Passive compensation of a tram's stray magnetic fields

A 2 kilometer stretch of Delft’s tramline 19 is the showcase of the world’s first compensation system for a tram’s stray magnetic fields, preventing sensitive measurements in TU Delft’s nano physics & electronics labs from being disturbed.

The tram (or ‘streetcar’) is a popular means of public transportation. Compared to buses, trams can accommodate more passengers, they move and load faster and they’re more environmentally friendly. That’s why TU Delft gladly welcomed the construction of a tramline that would connect the university to the city centre and the Technopolis Innovation Park.

When researchers expressed concerns about the tram’s stray magnetic fields, it was decided to look for a solution that would allow TU Delft’s renowned nano research and the convenient tram to peacefully coexist. The win-win outcome was an interference-free tram on campus and a TU Delft patent for the compensation system that made it possible.

Overhead wire segmentation

When a tramcar accelerates, it uses up to a 700 Ampere current, supplied by overhead lines and returning to earth through the rails. It’s this current loop that causes the disturbing magnetic fields - and the bigger the loop, the larger the magnetic field. An existing solution (known from literature) is to minimise the surface loop area, which can be done by bringing the feeding wire and the returning wire closer together.

To realise this, the overhead lines are divided into segments that are insulated from each other, and the feeding line is placed underground between the two tramway tracks. As the overhead wire is charged segment by segment through a vertical cable, the resulting loops can never be bigger than the overhead wire’s height multiplied by the segment’s width. However, this solution does not reduce the fields sufficiently, certainly not at distances from the track comparable to the segment’s width.

Two-way connection

The TU Delft inventors had the additional idea to connect the overhead line segments on both sides to the underground power line and use the relatively high resistance overhead line as a voltage divider. This way a tramcar is fed from two sides of the segment, creating two current loops in opposite directions. When the tram is in the middle of a segment, the two currents are equal, and as the two current loops’ contributions to the total magnetic field at some distance are equal but reversed, they neutralise each other.

When the tram is proceeding towards the end of a segment the stray magnetic fields are also compensated, but here the explanation is a bit more complex. The relative currents in the two loops are inversely proportional to the relative resistance of the overhead line sections that feed the tram. Since the resistances are proportional to the lengths of these sections, the short section provides most of the current. The magnetic field strength is the product of current and surface area, so both loops still give the same, but opposite contribution to the total field.

No local compensation needed

The compensation system is expected to reduce a tram’s stray magnetic fields at 50 meter distance by a factor of almost 10. This will save researchers the trouble of locally generating compensating magnetic fields around sensitive equipment. The invention doesn’t compromise the tram’s power supply security, it doesn’t require adjustments to the tramcars, and it can be built with conventional materials and equipment. As the system is built with passive components only (meaning no electronics are involved), it offers easy maintenance and a long life time.

The compensation system’s advantages in a nutshell

• reduces a tram’s stray magnetic fields at 50 meter distance by a factor of almost 10
• no need of locally generated compensating magnetic fields around sensitive equipment
• doesn’t compromise the tram’s power supply security
• doesn’t require adjustments to tramcars
• can be built with conventional materials and equipment
• easy maintenance and long life time (built with passive components only, no electronics)