Passive compensation of a tram’s stray magnetic fields:
No more disturbing effects to sensitive measurements!
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.
Illustratie: Coert Smeenk
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
Illustratie: Coert Smeenk
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)
Ref. TU Delft OCT-07-014
Official nr: EP 08159353.3
TU Delft / Valorisation Centre
Mekelweg 2
2628 CD Delft
T: 015-2786751
www.patent.tudelft.nl