Muting the tram’s magnetic field

The tramway under construction on campus will feature a patented, world’s first compensation system for the tram’s stray magnetic fields. Here’s how it works.

Professor Lou van der Sluis (electrical power systems, EEMCS) remembers being at a reception with TU’s Executive Board when concerns about the tramway’s stray magnetic field were discussed. Concern arose over to what extent would magnetic fields caused by a tram on the Mekelweg interfere with sensitive measurements at the faculty of Applied Sciences across the road.

“It was simply a matter of being in the wrong place at the wrong time”, Van der Sluis says jokingly. It was at that reception he and his colleague, Prof. Pieter Kruit from Applied Sciences, took up the challenge to find a solution for the tramway’s interfering magnetic radiation.

They found out that trams run on direct current (DC) coming off overhead wires (called catenaries) and flowing back through the rails. The current loop generates a magnetic field that drops off with the distance squared. At 50 meters distance a 1000-Ampere current will cause a magnetic field of 400 nanoTesla; less than 1 percent of the earth magnetic field. But while measurements are compensated for the stationary earth magnetic field, this cannot be done with varying fields from a tramway.

Commissioning Stadsgewest Haaglanden (SGH) posed some conditions to the 2-kilometer stretch over the campus: whichever remedy taken should not endanger the security of the system’s power supply. Changes could only be made to the tram’s infrastructure so that any tramcar can be deployed on the track. Plus, modifications were to be built with tram company HTM’s existing materials and equipment.

The principle behind the compensation system is simple enough: reduce the current loop as much as possible by bringing the feeding and returning wire closer together. Overhead segments of the tramway are electrically insulated and fed through underground wires. Because the feeding cable is much closer to the return cable than catenaries and rails, the surface magnetic loop is greatly reduced and so is the magnetic field (being proportional to current and loop surface).

The tricky part is the segment in which the tram runs, since magnet fields will inevitably emerge there. However, the current to the tram is fed from two sides. And if the electric resistance in the feeding wires is small compared to that of the catenaries, the currents are inversely proportional to the tram’s distance to each of these segment ends. In other words, the tram draws most of its current from the closest end of the segment.

As the surface of each of the two magnetic loops is directly proportional to the distance to the end of the segment, it follows that the magnetic moments from the loops left and right from the tram are equal but reversed. So, in principle, they neutralise each other. More realistic calculations and measurements indicate that at 50 meters distance the DC magnetic field will be reduced by a factor of 10.

Dr. Frank van Overbeeke from EM Power Systems, the company that engineered the compensation system, says that specifications for masts and overhead wires are in preparation, but he cannot say when the actual compensation system will be built. No one else we spoke could either.

Prof. Kruit mentioned he had seen large rolls of red plastic tube at the edge of the digging site. Those tubes will house the feeding wires for the compensation system. So, the process is underway.

Does this mean that passing trams on the campus will not hinder researchers? Not necessarily. Next to the residual mechanical vibrations (dampened by a large sand bed) there may be noticeable effect of a large mass of iron moving through the earth’s magnetic field with possibly some permanent magnetism on board.

Van Overbeeke says such semi-permanent magnetic fields drop off steeply with distance and will only contribute to the ‘magnetic noise’ in the lab, just as passing cars on the A13 do.

Once the tram is operational, Professor Kruit will conduct measurements on the resulting magnetic field to validate the compensation system’s performance.

Other cities, like Utrecht and even Paris have shown interest in the compensation system as well, says Van Overbeeke. Although, he points out, since the design of the plan in 2008, the development of trams powered by batteries and supercapacitors has made considerable progress. Future trams may be able to bridge sensitive parts of town without stray magnetic fields and visually disturbing overhead wires.