A pantograph
is a device that collects electric current from overhead lines for electric
trains or trams. The term stems from the resemblance to pantograph devices for copying
writing and drawings.
A flat
side-pantograph was invented 1895 at the Baltimore & Ohio Railroad and in Germany 1900 by
Siemens & Halske.
According to
the late rail historian Harre Demoro, the pantograph was invented by the Key
System shops for their commuter trains in the East Bay section of the San Francisco Bay Area in
California. They appear in photographs of the first day of service in 1903. For
many decades thereafter, the same diamond shape was used by electric rail
systems around the world, and remains in use by some today.
However, the
most common type today is the so called half-pantograph (sometimes 'Z'-shaped),
which has evolved to provide a more compact and responsive single arm design at
high speeds as trains get faster. The half-pantograph can be seen in use on
everything from the very fastest trains such as the TGV to low-speed urban tram
systems. The design operates with equal efficiency in either direction of
motion, as demonstrated by the Swiss and Austrian railways whose newest
high performance locomotives, the Re 460 and Taurus respectively, operate
with them set in opposite directions.
The electric
transmission system for modern electric rail systems consists of an upper load
carrying wire (known as a catenary) from which is suspended a contact wire. The
pantograph is spring loaded and pushes a contact shoe up against the contact
wire to draw the electricity needed to run the train. The steel rails on the
tracks act as the electrical return.
As the train
moves, the contact shoe slides along the wire and can set up acoustical
standing waves in the wires which break the contact and degrade current
collection. This means that on some systems adjacent pantographs are not
permitted. Pantographs are the successor technology to trolley poles, which
were widely used on early streetcar systems
and still are
used by trolleybuses, whose freedom of movement and need for a two-wire circuit
makes pantographs impractical.
Pantographs
with overhead wires are now the dominant form of current collection for modern
electric trains because, although more expensive and fragile than a third-rail system, they
allow the use of higher voltages.
Pantographs easily
adapt to various heights of the overhead wires by partly folding. The tram line
pictured here runs in Vienna.
The electric transmission system for modern electric rail systems
consists of an upper weight carrying wire (known as a catenary) from
which is suspended a contact wire. The pantograph is spring loaded and pushes a
contact shoe up against the contact wire to draw the electricity needed to run
the train. The steel rails on the tracks act as the electrical return.
As the train moves, the contact shoe slides along
the wire and can set up acoustical standing
waves in the wires which break the contact and degrade
current collection. This means that on some systems adjacent pantographs are
not permitted. Pantographs are the successor technology to trolley poles, which were widely used on early
streetcar systems and still are used by trolleybuses, whose
freedom of movement and need for a two-wire circuit makes pantographs
impractical.
Pantographs with overhead wires are now the
dominant form of current collection for modern electric trains because,
although more expensive and fragile than a third-rail system,
they allow the use of higher voltages.
Pantographs are typically operated by compressed
air from the vehicle's braking system, either to raise the unit and hold it
against the conductor or, when springs are used to affect the extension, to
lower it. As a precaution against loss of pressure in the second case, the arm
is held in the down position by a catch. For high-voltage systems, the same air
supply is used to "blow out" the electric
arc when roof-mounted circuit breakers are
used.
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