04-03-2013, 03:56 PM
SYSTEMS OF TRAIN LIGHTING
INTRODUCTION:-
Train lighting is one of the important passenger amenities which influence the image of Railways. Train lighting system through axle driven dynamo pioneered by M/s. J. Stone & Co. came to Indian Railways only by 1930. Dynamo / Brushless alternator driven from axle through flat / ‘V’ belts, supplies the load when train is in motion and charges the batteries. The batteries supply the load when train is stationary. Following systems for train lighting are presently in use.
1. Axle generation system working on 24 V DC.
2. Axle driven system working on 110 V DC supply.
3. Mid on generation with 415 V 3 Phase generation AC 110 V utilization.
4. End on generation with 3 Phase 415 V/750 V generation and AC 110 V utilization.
A decision has been taken that all coaches now being built will have only 110 V systems. The coaches operated in 24 V systems have already been converted to 110 V systems.
AXLE GENERATION WORKING ON D.C. 110 V SUPPLY:-
This system has proved more reliable and capable of meeting future increase in load. It has been adopted as standard for all future builds of self generating coaches. In this system 4.5 KW brushless alternators are driven through V-belts from axle. Lead acid batteries 11O V, 120 Ah arranged from 3 cell Monoblock units, are provided in the B.G. coaches.
For BG AC coaches, 18 KW/25 KW brushless alternators are used. Two such alternators are used in AC-2T/AC-3T /Chair Cars and only an alternator is used in First AC coach. Batteries of 800 / 1100 AH capacity at 10 hr rating are used in I AC / AC-2T / AC-3T /chair car of B.G. Coaches. The output from rectifier cum regulator on the under frame is brought through cables on the coach. The load is fed through four rotary switches (RSW) and fuses connecting circuits LI, L2, F and SPM. LI feeds the essential lighting load like lavatories, gangways, doorways and up to 50% of light in each compartment/bays corridor lights and night lights, L2 feeds remaining lighting loads, F feeds the fan load and SPM feeds emergency feed terminals (EFT).
MID-ON-GENERATION:-
In this system a power car housing DG sets is used in middle of rake. This system is chosen for small branch line slow trains having long halts where batteries are likely to remain undercharged if conventional axle driven system is adopted. Capacity of DG set will depend on composition of rake (usually 30 KVA) and generation is at 415 V, 3 phase, 50 cycle and is stepped down to 110 V, 3 Phase, 50 cycles.. The lights and fans in coaches are operated 110 V AC through feeders on either side of Power Car. A schematic layout of power car for mid-on-generation is shown at Figure 1.2.
END-ON-GENERATION:-
Rakes of RAJDHANI/ SHATABDI express trains having heavy load of air-conditioned coaches Pantry cars with electrically operated cooking appliances use Diesel Generating Sets housed in coaches known as Power cars to meet the load. Normally 2 power cars, one on either side of rake, generate power at 750 V AC or 415 V AC, 3 phases, 50 cycles. All the coaches of power cars are interconnected with each other through couplers consisting of switchgear flexible cables. The power at 750 V/ 415 V is stepped down to 110 V AC for lighting and fan load in the coaches. Two feeders run all along the entire rake through I.V. couplers. Each coach on the rake is provided with the control, distribution and feeder changeover arrangements in the 750/ 415 V control panel. 750 V, 3 phase supply is stepped down to 415 V, 3 phases 50 cycles by a step down transformer to feed the A.C. equipments.
INTRODUCTION:-
D.C Dynamos, 32 V used earlier, have been replaced by brushless alternators driven from axle through ’V’ belts. No new D.C. dynamos are being procured and old dynamos have been phased out. However for academic purposes the DC Dynamo is being discussed.
D.C. DYNAMO:-
The D.C. dynamo used for train lighting is shunt wound, reversible and totally enclosed.
When connected to a battery, the generators have inherent self regulating properties due to the Utilization of armature reaction through the medium of a third brush and the arrangement of part of the shunt field connected to obtain decreasing strength with rising speed. An additional shunt field is connected across the main brushes tending to improve the characteristics of the generator. The output of the generator is thus at its maximum at low speeds and drops at high speeds making the generator fit for both slow and fast trains. In effect, the battery charge in ampere hours over a given period of time is the same for either type of service. Fig. shows internal connections.
OUTPUT ADJUSTER:-
This is variable resistance provided in the body of the dynamo with suitable enclosure to provide easy access and adjustment. The output of the generator will depend upon the strength of the 'B' field circuit while the nature of the characteristic will either be ‘flat’ or "drooping" depending on the resistance of the 'A' field circuit. In ‘TONUM’ generators, the output adjuster resistance is designed to get the output at live positions, from 60 to 100 A in steps of 10 A. In the ‘B’ field resistance is calibrated in terms of the percentage output of the dynamo from 70 to 100% in steps of 10%. The output adjuster is provided to adjust the dynamo output to the correct value depending on the demand, type of service i.e. fast or slow and the ratio of generating lime to idle time.
TERMINAL ASSEMBLY:-
The terminal assembly is made up of terminal base of Bakelite and terminal pillar studs. The flexible connections from the brush arms and field connections are brought to the field terminal base. Fuses are provided in the positive side and a fink in the negative side on the terminal assembly both for "Main" and "field" connections.
SILICON BLOCKS DIODE:-
A silicon diode with rated capacity of 150 A for BG and 100 A for MG with working voltage of 50 V and peak transient voltage of 100 V with reverse current not exceeding 50 mA and voltage drop of 1.0 V at full load has been provided between Dynamo+ and Battery+. The silicon diode is attached to heat sink. The whole assembly is enclosed in a metal frame with louvers to allow free flow of air. With the dynamo voltage above the battery by an amount of the voltage drop across the blocker, the dynamo starts charging the battery.
CURRENT Vs VOLTAGE CHARACTERISTICS:-
The equipment is designed to give DC side output upto a maximum current of 140 A. The horizontal portion is at constant voltage and shows the end of charged condition. It must be noted that due to the capacitors in the surge protection circuit, the voltage at open circuit conditions will be in excess of nominal voltage. But with a little charging current, the voltage comes back to the nominal value. The sloped portion of the graph is the current limited portion. The junction between the horizontal and sloped portion is a curve, which serves to protect the battery even if the generator is loaded on a flat battery.
PRINCIPLE OF OPERATION:-
In a charged lead acid cell positive active material consists of lead peroxide (PbO2) and the negative of spongy lead (Pb). Dilute Sulphuric acid (H2SO4 + H2O) serves as electrolyte. The overall reactions inside the cell during discharge and charge are represented most -conveniently by a reversible equation as follows:-
PbO2 + Pb + 2H2SO4 <=> 2PbSO4 + 2H2O
During discharge, the lead peroxide on the positive plates as well as the spongy lead on the negative plates is converted into lead SULPHATE (PbSO4). In this process, SULPHURIC acid (H2SO4) is consumed and water (H2O) is formed. Consequently, the specific gravity of the electrolyte falls, the extent of fall being proportional to the ampere-hours taken out. The process causes at first a slow, and then a faster voltage drop, until a permissible lower limit (final discharge voltage) is reached, which depends on the rate of discharge current. The amount of ampere-hours (constant current x time) taken out is called the capacity of the cell at this rate. The chemical process during charge is the reverse of that during discharge. The lead-SULPHATE on the positive plates is reconverted into lead peroxide and the lead SULPHATE in the negative plates into spongy lead. Sulphuric acid is formed and the water consumed. The specific gravity of the electrolyte rises. There is at first a slow, later a faster rise of cell voltage. From 2.4 volts onwards gassing sets in due to a strong decomposition of water into hydrogen and oxygen. Violent gassing is injurious to the plate material. So after reaching this gassing voltage the rate of the charging current must be limited to within safe permissible values. The nominal voltage of a lead acid cell is 2.0 volts. The true open circuit voltage, however, is dependent on the specific gravity of the electrolyte and varies between 2.02 to 2.05 volts. During discharge the voltage depends on the rate of the discharge current.