This page describes the on-board electrical services required by electric trains and how they are provided on the locomotive and passenger vehicles. A separate page is provided for Pneumatic Auxiliary Equipment. See also Multiple Unit Operation, Electronic Power, Electric Traction Pages Drives, Electric Traction Pages (DC Resistance Control) and Electric Traction Pages Glossary.
The modern passenger train provides a number of on-board services, both for passengers and control systems. They are almost all electrically powered, although some require compressed air and a few designs use hydraulic fluid. Since the train is virtually a self contained unit, all the services are powered and used on board. Their use and features can be summarised as follows:
Compressed air - almost always used for brakes and sometimes for powering automatic doors. Also once popular for powering traction power switches or contactors. Usually used for raising pantographs on overhead line systems. Always needs drying after compression to avoid moisture getting into valves. The compressor is normally driven directly from the main power source (the overhead line or third rail on electrified lines or the main generator on diesel powered vehicles).
Battery - Normally provided on locomotives and trains as a basic, low voltage standby current supply source and for start up purposes when livening up a dead vehicle. The battery is normally permanently charged from an on-board power supply.
Generator - the traditional source for on-board low voltage supplies. The generator is a DC machine driven by the diesel engine or, on electric locomotives, by a motor powered from the traction current supply. On a coach, the generator was often driven directly off an axle (a dynamo), batteries providing power for lighting when the train was stationary.
Alternator - the replacement for the generator which provides AC voltages for auxiliary supplies. AC is better than DC because it is easier to transmit throughout a train, needing smaller cables, and suffering reduced losses. Needs a rectifier to convert the AC for the battery charging and any other DC circuits.
Converter - the replacement for the alternator. This is a solid state version for auxiliary current supplies and can be a rectifier to convert AC to DC or an inverter to convert DC to AC. Both are used according to local requirements and some designs employ both on the same train. The name converter has become generic to cover both types of current conversion.
This page deals with compressed air supplies. For details of pneumatic auxiliary equipment, go here.
High Voltage and Low Voltage Systems
A locomotive or multiple unit is provided with two electrical systems, high voltage (HV) and low voltage (LV). The high voltage system provides power for traction and for the low voltage system. The low voltage system supplies all the auxiliary systems on the train like lighting, air conditioning, battery charging and control circuits. The two are separated because the high voltage required for traction is not needed for most of the other systems on the train so it is wasteful and expensive to use the high voltage.
Converting HV to LV
The current drawn by a locomotive from the overhead line or third rail supply can be supplied at voltages ranging from 25,000 volts AC to 600 volts DC. With the exception of heaters and compressor motors which, on the lower voltage DC railways are normally powered by the line current, all of these supply voltages are really too high to use efficiently with the comparatively small loads required by the on-board services on a train. The common approach therefore, has been to reduce the line voltage to a suitable level - generally below 450 volts and on some systems as low as 37.5 volts. Most systems have used a dynamo, a generator, an alternator or a current converter to get the lower voltages required. Usually, different voltages are used for different applications, the particular conversion system being specially designed to suit.
The first electric lighting provided on steam hauled trains was supplied from a large capacity battery contained in a box slung under the coach. The battery was recharged by a dynamo powered by a belt driven off one of the coach axles. Of course, this meant that the battery was recharged only when the train was moving and it had to have sufficient capacity for prolonged station stops, particularly at terminals. The voltage varied from system to system but was usually in the 12 to 48 volt range. Trains were heated by steam piped from the engine. If the locomotive was electric or diesel, a train heating boiler would be installed on the locomotive. Some European railways had train equipped with both steam and electric heating. More recently, all heating has become electric.
Electric trains originally used power obtained directly from the line for lighting and heating. The lamp voltage was kept to a low level by wiring groups of lamps in series. Each vehicle had its own switch which had to operated by a member of the crew. On some railways, where there were tunnels, daytime crews were instructed to switch on all the lights at the station before the tunnel and switch them off at the station after the exit. Such stations were provided with staff allocated to this job.
This diagram shows the basics for an early electric train set-up with a DC overhead line power supply system. The line voltage supplies the lighting, heating and compressor power requirements directly. The only reduction in voltage is achieved by wiring lamps and heaters in series. Each circuit has its own switch. The compressor would also have a governor, not shown here. This diagram shows a typical arrangement before about 1914. After that time, trains were equipped with multiple unit control of auxiliary services, where all cars were controlled from one position using separate control wires running along the train. Multiple unit control of traction equipment arrived in the UK from the US in 1903.
Batteries were still provided on some electric trains, especially those on underground lines, for emergency lighting. A small number of lamps in each car were connected to the battery so that some illumination was available if the main traction current supply was lost. The batteries were recharged through a resistance fed from the traction current supply.
In the mid 1930s, electric multiple units began appearing with on-board, DC generators to supply lights. This allowed lower voltages and reduced the heavy wiring required for traction current fed lighting. Outputs from these generators ranged from 37 to 70 volts, depending on the application. The generator was driven by a small electric motor powered by the traction supply. For this reason they were often referred to as "motor generators".
In this diagram (left) of a motor generator system, the train lighting and battery are fed from a generator driven by a motor at the line voltage. The return circuit is through ground, using the car structure like a road vehicle. A voltage regulator is provided to reduce the risk of damage through sudden changes in voltage caused by gaps in the current rail or neutral sections in the overhead line. If the MG stops, the battery is disconnected from the charging circuit and supplies a few emergency lights. In addition to supplying lighting, the LV circuit was used to supply all the train's control circuits. See Multiple Unit Operation.
By the late 1940s, fluorescent lighting was becoming popular and was recognised as better, brighter and requiring less current that tungsten bulbs. However, if DC is used, the lighting tubes become blackened at one end, so AC was adopted for lighting circuits on trains. At first, some systems used a DC generator with an alternator added to the drive shaft, a motor-generator-alternator. The DC output from the generator was used for control circuits while the AC output from the alternator was used for lighting. Emergency lighting was still tungsten, fed from the battery.
In the early 1960s, the motor alternator appeared. The appearance of silicon rectifiers allowed the AC output of the alternator to be converted to DC for battery charging and control circuits. The introduction of solid state electronics also saw the old mechanical voltage regulators replaced.
Modern auxiliary services on electrified railways are now mostly solid state systems, using power and control electronics, as shown in the diagram below:
The output from the DC to AC auxiliary converter is 3-phase AC at about 380 volts and is used for train lighting and the AC motors of air conditioning fans and compressors. The 3-phase is also converted to DC by the rectifier which provides current for battery charging and control circuits. The diagram on the left shows the set-up for a DC overhead system but it is similar for AC systems except for the addition of the transformer and rectifier as shown below.
On a locomotive hauled train, the individual coaches are provided with an on-board converter supplied from a train line carrying a 3-phase supply generated on the locomotive. On a diesel locomotive, this supply would come from an on-board alternator driven by the diesel engine.
A feature of electric railway operation is the gap or neutral section. Gaps occur in third rail systems and neutral sections in overhead line systems. See also Electric Traction Pages Power Supplies. The gap in a current rail is necessary at junctions to allow the continuity of the wheel/rail contact and at substations to allow the line to be divided into separate sections for current feeding purposes. Neutral sections in the overhead line are also used for this purpose.
Although they are always kept as short as possible, gaps will sometimes cause loss of current to the train. The train will usually coast over the gap but there will be a momentary loss of current to the on-board equipment - lights will go off for a second or two and ventilation fans will slow down or stop. On trains provided with generators or alternators, the momentum in the machine would often be sufficient to maintain some generation over the gap and lighting often remained unaffected. The only difference noticeable to the passenger was the change in the sound of the generator as it lost power and then regained it a second or two later.
Modern electronics has given us static inverters to supply on-board inverters but they have no inertia and stop output as soon as a gap is encountered. To prevent the lights going off at every little gap, all lights are connected through the battery. To prevent the battery becoming discharged too quickly, the inverter starts a "load shed" at about a 60 second delay. After this time, the main lighting is switched out and only emergency lights remain. Battery current is also used for emergency ventilation, essential controls and communications.
See also the Multiple Unit Operation, Electronic Power, Electric Traction Drives, Electric Traction Pages (DC Resistance Control), Pneumatic Auxiliary Equipment and Electric Traction Pages Glossary pages.