Block Diagram of Modern AC Electric Locomotive

The Electric Tracti on Glossary

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Introduction Electric railways are widespread throughout the world and there are many different varieties but all are based on either direct current or alternating curr ent supplies. supplies. The following following pages descr describe ibe the two two systems and and how they they are applied to differ ent railways. There There are also artic les on drive systems and various various control sy stems stems used on electric locomotives locomotives and trains. On this page there there is an Electric Traction Glossary. Glossary . Articles Articles on Electric Traction:

Electric Traction Power Supplies DC Traction Tracti on Motor Motor Sy stem stems s Electric Traction Drives Multiple Unit Operation Electronic Power for Traction Modern AC Electric Locomotive Diagram (on this page)

Electric Traction Glossary A selection selection of terms used in electric trac tion. If you y ou can't find it here, try our Modern our Modern Railway Glossary page or use the Search facility. Asynchronous Motor

Modern Modern traction tr action motor motor ty pe using three phase AC electrical supply and now the favoured favoured design for modern modern train traction systems. Can be used on DC and AC electrified railways with suitable control electronics and on diesel-electric locomotives. See the article on AC and DC Motors. Motors . Axle Axle Brush

The means means by which the power supply cir cuit cui t is completed with with the substation once power has been been drawn on the locomotive. Curr ent collected from fr om the overhead line or third thir d rail is retur ned via the the axle axle brush and one of the running rails. See also Power Supply - return. return. Balancing Speed

See here for description. Battery

All trains are provided with a battery to provide start up current and for supplying essential circuits, such as emergency lighting, when the line supply fails. The The battery battery is usually connected across the DC control supply circ c irc uit. Bucholz Relay

A device inserted in the oil cooling cir cuits of electric locomotive locomotive transformers transformers to detect low oil pressure. In this event event the relay trips out the power power system. Often a source of spurious cir cuit breaker trips if not carefully calibrated. Camshaft

Most DC electric traction power circ uits use a camshaft camshaft to open open or close the contactors contactors controlling c ontrolling the resistances of the traction motor motor power power circ ci rc uit. The camshaft camshaft is dri ven by an electri electric c motor or pneumatic pneumatic c ylinder. The The cams on the shaft shaft are arr anged to to ensure that that the contactors open and close in the correct sequence.

It is controlled by commands commands from the the driver's cab and and regulated regulated by the fall of current in the motor motor circ uit as each each section of

resistance is c ut out in steps. steps. The sound of this this camshaft stepping stepping can be heard und under er many older older (pre (pr e electronics) trains as they accelerate. See also Notching Relay. Relay . Cannon Box

Sleeve used to mount a traction motor on axle in electric power bogies and sometimes including an axle brush. brush. Chopper Control

A developme development nt in electr ic traction c ontrol which eliminates the need need for power power r esistors by causing the voltage voltage to the traction motors motors to be switched on

The Electric Traction Glossary

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An electric train is almost always provided with some sort of cir cuit breaker to isolate the power supply when there is a f ault, or for maintenance. On AC systems they are usually on the roof near the pantograph. There are two types - the air blast circuit breaker and the vacuum circ uit breaker or VCB. The air or vacuum part is used to extinguish the arc which occ urs as the two tips of the circ uit breaker are opened. The VCB is popular in the UK and the air blast circuit breaker is more often seen on the continent of Europe. Contactor

Similar to a relay in that it is a remotely operated switch used to control a higher power local circ uit. The difference is that contactors normally latch or lock closed and have to be opened by a separate action. A lighting contactor will have two, low voltage operating coils, one to "set" the contactor closed to switch on the lights; the other to "trip" off the lights. Click here for diagrams and more detail. Converter

Generic term for any solid state electronic system for converting alternating current to direct current or vice versa. Where an AC supply has to be converted to DC it is called a rectif ier and where DC is c onverted to AC it is c alled an inverter. The word originated in the US but is now common elsewhere. Cooling Fans

To keep the thyristors and other electronic power sy stems cool, the interior of a modern locomotive is equipped with an air management system, electronically controlled to keep all systems operating at the correc t temperature. The fans are powered by an auxiliary inverter produci ng 3-phase AC at about 400 volts. Creep Control

A form of electronically monitored acceleration control used very effectively on some modern drive systems which permits a certain degree of wheel slip to develop under maximum power application. The GM Class 59 diesel-electric locomotive built for the UK has this system. A locomotive can develop maximum slow speed tractive effort if its wheels are turning between 5% and 15% faster than actually required by the train speed. DC Link

Used on modern electronic power sy stems between the single phase rectif ier and the 3-phase inverter. It is easier to convert the single phase AC from the overhead line to the 3-phase required for the motors by rectifying it to DC and then inverting the DC to 3-phase AC. Dynamic Braking

A train braking system using the traction motors of the power vehicle(s) to act as generators which provide the braking effort. The power generated during braking is dissipated either as heat through on-board resistors ( rheostatic braking) or by return to the traction supply line (regenerative braking). Most regenerative systems include on board resistors to allow rheostatic braking if the traction supply system is not receptive. The choice is automatically selected by the traction control system. See also the Dynamic Brake section of our Brakes Page. Earth Fault Relay

See Ground Relay. Grid

Train or locomotive mounted expanded steel resistor used to absorb excess electrical energy during motor or braking power c ontrol. Often seen on the roofs of diesel electric locomotives where they are used to dissipate heat during dynamic braking. Ground Relay

An electric al relay provided in diesel and electric traction systems to protect the equipment against damage from earths and so-called "grounds". The result of such a relay operating is usually a shut-down of the electric al drive. Also sometimes called an Earth Fault Relay. GTO Thyristor

Gate Turn Off thyr istor, a thyristor which does not require a commutation (reverse flow circui t) cir cuit to switch it off. See Thyristor IGBT

Most recent power electronic s development. It is replacing the GTO thyristor as it is smaller and requires less c urrent to operate the switching sequences. See Transistor upon which the technology is based. Inverter



Jerk Limit

A means by which starting is smoothed by adjusting the rate of acceleration of a train by limiting the initial acc eleration rate upon starting. It could be described as limiting the initial rate of change of acceleration. Also used in dynamic braking. Line Breaker

Electro-mechanical switch in a traction motor power cir cuit used to activate or disable the circuit. It is normally closed to start the train and remains closed all the time power is required. It is opened by a c ommand from the driving controller, no-volts detected, overload detected and (were required) wheel spin or slide detected. It is linked to the overload and no-volt control circ uits so that it actually functions as a protective circ uit breaker. Master Controller

Driver's power control device located in the c ab. The driver moves the handle of the master c ontroller to apply or reduce power to the locomotive or train. Motor Blowers

Traction motors on electric locomotives get very hot and, to keep their temperature at a reasonable level for long periods of hard work, they are usually fitted with electric fans called motor blowers. On a modern locomotive, they are powered by an auxiliary 3- phase AC supply of around 400 volts supplied by an auxiliary inverter. Notching Relay

A DC motor power circuit relay which detects the rise and fall of current in the circuit and inhibits the operation of the resistance contactors during the acceleration sequence of automatically controlled motors. The relay operates a contactor stepping cir cuit so that, during acc eleration of the motor, when the current falls, the relay detects the fall and calls for the next step of resistance to be switched out of the cir cuit. See DC Resistance Control and Camshaft. No-Volt Relay

A power circuit relay which detected if power was lost for any reason and made sure that the control sequence was returned to the starting point before power could be re-applied. See Motor Protection. Overload Relay

A power circuit relay which detected excessive current in the circuit and switched off the power to avoid damage to the motors. See Motor Pr otection above. Rectifier

A converter consisting of thyristors and diodes which is used to convert AC to DC.

A modern locomotive will usually have at least two, one for the

power circuits and one or more for the auxiliary circuits. Relay

A remotely controlled switch which uses a low voltage control cir cuit. It will close (or open) a switch in a local cir cuit, usually of higher power. To see the principle of how it works, look here. See also Contactor . Resistance Control

Method of traction motor control formerly almost universal on DC electric railways whereby the power to the motors was gradually increased from start up by removing resistances from the power circ uit in steps. See more here.

Originally this step control was done manually but it was later automatic,

a relay in the circuit monitoring the rise and fall of current as the steps were removed.

Many examples of this system still exist but new builds now use

solid state control with power electronics. SEPEX

Short form of SEParate EXcitement of traction motors where the armature and field coils of an electric motor are fed with independently controlled current. Click here for diagrams. This has been made much more useful since the introduction of thyristor control where motor control can be much more precise. SEPEX control also allows a degree of automatic wheel slip control during acc eleration. Shoegear



Synchronous Motor

Traction motor where the fi eld coils are mounted on the drive shaft and the armature c oils in the housing, the inverse of normal practic e. Favoured by the French and used on the high speed TGV Atlantique trains, this is a single-phase machine c ontrolled by simple inverter. Now superseded by the asynchronous motor. Tap Changer

Camshaft operated set of switches used on AC electric locomotives to control the voltage taken off the main transformer for traction motor power. Superseded by thyristor control. Thyristor

A type of diode with a controlling gate which allows current to pass through it when the gate is energised. The gate is closed by the current being applied to the thyristor in the reverse direction. Thyristors (also referr ed to as choppers) are used for tr action power control in place of resistance control systems. A GTO (Gate Turn Off) thyri stor is a development in which curr ent is turned off is by applying a pulse of c urrent to the gate. Transformer

A set of windings with a magnetic c ore used to step down or step up a voltage from one level to another. The voltage differ ences are determined by the proportion of windings on the input side compared with the proportion on the output side. An essential requirement for locomotives and trains using AC power, where the line voltage has to be stepped down before use on the train. Transistor

The original electronic solid state device capable of controlling the amount of curr ent flowing as well as switching it on and off. In the last few years, a powerful version has been applied to railway traction in the form of the Insulated Gate Bipolar Transistor ( IGBT). Its princ iple advantage over the GTO Thyristor is its speed of switching and that its controls require much smaller current levels. Wheel Spin

On a steam locomotive, the driver must reduce the steam admission to the cylinder s by easing closed (or par tially closed) the throttle/regulator when he hears the wheels start to spin. On diesel or electric locomotives, the curr ent drawn by individual or groups of trac tion motors are compared - the motor (or group) which draws proportionally less amps than the others is deemed to be in a state of slip - and the power is reduced. Some systems - EMD Super Series for one - measure known wheel speed against ground speed as registered on a Doppler Radar. Many locomotives additionally use sand, which is applied to the wheel/rail contact point to improve adhesion - this is either controlled automatically, or manually by the driver (Foamer? No Way, 25 Apr 98).

See also Wheel Spin Relay.

Whee l Spin Relay (WSR)

A relay in older traction motor control circuits used to detect wheel spin or slide by measuring the current levels in a pair of motors on a bogie and compari ng them. The idea is to prevent motor damage by preventing an overspeeding motor causing an unacceptable rise in current in the other motor of the pair. If detected, the imbalance causes the control cir cuits to open the line breakers and reset the power control to the start position like a "no-volt" relay.

Electric Locomotive Parts



Click on the part names for a description. This diagram shows an AC electric locomotive, i.e a locomotive collecting AC power f rom an overhead line. The red lines on the diagram indicate the single phase AC circ uit, the green lines the DC circ uits and the purple lines the 3-phase AC ci rcuits. A locomotive using DC traction current is similar, except that there is no single phase AC circ uit or transformer. The current passes directly f rom the pantograph (or shoe) to the main and auxiliary inverters. See also the DC Traction Motor Systems, Multiple Unit Operation, Electric Traction Pages Drives, and Electronic Power pages. Tony Woof has written an excellent article on the basic principles of Electric Traction Technology for his site. To the Top of this Page. Home Page Last updated 30th September 2009 © Copyright Railway Technical Web Pages 1998 - 2008.

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Block Diagram of Modern AC Electric Locomotive

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