Electric Traction : Concepts

Various systems for Traction:

1. Steam Locomotive :

This consists of a steam boiler which produces superheated steam at a pressure of 10 to 15 kg/cm². This steam drives two double acting, non-condensing steam engines which provide the motive power for the train. The speed is controlled by regulating the flow of steam to the engine.
These engines have now become obsolete and are being gradually withdrawn from services.

2. Diesel Engines :

In these engines a multi cylinder diesel engine is. coupled to a dc generator which supplies power to the dc traction motors. These engines are available in 450 HP (shunter) to 2500 HP range. These are manufactured at Diesel Locomotive Workshops.
These engines can be easily started from cold conditions. Their availability is higher as compared to a steam engine. Also these engines have better a overall efficiency (around 25%) as compared to about 5 to 10 percent for steam locomotives.

3. Electric traction :

Here power is applied to the vehicle from an overhead wire suspended above the track.
Electric traction systems may be broadly categorized as those operating on :
1. Alternating current supply
2. Direct current supply.

In general following electric traction systems exist :

(A) AC 3 phase 3.7 kV system
(B) AC single phase 15/16 kV -161/25 Hz
(C) AC single phase 20/25 kV - 50/60 Hz
(D) DC 600 V
(E) DC 1200 V
(F) DC 1.5 kV
(G) DC 3 kV.

Electric Traction > Advantages :

Electrical transmission, which is usually applied to high power units, has following advantages:

1. It has smooth starting without shocks.
2. Full driving torque is available from standstill.
3. Engine can be run at its most suitable speed range. This given higher efficiency range.
4. Characteristics of traction motor and generator are so chosen that the speed of the traction unit automatically adjusts according to the load and gradient so as to maintain constant output and not to overload the diesel engine.
5. Electrical transmission docs not only work as torque converter but also works as reversion gear.

Traction System	Efficiency
Steam locomotive	5-7%
Gas turbine electric locomotive	10%
Diesel electric locomotive	26-30%
Electric locomotive with thermal power plant	34-36%
Electrical locomotive with Hydroelectric power plant	40-42%

Electric Traction > AC SINGLE PHASE SYSTEM :

In this supply is taken from a single overhead conductor with the running rails. A pantograph collector is used for this purpose. The supply is transferred to primary of the transformer through on oil circuit breaker. The secondary of the transformer is connected to the motor through switchgear connected to suitable tapping on the secondary winding of the transformer.

The switching equipment may be mechanically operated tapping switch or remote controlled contractor of group switches. The switching connections are arranged in two groups usually connected to the ends of a double choke coil which lies between the collections to adjacent tapping points on the transformer. Thus the coil acts as a preventive coil to enable tapping change to be made without short circuiting sections of the transformer winding and without the necessity of opening the main circuit.

DC Electric Traction System :

The transformation and high voltage generation of dc is very inconvenient to the dc supply used is at normally 600 V and this voltage is almost universal for use in urban and suburban railways. For direct current equipment, the series motor is universally employed as its speed-torque characteristics are best suited to traction requirements. Generally two or more motors are used in single equipment and these are coupled in series or in parallel to give the different running speeds required. The motors are initially connected in series with starting rheostats across the contact line and rails, the rheostats are then cut out in steps, keeping roughly constant current until the motors are running in full series. After this the motors are rearranged in parallel, again with rheostats, the rheostats are cut out in steps, leaving the motors in full parallel. The power input remains approximately constant during the series notching, then jumps to twice this value during the parallel notching. Thus a 4 motor unit will have three economical speeds when the motors are running in series, series - parallel connections. The rheostats are operated electro magnetically or electro-pneumatically.

Braking System:

When a locomotive is running at certain speed and if it is to be stopped within a short distance brakes are to be applied. For this purpose brake shoes are provided which are pressed against the wheels for retardation. Steam and diesel locomotives have pneumatic braking system. Some electrical methods of braking have also been devised which are used mainly to step electric motors. During electric braking the kinetic energy of the motor and the coupled mechanism is steadily dissipated in some form or other and the speed of the machine goes on reducing. Four method of electric braking are:

1. Magnetic braking :

In this case the excitation of the armature is disconnected from the supply but the excitation remains on. When the armature rotates in the fixed field, there is reversal of flux in the armature and the iron losses are fed from the kinetic energy of the rotating components and the machine retards. This method can be adopted for shunt, compound „nd synchronous motors. In case of series motors the field cannot stand the full rated voltage, so separate battery has to provided for excitation during braking.

2.Braking through Plugging :

In this case the connections of excitation are reversed. The motor tends to rotate in the reverse direction. Care should be taken to disconnect the motor when it has just stopped This method can be used for small motors and is not suitable for traction motors which are generally of large size.

3. Resistance braking :

In this the motor after switching off is made to run as a generator. The output of generator is consumed in resistance thereby causing retardation.

4. Regenerative braking :

In this method although motor is made to run as a generator but the current instead of being fed to a resistance is fed to the mains. The essential condition for this is that the induced emf should be slightly more than the supply voltage. This method of braking cannot be used for synchronous motors.

Requirements of braking system :

Before we deal with various systems of braking we will first enumerate various desirable requirements which a braking system should satisfy. These are:
1. The braking system should be robust, simple and easy for driver to control and operate. It should require less maintenance and should be reliable.
2. The system should apply brakes simultaneously over all the vehicles.
3. Brake actuation time should be as small as possible.
4. To avoid damage to the goods and discomfort to the passengers, normal service application of brakes should be very gradual and smooth.
5. In case of emergency braking, safety consideration is the prime most consideration. As such retardation rate would be maximum consistent with the safety, so as to make unfailing halt in the minimum possible distance.
6. In order to obtain uniform deceleration, braking force applied to the axle should be proportional to axle load.
7. The braking system should be inexhaustible i.e. repeated quick application of brake should be possible without needing any relaxation, recuperation or normalizing time in between consecutive operations.
8. Kinetic energy of the train should as far as possible be stored during braking which could subsequently be utilized for accelerating the train.
9. There should be automatic slack adjustment for constant piston stroke as a result of wear on the rim and the brake blocks in the case of mechanical braking.

Speed time curve for braking system :

The typical speed time curve for a locomotive is shown in Figure given above. The curve may be broadly split into the following periods :

1. Braking phase : Acceleration period :

From starting to the stage when locomotive attains maximum speed, the period is known as acceleration period, as the vehicle is constantly accelerated. This is represented by OA portion of the curve and time duration is t₁.

2. Braking phase: Free running :

During this period the motor develops enough torque to overcome the friction and wind resistance and hence the locomotive runs at constant speed. This is shown by the portion AB of the curve.

3. Breaking phase : Coasting :

When the locomotive is running at certain speed, if the motor is switch off, due to inertia the vehicle will continue to run, of course with little deceleration due to friction and windage.

4. Braking phase: Braking :

The locomotive is retarded to stop it within short distance and at a particular spot. The shape of the curve will change depending upon the distance between consecutive stations .

from ur's -- Bellapuri saikumar
( www.facebook.com/saikumar544 )

Monday, 15 June 2015