ELECTRIC TRACTION

ABSTRACT

A lot of engineers must have heard of electric trains, its speed and efficiency but know little or nothing on how it works. This paper shows / gives an introduction to how a train can be powered / energized electrically via Electric Traction. 

            

INTRODUCTION

Electric traction means locomotion in which the driving (or tractive) force is obtained from electric motors. It is used in electric trains, tramcars, trolley buses and diesel-electric vehicles etc. Electric traction has many advantages as compared to other non-electrical systems of traction including steam traction.

Broadly speaking, all traction systems may be classified into two categories. 

a. Non-electric traction systems

b. Electric traction systems 

(a)Non-electric traction systems:-they do not involve the use of electrical energy at any stage examples are steam engine drive used in railways and internal combustion-engine drive used for road transport. 

(b) Electric traction systems.

They involve the use of electrical energy at some stage or the other. They are further divided into two groups. 

1. First group consists of self-contained vehicles or locomotives examples are: battery-electric and diesel-electric drive etc.

2. Second group consists of vehicles which receive electric power from a distribution network fed at suitable points from either central power stations or suitably spaced sub-stations examples are: railway electric locomotive fed from overhead ac supply, tramways and trolley buses supplied with dc supply. 

BATTERY-ELECTRIC DRIVE.

In this case, the vehicle carries secondary batteries which supply current to dc motors used for driving the vehicle. Such a drive is well-suited for shunting in railway yards for traction in mines, for local delivery of goods in large towns and large industrial plants. They have low maintenance cost and are free from smoke, however, the scope of such vehicles is limited because of the capacity of the batteries and the necessity of charging them frequently. 

ELECTRIC TRACTION HAS THE FOLLOWING ADVANTAGES 

1. Cleanliness: - since it does not produce any smoke or corrosive fumes, electric traction is most suited for underground and tube railways. Also, it causes no damage to the buildings and other apparatus due to the absence of smoke and flue gases. 

2. Maintenance cost: - the maintenance cost of an electric locomotive is nearly 50% of that for a steam locomotive. Moreover, the maintenance time is also less. 

3. Starting time: - an electric locomotive can be started at a moment needed whereas a steam locomotive requires about two hours to heat up. 

. High starting Torque: - the motors used in electric traction have a very high starting torque. Hence, it is possible to achieve higher accelerations of 1.5 to 2.5km/h/s as against 0.6 to 0.8km/h/s in steam traction.

As a result, we are able to get the following additional advantages: 

1. High schedule speed.

2. Increased traffic handling capacity

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5. Braking:-it is possible to use regenerative braking in electric traction system. It leads to this advantage. 

 (i) Goods traffic on gradients becomes safer and speedier. (iii) Since mechanical brakes are used to a very small extent, maintenance of brake shoes, wheels, tyres and track rails is considerably reduced because of less wear and tear. 

               DISADVANTAGES OF ELECTRIC TRACTION.

•          The most vital factor against electric traction is the initial high cost of laying out overhead electric supply system. Unless the traffic to be handled is heavy electric traction becomes uneconomical.

•          Power failure for few minutes can cause traffic dislocation for hours.

•          Communication lines which usually run parallel to the power supply lines suffer from electrical interference .Hence, these communication lines have either to be removed away from the rail track or else underground cables have to be used for the purpose which makes the entire system still more expensive.

•          Electric traction can be used only on those routes which have been electrified. Obviously, This restriction does not apply to non-electric traction.

•          Provision of a negative booster is essential in the case of electric traction. By avoiding the flow of return currents through earth, it curtails corrosion of underground pipe work and interference with telegraph and telephone circuits. 

•          SYSTEMS OF RAILWAY ELECTRIFICATION.

•          Presently, four types of track electrification systems are available.

•          a. Direct current system:-600V,750V.

•          b. Single-phase ac system:-15-25kV,  25 & 50Hz.

•          c. Three-phase ac system :-3000-3500V at 25Hz 

        d. Composite system-involving conversion of single: - phase ac into 3-phase ac or dc.

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