PHYSICS PRINCIPLES BEHIND THE BULLET TRAINS OF JAPAN(PART-2)

(2) ELECTRICITY :

WHY ELECTRIC CURRENT IS NOT USED IN OVERHEAD LINES ?

Although electric current can be used for overhead lines as a source of power, each wire required should be the thickness of a human arm, now this is practically not possible because as the current density increases the diameter of the wire increases, hence is not feasible for long-distance trains as it is expensive.

to reduce this expense Shinkansen uses a 25,000 V AC overhead power supply (20,000 V AC on Mini-shinkansen lines) to overcome the limitations of the 1,500 V Direct current used on the existing electrified narrow-gauge system. Power is distributed along the axles of the train to reduce the heavy axle loads under single power cars.

The voltage is not dependant on the thickness of the wire and only a contact to the wire is required.

DEVICE FOR PROVIDING THE CONTACT : PANTOGRAPH

   A pantograph seen inside a test facility in Japan.

PROBLEM OF LACK OF CONTACT WITH THE OVERHEAD LINES

Due to the lack of contact between the pantograph and the overhead lines there could be a

plasma arc discharge which is extremely dangerous for the train’s body.

An electric arc between two nails, the same thing can be observed but in larger amounts at the overhead lines.

“An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally non-conductive media such as air”

ANCIENT PRINCIPLE EMPLOYED TO OVERCOME THIS LACK OF CONTACT: ARCHIMEDES LEVER

A LEVER

LEVER PRINCIPLE :

“ Levers can be used to exert a large force over a small distance at one end by exerting only a small force over a greater distance at the other “

   Good example of Arching at wire crossings.

If you’re on a train going through where the lines cross over and all the lights in the train go out briefly, this is why. Arcing is not a desirable phenomenon, and excessive arcing can sometimes be indicative of a very serious problem.

a pantograph connecting the overhead lines with the train’s body

CASE1: when the distance between the train roof and the overhead lines is less then the pantograph bends to provide sufficient contact.

CASE2: when the distance between the train roof and the overhead lines is more then the pantograph is almost vertical to reach the overhead lines.

Thus, this follows the Archimedes principle of Lever.