Between the pantograph and the traction motors of a TGV there is a whole
set of power electronics with the task of "processing" a fixed voltage at
the (single-phase AC or DC) catenary to produce a variable force at the
wheel treads. These power electronics fill most of the space inside a TGV
We follow the power chain from pantograph to wheels, in the specific
case of the TGV Atlantique 24000 series power car. The TGV 24000 does
not contain any particularly exotic components, and in principle shares
many of its features with most modern electric (and even
diesel-electric) locomotives. The drawing below shows a cutaway
view of the 24000, and is followed by a more detailed description. There
are two power cars per trainset; each develops 4400 kW (5900 hp) and
weighs just 68 tonnes (150,000 lb).
B. Bayle (SNCF Direction du Matériel) in Revue
Générale des Chemins de Fer, December 1986
GPU Pantograph: GPU means "Grand Plongeur Unique" (large, single
plunger). [A pantograph is a device used to draw electrical power from a
fixed overhead wire]. The GPU pantograph was specially designed, with a
top linkage member (holding the wiper) that operates like a hydraulic damper
with a short stroke to keep intimate contact with the overhead conductor
and keep bouncing to a minimum. Contact wire pressure is about 70 N (17
lbs). The bottom linkage, wich guides alignment with the contact wire, is
locked at a fixed height when operating under the fixed-height overhead on
high speed trackage.
Main transformer: takes 25kV 50Hz single phase overhead power
and converts this to 1500V 50Hz. For information on how transformers
work, pick up any college physics textbook. The transformer is one of the
heaviest components in the unit, weighing about 8 tonnes. It is located
in the lower frame of the unit, and sits in a bath of oil, circulated by
pumps and cooled with fans.
Thyristor controlled-rectifier bridge: as the name implies,
rectifies the ouput of the main transformer to make 1500V DC. The
thyristors allow a refinement beyond a simple diode bridge: they not only
rectify the current, but can act as a switch and "chop" (turn on and off)
the output power. This is why we speak of a "controlled rectifier".
There are two thyristor-diode bridges, one for each pair of traction
motors. From now on in the traction chain, in fact, there are two
separate and independent paths to each of the two power trucks (bogies).
This is to maximize reliability. (under 1500V DC overhead power, all that
is used up to here is a thyristor chopper.)
(A note about the thyristor: these are made use of extensively in
the TGV, just as in most modern electric
locomotives. The development of the thyristor has made possible the use
of frequency controlled AC traction motors and revolutionized traction
circuit design. The thyristor is basically a switch, albeit a very large
one, which resembles a common transistor in the way it works. A thyristor
passes current only in one direction, called the "forward" direction,
providing that a suitable voltage is applied to its control electrode, or
"gate". As long as this gate voltage is not present, current cannot flow
through the device.)
Common block: consists of the DC circuit breaker (two of them
working in tandem, actually) and the main filter capacitor, which smooths
the chopped 1500V waveform to a lower DC voltage, depending on the duty
Traction inverters convert their DC input into a
computer-controlled three phase, variable frequency AC waveform, in order
to conrol the traction motors. There is one inverter per traction motor.
The inverters are thyristor-based. For each truck (bogie), the two
inverter/motor pairs are connected in series. The power electronics
physically associated with one truck (bogie) correspond to a "motor block"
or "power pack". There are thus two such power packs installed in each
power car. If one of them experiences a fault, it automatically isolates
itself. The driver can then switch it back on, without leaving his
station, by resetting the circuit breaker. But only once-- if the fault
persists, the power pack will again trip out and, this time, stay down.
This is not a problem in practice, since there is enough spare traction
power available that the train can continue on its journey, on three packs
out of four total. (Recall that TGVs have two power cars; one on each end
of the train.)
Synchronous AC traction motor: the motor is excited at a frequency
proportional to its rotational speed. There is no collector as on DC
motors, which allows a reduction of wear and maintenance costs. (Note:
the synchronous AC traction motor is different from asynchronous AC (induction)
traction motor. Whereas the latter has a simple cage rotor with no
power connections, the synchronous motor has rotor coils fed through
slip rings.) In an unusual arrangement considered to be one of the TGV
design's strong points, the traction motors are slung from the vehicle
body, instead of being an integral part of the Y230 power truck (bogie).
This substantially lightens the mass of the truck (each motor weighs 1460
kg), giving it a critical speed far higher than 300 km/h (186 mph) and
exceptional tracking stability. The traction motors are still located
where one would expect them: in between the truck (bogie) frames, level
with the axles, but just suspended differently. Each motor can develop
1100 kW (when power comes from 25kV overhead) and can spin at a maximum
rate of 4000 rpm.
Mechanical transmission: the output shaft of the motor is
connected to the axle gearbox by a tripod transmission, using sliding
cardan (universal-joint) shafts. This allows a full decoupling of the
motor and wheel dynamics; a transverse displacement of 120 mm (5 inches)
is admissible. The final drive is a gear train that rides on the axle
itself and transfers power to the wheels. This final drive assembly is
restrained from rotating with the axle by a reaction linkage.
Drawing of the Y230 power truck (bogie).
Schematic of the TGV's tripod transmission.
Sensors continuously compare motor speed to axle speed. A discrepancy
between the measured speeds indicates a tripod driveshaft failure condition,
which is indicated in the cab. If the tripod transmission develops a
dangerous vibration, the shaking reaction linkage can strike a pneumatic valve
that automatically dumps the main brake pipe and stops the train.
Besides the main traction chain, there is of course a whole suite of
auxiliary equipment inside the TGV power car.
Braking rheostat: large air-cooled resistors, located in the
roof of the unit, used to dissipate braking power generated by the
traction motors while braking. Also known as "dynamic brake"; in this
application it is used exclusively at high speeds, and combined with wheel
brakes as the speed drops below a predetermined level. Overall they can
take up almost half of the braking energy in stopping a trainset from full
speed. There are two sets of rheostats, one for each power pack. Their
effective resistance can be modulated by the chopper to vary braking
Pneumatic block and wheel brakes: the main compressor is used to
fill the air tanks used for the braking system. These tanks are located
underneath the frame of the unit. There are two brake lines running the
length of the trainset, as is common for the electropneumatic brake system
used on most passenger trains. The first, the "principal line", is
maintained at a pressure of 8 or 9 atmospheres at all times and is used to
fill the auxiliary brake reservoirs on each vehicle in the trainset. The
second, the "general line", modulates the wheel braking level between full
application (3.5 atmospheres) and full release (5 atmospheres).
Auxiliary power supply unit: a static converter that generates
head-end electrical power for the rest of the train. HEP ("hotel power")
is 380V 50Hz, while interior lighting is supplied with 72V DC. Output of
the converter also runs some equipment in the power car: the transformer oil
pumps and cooling fans, the brake rheostat cooling fans, the
thyristor cooling fans, etc.
Automatic coupler: the Scharfenberg-type coupler makes pneumatic
and electrical connections without external intervention. It allows to
couple two TGV trainsets nose to nose, either for normal multiple unit
operation (even at high speeds) or for towing. When not in use, the
coupler is concealed by two fiberglass clamshell doors that form the nose
of the unit. These can open away to each side to reveal the coupler.
Impact absorption block: is an impact shield to defend the cab
cubicle. The deformation of this thick aluminum honeycomb block absorbs a
part of the collision energy if a large object is struck.
Frame: Primary structural members, made of high tensile strength
steel. The main structure of the unit is a rigid space frame. In more
recent designs, crashworthiness has been improved by including sacrificial
frame members that collapse and absorb energy in the event of a large
Sinalling antennas: mounted under the front air dam, two
antennas read TVM 300 (and more recently TVM 430) cab signal information
from the rails, and relay them to the train's central computer (which in
turn displays them to the engineer). There is a more detailed description of the signalling system
On-board computer: manages all the subsystems. The computer can
help to diagnose faults, and can even generate a maintenance report which
is transmitted by radio to the shops prior to the train's arrival.
Because the computer is so closely involved in every aspect of the
operation of a TGV Atlantique unit, software glitches can cause annoying
problems. Early teething problems in the computer systems actually made
the TGV Atlantique initially less reliable than its older,
of a TGV trailer articulation (by B. Bayle).
A TGV power car isn't the only place in a trainset to find interesting
mechanical features. The diagram above shows a cutaway of the articulation
between two TGV trailers. This cutaway is somewhat out of date because the
suspension was since redesigned. The current design differs mainly in the
arrangement of shock absorbers and the replacement of the big secondary
suspension spring by a pneumatic spring. For a detailed description of
the new SR10 pneumatic suspension, see article.
Some pictures taken at the factory and
in maintenance will show you some more
details on how they are put together. You will be able to recognize
several components as mentioned above.
Also don't miss the excellent summary of how railway vehicles are manufactured,
by Paul Berkley and Piers Connor on the Railway Technical
Web Pages. While general in nature, this document has some detail relating
to TGV high speed trains.
This document was created with the help of Prof. Miguel
R. Bugarin at the University of La Coruna, Spain. The tripod transmission
diagram was provided by Jean-Emmanuel Leroy.
Last modified: March 2000