Note : DC traction -e4102

6.0 Introduction
 
Did you know what is dc traction motors? Traction is meant here is an electric motor system that is used as the vehicle transporting the goods or public vehicles such as train, LRT and electric bass. The motors are used is because the dc series motor has a torque-speed characteristic is good. The old system uses two series motors system which will produce a jump (JOLT) during the switching process or the process of braking.
To overcome the problem of a leap to escort train, the system uses chopper control method. Brake systems commonly used type of dynamic braking and air braking.

6.1 The advantages of electric railway systems
1. net
This system does not produce smoke, net of diesel and coal, and is suitable for use underground.
2. maintenance costs
Overall cheaper maintenance costs (electric system) when compared with the steam or petrol. For example the price of electric railway heads 50% cheaper than the steam or petrol.
3. Short initial time needed to start or run
electric train. Meanwhile, the steam train needs at least two hours of heated before operation.
4. High starting torque. Series motors are used with high torque and allows it to achieve higher speed limit.
5. Braking systems more easily.

6.2 Disadvantages
1. High initial costs. For example, construction of the railway station, and installation of power cable lines.
2. Power outage will cause the system to become chaotic traffic.
3. May cause interference (electrical inteference) to the lines of communication.

Chopper Control System for Electric Railway


Dc traction on railways usually use higher supply up to 700V. Thus a LC filter is used to control the line current pulse input (Is). Pemenggal used to control the ON / OFF and frequency. This will avoid the ripple current in the motor.

There are three methods of control.

    1. Time on fixed and time off to change.
    2. Fixed off time and time on changing.
    3. Time on and off changes in other words the frequency change.


Chopper output voltage can be expressed in the following equation;

Figure 6.2 chopper output voltage waveform

	Ton Ton + Toff

 

                                         Va ₁  = Va (                      )

           

Va₁ = Va f Ta   

dimana    f   -     Frekuensi penggal

  Ta -     masa ON

 Tb -     masa OFF

Figure 6.3 Chopper control circuit.

 Referring to Figure 6.3, the LC network as a whole serves to stabilize the voltage. Among other LC functions are;
i. Reduce the inductive reactance of the components found in the AU.
ii. Functions as a filter for filtering the ripple voltage.
iii. Can control the input current pulse to stabilize the voltage.

 Electric Railway System
 

6.4There are two types:
1. DC system
2. AC system

6.4.1 DC system
Used in cities and rural areas with a 600V voltage up to 750V. The main channel electric railway system using a 1500V to 3000V. Many substations (sub-station) should be built to provide supplies to the rail. Line or substation distance is 30-40 km.

6.4.2 AC System
AC systems are commonly used in European countries and is divided into three types.
i) 3-phase system where the voltage is 11 to 15 KV - Step down transformer with three phase to 1-phase.
ii) a phase, low frequency (16.66HZ - 25Hz)
iii) a phase, frequency standard (50 - 60Hz)

6.4.3 DC System Benefits Compared With AC
i. DC motor suitable for variable speed.
ii. Most of the dc component is lighter and more efficient.
iii. AT system does not interfere with communication.

Example: -
1. A series motor rated 150hp, 1500rpm, 600V is used to move the train. Full load current is 200A and the total resistance of the armature and the field is 0.1 ohm. It receives supplies and 700V line. a pemenggal used to control torque and speed have switching frequencies from 50Hz to 1600Hz  and ON time is set at 600 microseconds. Calculate:
i) Chopper Frequency .
ii) Current term when the motor is not moving and 240A load current.
iii) The frequency of the current term on the proportion of motor function.