Electric Traction

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

Traction Systems
Broadly speaking, all traction systems may be classified into two categories:
(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 electric energy at some stage or the other. They may be further
subdivided into two groups:
 UNIT III: ELECTRIC TRACTION-I
(i) Self-contained vehicles or locomotives. Examples are:
battery-electric drive and diesel-electric drive etc.
(ii) Electric vehicle fed from the distribution networks:
These 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 Steam engine drive
locomotive fed from overhead ac supply and
tramways and trolly buses supplied with dc supply.

Diesel-electric drive

Battery-electric drive Railway electric locomotive

 UNIT III: ELECTRIC TRACTION-I
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, dirt, 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.
 UNIT II: ELECTRIC TRACTION-I
ADVANTAGES OF ELECTRIC TRACTION
Electric traction system has many advantages compared to non-electric traction
systems. The following are the 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
 UNIT II: ELECTRIC TRACTION-I
drawbacks:
• 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.
 UNIT II: ELECTRIC TRACTION-I
Steam drive: In steam locomotives, the steam turbine is employed for driving engines.
Though losing ground gradually due to various reasons, steam locomotive is still the
most widely adopted means of propulsion for railway work. Invariably, the
reciprocating engine is employed because:
(a) It is inherently simple
(b) Connection between its cylinders and driving wheels is simple.
(c) Its speed can be controlled very easily.
However, the steam locomotive suffers from the following disadvantages:
(a) Since it is difficult to install a condenser on a locomotive, the steam engine runs
non-condensing and, therefore, has a very low thermal efficiency of about 6-8
percent.
(b) It has strictly limited overload capacity.
(c) It is available for hauling work for about 60% of its working days, the remaining
40% being spent in preparing for service , in maintenance and overhaul.
 UNIT II: ELECTRIC TRACTION-I
Self-contained locomotives.
In this type, the locomotives or vehicles themselves have a capability of generating
electrical energy for traction purpose. Examples for such type of locomotives are:
1. Diesel electric trains: A few locomotives employing diesel engine coupled to DC
generator used to feed the electric motors producing necessary propelling torque.
Diesel engine is a variable high-speed type that feeds the self- or separately excited DC
generator. The excitation for generator can be supplied from any auxiliary devices and
battery. Generally, this type of traction system is suggested in the areas where coal and
steam tractions are not available. The advantages and disadvantages of the diesel
engine drive are given below:
Advantages
(i) No modification of existing tracks is required while converting from steam to
diesel-electric traction.
(ii) It provides greater tractive effort as compared to steam engine which results in
higher starting acceleration
 UNIT II: ELECTRIC TRACTION-I
(iii) It is available for hauling for about 90% of its working days.
(iv) Diesel-electric locomotive is more efficient than a steam locomotive

Disadvantages
• For same power, diesel-electric locomotive is costlier than either the steam or
electric locomotive.
• Overload capacity is limited because diesel engine is a constant –kW output prime
mover.
• Life of a diesel engine is comparatively shorter.
• Diesel-electric locomotive is heavier than plain electric locomotive because it carries
the main engine, generator and traction motors etc.
• The regenerative braking cannot be employed for the diesel engine drives.
 UNIT II: ELECTRIC TRACTION-I
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 small capacity of the batteries and the
necessity of charging them frequently.

Advantages of Electric Traction

As compared to steam traction, 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 building 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 much less.
 UNIT II: ELECTRIC TRACTION-I
(3) Starting time: An electric locomotive can be started at a moment’s notice whereas a steam
locomotive requires about two hours to heat up.
(4) High starting torque: The motors used in electric traction have a very high starting torque.
Hence, it is possible to achieve higher acceleration of 1.5 to 2.5 km/h/s as against 0.6 to 0.8
km/h/s in steam traction. As a result, we are able to get the following additional advantages:
(i) High schedule speed
(ii) Increase traffic handling capacity
(iii) Because of (i) and (ii) above, less terminal space is required-a factor of great importance in
urban areas.
(5) Saving in High Grade Coal: Steam locomotives use costly high-grade coal which is not so
abundant. But electric locomotives can be fed either from hydroelectric stations or pit-head
thermal power stations which use cheap low-grade coal. In this way, high-grade coal can be
saved for metallurgical purposes.
(6)Lower Centre of Gravity: Since height of an electric locomotive is much less than that of a
steam locomotive, its centre of gravity is comparatively low. This fact enables an electric
locomotive to negotiate curves at higher speeds quite safely.
 UNIT II: ELECTRIC TRACTION-I
Disadvantages of Electric Traction
(1) 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.
(2) Power failure for few minutes can cause traffic dislocation for hours.
(3) 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.
(4) Electric traction can be used only on those routes which have been electrified.
Obviously, this restriction does not apply to steam traction.
(5) 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
 UNIT II: ELECTRIC TRACTION-I
SYSTEM OF RAILWAY ELECTRIFICATION
the following four types of track electrification systems are available:
1. Direct Current system--- 600 V, 750 V, 1500 V, 3000 V
2. Single-phase AC system--- 15-25 kV, 16.667 kV, 25 kV and 50 Hz
3. Three-phase AC system--- 3000-3500 V at 16.67 Hz
4. Composite system--- involving conversion of single-phase ac into 3-phase ac or dc.
 DIRECT CURRENT SYSTEM
In this system of traction, the electric motors employed for getting necessary propelling torque
should be selected in such a way that they should be able to operate on DC supply.
Examples for such vehicles operating based on DC system are tramways and trolley buses.
DC series motors are preferred for tramways and trolley buses.
DC compound motors are available where regenerative braking is desired.
The operating voltages of vehicles for DC track electrification system are 600, 750, 1,500, and
3,000 V.
Direct current at 600–750 V is universally employed for tramways in the urban areas and for
many suburban and main line railways, 1,500–3,000 V is used.
These substations receive AC power from 3-φ high voltage line or single-phase overhead
distribution network.
The operating voltage for traction purpose can be justified by the spacing between stations and
the type of traction motors available.
These substations are usually automatic and remote controlled and they are so costlier since
they involve rotary converting equipment.
The DC system is preferred for suburban services and road transport where stops are frequent
and distance between the stops is small.
 SINGLE-PHASE AC SYSTEM
In this system of track electrification, usually AC series motors are used for getting the
necessary propelling power.
The distribution network employed for such traction systems is normally 15–25 kV at
reduced frequency of 16Hz or 25 Hz.
The main reason of operating at reduced frequencies is AC series motors that are more
efficient and show better performance at low frequency.
These high voltages are stepped down to suitable low voltage of 300–400 V by means of
step-down transformer.
Low frequency can be obtained from normal supply frequency with the help of frequency
converter.
Low-frequency operation of overhead transmission line reduces the line reactance and
hence the voltage drops directly and
Single phase AC system is mainly preferred for main line services where the cost of
overhead structure is not much importance and rapid acceleration and retardation is not
required for suburban services
 THREE-PHASE AC SYSTEM
• In this system of track electrification, 3-φ induction motors are employed for getting the
necessary propelling power.
• The operating voltage of induction motors is normally 3,000–3,600-V AC at either
normal supply frequency or 16-Hz frequency.
• Usually 3-φ induction motors are preferable because they have simple and robust
construction, high operating efficiency, provision of regenerative braking without
placing any additional equipment, and better performance at both normal and reduced
frequencies. The induction motors suffer from some drawbacks; they are low-starting
torque, high-starting current, and the absence of speed control.
• The main disadvantage of such track electrification system is high cost of overhead
distribution structure.
• This distribution system consists of two overhead wires and track rail for the third phase
and receives power either directly from the generating station or through transformer
substation.
• Three-phase AC system is mainly adopted for the services where the output power
required is high and regeneration of electrical energy is possible.
 COMPOSITE SYSTEM
As the above track electrification system have their own merits and demerits,
• 1- φ AC system is preferable in the view of distribution cost and distribution
voltage can be stepped up to high voltage with the use of transformers,
which reduces the transmission losses.
• Whereas in DC system, DC series motors have most desirable features and
• for 3-φ system, 3-φ induction motor has the advantage of automatic
regenerative braking.
• So, it is necessary to combine the advantages of the DC/AC and 3-φ/1-φ
systems.
• The above cause leads to the evolution of composite system.
Composite systems are of two types.
1. Single-phase to DC system.
2. Single-phase to three-phase system or kando system.
 Special features of traction motors
The general features of the electric motors used for traction purpose are:
1. Mechanical
2. Electrical features.

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 of traction motor
1. High-starting torque: it must have high-starting torque, which is required to start the motor
on load during the starting conditions in urban and suburban services.
2. Speed control: The speed control must be simple and easy. This is necessary for the
frequent starting and stopping of the motor in traction purpose.
3. Dynamic and regenerative braking: it should be able to provide easy simple rheostatic and
regenerative braking subjected to higher voltages so that system must have the capability of
withstanding voltage fluctuations.
4. Temperature: it should have the capability of withstanding high temperatures during
transient conditions.
5. Overload capacity: it should have the capability of handling excessive overloads.
6. Parallel running: In traction work, more number of motors need to run in parallel to carry
more load. Therefore, the traction motor should have such speed–torque and current–torque
characteristics and those motors may share the total load almost equally.
7. Commutation: Traction motor should have the feature of better commutation, to avoid the
sparking at the brushes and commutator segments.
 TRACTION MOTORS
• No single motor can have all the electrical operating features required for traction. In
earlier days,
• DC motor is suited for traction because of the high-starting torque and having the
capability of handling overloads.
• But the speed control of the DC motor is very complicated through semiconductor
switches. therefore, the motor must be designed for high base speed initially by reducing
the number of turns in the field winding. But this will decrease the torque developed per
ampere at the time of stating. again regenerative braking is also complicated in DC series
motor;
• Therefore the separately excited motors can be preferred over the series motor because
their speed control is possible through semi-controlled converters.
• Also dynamic and regenerative braking in separately excited DC motor is simple and
efficient.
• DC compound motors are also preferred for traction applications since it is has
advantageous features than series and separately excited motors.
 Traction motors
But nowadays squirrel cage induction and synchronous motors are widely used for traction
because of the availability of reliable variable frequency semiconductor inverters. The
squirrel cage induction motor has several advantages over the DC motors.
The squirrel cage induction motor has several advantages over the DC motors.
1.Robust construction. 2. reliable. 3. Low maintenance and low cost. 4. High efficiency.

Synchronous motor features lie in between the squirrel cage induction motor and the DC
motor.
The main advantages of the synchronous motor over the squirrel cage induction motor are:
1. The synchronous motors can be operated at leading power by varying the field excitation.
2. Load commutated thyristor inverter is used in synchronous motors as compared to forced
commutation thyristor inverter in squirrel cage induction motors. Even though such
forced commutation reduces the weight and volume of induction motor, the synchronous
motor is less expensive.
 DC Series Motor
Following features of series motor make it suitable for traction.
1. DC series motor has high-starting torque and has the capability of handling overloads
that is essential for traction drives.
2. These motors has simple and robust construction.
3. The speed control of the series motor is possible by series parallel control.
4. Sparkless commutation is possible, because the increase in armature current increases the
load torque and decreases the speed so that the emf is induced in the coils undergoing
commutation.
5. Series motor flux is proportional to armature current and torque. But armature current is
independent of voltage fluctuations. Hence, the motor is unaffected by the variations in
supply voltage.
 DC Series Motor
6. Remember;

and
But for series motor
Therefore,

The power output of the motor is proportional to the product of the torque and speed.

That is motor input drawn from the source is proportional to the square root of the torque.
Hence, the series motor has self-retaining property.
7. If more than one motor are to be run in parallel, their speed–torque and current–torque
characteristics must not have wide variation, which may result in the unequal wear of
driving wheels.
 AC SERIES MOTOR
Practically, AC series motor is best suited for the traction purpose due to high starting torque.
When DC series motor is fed from AC supply, it works but not satisfactorily due to some of
the following reasons:
1. If DC series motor is fed from AC supply, both the field and the armature currents
reverse for every half cycle. Hence, unidirectional torque is developed at double
frequency.
2. Alternating flux developed by the field winding causes excessive eddy current loss,
which will cause the heating of the motor. Hence, the operating efficiency of the motor
will decrease.
3. Field winding inductance will result abnormal voltage drop and low power factor that
leads to the poor performance of the motor.
4. Induced emf and currents flowing through the armature coils undergoing commutation
will cause sparking at the brushes and commutator segments.
Hence, some modifications are necessary for the satisfactory operation of the DC series motor
on the AC supply and they are as follows:
1. In order to reduce the inductive reactance of the series field, the field winding of AC series
motor must be designed for few turns.
2. The decrease in the number of turns of the field winding reduces the load torque, i.e., if
field turns decrease, its mmf decrease and then flux, which will increase the speed, and
hence the torque will decrease. But in order to maintain constant load torque, it is
necessary to increase the armature turns proportionately.
3. If the armature turns increase, the inductive reactance of the armature would increase,
which can be neutralized by providing the compensating winding.
4. Magnetic circuit of an AC series motor should be laminated to reduce eddy current losses.
5. Series motor should be operating at low voltage because high voltage low current supply
would require large number of turns to produce given flux.
6. Motor should be operating at low frequency, because inductive reactance is proportional to
the frequency. So, at low frequency, the inductive reactance of the field winding decreases.
 Three-phase induction motor
The three-phase induction motors are generally preferred for traction purpose due to the
following advantages:
1. Simple and robust construction
2. Trouble-free operation.
3. The absence of commutator.
4. Less maintenance.
5. Simple and automatic regeneration.
6. High efficiency.

Three-phase induction motor also suffer from the following drawbacks.

1. Low-starting torque.
2. High-starting current and complicated speed control system.
3. It is difficult to employ three-phase induction motor for a multiple-unit system used for
propelling a heavy train.
 Synchronous motor
Synchronous motor
The synchronous motor is one type of AC motor working based upon the principle of
magnetic lacking. It is a constant speed motor running from no-load to full load. The
construction of the synchronous motor is similar to the AC generator; armature winding is
excited by giving three-phase AC supply and field winding is excited by giving DC supply.
The synchronous motor can be operated at leading and lagging power factors by varying
field excitation. The synchronous motor can be widely used various applications because of
constant speed from no-load to full load.
• High efficiency.
• Low-initial cost.
• Power factor improvement of three-phase AC industrial circuits.