Electrical Traction System:
Traction: The act of drawing or pulling by power is known as traction.
This system uses D.C series motor or single phase A.C series motor or a three-phase induction motor. The electrical input is directly fed to the motor and torque is generated.
A.C Electric Locomotive
It is used a 25KV industrial frequency A.C. supply collected from overhead equipment and stepped down by a transformer in the locomotive.
This supply is then converted to D.C supply and used for traction purpose.
The block diagram of the electric locomotive is as shown in Fig. The pantograph collects the current through sliding contact with the overhead conductor or catenary.
Air circuit breaker is provided at the top of the locomotive which disconnects the supply in case of over voltage.
Step down transformer with tap changer on HV side is provided in the locomotive.
The taps can be changed as per the speed requirement of the traction motor.
The traction motor is D.C. so the D. C. Supply is provided through rectifier unit.
A filter or smoothing choke is also provided so that pure D. C supply can be fed to the motors. The filter reduces the undesirable effects such as bad commutation and heating ( which are produced because of alternating component of rectified A. C.).
Advantages and Disadvantages of traction system.
Advantages:
1. Due to high voltage current is less therefore cross-sectional area of conductor is small so wiight of overhead equipment reduces. So cost of foundations supporting structures also reduces.
2. Erection and maintanace of overhead equipment is easier.
3. Saving is substations.
4. Large spacing between substation
5. Capacity of A.C substations is higher than the D.C substatons.
6. Substation used are simple and cheap.
7. Coeffiecent of adhesion is more in this system.
8. Starting of A.C locomotive is higher than that of D.C locomotive.
9. The KW demand during starting is less in case of A.C locomotive than D.C locomotive.
Disadvantage:
1. Interference with the communication line.
2. Current and voltage imbalance is produced.
Current collection system in Traction:
There are mainly two correct collection systems namely current collection from conductor rail ( or third rail) and current collection from overhead equipment.
The current collection from overhead wire is far superior compared to third rail arrangement.
This is because both theoretically and experimentally current collection is more difficult from a rigid body than from an elastic one.
1.conductor rail system ( Third Railway System) :
In this system, electric supply is collected from an insulated rail running parallel to the track at a distance of 0.3 to 0.4 metre from the adjacent running rail. The return path is through running rails.
In underground system of traction in large cities, the return rails(running rails) are insulated. These are insulated to eliminate any electrolytic action due to return current( on the U. G pipes, cables etc.)
In majority cases, the current collection is from the top surface but safety factor in this case is less because conducting surface st the top is cannot be insulated, hence it is open.
The side collection system more safe and protective against accidental contact.
The conductor rails are not hard as that of running rails, they contain small amount of carbon(0.05 %) and manganese (0.02%). To reduce voltage drop at joints, conductor rails are bonded together by Cu conductors rivetted or welded to the rails.
The current is collected by steel shoes. The necessary contact pressure is obtained by gravity and spring in case of the top contact and by means of only springs in case of side contact type.
The conductor rails have a definite length and a small clearance is there between consecutive conductor rails.
hence, two collecting shoes/brushes are provided so that continuity can be maintained. In many cases, the conductor rail alters between one side of the tract and the other. This also reduce the wear and tear of the collector shoes.
Overhead current collectors:
The contact between current collector and overhead equipment should be maintained under any circumstances. The contact wire in all practical installations is never perfectly horizontal.
It may raises or fall depending upon the weight of contact wire and distance between the droppers.
The contact wire comes very low under bridges and tunnels and raises high over public crossings.
The current collecting system/Gear has to accordingly raise and fall in order to maintain proper contact with overhead equipment.
The choice of overhead current collector depends on speed of vehicle and construction of overhead equipment.
There are three type overhead current collector
1. Pole collector
2. Bow collector
3. Pantograph collector
