Generators and Transmission

Describe the main components of a generator

All generators consist of two parts- a stator and a rotor. The stator consists of magnets, either permanent magnets or electromagnets that are arranged around the centre of the motor, such that the magnetic field produced immerses the rotor. The rotor is the rotating part of the motor and is attached to a spinning energy source, for example a windmill or a petrol engine. The rotor consists of a coil that rotates in a magnetic field, so that it experiences changing flux and therefore has an induced emf. AC generators have slip rings, where a ring is connected to each end of the rotor wire. The rings rotate with the rotor, and current is passed to a circuit from the rotor by brushes that are in contact with the slip rings. A DC generator has a commutator identical to that found in a motor, so that the direction of emf is always the same. The commutator is a ring split into two parts, each part connected to one end of the rotor wire and in contact with brushes that connect the rotor to the circuit via the commutator.

Remember- Generators consist of a stator and a rotor. They also have either slip rings or a commutator.

Describe the differences between AC and DC generators

Bear in mind that the DC generator does not produce a constant current. The current in a DC generator also fluctuates because the rate of change of flux is not always constant. The key point is that DC always flows in the same direction.

AC generators produce alternating current that switches direction periodically, while DC generators produce a current that, while varying in magnitude, always flows in the same direction. This stems from the fact that AC generators use slip rings with brushes to transfer electricity from the rotor to a circuit, while a DC generator uses a split-ring commutator to reverse current direction every half-turn.

Remember- DC generators produce current that always flows in the same direction, and they use a commutator instead of slip rings

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

In terms of structure, standard AC motors and generators are identical, and DC motors and generators are identical. They consist of the same parts connected in the same way. The difference between them lies in function. The role of a motor is to turn electrical energy into kinetic energy, and the role of a generator is to convert kinetic energy into electrical energy. In a motor, electrical energy is fed in, in a generator, electrical current is extracted. A motor is connected to an object that the operators wants to rotate (such as wheels), while a generator is connected to a rotating object (such as a steam turbine). Essentially, motors and generators have the same structure but function in opposite directions.

Remember- Motors and generators have the same structure, but perform opposite tasks.

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

AC and DC generators produce electricity that is very different, and so each has its own advantages and disadvantages.

AC generators have several advantages, including the ability for the output voltage to be changed easily, as well as greater durability because slip rings encounter less wear and tear than a commutator. On the other hand, the output from an AC generator needs to be constantly shielded so that energy is not lost to the environment by induction (as AC current is always fluctuating, and therefore causes changing flux around the wire). Also, wires carrying high voltages are subject to arcing, and so are more dangerous to their surroundings. Stronger insulation is required compared to DC generated power.

DC generators have the advantage of producing electricity that doesn’t induce emf in its surroundings, so less insulation and separation is required, as well as the fact that DC cable insulation can be lighter and therefore cheaper. However, the commutator in a DC generator is subject to wear and tear and is more prone to breakage. The commutator also undergoes sparking as it rotates, leading to further power loss and additional wear and tear. Since DC generators tend to produce low-voltage high- current electricity, a great deal of energy is wasted in power lines as heat.

Further, it is extremely difficult to change the voltage of DC power, which means it is difficult to reduce losses in transmission. These advantages and disadvantages mean that AC generators are used for large scale power generation where power is transmitted long distances, while DC generators are used for small-scale applications such as generators in vehicles.

Remember- AC can have its voltage transformed and can be transmitted efficiently, but it can also cause unwanted induction. It is hard to change DC voltage, and DC often has high current, but it is useful in small applications because induction is less of a problem.

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

Don’t forget to include relevant formulae in your answer to this dotpoint.

Moving electrical charge through a conductor wastes energy, because some of the electrical energy is converted to heat. The resistance of an object is essentially its capacity to convert electrical energy into undesired forms such as heat. This is because electrons collide with other atoms causing them to vibrate, resulting in heat, and in a loss of electrical energy. This is particularly problematic for power lines, because large amounts of electricity are passed through them for long distances. A portion of electricity passed through a power line is converted to heat, and this is how electricity is lost through transmission. The cost of lost energy can be very significant.

However, the formula P = I2R shows that the amount of energy lost (power consumed by the wire) is related to how much current is passing through the wire. By reducing the current, energy losses between the generator and the consumer can be minimised. This is done through the use of transformers, because transformers can be used to alter the voltage to current ratio without changing the total amount of energy.

From the generator, the voltage is stepped up by a transformer. According to P = IV , since the total power is constant, I must decrease when V increases. This means that the step up transformer produces electricity with low current, and this minimises heat losses through transmission lines. At the consumer’s end, a step down transformer lowers the voltage so that it is appropriate for use by the consumer.

Further, there are additional losses in the transformer, largely due to induction producing eddy currents in the iron core. Not only is the induction of eddy currents inefficient because emf has been used to produce them, but the resulting eddy currents heat the iron core and therefore the transformer coils, increasing their resistance. These are dealt with firstly by laminating the iron core, and by using cooling fans to keep the transformer cool. In this way, transmission losses are minimised between the generator and the consumer.

figure 21

Remember- Resistance in the wire causes heating, transformers are used to minimise current and thereby minimise energy loss.

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

Make sure you cover the key points where AC had effects- transmission, economies of scale, the location of the generators closer to fuel and pollution moved outside the city.

AC generation and its ability to have its voltage changed by transformers has revolutionised society and had an environmental impact. The major problem with DC power was that it could not effectively be transmitted- as DC power could not have its voltage switched, large volumes of current had to be passed through wires, resulting in huge energy losses. This meant that DC generators would have to be close to consumers, and there would have to be many generators as the electricity could not be transmitted long distances. However, AC power can be transmitted long distances by altering its voltage. This meant that electricity generation could be shifted outside urban areas where consumers were located and instead located close to the natural resources required to run the generator, such as coal. This helped to bring down the price of electricity. Further, AC generation could be carried out on a large scale and then distributed over long distances to many people. This meant that economies of scale could be achieved, resulting in dramatically cheaper electricity as well as increased efficiency. This placed electricity within the reach of the majority of the population, rather than the rich, privileged minority who would have been the only people capable of affording electricity under a DC grid. Therefore, the uptake of electricity was rapid and widespread, dramatically changing society with its labour-saving benefits (although in some cases making unskilled jobs redundant).

In terms of the environment, by placing the generator away from the city, pollution levels in urban areas were reduced. Further, switching to electricity reduced the need to burn fuels in the home, further reducing pollution levels. So while coal was still burnt, and likely in greater quantities than before the uptake of AC generation, pollution was shifted away from urban areas and out into the rural areas where the generators are located, resulting in a cleaner urban environment. However, the pollution resulting from large-scale electricity generation is a significant contributor to global warming. Overall, AC generators dramatically changed society because they made the labour-saving benefits of electricity available to the majority of the population. While AC generators have resulted in a cleaner urban environment, they are still a large source of pollution and have a significant impact on global warming.

Remember- AC made electricity available to almost all the population, not just the rich people. However, it generated more pollution as electricity consumption rose. However, this pollution was not in cities but in other areas.

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

Westinghouse and Edison were in direct competition to supply electricity to cities. Edison, who had invented appliances for DC power, planned out a DC power grid and advocated DC as a solution for powering cities. Westinghouse on the other hand owned the rights to the transformer and advocated AC power. The main problem with DC power was its inability to be transmitted- the furthest it could be sent at the time was 14km and that was with 38% of the energy lost to heat. Westinghouse’s AC grid used transformers to slash losses to merely 1% by using high voltages unattainable with DC power to optimise transmission. Further than this, because of its inability to be transported, DC electricity would have to be generated by multiple generators throughout the city. This would have resulted in infrastructure difficulties in bringing fuel into cities, high levels of pollution, and more costly electricity because economies of scale could not be realised. Because AC power could be transported, it could be generated near the source of fuel on a large scale, resulting in much cheaper electricity. Edison temporarily achieved a propaganda victory by claiming AC power lines were unsafe, but the lines were kept out of reach and substations fenced off, and eventually the high efficiency and economies of scale made AC the victor.

Remember- AC could have its voltage changed, which made it easy to transport efficiently and gen- erate on a large scale.

Gather and analyse information to identify how transmission lines are insulated from supporting structures and protected from lightning strikes

Power lines have two protective devices- insulation from supporting towers and protection against lightning strikes. In dry air, sparks can jump around 33cm from a 330kV source. This means that wires need to be held at least that far away from the supporting towers it is strung from. This is achieved by using disk-shaped ceramic insulators. The disks are stacked on top of each other, so that if it rains some of the disks remain and therefore do not conduct. Also, the disk shape means that current has a longer distance to traverse (since the current must go around the disks, instead of in a straight line), increasing safety. In terms of lightning, on power lines there is another single line strung at the very top of power poles, above the conducting wires. This wire is known as a shield conductor. In the event of a lightning strike, lightning hits points as high as possible, and so the shield conductor at the top will be hit instead of the lower conducting lines. The shield conductor is periodically earthed by having a connection to a wire that runs from the top of a power pole right down to the ground (known as an earth wire), so that lightning can travel from the sky to the ground via shield conductors rather than power lines. For high-voltage towers, the tower itself is taller than the height of the wires, so lighting will strike the top of the tower then travel down to the ground via the metal structure, thereby not interfering with the power lines.

Remember- Stacked ceramic disks isolate wires from towers, and a shield conductor above the trans- mission wires protects against lightning strikes.