In recent years the importance of environmental problems has increased. Because of this electrical traction has replaced the internal combustion engine for mass transit systems. At first DC locomotives were used but they have many drawbacks, such as commutation, sparks on the brushes and difficulty in maintenance. However dc motors have now been replaced by 3-phase squirrel cage induction motors. To allow speed control the dc bus bar is available (+750Vdc) with a digital inverter built into the train. Pulse width modulation (PWM) is commonly used to convert a dc voltage to three symmetrical ac components. Moreover the GTO thyristors is changed to an Insulated Gate Bipolar Transistor (IGBT Technology) for its many advantages. Most metro systems in the world now use this approach. In this paper the mathematical modelling and simulation results are presented and commented on.
The population of big cities needs major projects to solve their transportation problems. The provision of mass transit systems is considered to be the most effective way of overcoming this critical situation. These projects use electrical traction criteria to obtain the higher forces necessary to provide higher speeds [i]. So ac and dc electrical drives are considered to be the backbone of any metro project. At first dc motors were used in the locomotives, but now many countries have changed to the use of ac locomotives.
There are many well-known advantages of AC locomotives over DC ones, such as simplicity, maintenance and low costs [ii]. On the other hand ac motors require an additional unit, the inverter, to convert the dc supply to an ac input for the motor. The first generation of these inverters used GTO thyristors. In modern designs these GTO inverters have been replaced by IGBT technology because of its advantages, such as increased reliability [iii]. IGBTs also have a higher rate of current carrying capacity, which is required for electrical traction.
The reasons for using IGBT technology in power and electrical traction circuits are:
- It can produce the ideal 3-phase sine waveform current to the induction motor, so reducing the noise occurring from the motor.
- The applied voltage on one piece of equipment is half the power source voltage.
- Equipment can be smaller because of high frequency switching.
- Drive power is very small.
- No limitation of di/dt or dv/dt in the power circuit.
In the 1500Vdc rated catenary its change width is from 900V to 1900V, so IGBT can stand the average voltage equal to 1300V or 1500V repeating voltage.