Electric Locomotives | The Railway Technical Website | PRC Rail Consulting Ltd

Introduction

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.

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 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.

Camshaft

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.

Contactor

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. 

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 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.

Grid

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)

IGBT

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.

Inverter

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.

Pantograph

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.

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, a “Main
Rectifier” for the power circuits and one or more for the auxiliary
circuits.

Relay

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.

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. 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.

Shoegear

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.

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 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.

Transformer

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.

Transistor

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