ELECTROMAGNETIC & EDDY CURRENT BRAKING SYSTEMS

Andrew Sponsler
Sean Kurtz
Moving Electricity
The entire basis of the circuitry and construction of
electrically actuated braking systems is in the concept of
electric current. The formal definition of current is:
= (1) [4]
where i is current, q is charge and t is time [4]. Current is the
change in charge over a change in time. Many of the
relevant applications of electricity, such as the discovery of
electromagnetism and induction of charge, have their roots
in experiments with current.
The entire notion of the feasibility of eddy current
braking systems is predicated on the laws of
electrodynamics. ECB use currents and induced magnetism
to attract a rotating surface, thus slowing down a moving
system [3]. The system relies heavily on the use of eddy
currents to retard any given system.
A note regarding the formal definition of current: in the
context of electromagnetism, it is especially important to
distinguish the definition of current to be a change in the net
charge through an area. For example, take a solitary rod of
copper and imagine a cross-sectional slice anywhere along
the rod perpendicular to the length. Many electrons pass
through this planar intersection at any given moment.
However, since the electrons are passing through the plane
in both directions, there is no net change in charge in any
direction. If the same copper rod were attached to a rod, the
presence of an electric potential would then create a flow of
electrons through the cross-section and the rod would be said
to be experiencing a current.
Eddy Currents
When a non-conducting material passes through a
magnetic field, reactionary currents stir up beneath the
surface of the metal. These circulating currents, called eddy
currents, apply themselves against the magnetic field
creating a force that is contrary to the field [4]. This
principle is a product of Faraday’s law of induction.
In order to provide a clearer basis of understanding, a
brief overview of induction should be given consideration.
Similar to an electric field, a magnetic field can pass through
or be contained within an object. Thus how much of the field
is contained within an object must be defined. The quantity
of the magnetic field enclosed by an object is called the
magnetic flux. The magnetic flux through area A is given by
this equation.
Φ = ⋅ (2)[4]
Once the amount of magnetic flux has been
quantitatively analyzed the flux equation can be used to
formally state the Faraday-Lenz law of induction for a closepacked
coil of N turns,
= − (3)[4]
The clearest example of an eddy current being induced
in a conducting metal may be demonstrated by dropping a
magnet through a vertically oriented copper pipe. The
relative motion of the magnetic field, created by the magnet,
past the conducting metal, the copper pipe, will create eddy
currents inside the copper pipe and will slow the fall of the
magnet. The magnet will quickly reach terminal velocity and
will fall at a constant rate much slower than free fall. The
currents are stirred up beneath the surface of the metal
passing through the magnetic field. The swirling currents
point in every direction in actuality; however, since the
metal is in motion through the electric field, the only force
that is “felt” is the force opposite of the direction of motion
[5]. This antagonist force is the basis for an eddy current
brake system.
Levin et al. performed a study on eddy currents using
the example of a magnet falling through a copper pipe to
create an accessible model demonstrating Faraday’s theory
of electromagnetic induction [5].
FIGURE 1
Diagram of elements involved in Levin’s experiment [5]
In the study, a cylindrical magnet was dropped through
a copper pipe of a certain radius. The study took record of
the time the magnet took to travel through the length of the
pipe. When a second magnet of equal mass to the first was
attached to the first, the time required to travel through the
pipe increased. It is assumed that a magnetically and
electrically neutral object dropped through the copper pipe
will have the fastest descent of all of these objects. From
their measurements, Levin was able to calculate velocity,
using the length of the pipe and the time of travel in the
equation:
= / (4)[5]
University of Pittsburgh
Swanson School of Engineering April 13, 2013
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