26-09-2016, 12:48 PM
1456201131-ElectricTractionSystem.pptx (Size: 1.54 MB / Downloads: 6)
Introduction
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
Existing Tractions System
Existing tractions system uses D.C. motors.
The 25 KV over head voltage is step down to 2000 V with the help of step down transformer.
Rectifier rectifies this A.C. voltage to D.C. voltage.
This rectified D.C. voltage is used to operate the D.C. motors in existing system engine.
Causes favouring 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.
1) D.C. motors commutator which prove to failure because of vibrations and shock. This results in lots of sparking and corrosion.
2) 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
There are three main stages in power circuit of three phases locomotive.
Input converter
A transformer section step-down the voltage from the 25 KV input.
This converter rectifies AC from catenary to as specified dc voltage using GTO thyristors.
It has filter and circuitry to provide a fairly smooth and stable dc output, at the same time attempting to ensure good power factor.
The input converter can be configured to present different power factor to power supply.
DC link
This is essentially a bank of capacitor and inductor or active filter circuitry to further smooth.
Also to trap harmonics generated by drive converter and traction motors.
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
Drive converter
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.