AC or DC? Which one is best for electric traction motor trains?
Have you ever wondered which type of electrical current is used to power electrified trains? If AC or DC is preferred, why is that the case?
Alternating current, aka AC, is an electrical current (flow of electrons) that reverses its direction many times a second at regular intervals. It is typically used in power supplies in many nations around the world.
The standard current used in places like the US flips 60 times a second and is said to have a frequency of 60 Hz. In Europe and most other parts of the world, AC current typically has a frequency of 50 Hz or flips 50 times a second.
Source: The Franklin Institute
One of the biggest advantages of AC over DC is that it is relatively cheap to change the voltage of any current. It is also best suited for transporting electrical current over long distances when compared to DC, as energy losses are considerably reduced.
Alternating current was first demonstrated by Hippolyte Pixii in 1832. Called an alternator, Pixii's machine was based on the principles of electromagnetic induction developed by Michael Faraday.
Pixii would later add a commutator to his device to produce direct current instead of alternating current. The first practical application of AC was made by Guillaume Duchenne in the 1850s.
Developed for electrotherapy, Duchenne believed that AC was superior to DC for the electrotherapeutic triggering of muscles. AC was further refined throughout the latter half of the 1800s by the works of William Thomson, Charles P. Steinmetz, and Galileo Ferraris.
Other elements of an AC system, like the generator, transformer, and power transmission systems, were all developed by a series of engineers from many different countries. One notable example is, of course, Nikola Tesla, whose work on AC power transmission would prove pivotal in the mass adoption of AC today.
Direct current, or DC, is another form of electrical current that flows consistently in a single direction, hence the name. Technically defined as a constant flow of electrons from an area of high electron density to an area of low electron density, DC is used in any battery-powered electronic device you can think of.
DC is also typically used to charge batteries, so rechargeable devices like laptops and cell phones come with an AC adapter that converts alternating current to direct current
While Thomas Edison is commonly cited as the inventor of direct current, the accolade should probably really go to an Italian physicist called Alessandro Volta. His voltaic pile, an early battery, was the first device in history to produce direct current.
However, the first widespread application of DC for practical use was in electrical lighting. Shortly after developing his first practical incandescent light bulb, Edison sought to find a way to develop an entire power generation and distribution system to enable his light bulbs to be used on a large scale.
To this end, Edison, and his company, installed their first system in Pearl Street Station in downtown Manhattan. Using it, they were able to supply a few square blocks of the city with electrical power primarily for artificial lighting.
Since the invention of locomotives in the 1800s, the way trains are powered has changed beyond all recognition. Formerly powered by burning solid fuel to generate steam, trains today run on a mix of either pure electric, diesel-electric, or gas-turbine engines.
Of the three, diesel-electric trains are by far the most common and are widely considered the most efficient and cost-effective. This is because they require less human effort and consume less fuel compared to other alternatives.
They were first introduced in the 1930s, and would quickly replace many older steam locomotives on various railroads around the world. One of the first commercially successful diesel-electric locomotives was the Electro-Motive Division’s (EMD) E-series of locomotives. A six-axle locomotive, they were used primarily for passenger services.
Diesel-electric trains are equipped with a powerful diesel 'prime mover', which generates electrical current for use on the electric traction motors to actually turn the train's axles. Depending on the design of the train, it can either produce AC or DC current using a generator powered by the diesel engine.
Because of various electrical connections, multiple locomotives can be operated by a single lead until operated by a single crew.
Modern AC locomotives tend to have better traction and offer better adhesive to the tracks than earlier models. AC diesel-electric trains are most commonly used for carrying heavier loads. However, DC diesel-electric trains are still very popular as they are relatively cheaper to manufacture.
Source: Let's Grow Together/YouTube
Today, they can be found in many parts of the world and run as both freight and passenger services.
Electrical trains rose to prominence in the early 20th century. Some of the first appeared in about 1910 with the opening of the Hudson River Tunnels on the Pennsylvania Railroad’s Philadelphia-New York mainline.
As these tunnels were so long, steam locomotives were prohibited from being used, due to the dense fumes they generate. An alternative way of moving trains was needed, and the electric train was born.
Over the next few decades, electrical trains became more popular around the world and were notably used for various high-speed projects around the world (like the Bullet Trains in Japan, or TGV in France). This is because electric trains are highly efficient, and have a superior power-to-weight ratio than their alternatives.
Maintenance costs are also considerably lower.
16 kV AC Type Rc4 electric trains in Sweden. Source: Henrik Sendelbach/Wikimedia Commons
Most electrical trains operate by collecting current from an outside source, rather than generating it themselves. This can either form a DC third rail, or fourth rail (as in the London Underground), or overhead power lines (the most common type).
The form of electrical current used can vary depending on the region, with AC preferred in many countries in Europe. Whether AC or DC, the current enters a transformer, which is then sent to a rectifier to convert the current to low-voltage DC for use in the train itself.
Depending on the design of the train, this DC current can either be used directly to power the train's traction motors or will need to be converted to three-phase AC using inverters. The main difference between all DC and AC train systems is the location that the current is converted to DC – at a substation or onboard the train.
Any unused current is then usually returned to the power lines, completing the circuit.
To date, there are about six standard voltages commonly used around Europe and the world. These are:
Power conversion for DC systems tends to take place at a railway substation using large, heavy, and more efficient hardware compared to AC systems. AC systems, on the other hand, tend to convert current to AC onboard the train where space is limited and losses can be significantly higher.
However, the costs are offset somewhat by the more efficient transmission of AC current over long distances, allowing the need for fewer substations and more powerful locomotives. Whichever is chosen is usually a trade-off between these considerations, but can also be dictated by existing infrastructure.
The London Underground uses both third and fourth rails to transmit electrical current to the train. Source: Chris McKenna/Wikimedia Commons
So, which is most common around the world? Direct current, either directly supplied, or converted from AC onboard a train, is the most commonly used.
This is because, according to railsystem.net: "DC consumes less energy compared to an AC unit for operating the same service conditions. The equipment in the DC traction system is less costly, lighter, and more efficient than an AC traction system. It also causes no electrical interference with nearby communication lines."
So now you know. You'll never look at an electric, or diesel-electric, train the same way again!
This article was written by Christopher McFadden and first appeared in Interesting Engineering.