April 14, 2018
Part one of this series, “Of Pantographs & Wires”, discussed the history of the electric locomotive, the evolution of catenary technology to our present day, and the idea of a Pantograph Barrier. Part two of this four part series will examine the reasons why the Pantograph Barrier exists and its effects and consequences.
The speed of an electric locomotive is limited by several components: horsepower, the quality of the tracks, the presence of freight operations, the layout of the tracks, the signaling technology, and the Pantograph Barrier. These limitations, with the exception of the Pantograph Barrier, can be addressed by building more powerful locomotives and straighter tracks, but the Pantograph Barrier has yet to be overcome by simply improving current technology. The Pantograph Barrier is a limitation that is related to the speed of the train and the current pantograph design. The faster the train goes the worse this phenomena becomes. Currently, the Pantograph Barrier limits the speed of electric locomotives to about 220 MPH.
The Pantograph Barrier can be examined by pushing speed limitations under controlled conditions. In 2007, the French TGV-V150 train reached a speed of 357 MPH/574.8 KM/H in non-revenue service. This pushing of the envelope was accomplished by drastically increasing the available Horsepower, increasing the traction power voltage and, most significantly, increasing the horizontal tension on the overhead catenary beyond conventional design limits. Over the 40 or so mile long test track, the TGV-V150 exceeded 220 MPH. However, it only pushed the Pantograph Barrier further out. It did not demonstrate a way to get past the barrier.
The barrier is integral to the way that the current pantograph interacts with the contact wire. The pantograph pushes up on the wire, typically applying between 15 and 30 pounds of force on the bottom of the wire. This force causes the wire to be vertically displaced about 1 to 3 inches. There is a considerable body of academic literature analyzing the optimal design requirements for the wire and for the pantograph. However, there is a much smaller body of work that treats them as an interrelated set. All of this analysis confirms the simple physical reality that if you push on a wire, the wire will vibrate. When the pantograph is moving along the wire the upward pressure it exerts causes waves in the wire. The more the wire is restrained from moving the more chaotic the vibrations become. The faster the pantograph moves, the more severe the vibrations become. These vibrations damage the catenary over time and cause it to lose tension, which magnifies the problem. As we saw in the example of the TGV-V150, the vibration can be managed by increasing the horizontal force. This diminishes the amount the wire is deflected by the catenary and, by extension, limits the vibrations. But this does not resolve the underlying vibration problem.
Increasing horizontal force isn’t the only method to limit vibrations. The reduction of vertical force imparted to the wire by the pantograph is an option to reduce vibration as well. The problem is that reducing the upward force of the pantograph makes the connection between the wire and pantograph weaker. This affects the ability of electricity to conduct and reduces the performance of the train. The top speed of a train cannot be increased without increasing horsepower. The increase in horsepower requires an increase in necessary electrical current and exacerbates the consequences of a weaker electrical connection between the pantograph and the wire. Therefore, reducing the contact force is not an attractive option. A better solution to the pantograph barrier would be to find a way to gain traction power from an overhead wire system to the locomotive. This could potentially decrease vibrations and increase the top possible speeds of electric locomotives.
Part three of this series will discuss in more detail why increasing train speeds beyond 220 MPH is important. Part four will propose a possible solution to the Pantograph Barrier problem called the Balanced Force Pantograph.
To learn more about this topic, and to get a preview of the Balanced Force Pantograph solution, attend Frank J. Smith’s presentation, “Balanced Force Pantograph and OCS” at the Joint Rail Conference in Pittsburgh, PA on April 19, 2018
About the Contributor
Frank has over 45 years of diverse experience as a Professional Engineer and is registered in 17 states. His experience includes: electric power generation and distribution, microwave communications, public safety radio, SCADA, fiber optic communications and railroad communications. Currently, Frank is a Lead Consultant with MACRO, a division of Ross & Baruzzini, in Chalfont, Pennsylvania. Over the last decade and a half he has provided consulting and engineering services to: SEPTA, AMTRAK, PANYNJ, Caltrain, NJ Transit, Delaware Port Authority, San Diego Transit and many others. In addition, he has 4 patents relating to railroad technology.