Locomotives

Conventional Locomotives: Single phase 25 KV is stepped down and converted in to DC, then this DC supply is used to run DC traction motors equipped in Conventional Locos.

3 Phase Locomotives: Single phase 25 KV is stepped down and converted into DC, now this DC is again converted into 3 phase AC and used to run three Phase Induction motors equipped in 3 Ph Locos.

Electric trains can run on phase to neutral (3rd rail) or phase to phase (3rd & 4th rail). The power can be supplied via rail or by overhead. The voltages vary widely between different places and it can be run on AC or DC.

Given heat dissipation and insulation is much better on overhead lines, the voltage can be increased significantly. The UK west coast mainline runs on 11kv AC phase to phase and the London tube runs on          +500v, -130v DC.

The AC power is then taken into the train and via a traction transformer down to a manageable level then converted to DC before being converted back to AC at a variable frequency for speed control.

If DC power is being used, then it is converted directly to a variable frequency AC for speed control.

Each carriage generally has its own conversion and traction kit which means you can basically make a train as long as you desire. If you have one locomotive at the front, it must get bigger as the train gets longer until eventually the loco gets too big to pull itself and the tracks to handle!

If you get a fault, a connection between earth and a phase or between 2 phases, there are protection devices that should disconnect the supply, however a load of energy will flow before the protection devices see it as a fault and disconnect it.

Also as the voltage on over head lines is so high, the voltage will jump if something earthed gets close enough. That is how some youngsters died a few years ago when they broke in to a rail depot and climbed on the cars, very sad, very dangerous.

In both the diesel as well as the electric engine, the primary intention is to supply electricity to traction motor (which may either be a 3 phase Induction Motor or a DC motor). These traction motors are connected to wheels, which make the train move

The first stage of every electric engine is same: Pantograph

Pantograph consists of collector bars, which touch the overhead lines and collect electricity. This then runs over bus bar on the roof of locomotive, through surge arrester and vacuum circuit breaker (These are devices used to protect the locomotives from short circuit/ overload currents/ effects of lightning etc)

The output from circuit breaker is fed to transformer

From here, depending on whether your locomotive has DC motors (and tap changer operation) or Induction motor (and VVVF controls) the process varies.

Locomotives with Tap changer

In locomotives which has tap changer, there are two transformers, wound on same core. The first one is an auto transformer with 31tappings. Depending on where you tap, the output voltage from auto transformer varies and this varying of taps is done in the locomotive cab. The output of this auto transformer is fed to another transformer, which is a step down transformer with fixed ratio. It steps down the voltage to a safe, operating level for the semiconductor devices to whom the output of transformer is to be fed

From here the output of transformers is send for rectification (that is conversion from AC to DC) as we have Dc motors. The output of this rectifier blocks is fed to filters where the ripples from rectifier output are smoothened out and fed to DC switchgear and combination control of traction motors (as there are multiple traction motors) and then to the traction motors which are operated by DC.

The control of speed/torque is by means of tap changer which changes voltage (and thereby current) that is fed into the system

Locomotives with VVVF control

Here the output of transformer is fed to a power converter stage, which concerts AC to DC (convertor is another name used for rectifier). These convertors utilize either GTO's (old technology) or IGBT (current technology) for this purpose. The output is filtered by another circuit to provide a fairly flat Dc output. We further have a stage to trap harmonics arising in the system and stages to improve power factor of the system.

After this the output is fed to an inverter stage which converts the DC to a three phase Ac, which is then fed to the traction motors

Here control is by means of VVVF controls, which varies both frequency as well as voltage. Control on Induction motors is quite difficult, but the recent advent of power electronic devices has made frequency control quite easy

Final stage

The output of traction motors is not directly coupled to wheels. We have reduction gears placed over there which reduce speed (while increasing torque). The variation in ratio of reduction gears enables us to use same traction motor for both freight and passenger locomotives.

Railway electrification using 25 kV, 50 Hz AC has become an international standard. There are two main standards that define the voltages of the system:

    EN 50163:2004 - "Railway applications. Supply voltages of traction systems"[7]

    IEC 60850 - "Railway Applications. Supply voltages of traction systems"[8]

The permissible range of voltages allowed are as stated in the above standards and take into account the number of trains drawing current and their distance from the substation.

Electrification system 25,000 V, AC, 50 Hz

Lowest non-permanent voltage 17,500 V

Lowest permanent voltage 19,000 V

Nominal voltage 25,000 V

Highest permanent voltage        27,500 V

Highest non-permanent voltage 29,000 V

 'A' indicates 'AC traction' while 'C' indicates 'DC traction (not 'D', so as to avoid any confusion with diesel). One can, therefore, find 'WCG' class locos (Broad Gauge, DC traction, Goods duty) operating on the Mumbai Division, while their counterparts 'WAG' locos haul broad gauge goods trains on AC traction elsewhere on LR. Similarly, trains, such as Mumbai-Pune Deccan Queen Express were, till recently, hauled by gigantic 'WCM-l' and 'WCM-5' ('M' for 'Mixed' service, just as in 'WDM-2'), while Mail/Express trains in the other parts of the country are powered by AC electric locos such as 'WAM-4' (Broad Gauge, AC Traction, Mixed service).

DC motors fed by DC overhead supply, such as for WCM, WCG locos (Electric locos, DC traction)

    AC overhead supply, stepped down through a multi-winding transformer, rectified by static rectifiers, whose output was fed to the DC motors. Locos such as WAM, WAP, WAG are equipped with this technology. (Electric locos, AC traction).

    Diesel fuel driving an engine, which operates an alternator, whose AC Electric output was rectified and fed to the DC motors, in locos such as WDM-2, WDP, WDG. Due to this the diesel locos are actually 'diesel-electric locos. These locos were actually with an on-board moving powerhouse operated on diesel fuel.