Motors & Generators d

PHYSICS TUTORIAL NOTES

Motors & Generators

SYLLABUS TOPIC 9.3

These notes are meant as a guide only and are designed to focus your thoughts on the dot points mentioned in the syllabus. They give a very brief overview of the topic and should be used in conjunction with your class notes, your textbooks and your research from other areas such as the library and the Internet.

This is a draft and will be updated soon.

Thank you for your patience.

Notes compiled by:

CARESA EDUCATION SERVICES

Motors and Generators

Contextual Outline

Modern industrialised society is geared to using electricity. Electricity has characteristics that have made it uniquely appropriate for powering a highly technological society. There are many energy sources that can be readily converted into electricity. In Australia, most power plants burn a fuel, such as coal, or use the energy of falling water to generate electricity on a large scale. Electricity is also relatively easy to distribute. Electricity authorities use high-voltage transmission lines and transformers to distribute electricity to homes and industries around each state. Voltages can be as high as 5 x 10⁵ volts from power stations but by the time this reaches homes, the electricity has been transformed to 240 volts. While it is relatively economical to generate electric power at a steady rate, there are both financial and environmental issues that should be considered when assessing the long-term impact of supplying commercial and household power.

The design of a motor for an electrical appliance requires consideration of whether it will run at a set speed, how much power it must supply, whether it will be powered by AC or DC and what reliability is required. The essentials of an electric motor are the supply of electrical energy to a coil in a magnetic field causing it to rotate.

The generation of electrical power requires relative motion between a magnetic field and a conductor. In a generator, mechanical energy is converted into electrical energy while the opposite occurs in an electric motor.

The electricity produced by most generators is in the form of alternating current. In general AC generators, motors and other electrical equipment are simpler, cheaper and more reliable than their DC counterparts. AC electricity can be easily transformed into higher or lower voltages making it more versatile than DC electricity.

This module increases students’ understanding of the applications and uses of physics and the implications of physics for society and the environment.

“1. Motors use the effect of forces on current-carrying conductors in magnetic fields.” (syllabus)

“Students learn to 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” (syllabus)

The effects of these variations can be summarised in the equation: F = B I l sinq

* By varying the strength of the magnetic field in which the conductor is located it can be shown that the force is directly proportional to the strength of the magnetic field. F a B

* By varying the magnitude of the current in the conductor it can be shown that

the force is directly proportional to the current. F a I

* By varying the length of the conductor in the external magnetic field it can be shown that the force is directly proportional to the length of the conductor. F a l

* By varying the angle between the direction of the external magnetic field and the direction of the length of the conductor it can be shown that the force is directly proportional to the sine of the angle between the magnetic field and the conductor i.e. it is proportional to the perpendicular component of the length in the magnetic field.

F a sinq

“Students learn to describe qualitatively and quantitatively the force between long parallel current-carrying conductors:

” (syllabus)

Qualitatively: The force per metre between two parallel current carrying wires increases as the current increases and decreases as the distance between the wires increases.

Quantitatively: The force between two long, parallel conductors is directly proportional to the product of the currents in the wires and the parallel length of the wires and inversely proportional to the distance between the wires.

Parallel wires carrying currents in the same direction attract each other while parallel wires carrying currents in opposite directions repel each other.

“Students learn to define torque as the turning moment of a force using: ” (syllabus)

The torque is proportional to the product of the perpendicular component of the force and the distance from the turning axis. t = Fd

Draw a line between the axis of rotation and the point of application of the force. The length of this line is d. Now use vector components to determine the component of the force perpendicular to this line. Call the perpendicular component F. The torque is given by the product Fd.

Consider a bar AB pivoted at A. Suppose a force F is applied at B at an angle q to the bar. The perpendicular component of the force F is F sinq so the torque about A is F sinq d or t = Fd sinq

“Students learn to identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field” (syllabus)

The motor effect refers to the force experienced by a conductor carrying a current in a magnetic field. It occurs in motors, current balances and moving coil meters as well as between parallel current-carrying wires.

“Students learn to describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces” (syllabus)

Consider a single loop of wire, WXYZ, of length l and breadth b, situated between the poles of a magnet so that it is parallel to a magnetic field of flux density B. A current I flows through the loop in the direction ZYXW.

The force on side WX is given by F = BIl and the torque on side WX is given by t = BIlb/2

The force on side YZ is given by F = BIl and the torque on side YZ is given by t = BIlb/2

While the forces are equal and opposite and produce a resultant force of zero they are not in the same line of action and so produce a torque.

The total torque on the loop is BIlb/2 + BIlb/2 = BIlb = BIA

Suppose the loop was rotated so that it was vertical to the magnetic field.

The new force on side WX is still F = BIl, however the new torque on side WX is zero. Similarly the torque on ZY is zero and the total torque on the coil is zero.

Suppose the loop was rotated so that it was inclined at an angle q to the magnetic field.

The force on the side WX is still F = BIl but the component of the force on the side WX that is perpendicular to WZ is given by F = BIl cos q

The torque on side WX is t = BIl cos q b/2 = BIlb cos q /2

Also the torque on side YZ is t = BIl cos q b/2 = BIlb cos q /2

The torque on the side WX + the torque on side YZ when the coil is inclined at an angle q to the magnetic field is given by t = BIlb cos q = BIA cos q

If WXYZ consisted of n turns instead of being a single loop, the torque is

t = nBIA cos q

“Students learn to describe the main features of a DC electric motor and the role of each feature” (syllabus)

A coil of wire is balanced on an axle between the poles of a magnet. When a current passes through the wire it cuts lines of magnetic flux and the wire experiences a force, causing it to move.

A coil in an electric motor consists of several hundred loops of wire i.e. the wire is wound around several hundred times. However to simplify the explanation below a single loop ABCD from the diagram above is considered here.

The current will flow from positive to negative, hence its path will be from X toA toB toC toD toY. As the current passes from A to B a force is induced in the wire pushing it down (from Fleming’s left hand rule or the right hand palm rule). As the current flows from B to C it is flowing parallel to the magnetic field and is not cutting lines of flux. Consequently BC will not experience a force. As the current flows from C to D the wire will experience a force upwards.

So if we look at the coil from the front the left side will be pushed down and the right side will be pushed up which will tend to make it rotate anticlockwise i.e. it will experience an anticlockwise torque when viewed from the front.

If the wire AB rotated half a turn so that is was now on the right side of the coil with the current flowing from A to B it would still experience a downward force. Similarly wire CD would still experience an upward force. The result of this would be that the coil would come to rest in a vertical position with AB experiencing a downward force and CD experiencing an upward force.

To solve this problem a split-ring commutator is used. The operation of the commutator is explained below and this allows the coil of the motor to rotate.

The split-ring commutator: The split ring consists of two semi-circular pieces of conducting material such as brass or copper. Current is transferred to the semi-circular conductors by means of brushes; not brushes of hair or fibre but brushes made of a conducting material such as copper or graphite. As the coil passes the vertical position its momentum is sufficient to carry it a little further. As it does so the split-ring commutator rotates so that the brushes touch the opposite sides of the commutator. This changes the direction of the current through the coil so that instead of flowing from A to B in the diagram, it now travels from B toA. This causes the torque to always be in the one direction (in this case anticlockwise when viewed from in front) so that the coil rotates in the one direction.

The brushes: The brushes conduct current from the power source to the commutator and from the commutator back to the power source.

The magnets are usually curved: The curved magnets provide a radial magnetic field and this provides a constant torque when a current flows through the coil while it is in the magnetic field.

Since t = BAIn cosq and q = 90^o while the coil is in the radial magnetic field then the torque is constant.

Motors often have a soft-iron core: The soft-iron core concentrates the magnetic field i.e. it causes the lines of flux to line up in the same direction.

“Students learn to identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets” (syllabus)

The required magnetic fields in DC motors can be produced either by current-carrying coils (electromagnets) or permanent magnets.

“Students solve problems using:

” (syllabus)

Q. Two parallel wires, each 50 cm long each carry a current of 4.0 amps in opposite directions. If the wires are 2.0 mm apart, what is the force between them?

A. F = kI₁I₂l / d = 2 x 10^-7 x 4 x 4 x 0.5 / 0.002

= 8.0 x 10^-4 N repulsion

“Students perform a first-hand investigation to demonstrate the motor effect” (syllabus)

The motor effect can be demonstrated with a current balance. A current flowing through the balance when it is in a magnetic field will produce a torque and so demonstrate the motor effect. The motor effect can also be demonstrated by suspending a taut length of fine wire between the poles of a horseshoe magnet. When a current flows through the wire the wire will “twitch” as it is subjected to a force. The direction of the force can be observed and the right hand palm rule can be verified.

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

” (syllabus)

Q. A wire carrying a current of 5.0 Amps cuts a magnetic field of flux density 4.0 Tesla. The wire is inclined at 60^o to the magnetic field and the length of wire in the field is 20 cm.

Calculate the force on the wire.

A. F = BIl sinq = 4 x 5 x 0.2 x sin 60^o = 3.46 N out of the page

“Students solve problems and analyse information about simple motors” (syllabus)

Q. Calculate the torque on a rectangular coil of 200 turns 20 cm long and 10 cm wide carrying a current of 5 amps in a magnetic field of 2.0 T when the coil is inclined at 30^o to the magnetic field.

A. t = nBIA cosq = 200 x 2.0 x 5 x 0.1 x 0.2 x cos 30^o = 34.64 Nm.

“Students identify data sources, gather and process information to qualitatively describe the application of the motor effect in:

– the galvanometer

– the loudspeaker” (syllabus)

The galvanometer: The galvanometer has a coil mounted on a soft iron core between the poles of a magnet. The coil is free to turn and when a current flows through the coil it produces a torque due to the motor effect. The magnitude of the torque is proportional to the current. The spiral spring provides a counter torque so that the deflection of the pointer is proportional to the current.

The loudspeaker: The loudspeaker consists of a powerful magnet with light cylindrical coils, known as the voice coils, mounted over the central pole piece without touching it. The output current from the radio fluctuates as it passes through the voice coil. Since the voice coil is in a magnetic field it experiences a fluctuating force that causes it to vibrate due to the motor effect. The rigid paper cone is attached to the voice coils so the vibration is transmitted to the paper cone. This causes the air around it to vibrate and this is detected as sound.

“2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage” (syllabus)

“Students learn to outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet” (syllabus)

In 1831, Faraday found that moving a bar magnet into and out of a coil of insulated wire produced a current. He connected a coil of insulated copper wire to a galvanometer and found that when the magnet was moving within the coil, a current was recorded by the galvanometer.

“Students learn to define magnetic field strength B as magnetic flux density” (syllabus)

The following has been copied from the superseded 4-unit science syllabus that was discontinued at the end of 2000.

This is also what is emphasised in several university textbooks. Consequently the terms “magnetic flux density” and “magnetic induction” can be used for the term B.

“Students learn to describe the concept of magnetic flux in terms of magnetic flux density and surface area” (syllabus)

Magnetic flux refers to the total flux, i.e. the total lines of flux and is measured in webers, Wb.

Magnetic flux density refers to the concentration of lines of flux per unit area. It is measured in webers per square metre (Wbm^-2), i.e. tesla (T).

“Students learn to describe generated potential difference as the rate of change of magnetic flux through a circuit” (syllabus)

This is known as Faraday’s Law which states that the emf produced in a conductor is proportional to the rate of it cutting lines of magnetic flux.

It can be expressed mathematically as e = - DF / Dt = - DBA / Dt

“Students learn to account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors” (syllabus)

Lenz’s law states that the direction of the induced e.m.f. opposes the change that caused it.

If the direction of the induced e.m.f. was such as to enhance rather than oppose the change that caused it then an increase in energy would result, leading to a greater energy output than input. This violates the principle of conservation of energy and can not occur.

In an electric motor an emf is supplied and this produces a current that makes the coil turn. (motor effect) Since we now have a conductor moving through a magnetic field an emf is produced. Lenz’s law says that the induced emf opposes the change that caused it hence the induced emf is in the opposite direction to the supply emf. Since the induced emf goes back the other way it is known as a back emf.

A current in an electric motor causes the coil to turn. As the coil turns it cuts lines of flux and this induces an e.m.f. in the opposite direction to the supply e.m.f. This is known as a back e.m.f.

“Students learn to explain that, in electric motors, back emf opposes the supply emf” (syllabus)

“Students learn to explain the production of eddy currents in terms of Lenz’s Law” (syllabus)

Eddy currents are “whirlpools” of induced current when a flat or 3-D solid conductor is situated in a changing magnetic field.

When there is relative motion between a flat or 3-D solid conductor and a magnetic field, an e.m.f. will be induced to oppose this relative motion. This opposition to change is an application of Lenz’s law.

“Students perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet” (syllabus)

Connect a solenoid to a galvanometer. Insert the magnet into the solenoid and note any change to the galvanometer reading. Withdraw the magnet from the solenoid and note any change to the galvanometer reading. Reverse the polarity of the magnet and repeat the procedure. Repeat the procedure by keeping the magnet stationary and moving the solenoid towards and away from the magnet and observing any change to the galvanometer reading.

“Students 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” (syllabus)

Distance between the coil and magnet: Connect a coil to a galvanometer. Move a magnet towards the coil at constant speed and stop when the magnet is 10 cm from the coil. Note the reading on the galvanometer as the coil is moving. Repeat the procedure by moving the magnet towards the coil at the same constant speed and stop when the magnet is 1 cm from the coil. Note the reading on the galvanometer as the coil is moving.

The galvanometer should give a higher reading as the magnet gets closer to the coil since more lines of flux will be cutting the coil.

Strength of the magnet: Connect a coil to a galvanometer. Move a magnet towards and into the coil at constant speed. Note the reading on the galvanometer as the magnet is moving. Now get two magnets and use sticky tape to join them together with like poles together. Move the pair of magnets towards and into the coil at the same constant speed. Note the reading on the galvanometer as the magnets are moving.

The galvanometer should give a higher reading with two magnets as more lines of flux will be cutting the coil.

Relative motion between coil and magnet: Connect a coil to a galvanometer. Insert the magnet slowly, north-pole first, into the coil. Note the magnitude and direction of the deflection of the galvanometer. Pull the magnet out slowly and again note the deflection of the galvanometer. Repeat the procedure but insert and remove the magnet quickly. Again note the magnitude and deflection of the deflection of the galvanometer.

The galvanometer should record currents in opposite directions when the magnet is inserted and when it is removed. Also it should give a higher reading at the speed of the magnet is increased as the rate of cutting lines of magnetic flux is increased.

Connect a coil to a galvanometer. Insert the magnet, south-pole first, into the coil. Note the magnitude and direction of the deflection of the galvanometer. Pull the magnet out at the same speed and again note the deflection of the galvanometer.

The galvanometer should record a current in the opposite direction as the polarity of the magnet is reversed.

Connect a coil to a galvanometer. Hold a magnet nearby and move the coil towards the magnet. Note the deflection on the galvanometer as the coil moves towards and then away from the magnet.

The galvanometer should record the same readings whether the coil moves towards the magnet or the magnet moves towards the coil since the rate of cutting lines of magnetic flux is the same.

“Students gather, analyse and present information to explain how induction is used in cooktops in electric ranges” (syllabus)

Coils underneath the glass or ceramic cooktop have alternating current flowing through them. The changing magnetic field produced by the alternating current cuts the base of a metallic saucepan and induces eddy currents in its base. The eddy currents produce heat in the base of the saucepan so that the saucepan gets hot but the cooktop doesn’t.

http://stuweb.ee.mtu.edu/~mtromble/induction/cooktops.html#work

http://www.wolfstoves.com/Iwatani/Induction.html.BAK100902

http://www.cookpower.com/

http://www.choice.com.au/viewArticle.aspx?id=103867&catId=100168&tid=100008&p=1

http://www.phys.unsw.edu.au/~jw/FAQ.html#eddy

“Students gather secondary information to identify how eddy currents have been utilised in electromagnetic braking” (syllabus)

Electromagnets are placed next to a metal wheel. When the electromagnets are switched on, they induce eddy currents in the rotating wheel. The eddy currents induce a magnetic field opposing the original so that a force of attraction is produced and the wheel is subjected to a force that slows it down.

http://www.phys.unsw.edu.au/~jw/FAQ.html#eddy

http://www.chu.cam.ac.uk/~EF242/

3. Generators are used to provide large scale power production” (syllabus)

“Students learn to describe the main components of a generator” (syllabus)

Typical A.C. generator.

The main components of the generator are the same as those of the motor. Both have a coil on an axle that rotates between the poles of a magnet. Both have brushes to transfer the charge. A.C. generators and motors have slip rings while D.C. motors and generators have a split-ring commutator.

“Students learn to compare the structure and function of a generator to an electric motor” (syllabus)

The structure of the generator is very similar to that of the motor. Both have a coil in a magnetic field. Both have brushes making contact with either a commutator or slip rings. Whereas the motor is attached to a mechanical load has a source of potential difference, the generator has a mechanical device to turn the rotor and it produces a potential difference.

The generator converts mechanical energy into electrical energy whereas the motor converts electrical energy into mechanical energy.

“Students learn to describe the differences between AC and DC generators” (syllabus)

The direction of the current in the coil changes direction every half turn. The A.C. generator uses slip rings and these transfer the current to the brushes so that the current in the brushes changes direction with every half turn of the rotor, i.e. alternating current in the brushes and the external circuit. The D.C. generator uses a split ring commutator so that as the current changes direction in the coil the commutator is also changing the side of the coil that is feeding current to each brush The result is that current in the brushes and external circuit always flows in one direction i.e. direct current.

“Students learn to discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer” (syllabus)

Energy is converted to heat in the transmission wires and this heat is lost to its surroundings. There is also some energy loss in induction as the changing magnetic field produced by the alternating current cuts external conductors.

“Students learn to assess the effects of the development of AC generators on society and the environment” (syllabus)

Society: A.C. and D.C. generators have led to the technological society we have today. Electric motors are cleaner and more efficient than their petrol counterparts and would not be possible without a continuous and economical source of electricity. Electrolysis is important in the production of metals such as aluminium and would not be possible on a large scale if it were to rely on cells to provide the electricity. Many household appliances such as washing machines, dishwashers and vacuum cleaners use electricity and would not be feasible without generators.

Environment: While power stations that operate electric generators are responsible for the emission of large amounts of greenhouse gases and fine particle pollutants it can be argued that the effect is less damaging than it would be if there were no generators. Without generators, each household or factory would produce its own energy source, in most cases wood fired or coal fired boilers. The amount of greenhouse gases and atmospheric pollutants from these would be many times what we have from the coal fired power stations that power the generators. The coal fired power stations are located away from regions of high population so any pollutants have a chance to dissipate before they reach populated areas. Mining for coal to power the generators scars the landscape but the land can be filled in and later used as farmland. Natural gas can be used to provide energy to the power stations to convert to electricity. While it is cleaner than alternate fuels it is also in limited supply and would soon run out if it was our major source of supplying energy to electric generators.

“Students plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current” (syllabus)

The simplest method of demonstrating the production of A.C. power is to use a hand held generator. The generator can be connected to a cathode ray oscilloscope to demonstrate the pattern of the current produced. Connecting a coil to a galvanometer and inserting and withdrawing a bar magnet many times as quickly as possible will show the needle of the galvanometer oscillate back and forth as current oscillates in the coil in a manner that represents alternating current.

“Students gather secondary information to discuss advantages/disadvantages of AC and DC generators and relate these to their use” (syllabus)

The split ring in the commutator of a D.C. generator causes more wear on the brushes than the slip rings of an A.C. generator. It also results in a short interval (twice on each rotation of the coil) when no current is flowing. A.C. generators are usually preferred to D.C.

A.C. has the advantage of the voltage being able to be stepped up or down quite simply by means of transformers. This enables it to be transported long distances with minimum power loss. Transformers also allow 240V A.C. to be easily converted to lower A.C. voltages, and by the addition of a solid state rectifier can be converted to low voltage D.C. for use in devices such as portable C.D. players or portable telephones. D.C. has the advantage of producing a constant, rather than a changing, magnetic field, and this would result in less power loss by induction. As part of a power grid, D.C. would have the advantage that it would not have to be synchronised. If A.C. from two sources is out of phase then the two voltages would cancel each other and produce no potential difference.

“Students analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities” (syllabus)

The story begins in the 1870s as companies owned or controlled by Thomas Edison built a large number of D.C. power stations. Because of transmission difficulties, these power stations were only able to cater for customers who were within a radius of about 1 ½ km from the power station.

In the Early 1880s Nikola Tesla began working for the Edison Company in Paris. His talent was recognized and in 1884 he transferred to the Edison Company branch in New York City. He made several improvements to the dynamo but left the Edison Company in 1887 after a quarrel over a disputed bonus. Tesla set up his own company and in December 1887 patented the polyphase A.C. motor (an induction motor) and an A.C. power transmission system. George Westinghouse bought Tesla’s company and its patents in 1888 and retained Tesla to continue research and development.

The turning point for the acceptance of A.C. came at The Chicago World’s Fair (also known as the Columbian Exhibition) in 1893. Westinghouse won the contract to provide lighting for the exhibition with his tender being less than half that of the Edison company. Because D.C. uses lower currents than A.C. it requires thicker wires for transmission. Because of a huge rise in the price of copper and the large amount of wire involved the Westinghouse bid was able to make a large saving by using less copper. The lighting for the exhibition was a huge success and influenced many previous supporters of D.C. about the advantages of A.C. Among these supporters to change sides was Lord Kelvin.

With the success of the Chicago World’s Fair, Westinghouse won the contract to harness the energy from Niagara Falls to provide Energy and lighting for the town of Buffalo, about 20 km away. Three phase A.C. generators were used with electricity first being produced at Niagara Falls in 1895. The transmission wires to Buffalo were completed the following year with transmission of electricity to Buffalo beginning on November 15^th, 1896.

Suggested websites:

http://www.pbs.org/tesla/ll/ll_warcur.html

http://www.pbs.org/tesla/ll/ll_niagara.html

http://ublib.buffalo.edu/libraries/projects/cases/niagara.htm

“Students gather and analyse information to identify how transmission lines are:

– insulated from supporting structures

- protected from lightning strikes” (syllabus)

Supporting structures: Power lines are attached to supporting structures by large, disk shaped, ceramic insulators.

Lightning strikes: Transmission lines are protected from lightning strikes by suspending an earth wire above the transmission cables. This wire is connected to earth at regular intervals. Examination of most telegraph poles will reveal the insulated connection between the earth wire and earth.

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

“Students learn to describe the purpose of transformers in electrical circuits” (syllabus)

A transformer raises or lowers the voltage in an A.C. circuit.

“Students learn to compare step-up and step-down transformers” (syllabus)

A step up transformer produces a higher voltage in the secondary coil than was provided to the primary coil whereas a step down transformer produces a lower voltage in the secondary coil than was provided to the primary coil.

“Students learn to 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” (syllabus)

The ratio of the number of turns in the primary coil to the number of turns in the secondary coil is the same as the ratio of the voltage in the primary coil to the voltage in the secondary coil. n_p/n_s = V_p/V_s

“Students learn to explain why voltage transformations are related to conservation of energy” (syllabus)

The ratio of the changes in current is the reciprocal of the ratio of the changes in voltage so that the power input is equal to the power output, provided there are no energy losses. In this way energy is conserved.

“Students learn to explain the role of transformers in electricity sub-stations” (syllabus)

Transformers in the transmission sub-station step up the voltage so that it can be transmitted at a high voltage to a zone sub-station in the region where it is going to be used. Here it is stepped down to a lesser value and transmitted to a pole or street transformer near where it is to be used and here it is reduced even further to supply households, industry and commerce.

“Students learn to discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer” (syllabus)

Some appliances require a much higher voltage than the 240 volt mains supply. Examples are; Computer monitor (step up transformer used in electron gun for screen), Television (step up transformer used in electron gun for picture tube).

Some appliances require a lower voltage either for safety reasons or to make them compatible with the low voltage delivered by batteries. Examples of appliances that use a step-down transformer to lower the voltage include tape recorders, Christmas tree lights, cordless phones and answering machines.

“Students learn to discuss the impact of the development of transformers on society” (syllabus)

Transformers have enabled power stations to be built away from cities and towns where the power is used. By enabling the power to be transmitted at high voltages, energy losses can be kept to a minimum. Transformers have enabled high voltages to be produced to operate the electron guns used in cathode ray oscilloscopes, T.V. screens and computer screens. They have enabled high voltages provided by the mains to be reduced to the safer low voltages to operate tape recorders, portable phones and party lights.

“Students perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced” (syllabus)

Obtain an iron ring (a tightly wrapped coil of soft iron wire will do nicely) and wrap insulated wire around one side of it many times to make the primary coil. Connect an A.C. voltmeter in series with the wire and the ends of the wire to the A.C. terminals of a power pack. Wind insulated wire around the other side of the ring, and connect the ends of the wire to an A.C. voltmeter. An alternating current in the primary coil will induce an alternating current in the secondary coil.

“Students solve problems and analyse information about transformers using:

” (syllabus)

Q.1. A transformer has 50 turns in the primary coil and 2000 turns in the secondary coil. What voltage in the primary coil would produce an A.C. voltage of 240V in the secondary coil?

A.1. 50 / 2000 = V_p /240 V_p = 50 x 240 / 2000 = 6V

Q.2. A transformer has 5000 turns in the primary coil and 100 turns in the secondary coil. If an A.C. potential difference of 200 V is applied across the primary coil, what is the potential difference across the secondary coil?

A.2. 5000/100 = 200/V_s V_s = 200 x 100 / 5000 = 4V

“Students gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome” (syllabus)

Laminated sheets of soft iron can be used for the core instead of a solid ring of soft iron.

“Students 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.” (syllabus)

Transmitting electricity at high voltage reduces energy losses due to conversion of electrical energy to heat energy. Transformers are needed to step up the voltage near the power station and to step it down again near the consumer.

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

“Students learn to describe the main features of an AC electric motor” (syllabus)

The two types of A.C. electric motors are the synchronous motor and the induction motor.

A synchronous A.C. electric motor is similar in construction to the D.C. motor, except that it has slip rings instead of a split ring commutator. It has a coil in a magnetic field, the same as a D.C. motor but the current if fed to the coil by means of slip rings. It rotates at the same frequency as the A.C. i.e. 50 Hz.

Induction motors have a stator that contains at least two electromagnets that have a phase difference between them, and a rotor consisting of two end rings joined by several conducting bars. The stator of a 3 phase induction motor has three pairs of electromagnets with the two magnets of each pair opposite each other. When an electromagnet has a current flowing through it an emf is induced in the bars of the rotor. This produces a magnetic field that opposes this change and is repelled by the electromagnet, causing the rotor to turn. Three phase power causes the electromagnets to switch on in turn so the rotor’s magnetic field is constantly being pushed by the stator’s magnetic field, causing the rotor to rotate in the same direction as the magnetic field.

“Students perform an investigation to demonstrate the principle of an AC induction motor” (syllabus)

Mount an aluminium disk on an axle so that spins freely. Hold a strong magnet near the disk and move the magnet in a circular motion with a radius about ¾ that of the disk. The magnet should induce eddy currents in the disk and cause it to rotate in the same direction as the path of the magnet.

“Students 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” (syllabus)

Electrical energy is transformed to mechanical energy in devices containing electric motors in the home or industry. These devices include food mixers and blenders, fans, including those in computers, heaters and hair driers, washing machines and clothes driers, as well as power tools and industrial machines, just to name a few. Electrical energy is also transformed into heat energy in these devices either as a desired outcome in such things as heaters, clothes driers or hair driers or as lost energy in such things as washing machines and power tools. These devices also make a noise as electrical energy is transformed to sound energy and some electrical energy is transformed to magnetic energy, producing eddy currents. It is converted into light energy in lights and sound energy in electric bells.

THE A.C. INDUCTION MOTOR

Before considering induction motors consider the following problem based on Lenz’s Law.

A solenoid is mounted on a trolley that is free to move. A magnet is moved away from the solenoid as shown. The ends of the solenoid are joined by a conductor to complete the circuit. Which way will the trolley move?

Since the moving magnet will cause lines of flux to cut the windings of the solenoid there will be an emf induced in the solenoid and this will produce a current. Lenz’s Law says that the induced emf will oppose the change that caused it. Since the north pole of the magnet is closest to the solenoid, as it moves away it will induce an emf in the solenoid that will make the side closest to the magnet a south pole to attract the magnet back i.e. oppose the change that caused it. Since there is a force of attraction between the solenoid and the magnet, the solenoid will follow the magnet i.e. move to the right.

Now consider an aluminium disk that is free to rotate. What happens when a magnet is moved in a circular motion around the circumference of the disk?

The movement of the magnetic field across the disk will produce eddy currents. The eddy currents will oppose the change that caused it and produce a magnetic field that attracts the magnet. Since the disk can rotate and is attracted to the magnet it rotates in the same direction as the movement of the magnet i.e. it follows the magnet.

An induction motor works in a similar fashion. Induction motors were invented by Nickola Tesla in 1888. While induction motors work best with three phase A.C., most homes only have single phase A.C. The induction motor is the most common motor used in household appliances so it is the one that will be dealt with here.

Like conventional motors, induction motors have a stator and a rotor.

The stator (the stationary part) consists of the magnets. While the magnetic field “moves” as the magnets change polarity, the magnets themselves are stationary.

The rotor consists of a “squirrel cage” that takes the place of a coil in a conventional electric motor. The squirrel cage is made of conducting material such as aluminium or copper and consists of two end rings connected by several bars. The end rings allow current to flow from one bar to another.

The squirrel cage is usually encased in laminated iron to enhance the magnetic field. The laminations reduce eddy currents. The rotor is mounted on an axle.

In its simplest form the induction motor consists of electromagnets mounted on opposite sides of the squirrel cage. The magnets are wound so that they are of opposite polarity and are connected to the same A.C. power source so that they undergo simultaneous pole reversal.

The interaction of the magnetic fields of the rotor and stator will cause the rotor to rotate.

The problem with this type of motor is the initial start up. When the current is first switched on the eddy currents in the squirrel cage produce no torque and so the squirrel cage will not rotate.

To overcome this problem a second pair of electromagnets is sometimes added at right angles to the first. A capacitor in the circuit of the second magnet pair produces a slight delay so that it cuts in slightly later than the first pair. This produces the effect of the magnetic field rotating in four steps around the squirrel cage.

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