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AC DC difference
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If you are training to drive electric trains that may run over tracks of both AC & DC, you will learn that trains behave differently between the two supplies. There are also many different breakdown issues and weather impacts.
Nym
Established Member
Except those pesky lines in the North East, electrified as 1500V DC OHLE...
Thanks on Monday I will be doing my PTS training and wanted a heads up on what the 2 meant.
Cheers
With 1500 volts DC overhead wiring as on the former Woodhead line and currently on the Tyne & Wear Metro you can have what is known as "Regenerative Braking" where when the train is coasting the motors generate DC power and return it to the OHLE. As far as I know you cannot do this with AC though I fully expect someone to tell me I am wrong.
Nym
Established Member
You are indeed wrong...
Whats the difference between AC and DC?
Alternating current (AC) and direct current (DC) are notable for inspiring the name of an iconic metal band, but they also happen to sit right at the center of the modern world as we know it. AC and DC are different types of voltage or current used for the conduction and transmission of electrical energy. Quickthink of five things you do or touch in a day that do not involve electricity in any way, were not produced using electricity, and are not related to your own body's internal uses of electricity Nice try, but no way, you cant do it. (Or send us a list if you think you can; well check it.)
Electrical current is the flow of charged particles, or specifically in the case of AC and DC, the flow of electrons. According to Karl K. Berggren, professor of electrical engineering at MIT, the fundamental difference between AC and DC is the direction of flow. DC is constant and moves in one direction. A simple way to visualize the difference is that, when graphed, a DC current looks like a flat line, whereas the flow of AC on a graph makes a sinusoid or wave-like pattern, says Berggren. This is because AC changes over time in an oscillating repetitionthe up curve indicates the current flowing in a positive direction and the down curve signifies the alternate cycle where the current moves in a negative direction. This back and forth is what gives AC its name.
Leaving aside lines and graphs for a moment, Berggren offers another way to distinguish between AC and DC by looking at how they work in the devices we use. The lamp next to your bed, for example, uses AC. This is because the source of the current came from far away, and the wave-like motion of the current makes it an efficient traveler. If you happen to be a read-by-flashlight kind of person, you are a consumer of DC power. A typical battery has negative and positive terminals, and the electrical charge (its those electrons) moves in one direction from one to the other at a steady rate (the straight line on the graph).
Interestingly, if youre reading this on a laptop, you are actually using both kinds of current. The nozzle-shaped plug that goes into your computer delivers a direct current to the computers battery, but it receives that charge from an AC plug that goes into the wall. The awkward little block thats in between the wall plug and your computer is a power adapter that transforms AC to DC.
Berggren explains that AC became popular in the late 19th century because of its ability to efficiently distribute power at low voltages. Initially, power is conducted at very high voltages. In order to get these high voltages down to the low voltages necessary to power, say, a household light bulb, its necessary to transform the current. A transformer, which is basically two loops of wire, gets AC down from hundreds of thousands of volts to distributions of reasonable voltages (in the hundreds) to power most day to day electronics. The ability to transform voltages from AC meant that it was possible to transmit power much more efficiently across the country.
According to Berggren, theres a funny history of rivalry between AC and DC. In the later 19th century, there was a giant war between Edison and Westinghouse over AC and DC. Edison had patents in place that made him invested in the widespread use of DC. He set out to convince the world that DC was superior for the transmission and distribution of power. He resorted to crazy demonstrations like killing large animals with AC in an attempt to prove its terrible dangers. For a time, he was successful and most municipalities utilized local power plants with DC supply. However, getting power to less populated, rural communities all over the country with DC proved very inefficient, so Westinghouse ultimately won out and AC became the dominant power source. Elizabeth Earley
Source: MIT
Alternating current (AC) and direct current (DC) are notable for inspiring the name of an iconic metal band, but they also happen to sit right at the center of the modern world as we know it. AC and DC are different types of voltage or current used for the conduction and transmission of electrical energy. Quickthink of five things you do or touch in a day that do not involve electricity in any way, were not produced using electricity, and are not related to your own body's internal uses of electricity Nice try, but no way, you cant do it. (Or send us a list if you think you can; well check it.)
Electrical current is the flow of charged particles, or specifically in the case of AC and DC, the flow of electrons. According to Karl K. Berggren, professor of electrical engineering at MIT, the fundamental difference between AC and DC is the direction of flow. DC is constant and moves in one direction. A simple way to visualize the difference is that, when graphed, a DC current looks like a flat line, whereas the flow of AC on a graph makes a sinusoid or wave-like pattern, says Berggren. This is because AC changes over time in an oscillating repetitionthe up curve indicates the current flowing in a positive direction and the down curve signifies the alternate cycle where the current moves in a negative direction. This back and forth is what gives AC its name.
Leaving aside lines and graphs for a moment, Berggren offers another way to distinguish between AC and DC by looking at how they work in the devices we use. The lamp next to your bed, for example, uses AC. This is because the source of the current came from far away, and the wave-like motion of the current makes it an efficient traveler. If you happen to be a read-by-flashlight kind of person, you are a consumer of DC power. A typical battery has negative and positive terminals, and the electrical charge (its those electrons) moves in one direction from one to the other at a steady rate (the straight line on the graph).
Interestingly, if youre reading this on a laptop, you are actually using both kinds of current. The nozzle-shaped plug that goes into your computer delivers a direct current to the computers battery, but it receives that charge from an AC plug that goes into the wall. The awkward little block thats in between the wall plug and your computer is a power adapter that transforms AC to DC.
Berggren explains that AC became popular in the late 19th century because of its ability to efficiently distribute power at low voltages. Initially, power is conducted at very high voltages. In order to get these high voltages down to the low voltages necessary to power, say, a household light bulb, its necessary to transform the current. A transformer, which is basically two loops of wire, gets AC down from hundreds of thousands of volts to distributions of reasonable voltages (in the hundreds) to power most day to day electronics. The ability to transform voltages from AC meant that it was possible to transmit power much more efficiently across the country.
According to Berggren, theres a funny history of rivalry between AC and DC. In the later 19th century, there was a giant war between Edison and Westinghouse over AC and DC. Edison had patents in place that made him invested in the widespread use of DC. He set out to convince the world that DC was superior for the transmission and distribution of power. He resorted to crazy demonstrations like killing large animals with AC in an attempt to prove its terrible dangers. For a time, he was successful and most municipalities utilized local power plants with DC supply. However, getting power to less populated, rural communities all over the country with DC proved very inefficient, so Westinghouse ultimately won out and AC became the dominant power source. Elizabeth Earley
Source: MIT
scott118
Member
don't cl 360/379's, have this regenerative braking system?
NSEFAN
Established Member
I believe that almost all post-privatisation EMUs will have regenerative braking.
edwin_m
Veteran Member
Regeneration is more complicated with AC because the electronics on board the train has to generate AC that is at the same frequency and "in phase" with the supply before feeding it back into the wire. This only really became practicable with modern inverter drives as used in the UK from about 1990 onwards.
With DC it is in principle just a matter of feeding power back into the rail or wire but stopping if the line voltage gets too high (which indicates that there is no other train taking power near enough to use it) or too low (which could indicate that the supply has been isolated and it would be unsafe to energise it again). But although regeneration was used on Woodhead as far as I'm aware no third rail units could regenerate until, again, they started to be fitted with inverter drives. This was probably because of the extra electromechanical switchgear that would be needed. Interestingly most traditional trams could regenerate.
However because AC is generally at higher voltage and a lower current , it can travel further without being reduced so much by resistance losses. Hence it can be used to supply trains needing power further away, so if other things are equal regeneration should recover more energy in an AC system than a DC system.
When these inverter drive units were first introduced they were not allowed to regenerate on either electrification system. However since then Network Rail has made some changes to the supplies and solved the safety issues which might apply such as if a unit regenerated power into a dead section where people were working on the line. Today I believe all units that are capable of regenerating are permitted to do so.
With DC it is in principle just a matter of feeding power back into the rail or wire but stopping if the line voltage gets too high (which indicates that there is no other train taking power near enough to use it) or too low (which could indicate that the supply has been isolated and it would be unsafe to energise it again). But although regeneration was used on Woodhead as far as I'm aware no third rail units could regenerate until, again, they started to be fitted with inverter drives. This was probably because of the extra electromechanical switchgear that would be needed. Interestingly most traditional trams could regenerate.
However because AC is generally at higher voltage and a lower current , it can travel further without being reduced so much by resistance losses. Hence it can be used to supply trains needing power further away, so if other things are equal regeneration should recover more energy in an AC system than a DC system.
When these inverter drive units were first introduced they were not allowed to regenerate on either electrification system. However since then Network Rail has made some changes to the supplies and solved the safety issues which might apply such as if a unit regenerated power into a dead section where people were working on the line. Today I believe all units that are capable of regenerating are permitted to do so.
Maybe its an American way of explaining science for lay people but I can't see the point of this statement, it just doesn't relate to the nature of power transmission. For a given voltage, AC is (very slightly) less 'efficient' than DC over a long circuit length owing to skin effect. Both are subject to resistance loss in the conductors, hence the use of a high voltage low current circuit. Not sure what is meant by "wave-like motion of the current makes it an efficient traveler"....
Ironically, DC is now almost exclusively used for the longest circuits in the US as part of their grid system. The problem is that a network will have different path lengths when measured over different routes. That isn't a problem for DC systems as the current would equalise despite minor differences in path resistances. The big problem for an AC circuit is that although electric current flows in a conductor at a high speed (approaching 300,000KM per second depending on the cable characteristics), where path lengths are in the order of 1000s of KM, minor differences between two routes as little as a few tens of KM mean that at the point of combining the two routes, there is a small time difference. In an AC supply that time difference means that at the joining point, the two supplies cause large, possibly damaging currents, which also reduce the efficiency of the supply to drive motors etc.. as the sinewave waveform is distorted.
edwin_m
Veteran Member
Traditionally the whole point of AC is not because alternating current is intrinsically better, but because it can be fed into a transformer and the voltage stepped up and down at will. The higher voltage has less resistance loss over longer distances including to AC trains from the relatively widely spaced feeder stations on AC rail networks. Traditionally it was impossible to step DC voltage up and down other than by a heavy and inefficient rotary converter.
I agree in the last few years high voltage DC has become viable for long-distance transmission of power, and indeed is essential in some cases (another one is linking separate transmission networks where the AC is not synchronised). The main reason it is now possible is the development of solid-state converters to turn the DC back into AC at the other end. These are essentially giant versions of the traction inverters fitted to modern electric trains.
I agree in the last few years high voltage DC has become viable for long-distance transmission of power, and indeed is essential in some cases (another one is linking separate transmission networks where the AC is not synchronised). The main reason it is now possible is the development of solid-state converters to turn the DC back into AC at the other end. These are essentially giant versions of the traction inverters fitted to modern electric trains.
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