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. Linear

Metal disc, on the end of a rotor assembly, placed between two electromagnets

Static bank of magnets/conductors located on the tracks with conductors/magnets on the car

Advantages Disadvantages

Quiet Almost no wear

Wear only if coolant system failure

Essentially zero maintenance

Produce no chemical pollution Dust Smell Physical waste No toxic chemicals

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=AzOSYJmYLTg&feature=watch_response Induction heater levitation molten aluminum

http://www.youtube.com/watch?v=Q6Zrnv4OtbU&feature=related

Worlds Strongest Magnet Shot in the MagLab http://www.youtube.com/watch?

v=QGytW_C6hR8&feature=watch-vrec

References

1. "Diamagnetism." Wikipedia. Wikimedia Foundation, 16 Oct. 2012. Web. 19 Oct. 2012. <http://en.wikipedia.org/wiki/Diamagnetism>.

2. "Eddy Current Brake." Wikipedia. Wikimedia Foundation, 10 Nov. 2012. Web. 19 Oct. 2012. <http://en.wikipedia.org/wiki/Eddy_current_brake>.

3. "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>.

4. "Eddy Current." Wikipedia. Wikimedia Foundation, 16 Oct. 2012. Web.19 Oct. 2012. <http://en.wikipedia.org/wiki/Eddy_current>.

5. "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>.

6. "Lenz's Law." Wikipedia. Wikimedia Foundation, 16 Oct. 2012. Web. 19 Oct. 2012. <http://en.wikipedia.org/wiki/Lenz's_law>.

7. “Neodymium Disk Magnets" K&J Magnetics. N.p., n.d. Web. 19 Oct. 2012. <https://www.kjmagnetics.com/proddetail.asp?prod=D81>.

8. "Regenerative Brakes." How Do Regenerative Brakes Work? N.p., n.d. Web. 19 Oct. 2012. <http://www.explainthatstuff.com/how- regenerative-brakes-work.html>.

Diamagnetism (wiKi) Believed to be caused by the alteration of the electron orbital velocities

Alters magnetic dipole moment Field will oppose the the magnetic field changes due to the external field

Caused by quantum mech (Landau Levels) Copper – Diamagnetic Diamagnetism primarily responsible for the Lenz Law interaction Superconductors completely repel external magnetic fields

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