Electric Traction

Locomotion in which driving force

obtained from electric motors
•Electric trains
•Tram cars
•Trolley buses diesel – electric vehicles

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 Requirements of Ideal Traction
Systems
 Maximum tractive effort – rapid
acceleration
 Capable of overloads for short duration
 Wear on tracks should be minimum
 Locomotive or train unit should be self –
contained, able to run on any route
 Braking without excessive wear on brake
shoe, also regenerative braking
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 Systems of Traction
• Non – Electric Traction systems
– Direct Steam – Engine Drive
– Direct Internal – combustion Engine Drive
• Electric Traction systems
– Self contained vehicles or locomotives
• Battery – Electric Drive
• Steam – Electric Drive
• Diesel – electric Drive
• Internal – Combustion Electric Drive
– Vehicles receives power from distribution network
or suitably placed Sub-Stations
• Track Electrification

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 Steam Locomotive
• This consists of a steam boiler which produces superheated
steam at a pressure of 10 to 15 kg/cm2.

• This steam drives two double acting, non-condensing steam

engines which provide the motive power for the train.

• The speed is controlled by regulating the flow of steam to

the engine.

• These engines have now become obsolete and are being

gradually withdrawn from services.

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 Diesel Locomotive
• In these engines a multi cylinder diesel engine
is. coupled to a dc generator which supplies
power to the dc traction motors.

• These engines are available in 450 HP to 2500

HP range. These are manufactured at Diesel
Locomotive Workshops. These engines can be
easily started from cold conditions. Their
availability is higher as compared to a steam
engine.

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 Electric Traction
• Here power is applied to the vehicle from an overhead wire
suspended above the track.
• Electric traction systems may be broadly categorized as those
operating on :
– 1. Alternating current supply
– 2. Direct current supply.
•  In general following electric traction systems exist : 
(A) AC 3 phase 3.7 kV system
(B) AC single phase 15/16 kV -161/25 Hz
(C) AC single phase 20/25 kV - 50/60 Hz
(D) DC 600 V
(E) DC 1200 V
(F) DC 1.5 kV
(G) DC 3 kV.

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 Advantages
 Cheaper
 Free from smoke
 Rapid acceleration and braking
 Maintenance cost is less
 High Starting Torque
 Regenerative braking
 Passenger carrying capacity at higher
speeds

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 Disadvantages
 Capital cost is high
 Failure of supply
 Interference with adjacent telephone lines

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 Electric Traction
• The traditional DC (Direct Current) electric motor driving a train or
locomotive is a simple machine consisting of a case containing a
fixed electrical part, the stator and a moving electrical part, the rotor.

• As the rotor turns, it turns a pinion which drives a gearwheel. 

• The motion of the motor is created by the interaction of the

magnetism caused by the currents flowing the stator and the rotor. 

• This interaction causes the rotor to turn and provide the drive. 

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Electric Traction

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Electric Traction

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 Electric Traction
• In OHE, or overhead electrification systems, the supply of
electricity is through an overhead system of suspended
cables known as the catenary.

• A contact wire or contact cable actually carries the

electricity; it is suspended from or attached to other cables
above it which ensure that the contact cable is at a uniform
height and in the right position.

• The loco uses a pantograph, a metal structure which can be

raised or lowered, to make contact with the overhead contact
cable and draw electricity from it to power its motors.

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 Electric Traction
• The pantograph has one or two blades, shoes or collector pans
that actually slide against the contact wire. The DC pantographs
generally have two shoes, while the AC pantographs have one
shoe, owing to the higher current carried by the DC pantograph.

• The pantograph structure may be in the form of a single arm — a

single open bent angle ('>') — or in a diamond (rhombus) form
('<>'). The diamond form was more common for the DC locos.
Newer locos almost always have the single arm pantographs.

• The return path for the electricity (the return current) is through the
body of the loco and the wheels to the tracks, which are electrically
grounded. Ground connections are provided from the rails at
periodic intervals.

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 Supply Systems for Electric Traction
D.C systems
-Series Motors
-For Tramways –Compund Motors
-Tramways and Trolleybuses-600V
-For Trains-1500 V and 3000V
-Spacing between substations 30-40 km
-Substation input 33KV and 110 KV A.C.
-Using Transformers and Converters the conversion a.c to d.c is done
Advantages
-Series motor –better characteristics when compared to a.c motor
-Maintenance low
-weight of d.c.motor is less
-Suitable for short distance
-overall cost is high

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 Supply Systems for Electric Traction
• Single phase A.C.systems

- Operating voltage 300-400V Frequency 25/16.67 Hz

- Energy obtained from 15 KV to 25 KV with frequency 50
Hz
- Operating radius is within 30Km the energy is supplied to
locomotive else to substations.
- reduction in structure cost
- used in main line rapid acceleration and deceleration is
not important.

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Supply Systems for Electric Traction

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 Supply Systems for Electric Traction
• Three phase A.C.systems
-3000V -3600 V
-Induction motor
-2 overhead wires and track acts as third conductor
-Employed in hilly areas where regeneration is possible
Advantages
-High efficiency
-Simple and robust
-Automatic regenerative braking
Disadvantages
Low torque
High starting current
Difficult to collect current from overhead conductors

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 Supply Systems for Electric Traction
• Composite systems 1 phase A.C.to D.C. system
-HV Overhead distribution
-dc series motor
-15 -25 KV 50 Hz
-Locomotive carries transformer and converters
Advantages
-line current is reduced because of high voltage
-substations simpler and cheaper
-starting efficiency high when compared to d.c system
-A.C. locomotive has less KW demand at starting
Disadvantages
-Unbalance power supply system
-Interference with neighboring communication lines

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 Supply Systems for Electric Traction
• Composite systems 1 phase A.C.to 3 phase A.C

-single phase high voltage for distribution and 3 phase induction motor
for motive power
-locomotive carries a converter to convert 1 phase A.C.to 3 phase A.C
at low frequency

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 Braking
1. The braking system should be robust, simple and easy for driver to
control and operate. It should require less maintenance and should be
reliable.
2. The system should apply brakes simultaneously over all the vehicles.

3. Brake actuation time should be as small as possible.

4. To avoid damage to the goods and discomfort to the passengers, normal

service application of brakes should be very gradual and smooth.

5. In case of emergency braking, safety consideration is the prime most

consideration. As such retardation rate would be maximum consistent
with the safety, so as to make unfailing halt in the minimum possible
distance.

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 Braking
6. In order to obtain uniform deceleration, braking force applied to the
axle should be proportional to axle load.

7. The braking system should be inexhaustible i.e. repeated quick

application of brake should be possible without needing any
relaxation, recuperation or normalizing time in between consecutive
operations.

8. Kinetic energy of the train should as far as possible be stored during

braking which could subsequently be utilized for accelerating the
train.

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 Requirements for Electric Traction
Main Line service
-Minimum cost of overhead structure
-High maximum speed
-Acceleration and retardation not important

Suburban railway service

-Rapid acceleration
-Less voltage fluctuations
-No interference with communication lines

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Speed Time Curve for Electric Traction
1. Acceleration period :
From starting to the stage when
locomotive attains maximum speed, the
period is known as acceleration period,
as the vehicle is constantly
accelerated. This is represented by OA
portion of the curve and time duration is
t1. 

2. Free running :
During this period the motor develops
enough torque to overcome the friction
and wind resistance and hence the
locomotive runs at constant speed.
This is shown by the portion AB of the
curve.
 

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Speed Time Curve for Electric Traction
3. Coasting :
When the locomotive is running at
certain speed, if the motor is switch off,
due to inertia the vehicle will continue
to run, of course with little deceleration
due to friction and windage.
 
4. Braking :
The locomotive is retarded to stop it
within short distance and at a particular
spot. The shape of the curve will
change depending upon the distance
between consecutive stations .

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Speed Time Curve for Electric Traction
Crest Speed
Maximum Speed attained by the train

Average Speed

Maximum Speed maintained by the

train during the run

Va =D/T

D=Distance between stops in Km

T=Actual time of run in hour

Scheduled Speed

Scheduled speed=D/(T+Tstop)
Tstop=Stop time in hour

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Speed Time Curve for Electric Traction

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Speed Time Curve for Electric Traction

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Speed Time Curve for Electric Traction

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Speed Time Curve for Electric Traction

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