Molly Ayala
Subways are complex systems that involve many unseen components that work together to ensure the safe and efficient transport of people. One of the key aspects of subway systems is their use of electricity and magnetism. Early subways primarily used steam engines, but modern subways have adopted electric power for their trains, tunnel lights, and station equipment. This shift to electricity offers advantages such as improved energy efficiency, reduced emissions, and lower operational costs. The electrical supply for subways typically involves alternating current generation and distribution, with direct current operation of train motors. This electricity is generated in large power plants and transmitted to the subway network, powering the trains through overhead wires or an electrified third rail. The third rail, located outside or between the tracks, supplies power to the train's electric motor via a wheel, brush, or sliding shoe. Additionally, subways utilize magnetism in their electrical systems, with field magnets constituting the periphery of the revolving field and playing a crucial role in the functioning of alternators. The adoption of electricity and magnetism in subway systems showcases the evolution of transportation technology, offering enhanced efficiency, safety, and environmental benefits.
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Electric propulsion
The New York City subway system, for example, utilizes a third rail that carries 625 volts of electricity. The original lines required their own power plant, with electricity transmitted through a series of cables and substations to the third rail. The third rail distribution system was chosen due to the limited headroom in the subway, which prevented the use of an overhead conductor system. This electrical supply system also allows for interchangeable operation with the Manhattan Railway system.
The electrical equipment in subway trains is designed to ensure safe and efficient operation at high speeds. The power delivered to the various motors throughout the train is controlled and regulated by a motorman at the head of the train, using a system of electric circuits, including a small drum controller and actuating circuits. This system enables simultaneous control of multiple motors, enhancing operational efficiency.
To ensure safety, important power wires beneath the train cars are placed in conduits made of fireproof materials, primarily asbestos. Additionally, the vulcanized rubber insulation of the wires is covered with a special asbestos braid, reducing the amount of combustible insulating material. While insulation is necessary, it is also combustible and can produce smoke when burned. Therefore, precautions are taken to minimize potential accidents and address issues like electrolysis, where rubber insulating pieces protect the lead sheaths of cables.
Furthermore, subway systems may employ automated trains with surveillance systems, such as closed-circuit TV, allowing for remote monitoring and control. Sensors play a crucial role in detecting objects near the train and in doorways, enhancing passenger safety. These advanced electrical propulsion and control systems contribute to the overall efficiency and safety of modern subway networks.
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Overhead wires
One advantage of overhead wires is that they enable trains to achieve higher speeds compared to third-rail systems due to the lack of mechanical limitations on contact with the power source. However, a disadvantage of overhead wires is their susceptibility to lightning strikes during thunderstorms, which can cause power surges or breaks in the wires and disable trains.
Some countries, such as Japan, South Korea, and Spain, are more inclined to adopt overhead wiring for their urban railways. In contrast, other countries continue to build new third-rail systems, including technologically advanced cities like Copenhagen, Taipei, and Wuhan.
A combination of both systems is also employed in certain situations. Several British trains can operate on both overhead and third-rail systems. Additionally, some railways use overhead wires for sections of the route and then switch to third-rail or diesel power for the remainder of the journey due to historical reasons or the connection of separately owned railways. In the New York City subway system, the limited headroom available prohibited the use of an overhead system of conductors, leading to the adoption of the third-rail direct current system.
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Third rail
The third rail system, also known as the contact rail system, is a method of railway electrification that provides power to trains through a conductor rail placed alongside or between the running rails. This additional rail, also known as a live rail, carries electricity that is transmitted to the train through metal contact blocks or 'contact shoes'. The third rail usually carries 600 or 750 volts of electricity, although this can vary, with the New York City Subway's third rail carrying 625 volts, and the Bay Area Rapid Transit in San Francisco using 1000 volts.
The third rail system is commonly used in urban transit systems, particularly in metro systems and underground railways, where it is ideal due to its low clearance height requirements. Unlike overhead wiring, the third rail does not require additional structures to carry wires and does not need to be erected under tension, making it less prone to breakage accidents. The third rail is also generally more affordable to install and maintain than overhead wiring, as it requires fewer infrastructure modifications.
One advantage of the third rail system is its environmental benefit. As it is purely electric, no fossil fuels are burned and no exhaust is produced, making it a clean or semi-clean energy source. Electric trains are also cheaper to manufacture and maintain, and they cause less wear to tracks due to their lighter weight. The third rail system is also highly reliable, with a low susceptibility to failures and outages.
However, one of the primary concerns with the third rail system is safety. As the third rail carries a high voltage, it poses a significant risk of electric shock to individuals who come into contact with it. Adequate safety measures must be in place to prevent unauthorised access to the tracks. Additionally, the third rail system has speed limitations due to the mechanical impact on the contact shoe and the gaps in the conductor rail at level crossings and other locations.
Maintenance is another major concern with the third rail system, as it is subject to mechanical wear and tear during daily operations. Electrical erosion can also occur due to current arcing between the conductor shoe and the rail, resulting in electrical erosion of the conductor material. Regular upkeep and maintenance are essential to prevent this issue and ensure the smooth and reliable operation of mass transit systems.
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Alternating current
The New York City subway system, for example, uses a combination of alternating current (AC) for power generation and distribution and direct current (DC) for the operation of car motors. The third rail carries 625 volts of electricity to power the trains, tunnel lights, and station equipment.
The rotating magnetic field produced by AC is essential for the operation of electric motors used in subway trains. By adjusting the frequency of the AC current, the speed of the electric motors can be controlled. This is because the higher the frequency of the AC current, the faster the magnetic field rotates, which in turn increases the speed of the motor.
In addition, AC is commonly used in industrial motors, with three-phase AC motors being the most prevalent type. These motors utilise three different AC waveforms of the same frequency that are 120 degrees out of phase. This means that when one phase reaches its maximum, the other two phases are either increasing or decreasing. By arranging three electromagnets radially, a rotating magnetic field can be created. This rotating magnetic field enables the motor to rotate and generate mechanical power, driving the subway trains forward.
The use of AC in subway systems offers advantages such as efficient power transmission and the ability to control motor speed through frequency adjustment. However, it's important to note that the specific implementation details may vary across different subway systems, and some systems may employ different combinations of AC and DC power utilisation.
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Direct current
The New York City subway system uses a combination of alternating current generation and distribution, along with direct current to power the car motors. The direct current is supplied to the trains via an electrified rail known as the third rail. This rail carries 625 volts of electricity and powers the train's electric motor.
The use of direct current in the New York subway system is a result of several factors. Firstly, the limited headroom in the subway tunnels prevented the use of an overhead conductor system. Additionally, the desire for interchangeable operation with the Manhattan Railway system made tri-phase traction systems impractical. The direct current system, with its third rail, provided a viable solution that accommodated these constraints.
The use of direct current also has implications for safety and efficiency. In the event of derailment or accidents, direct current enables electrical insulation of specific tracks, allowing for uninterrupted service on other tracks with separate power channels. Additionally, direct current can reduce the intensity of arcs that may occur during short circuits, enhancing safety. Furthermore, direct current systems may have lower transmission losses compared to AC systems, as higher currents are needed in AC systems to provide the same power, resulting in greater energy loss through electrical resistance.
The New York subway system's electrical supply includes 11,000-volt circuits, with the use of oil to facilitate the making and breaking of circuits. The system also utilizes asbestos and vulcanized rubber insulation to protect the power wires and reduce the risk of fire.
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Frequently asked questions
Electric railways offer better energy efficiency, lower emissions, and lower operating costs compared to diesel engines. Electricity can also be generated from diverse sources, including renewable energy.
Electricity is typically generated in large and relatively efficient generating stations and transmitted to the railway network. Some electric railways have their own dedicated generating stations, but most purchase power from an electric utility.
A third rail is an electrified rail that supplies power to trains. The third rail lies outside or between the subway tracks, and a wheel, brush, or sliding shoe carries the power from the rail to the train's electric motor. The third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, making it a good option for subways.
The voltage used in subway systems can vary. For example, the third rail in the New York City subway system carries 625 volts of electricity, while the London Underground uses a four-rail system with +420 V DC and -210 V DC rails.
