Accessories Used for Track Electrification | Electrical Engineering

The following points highlight the four main accessories used for track electrification. The accessories are:- 1. Overhead Contact Wire 2. Conductor-Rail System 3. Current Collecting Shoes 4. Current Collectors for Overhead Contact Wire.

1. Overhead Contact Wire:

The design of overhead contact wire as used for tramways, trolley buses and for railways of 1500 V and above is highly difficult because it is not sure that the collector and contact wire remain in contact at a fairly even pressure. The materials employed for overhead wire are hard drawn copper, Cadmium-Copper or silicon bronze. Due to higher electrical resistance and tensile strength, silicon-bronze is mostly used.

2. Conductor-Rail System:

It is employed at 600 V for suburban services. It is cheaper, easy to inspect and required less maintenance. Moreover the supporting structure does not interfere in the visibility of the signals. In this system, the current is supplied to the electrical operated vehicle either through one rails conductor or two rail conductors.

In case of one rail conductor the track rail is employed as the return conductor. The conductor rail is place on insulators at the side of the track as shown in figure 7.4 while in some lines a rail with a side running contact is used.

These features make the rail easier to protect against accidental contact. The conductor rail is not fixed rigidly to the insulator, so as to take care of the contraction and expansion of rails. To prevent creepage on account of friction of the collector shoes, the conductor rail is anchored at intervals of 100 to 150 m.

3. Current Collecting Shoes:

The current is conveyed from the conductor rail to the train equipment by means of collector shoe. The collector shoe is made from flat steel about 20 cm in length and 7.5 cm in width and presses on to the rail with a force of about 15 Kg.

The current which can be collected is about 300 to 500 A. Both sides of the locomotive or train are to be fitted with conductor shoes to avoid any possibility of interruption of current. The shoe should not fall more than 2.5 to 5 cm when not in contact with the rail.

4. Current Collectors for Overhead Contact Wire:

i. The Trolley Collector:

The trolley collector is universally employed with trans-ways and trolley buses. It consists of grooved gun metal wheel carried at the end of a light pole. The other end of this pole is hinged to a swivelling base fixed to the roof of the vehicle.

Necessary upward pressure for the pole and current collector is achieved by means of springs. As the two trolley wires are required for trolley bus a separate trolley collector is provided for each wire. The pressure for wheel is approximately 10 Kg.

Necessary upward pressure is obtained by springs. The length of the pole is kept sufficient to have lateral movement along the mean path of trolley bus. Separate bow collectors are used for forward and reverse running. The actual contact strip is made of aluminium or copper. This arrangement enables vehicle to run at higher speed without danger of jumping off the wire.

ii. Bow Collector:

The current may be collected by a bow collector instead of a trolley wire. The bow collector is a light metal strip or bow 60-80 cm in width attached to a framework mounted on the roof of the car as shown in figure 7.7.

iii. Pantograph:

The pantograph employed in railways for collection of current where the operating speed is high (as high as 110 or 130 Km p.h.) and currents to be collected as large as 2000 or 3000 amperes). Pantographs are mounted on the roof of the vehicles and usually carry a sliding shoe for contact with the overhead trolley wire.

These consist of a joined frame usually of steel tubing. The contact shoes are usually about 1.2 metres long. There may be a single shoe or two shoes on each pantograph. The shoe may be straight throughout their lengths or cambered slightly or may be in the form of an auxiliary bow.

Material used for pantograph is often steel with sometimes, wearing plates of copper. The pressure varies from 5 to 15 Kg.

The pantograph is raised or lowered from the driver cabin by one of the following methods or with some modification of it:

(i) Air raised, gravity lowered,

(ii) Air raised, spring lowered,

(iii) Spring raised, air lowered.

The pantographs should be of such construction that they maintain continuous contact with the overhead wire at reasonably constant pressure at any changes height. They must be light enough in order to follow the overhead wire as it changes height above. The rails to pass under overhead crossing etc., with minimum inertia effects even at high speeds.

They must be sufficiently strong and rigid air pressure both head on, due to speed of train, and transverse due to wind. If, however, there is a serious trouble on the overhead system, the pantograph must be sufficiently pliable so that it will be damaged rather than pulling down the overhead structure.

The above arrangement has the following advantages over other types of collectors:

(i) It can operate in either direction of motion.

(ii) There is no risk of leaving the wire junction etc.

(iii) The erection of the overhead network is very simple due to absence of points, and grooved crossing required for bows.

(iv) Its height can be varied from the driver’s cabin by carrying out simple operations.