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The Physics of Electric Locomotives

I intend to use the principles of physics in this research paper to help explain the processes involved in transforming electrical energy to an electric locomotive. This paper may include the railway electrification system but may not necessarily be valid for all locations around the world.

Origin and Background

            Locomotives may be defined as tractors that are propelled on rails in order to tow a certain number of carriages or wagons. The first locomotive, known as the steam engine and once called “the iron horse” was built by Richard Trevithick and was put in service in February 1804. Note that James Watt was responsible for inventing the heat engine rather than the steam locomotive. Robert Davidson, the Scottish engineer, is responsible for inventing the first electric locomotive which ran on the Edinburgh-Glasgow Railway in 1835. However, the cost of producing electricity was expensive. In 1879, the first electric railway that was run using a generator was demonstrated in Germany.  In the United States, the first electric railway was demonstrated in 1880 by the scientist, Thomas Edison. Edison was the first to construct a locomotive (illustration below) such that the tracks carried the current. This was demonstrated at Menlo Park, New Jersey (Getting Electricity to work for Man). Edison’s firm, the Thomas-Huston Company later became known as the General Electric Company went on to produce a 30 ton locomotive capable of 12000 pounds tractive effort. The General Electric Company along with the American Locomotive Company (ALCO) produced direct current locomotives. During the same time, the Baldwin Locomotive Works built locomotives unlike those of General Electric and used alternating current (Steamtown Special History Study)

            Frank J. Sprague commenced the Union Passenger Railway in 1887 in Richmond, Virginia. The 1940’s marked some major electrification projects in Europe. The TGV (Train a Grande Vitesse) was established in 1981. Currently in the United States, only about 2000 miles out the 250000 miles of railroad are electrified (World Book Encyclopedia, 185-187). The following table gives a percentage of railroads electrified in selected countries. It is clear that the United States has among the least electrified networks where as Austria, Sweden and India have among the highest.

Country Percentage electrified (%)
U.S.A 0.9
Canada 0.1
Australia 9.6
China 15.6
France 44
India 44
Italy 59
Sweden 59
Austria 59

(Taken from http://irfca.org/articles/diesel-vs-electric.ppt#263,8,World Railways - Status of Electrification)

           

       

(Taken from http://www.luminet.net/~wenonah/history/edpart2.htm)

            The use of electric locomotives primarily came in order to reduce problems of smoke pollution. Though electric locomotives are more efficient than steam and diesel locomotives, electrification of railway lines is expensive. For this reason, electric locomotives are used mainly for busy passenger lines (Physics Daily).   

 

 

Power Transmission

            There are two kinds of power transmission that are used for electric locomotives. These include alternate current and direct current. Alternate current, as the name suggests, flows in more than one direction where as DC is restricted to one direction only (Railway Technical).

 

(Taken from http://www.railway-technical.com/tract-02.html)

            Alternating current has an advantage over DC current such that it can be used over large distances even through a wire that has a smaller radius. Direct current locomotives typically require wires with greater diameter and in some cases a third rail. At present, alternating current locomotives haul trains over large distances or main line areas where as DC locomotives are restricted to shorter distances. The voltage difference is also great as AC is varied between 15KV to 50KV while DC is confined to below 3KV (Railway Technical).

 

            There are different methods in which the current is transferred. One way is through a third rail and the other is through overhead wires. The third rail is most popular with DC electric systems. Examples of trains that use the third rail are usually metro rail systems but they can be found in main-line routes in southern England to provide speeds of up 100 miles per hour at 750V DC (Railway Technical).

 

            There are certain disadvantages of the third rail. The first one is that there is a safety hazard. If pedestrians were to trample on the rail, they are likely to have a shock hazard. Secondly, weather conditions such as snow can cause the system to not function (Railway Technical).

 

            Third Rail systems also have rail gaps in order to cope with excess voltage supply. For each gap, there is what is called a “substation”. These substations ensure continuity and provide voltage in the direction of propagation and are off support if one substation were to fail. Substations also give an indication for the train to continue on its route or to halt for any reason. In cases where there may be faults along the electric lines, there are switches to stop the flow of current which means that the train must stop before entering that section (Railway Technical).

(Taken from http://www.railway-technical.com/etracp.html)

            AC electric systems are more likely to be used overhead wires otherwise known as a catenary. The design of the overhead wires are created such that it is held in tension horizontally and also subjected to lateral pulling such that it can accommodate for curves. Wires are usually 1km to 1.5km long between poles depending on temperature changes.   

 

 

 (Taken from http://www.railway-technical.com/etracp.html)

            DC locomotives also use overhead wires. However, their wires are usually thicker and in some case double wires are used for severe loads. Examples of such systems include Hong Kong’s Mass transit and DC main lines in France, Belgium and Italy. 

 

            In order to makeup for interference along the catenary, there are booster transformers connected at frequent intervals throughout the route. The main purpose of this is to reduce any inequality of voltages that are induced by other electric lines that run parallel to the catenary. A return wire is connected to the track such that the current returns to the transformer. Without this, there can also be a safety hazard. 

            The following circuits show a circuit of an electric railway system used by the Indian Railways. The current IC flows through the catenary and then enters the locomotive through the pantograph. The departing current IR returns to the main power supply through the tracks. The second circuit diagram is an example of booster transformers used in Indian Railways.  Notice that the there are insulated rail joints to be sure that the current flows at the correct sections (IRFCA).                  

           

(Taken from http://irfca.org/docs/traction-schematics.html)

           

 

Circuits of electric locomotives

 

 

 (Taken from http://www.railway-technical.com/tract-02.html)

 

             Now, the various components of the AC electric locomotive will be discussed. The first one is a locomotive with a DC motor. As seen in the above diagram, the locomotive receives its power through the pantograph from the catenary. The voltage received is decreased by a step-down transformer. The amount of current is controlled by a device known as the “tap changer”. The “tap changer” is basically a camshaft set of operated switches and connects more sections of the transformer. The next device is the AC-DC rectifier. The kind of rectifier used in this circuit is the bridge rectifier. It is basically an arrangement of diodes that enable the current to flow in one direction only. The capacitor is used to decrease the fluctuation of voltage. The drop of voltage from the rectifier is eliminated by the voltage drop in the capacitor (Railway Technical).

  

 

 

 (Taken from http://www.railway-technical.com/tract-02.html)

            Instead of using a bridge rectifier, it is possible to use a thyrister. A thyrister is a kind of diode such that it allows current to flow when it receives a command through a third terminal (Physics Daily). The thyristor can be illustrated as an N-P-N and a P-N-P transistor connected to each other. As the current enters the base of the N-P-N transistor, it  causes a greater current to exit through both terminals. This current then enters the base of the P-N-P transistor causing a much greater current to leave the emitter. However, if current flows the other way, the initial command is deleted. The device is also turned off if the current is zero.

(Taken from http://www.physicsdaily.com/physics/Thyristor)

 (Taken from http://www.railway-technical.com/tract-02.html)

            When the thyristor is used to rectify from AC to DC, the sinusoidal wave display on the voltage and time graph is confined to above the positive axis. In order to create a steady supply of voltage and smoothen the delivery, a smoothening circuit is connected. The smoothening circuit may include inductors and capacitors (Railway Technical).

 

(Taken from http://www.railway-technical.com/tract-02.html) 

 

 

(Taken from http://www.railway-technical.com/tract-02.html) 

 

The above circuit diagram includes the use of a diode, an inductor and a capacitor along with a thyristor. A problem with the previous circuit is that once the thyrister is activated, there is not always a path that can cause the thyrister to be turned off. Hence the inductors and capacitor cause the current to move in the opposite direction (Railway Technical).

 

            The line filter elements in the circuit represent the combination of an inductor and a capacitor such that it prevents interference from entering the train’s main circuit. The free-wheel diode actually keeps the current flowing through the circuit using the motor’s self inductance while the thyrister is not activated (Railway Technical).  

 

            It is also possible to have AC electric locomotives with AC motors. As seen in the circuit below, there is both an AC-DC rectifier and a DC-AC inverter. The connection between the rectifier and the inverter is known as a DC link. The DC-AC inverter is necessary in order to provide input to the 3 phase traction motors. The speed of the motor also depends on the frequency submitted towards it. There may be more inverters connected to the DC link in other to serve other parts of the locomotive such as the compressor and cooling fans (Railway Technical).

 (Taken from http://www.railway-technical.com/tract-02.html) 

            The following diagram of the AC electric locomotive has almost the same circuit as shown above. Notice there are other inverters used and not just to run the 3-phase motors.

 

 (Taken from http://www.railway-technical.com/elec-loco-bloc.html)

The Future of Electric locomotives

Though electrification is expensive, it is possible that electric locomotives may eventually eliminate steam and diesel-electric locomotives, especially on passenger routes. Electric locomotives can provide more tractive effort than diesel locomotives (The World Book Encyclopedia, 420). Diesel-electric locomotives could possibly be restricted for freight use.  However, the United States has preserved diesel traction for the most part due to their popularity (Academic American Encyclopedia, 391). On the other hand, it is possible that diesel fuel may decline in availability in the future which can lead to rise in costs. This could enhance electrification to become more widespread in the United States.    

 

 

Works Cited

 

"Electric Railroad." The World Book Encyclopedia. 1989 ed. 1989.

"Locomotive." The World Book Encyclopedia. 1989 ed. 1989.

“Locomotive.” Academic American Encyclopedia. 1998 ed. 1998

"Electronic Power for Trains." Railway Technical Web Pages. 16 Dec. 2005. 27 Jan. 2006 <http://www.railway-technical.com/tract-02.html>.

Electric Locomotives." 06 Nov. 2005. Physics Daily. 27 Jan. 2006 <http://www.physicsdaily.com/physics/Electric_locomotives>.

"Getting Electricity to work for Man." The Era of Giants. 30 Jan. 2006 <http://www.luminet.net/~wenonah/history/edpart2.htm>.

"Electrification-Circuit Diagrams." IRFCA. 2002. Indian Railways Fan Club Association. 24 Jan. 2006 <http://irfca.org/docs/traction-schematics.html>.

"American Electric Locomotives." Steamtown Special History Study. 14 Feb. 2002. 27 Jan. 2006 <http://www.cr.nps.gov/history/online_books/steamtown/shs4.html>.