In many rail networks sections of the overhead catenary are neutral, requiring trains to stop drawing current from the overhead line. Ensuring that no power is drawn prevents the pantograph from arcing as it moves from the powered to unpowered catenary.
Craig Mulvay, technical and commercial development manager – APAC for Pandrol, explained how automatic power control (APC) magnets are used to regulate a train’s circuit breaker.
“Wherever there is an overhead catenary with a neutral section area that calls for the train’s pantograph circuit to be opened and closed before and after the train traverses the neutral section, then there is an application for APC magnets to signal the pantograph circuit breaker.”
Typically APC magnets are mounted on the field side of the rail. When trains pass the magnetic field created by the APC, an onboard sensor triggers a circuit breaker, stopping current from being drawn through the pantograph. A corresponding APC at the end of the neutral section restarts the electric connection between wire and train.
These traditional magnets are manufactured using ferrite magnet material. Despite being cheap, the magnets are large, heavy, and require regular checks as their strength degrades over time. Another drawback is that, due to the size of traditional magnets, specially extended sleepers have to be installed to accommodate fixation to the track. As Mulvay notes, in many networks, these requirements introduce extra costs.
“For the most part the technology used is still the larger ferrite type sleeper mounted magnet which requires long sleepers/bearers to be installed in the neutral section area. This creates additional design and site works to facilitate the installation.”
In addition to the cost of installing the extended sleepers, the magnets, which can weigh over 30 kilograms, need to be removed to allow tamping to take place. With the requirement for lifting equipment to remove these magnets, it is often common practice to not tamp these locations, leading to losses in track geometry quality.
To overcome these limitations, Pandrol has developed a White Super Strength Magnet. By utilising a neodymium-boron-iron magnet material, the APC’s magnetic field is more powerful and stable, and is encased in a compact, easy to install housing.
Now standard in the UK and weighing just 8kg, the magnet’s mounting mechanism can be attached to most flat-bottomed rail profiles, and most standard sleeper designs. The frame is made from 316 marine grade stainless steel to withstand harsh railway environments, and the ‘rare earth’ magnetic material is more stable at extreme temperatures. This entirely removes the need for extended sleepers to be installed at neutral sections of the overhead line.
“With our product the track design does not need to be customised to suit our rail mounted APC magnet,” said Mulvay. “Whereas if you use the older solution the track design needs to incorporate two long sleeper/bearers for each track magnet installed, positioning these adds work and complexity to the track/power system interface.”
In addition to the sound business case for installation, the low profile of the magnet allows for tampers and stone blowers to operate without its removal, reducing maintenance costs and building in a whole life benefit to the product use.
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