Traction is defined as ‘the act of drawing or pulling especially by a motive force‘ (drawing of carriage by horse constitute traction). In mechanical terms, it is defined as the ‘adhesive friction between the wheel and the surface on which it moves‘ (between a car tire and road). Traction system can be broadly classified as steam engine drive, internal combustion engine drive and electrical traction drives which drives the load on electricity such as diesel-electric drive, battery electric drive, and straight electric drive.
1. Steam Engine drive
Steam engine electric drives are popular in olden days because of simplicity in design, simplified maintenance, easy speed control, low initial cost. In olden days trains were driven by the steam engine where coal is burned in the furnace and steam is produced, the steam generated is used to drive the locomotive. It has some drawbacks because of the low thermal efficiency of the steam engine. Further, a fire has to be banked even when the locomotive is idle.
2. Internal Combustion engine drive
This is non-electric drive mostly used for road transport. Main advantages of the drive are low investment and flexibility of the routs and have the disadvantages of low overload capacity, uneconomical operation, high maintenance cost and short life.
3. Electric Traction drive
In electric traction drive, the drive is mainly by electric motors (dc series motors) which are fed from overhead distribution system generally. Supply to the locomotive constitutes 15 kV and frequency of about 16-2/3 to 25 cycles are used. High supply voltage reduces the current consumption and low frequency will reduce the impedance drop which enables substations to place at a distance of 50 to 80 kms. Some of the advantages of electric traction drives which out rule others are:
Advantages of Electric Traction Drives
- DC series motors are employed in electric traction whose starting torque is high. Due to high starting torque, it is possible to produce high acceleration rates.
- Electric locomotives require less maintenance compared to other drives.
- As it has no smoke it is well suited in underground transport services
- economical operation cost
- It is possible to apply regenerative braking in electrical locomotives. It has the advantage of about 80% of the energy spent during ascent can be pumped back during descent
- Supply of power to the locomotive is easier through the traction substations.
Disadvantages of Electric Traction Drives
- Requires high capital cost.
- Less flexibility of the routes.
- Can cause interference with nearby communication lines.