Electric Traction

The action of pulling something over a surface (especially a road or a track) is known as
traction. The action of drawing of vehicles by electric power derived from overhead wires,
third rail, storage batteries or diesel generators mounted on the vehicles is known as electric
traction. In simple words, the traction system which uses electric power for its operation is
known as electric traction.
The electric traction system is widely used in electric trains, tramcars, trolley buses and hybrid
vehicles, etc.
Advantages of Electric Traction:
The major advantages of electric traction system over other types of traction (such as steam
traction) are described below −
• Cleanliness − Electric traction is free from smoke and flue gases, etc. Therefore, it is
most suitable type of traction system to be used for underground and tubular railways.
• Less starting time − Electric locomotives can be stating without any loss of time.
• Less maintenance cost & time − The maintenance cost of an electric traction system is
about half of that of steam traction system and the time required for maintenance is
quite low.
• High starting torque − The motor used in electric traction is DC series motor or AC
series motor which has a very high starting torque. Hence, the electric locomotives have
very high acceleration.
• High traffic handling capacity − As the electric locomotives have high acceleration
which makes the traffic handling capacity almost double of that of the steam
locomotive.
• Require lesser terminal space − As the electric traction has high traffic handling
capacity. Therefore, it needs lesser terminal space because quicker disposal of
passengers to destination is there and hence gathering is reduced.
• Regenerative braking is possible − In case of electric traction system regenerative
braking can be applied which feeds back about 80% of electrical energy to the supply.
Also, due the presence of electric braking, there is less requirement of mechanical
braking. Thus, maintenance cost of brake shoes, wheel types and rails get reduced on
account of less wear and tear.
• Center of gravity − Height of electric locomotive is quite less as compared to steam
locomotive which results in lowering of center of gravity. This makes the electric
locomotive more suitable for taking curvy turns at higher speeds safely.
• Saving in high grade coal − The use of electric locomotive saves high grade coal which
is quite deficient in India. Electrical energy required for running electric locomotives is
taken either from hydro-electric power station or thermal power station which uses low
grade coal for electricity generation.
• Absence of unbalance Forces − Due to absence of unbalanced forces produced by the
reciprocating masses, the coefficient of adhesion is more for electric traction. This
reduces the weight to power ratio of locomotive.
• Electric traction has high power-to-weight to ratio.
• The electrification of railway encourages rural electrification.
Disadvantages of Electric Traction:
Although the electric traction has so many advantages, it suffers from many disadvantages
which are given below −
• Electric traction has high capital cost due to overhead equipment, so electric traction
becomes uneconomical unless heavy traffic is to be handled.
• In electric traction, a power failure for few minutes causes distortion in traffic.
• Electric traction can be used only at the places which are electrified.
• Electric traction is tied to electric routes.
• Power lines for electric traction causes disturbance in neighbouring communication
lines.
• In electric traction system, provision of negative booster is necessary, otherwise it
results in corrosion of underground pipes.
Efficiencies of Different Types of Locomotives:

Traction System Efficiency

Electric Traction System

An electric traction system is the type of traction system that uses electrical energy for the
movement of vehicle at any stage.
The electric traction system is classified into two groups as −
• Self-contained vehicles or locomotives (Ex. – battery-electric drive and diesel-electric
drive)
• Vehicles or locomotives which receive electric power from a distribution network or
suitably placed substations (Ex. - railway electric locomotives, tramways and
trolleybuses, etc.)
Here, we described different types of electric traction systems along with their advantages and
disadvantages –
1. I.C. Engine Electric Drive or Diesel-Electric Drive:
In diesel-electric drive, a diesel engine (internal combustion engine) is used which is
coupled to a DC generator. The speed of the diesel engine is maintained constant and
 this DC generator supplies power to the traction motors which are connected to the
driving axels.
Advantages of Diesel Electric Drive:
The advantages of the diesel electric drive are as follows −
• Low capital cost due to the absence of overhead equipment.
• It has high acceleration and retardation.
• No requirement of modification in the existing tracks while converting from non-
electric to diesel-electric traction.
• This locomotive and train is a self-contained unit, therefore, it is not tied to any route.
• The diesel electric drive can be put into service at any moment.
• Overall efficiency of this system is high.
Disadvantages of Diesel-Electric Drive:
The disadvantages of the diesel electric drive are given as follows −
• The diesel engine cannot be overloaded.
• This system has high running and maintenance costs.
• The life of a diesel engine is comparatively shorter.
• It is required to provide a separate cooling system for the diesel engine.
• The dead weight of this type of locomotive is more which requires more axels.
• In this type of traction system, the regenerative braking is not possible.

2. Battery-Electric Drive:
In battery-electric drive system, the locomotive carries the secondary batteries which supply
power to DC series motors employed for driving the vehicle. This type of traction system is
suitable for frequently operated services like daily run of 40 to 60 km for local delivery of
goods, shunting and traction in industrial works and mines. The motors and batteries used in
this system are flame proof.
Advantages of Battery-Electric Drive:
The main advantages of the battery-electric drives are as −
• Cleanliness, i.e., absence of fumes and ash, etc.
• Easy maintenance and the maintenance cost is low.
• The speed control is very easy and effective.
Disadvantages of Battery-Electric Drive:
The following are the disadvantages of the battery electric drives −
• The batteries require charging at regular interval due to small capacity.
• In battery driven vehicles, the speed range is also limited.
 3. Hybrid Drive:
In the hybrid drives, the power is taken partly from the overhead lines and partly from batteries.
In this system, the locomotive receives power from overhead lines for most of the journey and
at the end, the batteries are switched ON, so now locomotive is powered by the batteries. The
batteries are charged by taking the power from overhead lines. The hybrid vehicles are
generally used in mines where long tunnels are to be excavated.
4. Electric Drive:
In this type of electric traction system, the locomotive is driven by the electric motors which
are fed from the overhead distribution system.
Advantages of Electric Drive:
Now-a-days, the electric drives are widely used in traction work due to the following
advantages −
• Cleanliness − it has no smoke or ash; hence it is most suited for underground railways.
• The motors used in electric traction have a very high starting torque.
• These drives have high scheduled speed.
• This type of traction system provides increased traffic handling capacity.
• The electric locomotive requires easy maintenance and less maintenance cost.
• Regenerative braking is possible in case of this electric traction system.
Disadvantages of Electric Drive:
The disadvantages of the electric drive traction system are listed below −
• High capital cost due to overhead equipment.
• Power failure for few minutes can cause traffic distortion.
• Interference with the communication lines.
• This electric traction system is completely tied with electrified routes.