4.

ELECTRIC TRACTION
WHY ELECTRIC TRACTION SYSTEM?
 Requirements of ideal traction system
Normally, no single traction system fulfills the requirements of ideal
traction system, why because each traction system has its merits and
suffers from its own demerits, in the fields of applications.

The requirements of ideal traction systems are:

 Ideal traction system should have the capability of developing high

tractive effort in order to have rapid acceleration.

 The speed control of the traction motors should be easy.

 Vehicles should be able to run on any route, without interruption.

 Equipment required for traction system should be minimum with

high efficiency.
It must be free from smoke, ash, durt, etc.
 Regenerative braking should be possible and
braking should be in such a way to cause
minimum wear on the break shoe.
 Locomotive should be self-contained and it
must be capable of withstanding overloads.
 Interference to the communication lines
should be eliminated while the locomotive
running along the track.
 Advantages Of Electric Traction
 Electric traction system is more clean and easy to handle.

 No need of storage of coal and water that in turn reduces the

maintenance cost as well as the saving of high-grade coal.

• Electric energy drawn from the supply distribution system is sufficient

to maintain the common necessities of locomotives such as fans and
lights; therefore, there is no need of providing additional generators.

• The maintenance and running costs are comparatively low.

• The speed control of the electric motor is easy.
• Regenerative braking is possible so that the energy can be fed
back to the supply system during the braking period.
• In electric traction system, in addition to the mechanical
braking, electrical braking can also be used that reduces the
wear on the brake shoes, wheels, etc.
• Electrically operated vehicles can withstand for overloads, as
the system is capable of drawing more energy from the
system.
 Disadvantages of Electric Traction
• Electric traction system involves high erection cost of
power system.
• Interference causes to the communication lines due to
the overhead distribution networks.
• The failure of power supply brings whole traction
system to stand still.
• In an electric traction system, the electrically operated
vehicles have to move only on the electrified routes.
• Additional equipment should be needed for the
provision of regenerative braking, it will increase the
overall cost of installation.
 Supply Systems of Electric Traction

• The way of giving the power supply to locomotive

unit is generally referred as traction
electrification system.
• Presently, there are four types of track
electrification systems are available based on the
availability of supply. These are
• DC traction system
• Single phase AC traction system
• Three phase AC traction system
• Composite traction system
Systems of track electrification
• DC System
• Single Phase AC System
• Three Phase AC System
• Composite System
– Single Phase - Three Phase System or Kando System
– Single Phase - DC System
DC system
• In this system, D.C. series motors used for getting the necessary
motive power, D.C. compound motors are also used for tramways
and trolley buses where regenerative braking can be utilized.

• The operating voltage is from 600 V to 750 V for tramway and

suburban railways and from 1500 V to 3000 V for mainline service.
•
DC Traction
AC Traction
Single phase AC System

(a)In single phase AC system ac series motors are used for

getting necessary motive power
(b).The voltage employed for distribution network is 15 to 25
kV at 25 Hz, which is stepped down on locomotive to a low
voltage suitable for supplying to single ac series motor.
• The spacing of substation is 50 to 80 km.
• The change of supply frequency become
necessary because of
–Batter performance.
–Improves its commutation properties, power
factor and efficiency.
–Reduces the line reactance and hence the
voltage drop.
• AC single phase system is invariably adopted for
main line service.
Single phase to DC system
• This traction system is most popular and widely
used system everywhere. It combines the single
phase high voltage AC distribution at industrial
frequency with DC series motor traction.

• In this, the overhead line carries single phase,

25KV, 50 Hz supply which is then stepped down
to a desired range using step-down transformer
located in the locomotive unit itself.
• This single phase supply is then converted into
DC using rectifier (in the locomotive) and then
applied to DC series motor.

• The advantages of this system include higher

starting efficiency, less number of substations,
simple substation design and lower cost of
fixed installations.
Type of Track Electrification
Parts of Electric Locomotive:
Output waveform
Inverter
Special features of TRACTION MOTORS
Mechanical Features:
• 1. A traction motor must be mechanically strong and robust and it should
be capable of
• withstanding severe mechanical vibrations.
• 2. The traction motor should be completely enclosed type when placed
beneath the locomotive
• to protect against dirt, dust, mud, etc.
• 3. In overall dimensions, the traction motor must have small diameter, to
arrange easily beneath
• the motor coach.
• 4. A traction motor must have minimum weight so the weight of
locomotive will decrease.
• Hence, the load carrying capability of the motor will increase.
Electrical features
• High-starting torque
• Speed control
• Dynamic and regenerative braking
• Temperature
• Overload capacity
• Parallel running
• Commutation
D.C. Series Motor
 D.C. Series Motor
The aspects pertaining to the suitability of a dc series motor for traction duty are given
below:

• It generates high torque at low speeds and low torque at high speeds which is required
to accelerate the vehicle.

• Speed-torque and speed-current characteristics of a dc series motor are steep. So, the
difference in speed of motors due to different wear of driving wheels is less.

• As field flux is directly proportional to armature current, torque for a given current will
not be affected by voltage fluctuations.

• Series speed torque curve gives stable operation because as the speed increases,
torque decreases.

• If armature current increases (due to heavy load torque), then speed decreases.
Therefore, the emf induced decreases and spark-less commutation can be obtained.

• Since torque is proportional to the square of armature current, less current is needed
to increase the torque. Therefore, the series motor can withstand heavy load torque.
 D.C. Series Motor
• For a given increase in torque, the horsepower (power = torque x speed, as torque
increases, speed decreases) remains almost constant. This indicates the self-relieving
property of the series motor.

• Series motor is amenable to various speed control methods.

• Without special arrangements, a series motor cannot be used for regenerative braking.

• Since the series field time constant is low, the back emf becomes zero in case of power
failure. So, the initial rush of current on temporary interruption of supply is more in
the series motor.

Applications of Series Motor :

• Due to low weight and high starting torque, series motors can develop high starting
torque. Therefore, they are suitable for urban and suburban services where a high rate
of acceleration is required which can be met by a series motor. In a 1500V dc system,
the dc series motor may be operated either at 1500V or 750V by connecting them in
series permanently.
 Single Phase A.C. Series Motor
If an ordinary dc series motor is fed from an ac supply, it would operate as ac series
motor but not very satisfactorily owing to the following reasons:

• Since the field and armature currents both reverse every half cycle, the torque would
be exerted at a double frequency in one direction.

• The alternating flux set up by the field winding due to alternating current causes
excessive eddy current losses in fixed core and yokes, thereby increasing the motor
temperature and decreasing the operating efficiency.

• The inductance of field and armature winding decreases the power factor and causes
some abnormal voltage drops which in turn affects the performance of the motor.

• There will be heavy sparking at the brushes undergoing commutation.

• In order to reduce the reactance of the series field, ac series motors are built with as
few turns as possible.
 Single Phase A.C. Series Motor
• Reduction in the number of turns on the field winding results in the reduction of flux
per pole leading to an increase in the speed for a given current and therefore there
would be a reduction in the load torque available for a given current. Hence to develop
the required load torque, the number of armature conductor have to be increased
proportionately.

• The increase in armature conductors would increase inductive reactance of the

armature which can be neutralized by providing the compensating winding
(compensating winding neutralizes completely the armature MMF).

• The air gap is made very small because of a very weak field which is necessary to
obtain a high power factor.

• The yoke and field of the motor are laminated in order to reduce eddy current losses.

• To reduce sparking, brush width is decreased.

• Series inductive reactance is directly proportional to the frequency, so ac series motor

characteristics are better at low frequencies.
Single Phase A.C. Series Motor
 Single Phase A.C. Series Motor
Applications of Single Phase AC Series Motor :

• The operating characteristics of an ac series motor are similar to a series motor. In

which speed is inversely proportional to the armature current and the torque
produced will be equal to the square of the armature current.

• In a traction system, an AC series motor of several hundred KW is usually employed.

• Due to poor power factor at starting, AC series motor has low starting torque
compared to DC motor.

• So, they are not suitable for urban and suburban services where high starting torque
is needed. So they can be employed for mainline traction service.
 Three Phase A.C. Induction Motor
• Earlier three-phase induction motor was considered a constant speed machine. But
with the advent of power electronics, many characteristics of a three-phase induction
motor can be modified to suit the requirements of traction.

• The factors governing the suitability of a three-phase induction motor for traction
applications are given below.

• It has many advantages as simple, robust construction, trouble-free operation, less

maintenance, and high voltage operation requiring reduced current and easy braking.

• With the development of power electronic inverter circuits, the variable output
frequency can be obtained. This can be used to control the speed of a three-phase
induction motor.

• With variable-frequency input to the induction motor, good efficiency and power
factor can also be obtained by lowering the synchronous speed of the motor.

• Starting current of the motor can be decreased by starting the motor at a low
frequency using semiconductor converters.
Squirrel Cage Induction Motor

Slip Ring Induction Motor

𝒌𝒔𝑽𝟐 𝑹𝟐
𝑻= 𝟐 𝟐
𝑹𝟐 + 𝒔𝑿𝟐
Torque-Speed Characteristics of Three Phase Induction Motor
 Three Phase A.C. Induction Motor
Applications of Three-Phase Induction Motor :

• The traction systems employing three-phase induction motors consist of two overhead
conductors for two phases and a track rail for the third phase. This makes the overhead
structure complicated and may also lead to electric shock if any person gets in contact
with the third rail.

• This can be overcome by using the Kando system. In this system, a single-phase high
voltage supply is given by a single overhead conductor. The locomotive has phase
converters that will convert the single-phase to three-phase at the desired frequency
and feed it to the three-phase induction motor.
 Comparison between DC traction and AC traction Motors
Factor DC Traction AC Traction

Motor DC series motor. AC series motor.

Not as good as that used for DC
Performance Good performance.
traction.
Starting torque More. Less.

The speed control of DC series Motor

Speed control Wide range of speed control is Possible.
is limited.

DC system causes less interference It will produce more interference with

Interference
with Communication lines. Communication lines.

Overhead
distribution Heavier and more costly Comparatively. Lighter and less costly.

The number of substations required for

The number of substations required in
a given track distance on DC traction is
Substations AC traction is less.
More.

Weight of Weight of cu required per track km Weight of cu required per track km

Copper is more. is less.
Application Tramway, Trolley bus. Main Line Service.
 Mechanics of Train Movement
• It is the effective force acting on the wheel of locomotive, necessary to propel the train is
known as ‘tractive effort’. It is denoted with the symbol Ft.
• The tractive effort is a vector quantity always acting tangential to the wheel of a
locomotive. It is measured in newton.
• The net effective force or the total tractive effort (Ft) on the wheel of a locomotive or a
train to run on the track is equals to the sum of tractive effort:
1. Required for linear and angular acceleration (Fa).
2. To overcome the effect of gravity (Fg).
3. To overcome the frictional resistance to the motion of the train (Fr).
Mechanics of train movement
• The electric locomotive consists of pinion and gear wheel meshed with the traction motor
and the wheel of the locomotive.
• Here, the gear wheel transfers the tractive effort at the edge of the pinion to the driving
wheel.
Mechanics of Train Movement
Mechanics of Train Movement
 Mechanics of Train Movement
• Here, the armature of the driving motor has a pinion of the diameter d1 attached to it.

• The tractive effort at the edge of the pinion is transferred to the driving wheel by
means of a gearwheel.
Mechanics of Train Movement
 Mechanics of Train Movement
• The maximum frictional force between the driving wheel and the track is equal to
μW.
• Where, μ is the coefficient of adhesion between the driving wheel and the track
and W is the weight of the train on the driving axles, known as adhesive weight.
• The adhesive weight is defined as the total weight to be carried on the driving
wheels. The slipping will not take place unless
Ft > μW
• Therefore, for the motion of trains without slipping, the tractive effort should be
less than or at the most equal to μW.
• From the above discussion it is clear that the magnitude of the tractive effort that
can be employed for propulsion of train depends upon the weight coming over
the driving wheels and the coefficient of adhesion between the driving wheel and
the track.
 Coefficient of Adhesion
• The coefficient of adhesion is defined as the ratio of the maximum tractive effort
that can be applied without slipping of wheels to the adhesive weight, i.e.,

• Also, the coefficient of adhesion decreases with the increase in speed. The normal
value of the coefficient of adhesion with the clean and dry rails is 0.25 and with
wet or greasy rails it is as low as 0.08.
• Since the higher value of tractive effort can be used in electric traction so that the
electric train can be made to accelerate at a faster rate. This results in saving of
time, especially, when the distance between the stops is small.
Problem: An electric train has eight motors geared to driving wheels, each wheel is
80 cm in diameter. Determine the torque developed by each motor to accelerate the
train. The tractive effort required for accelerating the train to a speed of 50 kmph in
30 seconds is of 117590 N, the gear ratio is 4 to 1 and the gearing efficiency is 85%.