Physics

Home > Physics > Core > Motors and generators > Motors and generators: 3. Generators for large scale power production

9.3 Motors and generators: 3. Generators for large scale power production

Syllabus reference (October 2002 version)
3.Generators are used to provide large scale power production	Students learn to: describe the main components of a generator compare the structure and function of a generator to an electric motor describe the differences between AC and DC generators discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer assess the effects of the development of AC generators on society and the environment	Students: plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current gather secondary information to discuss advantages/ disadvantages of AC and DC generators and relate these to their use analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities gather and analyse information to identify how transmission lines are: insulated from supporting structures protected from lightning strikes

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 30June 09]

Prior learning: Preliminary module 8.3.

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

Plan this first-hand investigation by describing ways in which an AC current can be produced and explaining why this works. You should first think about how to produce a direct electric current, then about how to produce a current flowing in the opposite direction. You should also think about how you will know when a current is flowing and in which direction. Finally you should think about how to make the current alternate in direction in a regular manner.
You should choose equipment for this investigation by identifying items that would be suitable to produce an electric current. Choose an instrument that can be used to measure the size of current that will be produced and can also indicate the direction of the current. Consider using datalogging technology so that you can record the size and direction of the current over a period of time. Alternatively, you might consider using equipment that can display the voltage characteristics of the alternating current on the screen of a cathode ray tube.
You should then perform the investigation by carrying out the procedure you have planned. Experiment with different combinations of equipment until you are able to demonstrate the production of an alternating current. Try to identify variables that can be changed, and carefully record the results of any changes you make.

Sample investigation

An electric current can be produced by moving a wire in a circuit near a permanent magnet or by moving the magnet near the wire. The magnet can be either moved back-and-forth or rotated end-for-end. As the magnet is moved one way, a current is induced: as it is moved the other way, a current is induced in the opposite direction. The current can be detected by a galvanometer.

A coil shows the effect more clearly than a single wire, as the effect is enhanced by having the magnetic flux cutting the many turns of wire in a coil. The current produced is usually small and is best indicated with a microammeter or a galvanometer scaled in microamps (mA). A centre-reading galvanometer will also indicate the direction of the current.

Connect the ends of the coil to the terminals of the galvanometer. Stand the permanent magnet on its end on the bench and place the coil vertically over the magnet. Move the coil upwards and note the direction of movement of the galvanometer needle. Move the coil downwards and again note the direction of movement of the galvanometer needle. Now move the coil rhythmically up and down while observing the galvanometer needle. Record all of your observations. Movement of the galvanometer needle back and forth indicates the production of an alternating current.

Change the speed at which you move the coil up and down, and change the displacement with each movement. Repeat the investigation with a stronger magnet or with a coil of a different number of turns. Each time you make a change, record your observations and try to account for any changes you notice in the magnitude or the frequency of the alternating current you produce.

describe the main components of a generator

The following table contains a description of the main components of a generator.

Component of generator	Description
Rotor	In its simplest form, the rotor consists of a single loop of wire made to rotate within a magnetic field. In practice, the rotor usually consists of several coils of wire wound on an armature.
Armature	The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. 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 two slip rings (AC) or two opposite bars of a split-ring commutator (DC).
Stator	The stator is 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.
Field electromagnets	Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way 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.

compare the structure and function of a generator to an electric motor

Electric motors and generators have several structural features in common. Each consists of a stator that provides a magnetic field and a rotor that rotates within the magnetic field. In both motors and generators 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 and a DC generator are similar in that their rotor coils are connected to the external circuit through a split-ring commutator. 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.
The function of an electric motor is the reverse of the function of a generator. An electric motor converts electrical energy into mechanical (usually rotational) energy. A generator converts mechanical (usually rotational) energy into electrical energy. A motor rotates when current is supplied while a generator supplies current when rotor is made to rotate. It is possible to have a DC motor act as a generator by providing the energy to rotate the armature containing the coils.

describe the differences between AC and DC generators

The essential difference between an AC and a DC generator is the nature of the connection between the rotor coils and the external circuit.
In an AC generator, the brushes run on slip rings which maintain a constant connection between the rotating coil and the external circuit. This means that as the induced emf changes polarity with every half-turn of the coil, the voltage in the external circuit varies like a sine wave and the current alternates direction.

In a DC generator, the brushes run on a split-ring commutator which reverses the connection between the coil and the external circuit for every half-turn of the coil. This means that as the induced emf changes polarity with every half-turn of the coil, the voltage in the external circuit fluctuates between zero and a maximum while the current flows in one constant direction.

The following Internet site contains a simple animation of an AC generator: AC-DC: Inside the AC Generator The American Experience Online, Public Broadcasting Service (PBS), USA

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

In gathering your information you should ensure that you use a variety of sources including written texts and the Internet. You should also gather relevant information by interviewing people such as auto electricians or electrical engineers. You should make sure that the information you collect relates directly to how and where AC and DC generators are used and how each is best suited to its use. An auto electrician could probably explain, for instance, why the AC generator, or alternator, has replaced the DC generator in motor cars and give some good insights into the relative advantages of each of these devices.

Sample information

The relative advantages and disadvantages of AC and DC generators relate to two features of their design: DC generators use a split-ring commutator, while AC generators use slip rings; and in DC generators the output current is induced in the rotor, whereas the roles of the rotor and the stator can be reversed in an AC generator.

The commutator of a DC generator consists of a number of metal bars separated by narrow gaps filled with insulating material. As the brushes remain in contact with the commutator under spring pressure, they are constantly striking the leading edge of each successive bar. This wears the brushes and they need to be replaced regularly. The commutator bars also wear down until the insulating material between them prevents the brushes from making proper contact with the bars, reducing the efficiency of the generator. Pieces of metal worn from the commutator bars can become lodged in the gaps, causing a short between bars and reducing the output of the generator.

In contrast, the slip rings of an AC generator have continuous, smooth surfaces, allowing the brushes to remain continuously in contact with the slip ring surface. Thus the brushes in an AC generator do not wear as fast as in a DC generator. There is no possibility of creating an electrical short circuit between segments in an alternator because the slip rings are already continuous. An AC generator therefore requires less maintenance and is more reliable than a DC generator. Most commercial generators are AC generators.

In a DC generator the current is generated in the rotor and is then drawn from the windings through the commutator and out via the brushes. The larger the current required, the heavier the rotor coils must be, placing high demands on bearings and supporting structures. In addition, drawing large currents through the commutator-brush connection increases the likelihood of electric arcs forming as the brush breaks contact with each bar in turn. This reduces the efficiency of the generator and creates radio “noise”. This limits the usefulness of DC generators to relatively low current applications.

In an AC generator designed for high current applications, such as in a power station, the current is produced in the stator windings rather than in the rotor. The rotor is used to create the field magnetization that induces the AC current in the stator when the rotor is rotated. It is much easier to draw the current through a fixed connection in the stator rather than through a commutator from a moving rotor. Thus AC generators are better suited to high current demands than DC generators.

An advantage of a DC generator is that its output can be made smoother by the arranging many coils in a regular pattern around the armature. The brushes are arranged to make contact only with the commutator bars corresponding to the coils producing the greatest emf at a particular time. The result is an output voltage that “ripples” about a mean value rather than fluctuating between zero and the maximum twice per revolution. The more coils, the smoother the output DC voltage ripple. This is an advantage for use with equipment that needs a steady voltage rather than a sinusoidally varying voltage. This cannot be achieved with an AC generator without the addition of a rectifying and smoothing circuit.

An advantage of AC generators is that they can easily be designed to produce three-phase electricity by the use of six stator poles and a single electromagnet rotor. The coils are mounted in opposing pairs spaced evenly around the stator, and connected in pairs to the three phases of the power supply. The rotor induces alternating current in successive pole pairs. The sinusoidally varying voltages are then 120 degrees out of phase with each other. AC generators are ideal for generating electricity on a large scale for distribution over a wide area.

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

Your teacher may have provided you with information from secondary sources on the competition between Westinghouse and Edison. This may be in the form of reprinted articles from journals, information downloaded from Internet sites, an HSC Physics text book or other relevant information. To find further secondary information yourself, use an Internet search engine such as Google with all the words “Edison”, “Westinghouse”, “AC” and “DC”.
To analyse the information, first identify the major difference between Westinghouse and Edison in their technological approach to supplying electricity to cities. Next, describe advantages and disadvantages of each approach. Describe also other reasons why either party held firmly to their preferred method of generating electricity.

Sample analysis

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

Edison had the initial advantage that the technology for generating DC was well established and DC worked well over short distances. However, DC could only be generated and distributed at the voltages at which it was used by consumers. This meant that currents in conductors were large, leading to huge and expensive energy losses over distances of more than one or two kilometres. To supply a large city required many power stations throughout the city and an unattractive proliferation of wires to carry the required current.

The great advantage of AC was that, through the use of transformers the voltage could be stepped up or down as required. This meant that AC could be generated at moderately low voltages, stepped up to high voltages for transmission over great distances and stepped down again to lower voltages for consumers. The higher voltage meant that AC could be transmitted over greater distances than DC, with smaller energy losses. Power stations could be fewer and further apart and conductors could be lighter.

The economic advantages of AC, including the smaller energy losses and the economy of scale in needing fewer power stations further apart, along with the unattractive web of wires required for DC, supported Westinghouse’s solution to the supply of electricity over Edison’s. AC received a boost in popularity with Tesla’s invention of the induction motor which operates only on AC.

Competition was not always open and fair. Edison had a vested interest in DC as he owned hundreds of DC power stations and all of his many electrical inventions to that time ran on DC. Edison attempted to prove that AC was very dangerous by electrocuting animals on stage and convincing authorities to use AC for the first electric chair. He resorted to legal tactics in an attempt to have AC banned and to prevent its use with his inventions. Edison seems to have unreasonably shunned AC electricity. AC eventually came to be the dominant form in which electricity is generated world-wide.

But DC has the advantage of not causing losses through electromagnetic radiation or magnetic induction. With solid-state switching it is now relatively simple to change between DC and AC at high or low voltages. High voltage DC transmission is now practicable. Scientists are striving to develop super-conducting wires for power transmission. If they do, DC could become the preferred current for long distance transmission. There is already a 500 kV DC submarine transmission line carrying 2800 MW over 50 km between the two islands of Shikoku and Kansai in Japan.

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

There are two main types of energy loss occurring in transmission lines: those resulting from the resistance of the wires, and those resulting from the induction of eddy currents

Resistive energy losses

Heat is generated in transmission lines because of the resistance of the wires. The resistance per kilometre is small, but the resistance of a long transmission line is significant. Distances are often great, up to hundreds of kilometres, because power stations are often located in remote places, close to the primary energy source such as a major coal field or a system of dams for a hydroelectric scheme, rarely close to consumers in the city.
The power loss in transmission lines is given by the relationship: P= V I or P = I² R. Power loss is proportional to the square of the current. As the resistance of the conductor is relatively constant, power loss is affected most by the size of the current. Increasing the current by a factor of two increases the power loss by a factor of four.
Energy losses are kept to a minimum by transmitting the electricity at the highest practicable voltage, with the lowest practicable current. Generally, the greater the distance, the higher the voltage. Closer to the consumer, voltages are lower but energy losses are not substantial since distances are shorter and the current is shared by many separate distribution lines.
The type of electricity transmitted over long distances is predominantly AC, since AC can be changed easily to high voltages and correspondingly low currents by the use of a step-up transformer. With advances in solid state technology it is becoming easier to step DC voltages up and down, and DC is increasingly being used for long distance power transmission.
Energy losses can also be minimised through careful choice of materials and design of conductors. Transmission lines are typically made of either copper or aluminium, as these metals have low resistivity, that is, they are good conductors. Resistance is inversely proportional to the area of cross-section of the conductor, so the thicker a conductor, the lower the heat losses. However, heavier conductors require more expensive support structures. Aluminium has higher resistivity than copper but it is much lighter than copper, and less susceptible to corrosion. The smaller weight and lower maintenance costs more than compensate for the larger diameter of aluminium needed to carry a certain current. Recent experiments with superconducting materials show some promise for reducing energy losses from high voltage transmission lines even further in the future.
For energy losses to be minimised, the transmission voltage must be very high. This requires high poles or towers and large insulators. These are expensive to build and maintain and have an adverse effect on the visual environment. Trees must be kept well clear of high voltage transmission lines to avoid damage to the lines during storms and to reduce the possibility of a short to earth. This often requires a wide corridor to be cleared, sometimes through environmentally sensitive areas.

Sample calculations of power loss

Consider an imaginary transmission of electricity from a power station generating 1000 W of power through a transmission line with a resistance of 2 Ω km^-1. If the electricity is transmitted at 100 V and 10 A over a distance of 1 km, then the transmission losses will be:

P = I²R= (10 A)² x 2 Ω = 200 W.

This leaves 800 W of energy for the consumers from the original 1000 W. The 200 W of power dissipated in this transmission would have been converted into heat.

If the electricity is now transmitted at 10 000V and 0.1 A over a distance of 1 km, then the transmission losses will be

P = I²R= (0.1 A)² x 2 Ω = 0.02 W

for the same initial amount of electrical energy. This would leave almost all of the power generated available for consumers.

Inductive energy losses

Energy is also lost through the induction of eddy currents in the iron core of transformers. This applies both to step-up transformers at the power station and to step-down transformers at the sub-station and on power poles on suburban streets. The circulation of eddy currents in the transformer core generates heat because of the resistance of the iron. The heat represents an energy loss from the electrical system.
Transformer cores are usually made of laminated iron, consisting of many thin layers of iron sandwiched together, with thin insulating layers separating them. This limits eddy currents to the thickness of one lamina and reduces the corresponding heat loss. Eddy currents may be further limited in transformer cores made of granular ferrites, as used in some recent experiments. The ferrites allow the magnetic flux to change freely but have high resistance to the eddy currents.
Heat loss inevitably occurs in the core of a transformer. As overheating can damage the transformer, various cooling techniques are used to dissipate the heat. These include cooling fins on the outside of the transformer, radiator pipes to allow cooling oil to circulate by convection and transfer heat to the air, and electric fans to force cooling air to flow around the transformer.
The induction of eddy currents in metal parts of transmission towers is kept to a minimum by the distance at which the wires are held away from the tower by the insulators.

gather and analyse information to identify how transmission lines are:

insulated from supporting structures

protected from lightning strikes

Gather the information by inspecting the transmission lines through a pair of binoculars from a safe distance. Take note of how the conductors are supported off the ground and prevented from touching the support structures. Photograph or draw sketches of the structures you see for later study. Inspect both low tension transmission lines (11 or 22 kV) carried on wooden poles in a suburban street and high tension transmission lines (110 or 220 kV) carried across the country on tall steel towers. Take care to look for any structures capable of carrying a large electric current to the ground.
Analyse the information by making a simple model of a transmission tower or pole to illustrate and explain the insulation and protection strategies you observe. Compare the structures used for low tension and high tension transmission.

Sample information

High voltage transmission lines are kept away from their supporting structures by chain insulators to reduce the likelihood of a discharge between the conductor and the support structure. Insulator chains can be up to around 2 m in length: generally, the higher the voltage, the longer the chain.

Insulators are constructed either of ceramic segments joined together with metal links or of rubber discs with a fibre glass core. Their design reduces the possibility of charge leaking through the insulators themselves. The metal links in ceramic insulators are isolated from each other, and the fibreglass is a non-conductor, so there is no continuity of conduction. The insulator segments are designed to shed water and prevent dust from building up, as either moisture or dust can make a conductive path across the surface of the insulator. The disc-like shape of the segments, whether ceramic or rubber, ensures a long pathway for any spark discharge across the insulator.

Transmission lines and supporting structures have a number of protective features associated with their design. In the event of a transmission tower being struck by lightning, the metal tower itself acts as a conductor to take the charge to the ground. The towers are well earthed, with a large surface area of metal buried in the ground, enabling the charge from any lightning strike to dissipate harmlessly in the earth. Towers are widely spaced to ensure that, should one tower be struck, the adjacent towers suffer no damage from the lightning strike.

Not all the wires on a transmission tower carry the electric current. The uppermost wires are called shield conductors, as they are designed to reduce the chance of a lightning strike to the transmission wires. Shield conductors are connected directly to the transmission towers without the use of insulators so that they can conduct charge between the clouds and the earth as it builds up, to neutralise the charge distribution. If the shield conductors are struck directly by lightning the current is conducted safely to earth.

The following Internet site offers a downloadable audio file of an interview about substations, transformers and protection of transmission towers from lightning strikes: 9.3 Motors and generators Learning Materials Production Centre: OTEN-DE. [Requires browser plug-in capable of playing .ram files.]

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

The development of AC generators has led to the widespread application of some of the useful features of AC electricity. AC generators are simpler and cheaper to build and operate than DC generators. Because AC electricity can easily be transformed, it can be transmitted cheaply over great distances, allowing a wide range of primary energy sources to be exploited. This has allowed the development of extensive, reliable AC electricity networks for domestic and industrial use throughout much of the world. This in turn has had both positive and negative effects on society and the environment.
The affordability of electricity has promoted the development of a wide range of machines, processes and appliances that depends on electricity. Many tasks that were once performed by hand are now accomplished with a purpose-built electrical appliance and most domestic and industrial work requires less labour. Other new tasks can now be achieved that were formerly impossible, such as electronic communication. However, this has led to a reduction in the demand for unskilled labour and an increase in long-term unemployment. The ready availability of electricity has led to increasing dependency on electricity. Essential services such as hospitals are forced to have a back-up electricity supply, “just in case”. Any disruption to supply compromises safety and causes widespread inconvenience and loss of production. A major electricity failure can precipitate an economic crisis. The global electricity industry lobby is very powerful but is not always just. Social values may give way to economic pressures, especially in developing countries where often the poorest people lose their livelihood to make way for new energy developments.
AC power generating plants can be located well away from urban areas, shifting pollution away from homes and workplaces, thus improving the environment of cities. However, many environmental effects of the growth in the electricity industry are negative. Power transmission lines criss-cross the country with a marked visual impact on the environment, often cutting a swathe through environmentally sensitive areas. Remote wilderness areas can easily be tapped for energy resources such as their hydro-electric potential. Air pollution from thermal power stations burning fossil fuels may be a cause of acid rain. In addition it contributes to the global increase of atmospheric carbon dioxide which is linked to long-term global climate change. Nuclear power stations leave an environmental legacy of radioactive waste that will last many thousands of years.
The effects of the development of AC generators on society and the environment have been far-reaching. Some effects have changed the way people live, but not always for the better. Many people now enjoy increased convenience and leisure, many new industries flourish on new technologies made possible by electricity, but the dislocation and unemployment experienced by some can be devastating. Many aspects of the development of electricity have led to environmental degradation, often in remote areas where the long-term effects are poorly understood. These effects seem likely to be ongoing, as the compromise between economic interests and social and environmental values often favours the economic. We have not yet learned to live with AC electricity in a sustainable way.