Electric traction; current collection arrangements

Electric traction is the system of moving vehicles (trains, trams, trolley buses, electric locomotives) using electric motors, with energy supplied from a fixed distribution system along the track or from an on-board source.

For vehicles fed from a distribution line, a current collector keeps a sliding contact with the supply conductor while the vehicle moves.

1. Third (conductor) rail system

• An insulated steel rail laid beside or between the running rails; a collector shoe slides on it (top, side or bottom contact).
• Used for 600–750 V dc metros and suburban lines (e.g. London Underground, many older metros).
• It can collect high current, but only at low voltage (for safety).

2. Overhead system

An overhead wire is hung above the track; a collector on the roof presses against it.

a) Trolley collector (trolley pole)

• A pole with a grooved wheel or slider at its end, pressed upward by springs.
• Used in tramways and trolley buses (two poles, since the return is not through rails).
• Suitable only for low speed (about 30 km/h); it can leave the wire at junctions and must be reversed at route ends.

b) Bow collector

• A light metal strip (bow), about 1 m wide, on a frame, sliding on the wire.
• Used in tramways; speeds up to about 30–35 km/h; needs reversing with direction (or a reversible type).

c) Pantograph collector

• A hinged, diamond- or half-diamond (Z) frame raised by springs or air pressure, with a carbon/copper contact strip.
• Keeps uniform pressure over a wide range of wire heights; works in both directions and at high speeds (over 300 km/h); can collect large currents.
• Used on all modern ac and dc main-line locomotives and EMUs.

   ===== contact wire ==========
        \____/    pantograph head
         /  \
        /    \   frame (springs/air)
   ____/______\____ vehicle roof

Comparison

The pantograph is preferred today for its reliability, high-speed performance and ability to handle high voltage; the third rail remains common for underground metros where tunnel height is small.