor is starting, back emf is small and so current in the coil will be large. To prevent burning out

the motor, a starting resistance is placed in series with the coil, which is removed once the motor speeds up.

EXPLAIN THE PRODUCTION OF EDDY CURRENTS IN TERMS OF LENZ’S LAW

Eddy current: circular current induced in a conductor that is stationary in a changing magnetic field, or that is moving through a magnetic field.

Lenz’s Law: An induced emf always gives rise to a current that creates a magnetic field that opposes the original change in flux through the circuit

That is, the polarity of the magnetic field produced by the eddy current is such that it opposes the relative motion of the magnetic field that induced the eddy current.

E.g. a metal plate is being moved down, through a magnetic field directed into the page

Right-hand push rule: On the bottom part of B, positive charges are moving downwards, and magnetic field is into the page. Using right-hand push rule, it can be seen that the charges experience a force to the right. In the region of I, charged particles experience no force, so they’re free to move back left in this region. Hence a clockwise eddy current is created. Similarly, positive charges experience a force to the right in the upper region of B, contributing to an anticlockwise eddy current.

Lenz’s Law: In the bottom region, the plate is being removed from a magnetic field directed into the page. Applying Lenz’s Law and right-hand rule for coils, it can be seen that a clockwise current will create a magnetic field which opposes this change. In the upper region, an anticlockwise eddy current opposes the increase in magnetic field lines into the page.

The sides of the eddy current loops that are inside the magnetic field experience a force due to the magnetic field. Using the right-hand push rule, this force is determined to be upwards

o The direction of force on the eddy current due to external magnetic field is always a retarding force, which opposes any motion.

GATHER, ANALYSE AND PRESENT INFORMATION TO EXPLAIN HOW INDUCTION IS USED IN COOKTOPS IN ELECTRIC RANGES

Applying an alternating current through the induction coil sets up a rapidly changing magnetic field that induces eddy currents in the metal of the saucepan

Eddy currents cause an increase in temperature of the metalo Resistance heating: Due to resistance of saucepan, collisions between moving charges and atoms of the

metal causes heatingo Hysteresis loss: Atoms in the saucepan tend to line up with the magnetic field. The continuous

movement of the magnetic particles, as they try to align themselves with the high frequency alternating magnetic field, produces molecular friction. This, in turn, produces heat.

Heat produced in metal saucepan is used to cook the food Advantages:

o Without loss of thermal energy that occurs with gas cooking Does not warm the air around it Induction cooktop itself is not directly heated

o Induction cookers are more efficient (80%) than gas cookers (43%) Disadvantages:

o Cookware must be made of ferrous materialo Almost instantaneous heating may burn foodo Requires flat-bottomed pans

GATHER SECONDARY INFORMATION TO IDENTIFY HOW EDDY CURRENTS HAVE BEEN UTILISED IN ELECTROMAGNETIC BRAKING

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 magnets near the bottom of the ride, inducing eddy currents and associated magnetic poles in the copper plate. As the plate approaches, a like pole is induced which is repelled by the fixed magnet, resisting the ride capsule’s forward motion. As the plate leaves, an opposite pole is induced which is attracted to the fixed magnet, again resisting the ride’s forward motion. The ride slows down smoothly because the strength of the eddy currents, and hence that of the magnetic fields and forces produced, are directly proportional to the speed of the copper plate’s movement. As the ride slows, the braking force is reduced.

Some trains have electromagnets close to the metal rails to induce eddy currents in the rails. These eddy currents produce magnetic fields in the rails, a like pole ahead of each electromagnet and an opposite pole behind it. The interaction between the electromagnets and the induced magnetic poles opposes the forward motion of the electromagnets and the train to which they are attached. 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.

Triple beam balances commonly used in school laboratories have an aluminium plate fixed to the end of the beam. As the beam swings, the plate passes through the field of a permanent horseshoe magnet. Eddy currents are induced in the plate, setting up magnetic fields and damping the motion of the balance.

3. Generators are used to provide large scale power production

DESCRIBE THE MAIN COMPONENTS OF A GENERATOR

Component of generator

Description

Rotor ‘rotating part of an electrical rotating machine’Consists of a single loop of wire made to rotate within a magnetic field, or several coils of wire wound on an armature.

*In power station generators, rotor is field electromagnetArmature A cylinder of laminated iron mounted on an axle around which coils are wound.

Torque is applied to the axle to make the rotor spin.Coil Each coil usually consists of many turns of copper wire wound on the armature. The

two ends of each coil are connected either to 2 opposite bars of a split-ring commutator (DC) or to 2 slip rings (AC)

Stator ‘stationary functioning parts of an electrical rotating machine’The fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Alternatively, the magnetic field may be provided by two electromagnets.

*In power station generators, stator consists of coils wound around a circular iron core

Field electromagnets

Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are mounted such that opposite poles face each other and wrap around the rotor.

Brushes The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current

from the coils to the external circuit.

Generator primarily consists of a coil of wire that is forced to rotate about an axis in a magnetic fieldo As coil rotates, magnetic flux threading the area of the coil changes, which produces a changing emf

across the ends of the wire, in accordance with Faraday’s Law of Induction: The induced emf in a coil is equal in magnitude to the rate at which the magnetic flux through the coil is changing with time

o If the coil of a generator is forced to rotate at a constant rate, flux threading the coil varies in a sinusoidal manner

When plane of coil is parallel to magnetic field, magnetic flux is zero, change in magnetic flux is maximum/minimum, and emf is maximum/minimum

When plane of coil is perpendicular to magnetic field, magnetic flux is maximum, change in magnetic flux is zero, and emf is zero

*Just as a motor has a dead spot when coil is perpendicular to magnetic field Coil is often wound onto an iron core armature, which behaves like an electromagnet,

intensifying the changes in flux threading the coil, and increasing magnitude of emf that is induced

When number of turns of wire on armature is increased, emf induced increases because coil behaves like a number of individual coils connected in series

PLAN, CHOOSE EQUIPMENT OR RESOURCES FOR, AND PERFORM A FIRST-HAND INVESTIGATION TO DEMONSTRATE THE PRODUCTION OF AN ALTERNATING CURRENT

COMPARE THE STRUCTURE AND FUNCTION OF A GENERATOR TO AN ELECTRIC MOTOR

Similarities: Each consists of a stator that provides a magnetic field and a rotor that rotates within the magnetic field.

(unless power station generator) The magnetic field may be supplied either by permanent magnets or by electromagnets. The rotor in both an electric motor and a generator consists of coils of wire wound on a laminated iron

armature and connected through brushes to an external circuit. An electric motor (both DC and universal) and a DC generator are similar in that their rotor coils are connected

to the external circuit through a split-ring commutator.

Differences: An AC generator is different as its rotor coils are connected to the external circuit through slip rings. An AC induction motor is different from a generator as its rotor coils are not connected to an external circuit

and its field is always supplied by electromagnets. A power station generator has current-carrying coils in the stator, and the field electromagnet as the rotor,

whereas a motor’s stator produces the magnetic field, and rotor is comprised of current-carrying coils. Terminals of a generator go to a circuit with a load, whereas the terminals of a motor go to an emf source, and

the motor itself is the load.

Function An electric motor converts electrical energy into rotational kinetic energy. A generator converts rotational

kinetic energy into electrical energy. A motor rotates when current is supplied while a generator supplies current when rotor is made to rotate.

DESCRIBE THE DIFFERENCES BETWEEN AC AND DC GENERATORS

AC generator: coil is connected to external circuit by slip ringso Slip rings maintain a constant connection between the rotating coil and the external circuit; brushes

make contact with slip rings and transfer the current to terminals of the generatoro Since each terminal is continually connected to one long side of the coil, the emf produced periodically

alternates its direction Produces an alternating current (AC)

DC generator: coil is connected to external circuit by a split ring commutatoro Direct current produced varies with time, but when the plane of the coil is perpendicular to magnetic

field (emf is zero), the commutator reverses the linkage between the two terminals and two long sides of the coil

This ensures that current always flows in one direction in the external circuito Output from DC generator can be made smoother by including more coils set at regular angles on the

armature. Each coil is connected to two opposite segments of a multi-part commutator, and brushes only make contact with the segments connected to the coil producing greatest emf at the particular time (one that is parallel to magnetic field)

GATHER SECONDARY INFORMATION TO DISCUSS ADVANTAGES/DISADVANTAGES OF AC AND DC GENERATORS AND RELATE THESE TO THEIR USE

DC generator: Commutator consists of a number of metal bars separated by narrow gaps filled with insulating material

o Commutator bars wear down until insulating material protrudes, and prevents brushes from making proper contact with the bars, reducing generator’s efficiency

o Pieces of metal worn from commutator bars can become lodged in the gaps, causing a short between bars and reducing generator output

o Spring pressure keeps brushes in contact with multi-part commutator, and wears them down, so brushes need to be replaced regularly

DC generators require higher maintenance than AC generators, and so are less suited for industrial applications

Very large DC machines cannot be built with commutators, because of upper limits: brushes’ current density and maximum voltage of each commutator segment

Current is generated in rotorso The larger the current produced, the heavier the rotor coils must be, placing high demands on bearingso Drawing large currents through commutator-brush connection increases electric arc formation as brush

breaks contact with each bar in turn, reducing generator’s efficiency This limits usefulness of DC generators to low current applications

Output can be made smoother by arranging many coils in a regular pattern around the armature. Brushes arranged to make contact only with commutator bars corresponding to the coils producing the greatest emf at a particular time (the coil parallel to magnetic field)

o Results in output voltage that ‘ripples’ about a mean valueo Advantage for use with equipment that needs a steady voltage rather than a sinusoidally varying voltage

AC generator (alternator): Slip rings have continuous, smooth surface, allowing brushes to remain continuously in contact with slip ring

surfaceo Hence, brushes in AC generators don’t wear out as fast as in DC generatorso No possibility of creating electrical short circuit between commutator segments because slip rings are

already continuous Therefore, AC generators require less maintenance and are more reliable than DC generators,

so most commercial generators are AC In power stations’ AC generators, current is produced in stator windings rather than in the rotor; rotor is the

field electromagnet that spins with 50 Hz frequencyo Power generators have three sets of coils mounted at angles of 120⁰ to each other on stator

Produces three sets of voltage signals (three-phase power generation)o Much easier to draw current through a fixed stator connection, rather than through a commutator and

moving rotor AC generators are better suited to high current demands, and ideal for generating electricity on

a large scale for distribution over a wide area Output is always sinusoidal

o Cannot be used for equipment that needs a steady voltage without rectifying and smoothing circuit

Transmission of AC vs. DC Disadvantages of AC:

o Energy loss by electromagnetic radiationo Skin effecto Proximity effecto Needs heavier duty cable insulation than DC, and higher towers for transport of power, since AC is more

dangerous than DC of same voltage Disadvantages of DC:

o Heat loss because transformers cannot be used to step-up voltageo Voltage changes are difficult

DISCUSS THE ENERGY LOSSES THAT OCCUR AS ENERGY IS FED THROUGH TRANSMISSION LINES FROM THE GENERATOR TO THE CONSUMER

Simple Resistive Energy Losses: Power stations are usually located close to primary energy sources, far away from cities where consumers are

locatedo Transmission lines can have large resistance over a large lengtho Transmission lines are typically made of either (high purity) copper or aluminium, as these metals have

low resistivity Power lost in transmission line is given by formula: Ploss = I2R

o Power lost is proportional to square of current Using step-up transformers to increase voltage and reduce current before transmission, reduces power loss

o AC is the type of electricity generally transmitted over long distances, since transformers can be used to change voltage of AC currents

Step-down transformers are used closer to the consumer to step down the voltage for practical applications Generators output 23 kV, transformers step up voltage to 330 kV, major terminal substation steps down to 132

kV, terminal substation steps down to 33 kV, zone substation steps down to 11 kV, pole transformers step voltage down to 415V for industry and 240V for domestic consumption

o Carrying high voltage requires high poles or towers and large insulators. These are expensive to build and maintain and have an adverse effect on the visual environment.

Skin Effect: An AC current creates a changing magnetic field, which induces eddy currents in the conductor. These eddy

currents alter the uniform current density in a conductor. AC tends to travel through outer portion of a conductor, reducing effective cross-sectional area, and increasing resistance/heating losses.

o As frequency is increased, depth to which current flow can penetrate decreases

o DC does not suffer from this loss because direct current does not create a changing magnetic field.

Proximity Effect and Electromagnetic RadiationInductive Energy Losses:

Induction of eddy currents in metal parts of transmission towers is minimised by distance at which wires are held away from the tower by insulators

Energy losses in Transformers: (given in another DotPoint)

ASSESS THE EFFECTS OF THE DEVELOPMENT OF AC GENERATORS ON SOCIETY AND THE ENVIRONMENT

Ramifications of AC 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.

Society: Many tasks that were once performed by hand can now be accomplished with electrical appliances

o Domestic and industrial work now requires less labour Nevertheless, has not increased leisure time as people work to earn the money to buy the many

electrical appliances available Women are freed from household chores and able to work, yet this raises difficulties in

childcare Reduction in unskilled jobs Increase in unemployment

Ready availability of electricity has led to increasing dependency on electricityo Any disruption to power supply compromises safety, causes widespread inconvenience and loss of

production Hospitals are forced to have a back-up electricity supply “just in case” A major electricity failure can precipitate an economic crisis

Social values may give way to economic pressureso In developing countries, poorest people often lose their livelihood to make way for new energy

developments to power AC generators Increased rates of electrical fires and electrocutions due to the high voltages necessary for AC transmission

Environment: Air pollution from burning fossil fuels may cause acid rain

o Oxides of nitrogen and sulfur, particulates…

o AC power generating plants can be located far away from urban areas, shifting pollution away from homes and workplaces

Carbon dioxide released by burning fossil fuels to power AC generators has contributed to greenhouse effect Nuclear power stations leave radioactive waste that will last for a long time Power transmission lines cut through environmentally sensitive areas

Assess: Many people enjoy increased convenience and leisure, and many new industries flourish on new technologies

made possible by electricity For others, it has led to dislocation and unemployment Development of electricity has led to environmental degradation

ANALYSE SECONDARY INFORMATION ON THE COMPETITION BETWEEN WESTINGHOUSE AND EDISON TO SUPPLY ELECTRICITY TO CITIES

Edison favoured generating and supplying direct current (DC), while Westinghouse promoted the use of alternating current (AC) electricity

Edison had initial advantage Technology for DC generation was well established, and it worked well over short distances DC worked well for incandescent lamps which were standard load at the time, and motors DC systems could be used with storage batteries, as backup power supplies Edison had invented a meter to allow customers to be billed for energy proportional to consumption, but this

meter only worked with direct currentTesla devised a system for generation and transmission of AC power. He partnered with George Westinghouse to commercialize this systemEdison had vested interest in DC as he owned hundreds of DC power stations, all of his electrical inventions ran on DC, and the Edison Electric Light Company powered its lights using DC

Published a booklet entitled ‘A Warning’, describing fatal AC accidents Attempted to prove that AC was dangerous by electrocuting animals in a public demonstration using a

Westinghouse AC generator Lobbied against use of AC in state legislature Convinced authorities to use AC for the electric chair, to promote idea that AC was deadlier than DC

o Hired Harold Brown and his assistant Dr. Peterson to develop an AC electric chairo Dr. Peterson headed a committee that advised government on AC as best method of electrocution

Westinghouse’s AC current was able to be more efficiently transmitted over long distances, and ultimately came to be the dominant form in which electricity is generated world-wide

1) Successful distribution of AC was demonstrated in London, Italy, Paris2) In a demonstration, AC was transmitted 160km from Frankfurt with 77% efficiency (far more than with DC)3) Westinghouse offered to power the world’s first all-electric fair for half the previous price, and was given the

contract4) Westinghouse’s AC system was chosen over Edison’s DC to power the fairgrounds of the World’s Columbian

Exposition5) Against Edison’s proposal, Westinghouse won the international Niagara Falls Commission contract. AC system

was used to transmit electric power from Niagara Falls to Buffalo6) Tesla’s invention of the polyphase induction motor which operates only on AC and proved economical for

factories and consumer machines, supported Westinghouse’s AC current over Edison’s DC

Advantages of Westinghouse’s AC over Edison’s DC electricity Energy losses through transmission wires

o Transforming DC power from one voltage to another was difficult and expensive due to the need for a large spinning rotary converter or motor-generator set,

To counter energy losses through transmission wires with DC, Edison generated power close to where it was consumed, and installed large conductors

o AC could be generated at low voltages, stepped up to high voltages for transmission over large distances, and stepped down again for consumer use, all with simple transformer coils

High voltage/smaller current meant smaller energy losses over long distances Power stations could be further apart, and conductors could be lighter and cheaper, than with

DC current DC could only be generated/distributed at voltages at which it was used by consumers, but the voltage of an AC

current could be changed as required through transformerso DC operated at the same voltage level throughout; e.g. a 100 volt lamp at the customer's location would

be connected to a generator supplying 110 volts, to allow for some voltage drop in the wires DC voltage depends on speed of rotation of generator’s armature. High speeds were needed for

high voltages to allow for long distance transmission, but low voltages were needed by consumers

o AC could be generated at a voltage maximising the generator’s efficiency, stepped up to high voltages for transmission over large distances, and stepped down again for consumer use

The Return of Edison’s DC? DC doesn’t cause energy losses through electromagnetic induction or electromagnetic radiation AC current loses energy by releasing electromagnetic radiation Switching between DC and AC at high/low voltages is now possible through solid-state switching ‘Skin effect’ means AC tends to travel through outer portion of a conductor, reducing effective cross-sectional

area, and increasing resistance/heating losses. Three-phase AC transmission requires at least 3 conductors, whereas DC transmission requires only two

conductors, reducing cost of conductors over long distances AC generating stations must be synchronised so that they operate at the same frequency and are kept in phase

with each other Underground/underwater cables transmit DC more effectively than AC

DC could become the preferred current over long distances

GATHER AND ANALYSE INFORMATION TO IDENTIFY HOW TRANSMISSION LINES ARE:– INSULATED FROM SUPPORTING STRUCTURES– PROTECTED FROM LIGHTNING STRIKES

Insulated from supporting structures: Suspension insulators separate high voltage transmission lines from

metal support towerso Individual sections of insulators are disk shaped

To increase distance a current has to pass to reach the support structure

Less likely to get wet (water is a conductor) To minimise dust/grime accumulation (possible

conductors)o Metal links between individual sections are isolated from each

other, so there is no continuity of conductanceo The higher the voltage, the longer the insulation chain

Static dischargers are also placed between insulators and support structures

Protected from lightning strikes: If lightning strikes a transmission line, a power surge may occur through the transmission line, which could

damage a nearby substation Uppermost wires in a power tower are called shield conductors

o Normally carry no current, and are directly connected to transmission towers Shield conductors can conduct the charge between earth and clouds as it builds up, to

neutralise charge distribution In the event of a lightning strike, the current is safely conducted to the ground through the metal transmission

towero Towers are well earthed, with a large surface area of metal buried in the ground

Towers are widely spaced, so that in case one tower is struck by lightning, adjacent towers suffer no damage

4. Transformers allow generated voltage to be either increased or decreased before it is used

COMPARE STEP-UP AND STEP-DOWN TRANSFORMERS

Step-up transformer Step-down transformer

Consist of two inductively coupled coils wound on a laminated iron core

More turns in the secondary coil than the

primary coil

Fewer turns in the secondary coil than the primary

coil

Higher output voltage than input voltage Lower output voltage than input voltage

Lower output current than input current Higher output current than input current

Used at power stations to increase voltage and

reduce current for long-distance transmission

Used at substations and in pole transformers to

reduce transmission line voltage for domestic and

industrial use

Used in television sets and computer monitors

to increase voltage to operate cathode ray

tubes

Used in computers, radios, and CD players to

reduce household electricity to very low voltages

for electronic components

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

Transformers must have an iron core of high magnetic permeability so that almost all the magnetic flux produced in the primary coil threads the secondary coil

When an alternating current from the input line flows through primary coil, a constantly changing magnetic flux is created and threads secondary coil, producing an AC voltage at secondary coil terminals with same frequency as the supplied AC voltage (mutual induction)

The changing magnetic field creates a back emf in the primary coil that opposes the original current. This back emf is almost equal to the original emf in the primary coil, so that the ‘exciting current’ in the primary coil is very small. This ensures that the Law of Conservation of Energy is followed, as energy has been transferred from primary coil to secondary coil

Rate of change of flux through both coils is the same. Faraday’s Law is used to show that secondary voltage is found using:

V s=ns∆ϕ∆ t

Similarly, input primary voltage, is given by:

V p=np∆ ϕ∆ t

Therefore, relationship between ratio of number of turns in the primary and secondary coils and the ratio of primary to secondary voltage, is given by the transformer equation

V p

V s=n pns

In the transformer circuit schematic, the two lines represent the iron core

In actual transformers, windings are wound on top of each other, not on separate legs, to reduce leakage inductance

Step-up transformer: Provides an output voltage that is greater than the input voltage, and output current that is lower than input

current ns is greater than np, so vs will be greater than vp Secondary coil has more turns than primary coil, so secondary voltage will be greater than primary voltage

Step-down transformer: Provides an output voltage that is less than the input voltage, and output current that is higher than input

current ns is lesser than np, so Vs will be lesser than Vp

Secondary coil has less turns than primary coil, so secondary voltage will be smaller than primary voltage

EXPLAIN WHY VOLTAGE TRANSFORMATIONS ARE RELATED TO CONSERVATION OF ENERGY

Principle of Conservation of Energy: energy cannot be created nor destroyed, but it can be transformed from one form to another

o The rate of energy input to the primary coil must be greater than or equal to the rate of supply of energy from the secondary coil

i.e. input power ≥ output power

Therefore, in the ideal transformer: Pp=P s Vp Ip = Vs Is I sI p

=npns

Thus, voltage transformations in a transformer are related to conservation of energy because output power cannot be greater than input power. Therefore, when the voltage is raised from primary to secondary coils, the current is proportionally lowered.

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 consumed in homes and industry at 240V/415V. If there were no transformers, electricity would have to be generated and distributed at same voltages. To supply power demands at these low voltages, currents would have to be high

o Power lost in transmission line is given by formula: P loss = I2R Power lost is proportional to square of current

o Power stations are usually located close to primary energy sources, far away from cities where consumers are located. Transmission lines can have large resistance over a large length

o Therefore, without the use of transformers to step-up voltage before long-distance transmission, power losses would be significant

This would require many power stations spaced every few kilometres, separate power stations to produce different voltages, and an expensive, unsightly web of cables

Using step-up transformers to increase voltage and reduce current before transmission, reduces power losso AC is the type of electricity generally transmitted over long distances, since transformers can be used to

change voltage of AC currents Step-down transformers are used closer to the consumer to step down the voltage for practical applications

EXPLAIN THE ROLE OF TRANSFORMERS IN ELECTRICITY SUB-STATIONS

Generators output 23 kV, transformers step up voltage to 330 kV, major terminal substation steps down to 132 kV, terminal substation steps down to 33 kV, zone substation steps down to 11 kV, pole transformers step voltage down to 415V for industry and 240V for domestic consumption

The role of transformers in electricity sub-stations is to progressively reduce the voltage as it comes closer to the consumer. At each stage, the output voltage is chosen to match the power demand and the distances over which supply is needed.

DESCRIBE THE PURPOSE OF TRANSFORMERS IN ELECTRICAL CIRCUITS a n d DISCUSS WHY SOME ELECTRICAL APPLIANCES IN THE HOME THAT ARE CONNECTED TO THE MAINS DOMESTIC POWER SUPPLY USE A TRANSFORMER

A transformer is a magnetic circuit with two multi-turn coils wound onto a common core, designed to change the size of an alternating (AC) voltage, so that the single voltage produced by an e.m.f. source in a circuit can be changed as required

Domestic supply is 240V AC, and industrial supply is 415V AC, but printed circuit boards, semiconductor devices, and other electronics typically need 3-4V

o Step-down transformers used in electronic appliances to provide lower voltages, e.g. for amplifier circuits in radios, cordless telephones, laptop computers

Small appliances e.g. mobile phone chargers, require low DC voltages to recharge the batteries A step-down transformer-rectifier ‘power-cube’ may be built into plug of power supply lead

that connects to main supplyo Rectifiers convert AC to DC

Or normal power lead may connect mains to built-in power supply unit that contains a step-down transformer-rectifier

Also found in answering machines, digital cameras, computers, phones, and other devices Cathode ray tubes in television screens need up to 25kV, to accelerate electrons towards the screen

o Step-up transformers are placed between AC supply and the component to provide very high voltages to drive cathode ray tubes

Many appliances e.g. televisions, contain step-up and multi-tapped step-down transformers, capable of supplying a range of different voltages for various components

There may be external connections (taps) to various intermediate points on the winding to allow selection of the voltage ratio.

DISCUSS THE IMPACT OF THE DEVELOPMENT OF TRANSFORMERS ON SOCIETY

The development of transformers has made it possible to transmit electrical energy efficiently over great distances, and then stepped down at the point of use

Even very remote communities now have access to grid-supplied electricity which is stepped down locally by transformers

o Raised living standards in rural communities e.g. through electric lighting, refrigerationo Increased the scope of rural industries

Large cities have been allowed to spread, because electricity can be transmitted over long distances.o People don’t have to live close to a power source to have access to electricityo This has led to social dislocation as people have moved further from family, friends and workplaces.

Industry is decentralised, and no longer clustered around power stations or other sources of energy. o Facilitated the development of industrial areas away from residential areas.

Relocated pollution away from homes, but it means that many people now spend significant time travelling between home and work.

Ready availability of electricity due to transformers has led to increasing dependency on electricityo Electricity to every home has become an affordable necessity rather than a luxury.o Any disruption to power supply compromises safety, causes widespread inconvenience and loss of

production Hospitals are forced to have a back-up electricity supply “just in case” A major electricity failure can precipitate an economic crisis

Reduced pollution and cost of electricityo Without transformers to transmit electrical energy efficiently over large distances, many more power

stations with their associated pollution would be necessary to supply power to their local areas. o Without transformers, different industries requiring different voltages would have to build generators

to produce those specific voltages. Development of transformers was crucial in determining the success of the AC system over DC electricity, and in

the widespread proliferation of electricity grids

PERFORM AN INVESTIGATION TO MODEL THE STRUCTURE OF A TRANSFORMER TO DEMONSTRATE HOW SECONDARY VOLTAGE IS PRODUCED

SOLVE PROBLEMS AND ANALYSE INFORMATION ABOUT TRANSFORMERS USING:

V pV s

=npns

GATHER, ANALYSE AND USE AVAILABLE EVIDENCE TO DISCUSS HOW DIFFICULTIES OF HEATING CAUSED BY EDDY CURRENTS IN TRANSFORMERS MAY BE OVERCOME

Energy is lost through induction of eddy currents in iron core of transformers, as these eddy currents partially oppose the magnetic field in the transformer by Lenz’s law

o Circulation of eddy currents in transformer core generates heat because of the iron’s resistance. This heat is an energy loss from the electrical system, and excessive heating can damage the transformer

o As transformer starts to heat up, eddy current heat losses will be even greater, since resistance of metal increases with temperature

To minimise this energy loss: Transformer cores are made of laminated iron (many thin layers of iron sandwiched together with thin

insulating layers separating them) o This limits eddy currents to one lamina thickness, and

hence reduces heat loss

o Since Ploss∝ I 2 and I∝ dϕdt , hence dϕdt in each lamination

is decreased (to ¼ if 4 laminations), and Ploss in each lamination decreased to 1/16, and total Ploss is reduced to 25%

o Since laminations reduce width of each layer, each layer has a higher resistance to eddy currents

Transformer cores made of granular ferrites are used, which allow magnetic flux to change freely but have a high resistance to eddy currents (high magnetic permeability but low electrical conductivity)

Iron alloyed with 3% silicon to increase resistivity by 350%

To cool down the transformer so that it isn’t damaged by overheating, and to reduce resistance: Cooling fins on outside of transformer allow heat to dissipate faster over larger surface area Transformer case may be made of black material so that internal heat is efficiently radiated outside Electric fan to assist air circulation, to remove excess heat faster

o Thermostatically controlled, i.e. starts working at a certain temperature Transformer case may be filled with non-conducting oil in radiator pipes, transporting internal heat to the

outside where it can be dissipatedo Oil may circulate by convection, or may be assisted by a pump

Large transformers are located in well-ventilated areas to maximise airflow around them for cooling

Other Sources of Energy Loss in Transformers: Winding resistance (Copper Loss)

o Current flowing through windings causes resistive heating of conductors Leakage inductance

o Any flux traversing a path that goes outside the windings may give rise to eddy currents in support structures, causing heat loss

Hysteresis losso Magnetic domains of the core tend to line up with the magnetic field. The continuous movement of the

magnetic domains, as they try to align themselves with the changing magnetic field, produces a ‘molecular friction’ resulting in heat loss

Magnetorestrictiono Magnetic flux in the core causes it to physically expand and contract with each cycle of the magnetic

field, causing losses due to frictional heating

5. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy

DESCRIBE THE MAIN FEATURES OF AN AC ELECTRIC MOTOR

There are two main types of electric motors that run on AC: universal motors and induction motors.

Universal motor : a series-wound motor that may be operated on either AC or DC electricityo Used for small machines e.g. portable drills or food mixers

Similar in construction to a DC motor (uses a split-ring commutator)o Except the external magnetic field is supplied by stator electromagnets that are connected in series

with the coils of the armature via brushes Since current and magnetic field are both changing direction 50 Hz with AC input, the motor will

continue to rotate in the same directiono A variable resistor controls speed of the motor by varying the current through coils of the armature and

field coils of the stator. o A universal motor will always rotate in the same direction, regardless of which way it is connected to

the DC sourceo Universal motors have laminated stator cores, so that the changing magnetic field produced by the

stator does not induce eddy currents in the stator core itself

Parts of a DC motor (altered for universal motor)Feature Description RoleStator Electromagnets

Each stator coil (or “field” coil) is wound on a soft iron core attached to the casing of the motor. The coils are shaped to fit around the armature.

Each opposed pair of stator coils (connected in series with armature coils) produces an external magnetic field which interacts with the current in the armature cause the coils to rotate. The iron core concentrates the field.

Rotor Rotating part of the motor; consists of armature and coil

Armature Consists of a cylinder of laminated iron mounted on an axle. Axle protrudes from the casing, enabling movement of coil to be used to do work

The armature carries the rotor coils. The iron core greatly concentrates the external magnetic field, increasing the torque on the armature. The laminations reduce eddy currents which might otherwise overheat the armature.

Rotor coil(s) One or several coils, usually of several turns of insulated wire, wound onto the armature. Ends of the coils are connected to commutator

The coils provide torque, as the current passing through the coils interacts with the magnetic field. As the coils are mounted firmly on the rotor, any torque acting on the coils is transferred to the rotor and thence to the axle.

Split-ring commutator

A broad ring of metal mounted on the axle at one end of the armature, and cut into an even number of separate bars (two in a simple motor). Each opposite pair of bars is connected to one coil.

Mechanical switch for reversing the direction of current flowing the coil every half-turn so that coil continues rotating in same direction

Brushes Conducting contacts made of compressed carbon connected to external circuit; spring-loaded to contact commutator as it turns

Connect commutator to the source of emf. Necessary to stop connecting wires from becoming tangled

Axle A cylindrical bar of hardened steel passing through the centre of the armature and the commutator.

The axle provides a centre of rotation for the moving parts of the motor. Axle protrudes from casing, enabling movement of coil to be used to do work

Induction motor: An AC motor in which torque is produced by the interaction of a rotating magnetic field produced by the stator, and currents induced in the rotor

Stator of a three-phase induction motor consists of three sets of opposing coils, each of which is connected to a different phase of the mains electrical supply

o Iron core in each coil

o Therefore, the magnetic field in the cylindrical space inside the stator rotates at 50 Hz Squirrel-cage rotor consists of a few conducting bars made of aluminium or copper arranged to form a cylinder

o Two end rings at the ends of the bars ‘short-circuit’ the barso Bars and end rings encased in a laminated iron armature, which is mounted on an axle

How it workso As magnetic field is moving with respect to the rotor bars, a current is created in the barso Bars carrying this current in a magnetic field, experience a force

The force is always in same direction as movement of the magnetic fieldo Cage is constantly ‘slipping’ behind the magnetic field (especially when heavy load) so that there is

relative motion between the cage and magnetic field, so the cage experiences a torque Slip speed: difference between speed of the rotating magnetic field and speed of the rotor

Advantages of AC Induction Motors: Three-phase motors are self-starting No brushes/commutators

o Less frictional wear, no electrical discharges as brushes cross the gaps between rings Simpler to construct, easier to maintain, and more robust/reliable Run on AC Smaller and lighter for equivalent power output Make less noise when they run Large output, and therefore suited for industrial uses

Disadvantages of AC Induction Motors: Maximum speed is 50 Hz (due to AC current input) or 3000 rpm, limiting their uses in industry Single-phase motors are not self-starting, and have low efficiency Only operate on AC Starting torque is high, but they start slowly Induction motors lose efficiency when starting through production of larger eddy currents

PERFORM AN INVESTIGATION TO DEMONSTRATE THE PRINCIPLE OF AN AC INDUCTION MOTOR

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

Principle of Conservation of Energy: energy cannot be created or destroyed, but it can be transformed from one type to another

*Note o An energy transfer occurs when energy in one form moves from one object or location to another

object or location as the same form of energy, e.g. heat transferred by conduction from the electric heating element to the water in a domestic hot water system.

o An energy transformation occurs when energy is changed from one form to another form within an appliance, e.g. electrical energy changed into useful rotational kinetic energy in a food processor.

From Electrical Energy: Hair dryer (home):

o Some electrical energy is transformed into heat due to eddy currents in the laminated iron core, as parts of the motor are heated up

o Electric motor transforms electrical energy into mechanical energy (the rotor spins) Mechanical energy of rotor is transferred to axle and to fan.

o Mechanical energy of the rotor is transformed into sound, and transferred into the kinetic energy of air particles by a fan that is attached to the axle

o The air passes through a heating element where electrical energy is transformed into heat energy (i.e. kinetic energy of particles) and light energy.

o Heat energy is transferred out of the hair dryer by direct conduction to the air particles, and by convection as the air particles carry the energy from the dryer

Analysis e.g. “In electric kettles and toasters, current from the mains causes heating in a high-resistance element”

To Electrical Energy Solar cell (industry):

o Transforms radiant energy from the sun into electrical energy Coal-fired power plant (industry):

o Chemical energy of coal transformed into heat energy (kinetic energy of particles) as it is burnedo Heat energy transformed into kinetic (mechanical) energy as moving steamo Kinetic energy of steam converted into rotational mechanical energy of turbineo Mechanical energy of turbine converted into electrical energy via a generator

Microphone (home/industry) o Converts kinetic energy of air particles (sound) into electrical energy

Batterieso Transform chemical energy into electrical energy

Transfer of Electrical Energy: Transformer (home/industry): Electrical energy is transferred from the primary coil to the secondary coil

o Occurs in plug-in voltage adaptors for many small (home) appliances such as CD players, and in welders too

o Electrical energy in primary coil is converted into ‘magnetic potential energy’ by creating a changing magnetic field. As a result, charged particles in secondary coil experience a force and move as a result of their magnetic potential energy within the magnetic field, and so their magnetic potential energy is converted into electrical energy.

Induction motor (home/industry): Electrical energy is transferred by induction from the stator to the rotor Cathode ray tube (home): Transfers electrical energy from power supply, to the screen in the form of a beam of

electrons Hair dryer (home): Heat energy is transferred out of the hair dryer by direct conduction to the air particles, and

by convection as the air particles carry the energy from the dryer