Eddy Currents and
Induction Braking
By: John Norris
Date: Oct. 22, 2012
What is Induction?
 Faraday’s Law:

“The induced electromotive force in any closed circuit
is equal to the negative of the time rate of change of
the magnetic flux through the circuit.”
 Lenz’s Law
 Why circuits obey Newton’s third law along with the
conservation of energy
 “An induced electromotive force (emf) always gives
rise to a current whose magnetic field opposes the
original change in magnetic flux”
Induction Currents = Eddy Currents
 Called “Eddy” since they are analogous to fluid
eddies in formation and behavior
 Responsible for the opposing magnetic fields that
produce drag and heating effects
 The drag effects give rise to induction braking

Absent external potentials will result in the system
coming to a halt
 Heating effects are exploited by devices such as
induction cookers

If left unchecked, then it could result in serious damage
to mechanical/circuit components
Check on Heat Effects
 High voltage circuits are constructed as a
series of “laminations” to reduce eddy
currents
 Solid conductors would suffer increased
resistivity and large energy losses (heat)
Drag Effects: Magnetic Braking
 Not to be confused with stellar magnetic braking
 The primary reason for slow stellar rotations
 Utilized in many practical applications:
 Trains (Maglev and conventional) as a braking system
 Some roller coasters
 Braking Industrial equipment and power tools
 Some exercise equipment
 Rowing or Spin machines… to increase resistance
 Determining structural defects in conductive materials
 Metal Detectors
Circular
vs.
 Metal disc, on the end
of a rotor assembly,
placed between two
electromagnets
Linear
 Static bank of
magnets/conductors
located on the tracks
with conductors/magnets
on the car
Advantages
 Quiet
 Almost no wear
 Wear only if coolant
system failure
 Essentially zero
maintenance
 Produce no chemical
pollution




Dust
Smell
Physical waste
No toxic chemicals
Disadvantages
 Need conventional
brakes to hold a vehicle
stationary
 In reality certain
sections of railroad
tracks have lots of
brake activity


Excess heat would
cause structural issues
Reduce brake
effectiveness
 Increased cost
Regenerative Braking
 Still not widely used
 Basic idea has been around since the
industrial revolution

Flywheels (mechanical capacitor)
 Flywheels still used, but eddy currents can be
exploited for this end as well
 Capture the energy from the induced currents
instead of it just being allowed to dissipate as
heat
 Avoids problem of having to carry around big
flywheels
Demonstration
 Copper pipe & Neodymium
magnets
 Pipe Dimensions:

½” and ¾” diameter (outer)
pipes both ≈ 1/32” thick
 Magnets: (NdFeB)




½” diameter
N42 Axially magnetized
Pull Force: 3.1 lbs
Surface Field: 1601 Gauss
(0.1601 T)
 Magnets have been weakened
due to an living an abused life
 For increase braking effect:




More powerful magnet
Thicker contiguous pipe
Use more strongly conductive
diamagnetic material
Lower temperature (pipe not
magnet)
Great Videos:
 Liquid N2 and copper tube magnetic levitation
 http://www.youtube.com/watch?v=AzOSYJmY
LTg&feature=watch_response
 Induction heater levitation molten aluminum
 http://www.youtube.com/watch?v=Q6Zrnv4Otb
U&feature=related
 Worlds Strongest Magnet
 Shot in the MagLab
 http://www.youtube.com/watch?v=QGytW_C6
hR8&feature=watch-vrec
References
1.
2.
3.
4.
5.
6.
7.
8.
"Diamagnetism." Wikipedia. Wikimedia Foundation, 16 Oct. 2012.
Web. 19 Oct. 2012. <http://en.wikipedia.org/wiki/Diamagnetism>.
"Eddy Current Brake." Wikipedia. Wikimedia Foundation, 10 Nov.
2012. Web. 19 Oct. 2012.
<http://en.wikipedia.org/wiki/Eddy_current_brake>.
"Eddy Current Brakes." How Do Eddy-current Brakes Work? A Simple
Introduction. N.p., n.d. Web. 19 Oct. 2012.
<http://www.explainthatstuff.com/eddy-current-brakes.html>.
"Eddy Current." Wikipedia. Wikimedia Foundation, 16 Oct. 2012. Web.19 Oct.
2012. <http://en.wikipedia.org/wiki/Eddy_current>.
"Faraday's Law of Induction." Wikipedia. Wikimedia Foundation, 17
Oct. 2012. Web. 19 Oct. 2012.
<http://en.wikipedia.org/wiki/Faraday's_law_of_induction>.
"Lenz's Law." Wikipedia. Wikimedia Foundation, 16 Oct. 2012. Web. 19 Oct.
2012. <http://en.wikipedia.org/wiki/Lenz's_law>.
“Neodymium Disk Magnets" K&J Magnetics. N.p., n.d. Web. 19 Oct.
2012. <https://www.kjmagnetics.com/proddetail.asp?prod=D81>.
"Regenerative Brakes." How Do Regenerative Brakes Work? N.p.,
n.d. Web. 19 Oct. 2012. <http://www.explainthatstuff.com/howregenerative-brakes-work.html>.
Diamagnetism (wiKi)

Believed to be caused by the alteration of the electron orbital velocities






Caused by quantum mech (Landau Levels)
Copper – Diamagnetic
Diamagnetism primarily responsible for the Lenz Law interaction
Superconductors completely repel external magnetic fields




Alters magnetic dipole moment
Field will oppose the the magnetic field changes due to the external field
All conductors exhibit an effective diamagnetism when they experience a
changing magnetic field. The Lorentz force on electrons causes them to circulate
around forming eddy currents. The eddy currents then produce an induced
magnetic field which opposes the applied field, resisting the conductor's motion.
Joke: Of course if I could have had some gold piping the effect could have been
much stronger but I probably wouldn’t have brought it to class
Magnetic permeability < or = 1
All conductors exhibit an effective diamagnetism when they experience a
changing magnetic field. The Lorentz force on electrons causes them to
circulate around forming eddy currents. The eddy currents then produce an
induced magnetic field which opposes the applied field, resisting the conductor's
motion.
Super Conductors
 Pic: Neodynium iron boron mags
 What does 42 MGOe mean?

Mega Gauss Oersted – magnetic energy
product