INVERTERS AND CONVERTERS
• Act of drawing or state of being drawn propulsion of vehicle is called tractions. There are
various systems of traction prevailing in our country such as steam engine drive, electric
drive. These systems of tractions may be classified broadly into groups namely
• 1. The traction system which do not involve the use of electricity at any stage and called as
non-electric tractions system such as steam engine drive, IC engine drive etc.
• 2. The tractions system which involves the use of electricity at some stage and called as
electric tractions. System such a diesel electric drive, electric drive etc.
• In India electrification in tractions are conducted with three types of locomotives.
• 1. Using single phase A.C. series commutator motor.
• 2. Using D.C. motor with tapped transformer and rectifier.
• 3. Using phase converter and induction motors
What is traction system
• A system which causes the propulsion of vehicle in which tractive or driving
force is obtained from various devices such as diesel engine drives, steam
engine drives, electric motors, etc. is called as traction system. It can also be
defined as the railway vehicle that provides the necessary traction power to
move the train is referred as the traction or locomotive. This traction power
can be diesel, steam or electric power.
Block diagram of traction system
Working of traction system
• 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. In the following the term Catenary is loosely used even
when talking about the contact wire.
• 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 single arm types are generally oriented with the bend of the
pantograph pointing forwards (in the direction of motion) although this is not a strict
rule and locos exist with pantographs in both orientations. Compressed air is used to
raise the pantograph from its resting position to the raised position where its shoes
touch the contact wire.
• The return path for the electricity i.e. 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. Since the body of the
locomotive and the wheels are all metal they are quite conductive.
Axle brushes are used to electrically connect the rotating axles to the body of the
locomotive. Between the wheels and the rails, usually the return current flows well, but
conductivity may be reduced in cases of dirt and debris on the rails, or if embedded
particles of soot, coal dust, or films of oil, etc. form an insulating layer on the top of the
rails. The voltages and currents involved are such that such thin insulating films are easily
punctured and a conductive path established at the rail-wheel contact
• Earthing cables and earth bond conductors are provided periodically to keep the
rails firmly connected to earth and at earth potential and therefore prevent them
from developing a floating potential or step voltage that may be hazardous.
• Modern electric locos have some fairly sophisticated electronic circuitry to
control the motors depending on the speed, load, etc., often after first
converting the incoming 25 kV AC supply to an internal AC supply with more
precisely controlled frequency and phase characteristics, to drive AC motors.
Some AC locos (WAG-4, WAM-4) have DC motors, instead. Some AC locos (WAP-5
and WAG-9, both designs from ABB) generate 3-phase AC internally using a
thyristor converter system; this 3-phase supply is then used to power
After flowing from the wheels to the rails, the return current flows through the rails and
also partly through the earth beneath and along it. Bonding cables or bonding strips are
provided at rail joints connecting the rails on either side of a fishplated joint to ensure
continuity of return current flow in the rails in case the joint is not conductive because of
dirt, rust, and so on, and also to allow permanent way operations that involve loosening
asynchronous AC motors. 3-phase AC motors are somewhat more efficient, and
can generate higher starting torque.
EXISTING TRACTION SYSTEMS
• Existing tractions system uses D.C. motors.
• 1. The 25 KV over head voltage is step down to 2000 V with the help of step down transformer.
• 2. Rectifier rectifies this A.C. voltage to D.C. voltage.
• 3. This rectified D.C. voltage is used to operate the D.C. motors in existing system engine.
• Causes favoring the DC motors
• 1. D.C. series motors are less costly, however for some H.P more efficient and requires less
maintenance than A.C. series motor.
• 2. Rail conductor system of track electrifications which is less costly with D.C.
system than with A.C. system
Future Trends Of Tractions System
• There are some disadvantages of D.C. series motor used in system.
• D.C. motors commutator which prove to failure because of vibrations and shock. This results in
lots of sparking and corrosion.
• It is hard to use a D.C. motor for regenerative braking and for this purpose extra switchgear is
required, which adds to the bulks and increases the complexity of the locomotives.
• This short coming from this overcome by using three phase A.C. motor in locomotive.
Microprocessor technology and availability of efficient and compact power components have
given a new technology for A.C locomotive.
• In three phase A.C. locos, the single phase input signal from overhead equipment is rectified
• then three phase A.C. is generated with the help of three phase inverter, whose phase voltage and
frequency can be manipulated widely.
• The three phase induction motors are simple and robust in construction and have a high operating
efficiency and properly of automatic regenerative braking with requiring additional equipment.
There are three main stages in power circuit of three phases
1. Input converter
2. DC LINK
3. DRIVE CONVERTER
•1. A transformer section step-down the voltage from the 25 KV input.
•2. This converter rectifies AC from catenary to as specified de voltage using GTO thyristors.
•3. It has filter and circuitry to provide a fairly smooth and stable dc output, at the same time
attempting to ensure good power factor.
•4. The input converter can be configured to present different power factor to power supply.
•1.This is essentially a bank of capacitor and inductor or active filter circuitry to further
•2. Also to trap harmonics generated by drive converter and traction motors.
•3. The capacitor bank in this section can also provide a small amount of reserve power in
transient situations (e.g., pantograph bounce) if needed by the traction motors.
This is basically an inverter which consist of three thyristors based components that switch on
and off at precise times under the control of a microprocessor.
The three components produce three phase of A.C.
The microprocessor controller can vary the switching of thyristors and thereby produce A.C. of
wide range of frequency and voltage.
Advantages Of AC Motors Over DC Motors
1. DC motors use commutators which are prone to failure because of vibration and shock, and which
also result in a lot of sparking and corrosion. Induction AC motors do not use commutators at all.
2. High starting torque
3. In addition, DC motors tend to draw power with a bad power factor and injecting a lot of
undesirable harmonics into the power system.
4. AC motors have the advantage of a simpler construction.
Disadvantages of ac motors over Dc motors
. The starting torque of this motor is very less.
• At low speeds, it won't operate.
• Poor positioning control.
• Limited speed through the frequency supply.
• 25 kV AC single phase
• For traction substation (TSS) the incoming EH supply is
• 220/132/110/166KV through protective equipment it can be transformed by using traction transformer to 25
• KV AC single phases.
• o Spacing between TSS is 30 KM to 40 KM depending upon the traffic (load).
• o To avoid load on one phase and balancing the incoming supply grid, the section TSS is divided into
subsector through switching posts.
• 1. S.P:- Sectioning and paralleling Post.
• 2. S.S.P:-Sub-sectioning and paralleling Post.
Traction Inverter Technology
TRACTION INVERTER FEATURES
• Customized for input from Alternator, Transformer winding, or Third Rail supply (600 to 750
• Energy efficient solutions
• Forced air or liquid cooled
• In-built redundancy and reliability for improved availability
• Individual motor or bogie control
• Under-frame or on-board mounted, light weight solutions
• Mild Steel (MS), Stainless Steel (SS) or Aluminium (Al) enclosures with IP54 or better
• Proven in use modules packaged to meet space and weight restrictions
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