Electric Locomotives


This page shows the internal arrangement for a typical AC electric locomotive and offers a selection of terms used in electric traction. If you can’t find it here, try our Modern Railway Glossary

Electric Locomotive Parts

Figure 1: This diagram shows an AC electric locomotive, collecting power from an overhead line.  The red lines on the diagram indicate the single phase AC circuit, the green lines the DC circuits and the purple lines the 3-phase AC circuits. A locomotive using DC traction current is similar, except that there is no single phase AC circuit or transformer.  The current passes directly from the pantograph (or shoe) to the main and auxiliary inverters. Diagram: Author.

Asynchronous Motor

Modern traction motor type using three phase AC electrical supply and now the favoured design for modern train traction systems.  It can be used on DC and AC electrified railways with suitable control electronics and on diesel-electric locomotives.

Axle Brush

The means by which the power supply circuit is completed with the substation once power has been drawn on the locomotive.  Current collected from the overhead line or third rail is returned via the axle brush and one of the running rails.

Balancing Speed

The economical service speed at which the tractive effort of the train equals the train resistance and no further acceleration takes place.


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 battery is usually connected across the DC control supply circuit.

Bucholz Relay

A device inserted in the oil cooling circuits of electric locomotive transformers to detect low oil pressure. If low oil pressure is detected, the relay trips out the power system.  Often a source of spurious circuit breaker trips if not carefully calibrated.


Most DC electric traction power circuits use a camshaft to open or close the contactors controlling the resistances of the traction motor power circuit. The camshaft is driven by an electric motor or pneumatic cylinder. The cams on the shaft are arranged to ensure that the contactors open and close in the correct sequence. It is controlled by commands from the driver's cab and regulated by the fall of current in the motor circuit as each section of resistance is cut out in steps. The sound of this camshaft stepping can be heard under many older (pre electronics) trains as they accelerate.  See also Notching Relay.

Cannon Box

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

Chopper Control

A development in electric traction control which eliminates the need for power resistors by causing the voltage to the traction motors to be switched on and off (chopped) very rapidly during acceleration. It is accomplished by the use of thyristors and will give up to 20% improvement in efficiency over conventional resistance control.

Circuit Breaker

An electric train is almost always provided with some sort of circuit breaker to isolate the power supply when there is a fault, 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 circuit breaker or VCB. The air or vacuum part is used to extinguish the arc which occurs as the two tips of the circuit 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.


Similar to a relay in that it is a remotely operated switch used to control a higher power local circuit.  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. 


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 rectifier and where DC is converted to AC it is called an inverter. The word originated in the US but is now common elsewhere.

Cooling Fans

To keep the thyristors and other electronic power systems cool, the interior of a modern locomotive is equipped with an air management system, electronically controlled to keep all systems operating at the correct temperature. The fans are powered by an auxiliary inverter producing 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. 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 systems between the single phase rectifier 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 Brakes Page.

Earth Fault Relay

See Ground Relay.


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

Ground Relay

An electrical 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 electrical drive. Also sometimes called an Earth Fault Relay.

GTO Thyristor

Gate Turn Off thyristor, a thyristor which does not require a commutation (reverse flow circuit) circuit to switch it off.  See Thyristor (below)


Most recent power electronics development.  It is replacing the GTO thyristor as it is smaller and requires less current to operate the switching sequences.  See Transistor upon which the technology is based.


Electronic power device mounted on trains to provide alternating current from direct current. Popular nowadays for DC railways to allow three phase drive or for auxiliary supplies which need an AC supply. See also converter with which it is often confused.

Jerk Limit

A means by which starting is smoothed by adjusting the rate of acceleration of a train by limiting the initial acceleration 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 circuit 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 command 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 circuits so that it  actually functions as a protective circuit breaker. 

Master Controller

Driver's power control device located in the cab. The driver moves the handle of the master controller to apply or reduce power to the locomotive or train. Modern systems often have controllers that incorporate braking.

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 circuit so that, during acceleration 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 circuit.

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.

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 Protection above.


The current collection system used by locomotives and trains on routes electrified with overhead lines (Figure 2). The pantograph (often shortened to "pan") is held up by compressed air pressure. It is designed to collapse if it detects an obstruction. It can also be lowered manually to isolate the locomotive or train.

Figure 2: Nottingham tram showing the pantograph on the roof in contact with the overhead wire. Photo: R. Schwandl.


A converter consisting of thyristors and diodes which is used to convert AC to DC. A modern locomotive will usually have at least two, a "Main Rectifier" for the power circuits and one or more for the auxiliary circuits.


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

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 circuit 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.


Short form of SEParate EXcitement of traction motors where the armature and field coils of an electric motor are fed with independently controlled current. 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 acceleration.


Equipment carried by a train and used for current collection on track mounted (third rail) power supply systems.  Shoegear is usually mounted on the bogies close to the third rail.  It is often equipped with devices to enable it to be retracted if required to isolate the car or on-board system which it supplies.  See also the Power Supply page.

Synchronous Motor

Traction motor where the field coils are mounted on the drive shaft and the armature coils in the housing, the inverse of normal practice.  Favoured by the French and used on the high speed TGV Atlantique trains, this is a single-phase machine controlled 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.


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 referred to as choppers) are used for traction power control in place of resistance control systems.  A GTO (Gate Turn Off) thyristor is a development in which current is turned off is by applying a pulse of current to the gate.


A set of windings with a magnetic core used to step down or step up a voltage from one level to another.  The voltage differences 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.


The original electronic solid state device capable of controlling the amount of current 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 principle 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 cylinders by easing closed (or partially closed) the throttle/regulator when he hears the wheels start to spin.  On diesel or electric locomotives, the current drawn by individual or groups of traction 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.

Wheel 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 comparing 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 circuits to open the line breakers and reset the power control to the start position like a no-volt" relay

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