Eddy Currents

1
Eddy Currents

• Explore Lenzs Law

2
Eddy Currents

• Objectives
• At the end of this activity your apprentices
  should be able to
• State Lenzs Law
• State why eddy currents cause losses in
  transformers and motors

3
Eddy Currents

• When a magnet is dropped down a metallic tube,
  the changing magnetic field created by the
  falling magnet pushes electrons in the metal tube
  around in circular, eddy-like currents. These
  eddy currents have their own magnetic field that
  opposes the fall of the magnet. The magnet falls
  dramatically slower than it does in ordinary free
  fall in a nonmetallic tube.

4
Eddy Currents

• The first part of this presentation is to explain
  to instructors a way to set up this
  demonstration. The last part is for use in the
  classroom. Save this to a local drive under
  another name and delete slides as directed at the
  beginning of the student section.

5
Eddy Currents

• Parts
• 1-a strong magnet
• 2-copper bus
• 3-some aluminum ladder tray rail
• 4-aluminum rigid
• 5-pvc conduit
• 6-shims and rubber bands

6
Eddy Currents

• Parts
• 1- the magnets we use are from a Sonicare
  toothbrush head. Any rare earth magnet will do.
  Ceramic magnets are not strong enough.

7
Eddy Currents

• Parts
• 2- bus was from a 2000 amp switch we had, any
  thickness would do. These pieces are easy to
  stand up and space properly.

8
Eddy Currents

• Parts
• 3- Aluminum ladder tray rail was handy and free.
  Any aluminum plate would do.

9
Eddy Currents

• Parts
• 4- Aluminum rigid conduit, the closer the ID of
  the conduit is to the size of the magnet the
  better.
• 5- PVC the same size as the AL rigid if
  possible.
• 6-some shims and rubber bands to hold the tray
  or bus the right distance apart

10
Eddy Currents

• Lenzs Law
• When a magnetic field cuts a conductor a current
  will be induced, the magnetic field caused by the
  current will oppose the motion that caused the
  current.

11
Eddy Currents

• As the magnet falls, the magnetic field around it
  constantly changes position. As the magnet passes
  through a given portion of the metal tube, this
  portion of the tube experiences a changing
  magnetic field, which induces the flow of eddy
  currents in an electrical conductor, such as the
  copper or aluminum tubing. The eddy currents
  create a magnetic field that exerts a force on
  the falling magnet. The force opposes the
  magnet's fall. As a result of this magnetic
  repulsion, the magnet falls much more slowly.

12
Eddy Currents

• We plan on giving the apprentices basic
  directions, the parts, and asking them to make
  the observations and come up with the reason why
  the fall of the magnet is retarded.
• The original idea for this demonstration came
  from www.exploratorium.edu science snacks.

13
Eddy Currents

• You do not have to do all the demonstrations,
  using just one of the set-ups would work. The
  aluminum rigid versus pvc is easy to set up and
  the length of fall makes the lag time very
  obvious. The bus bar and aluminum tray work well
  because you can see the magnets path change.

14
Eddy Currents

• This is the end of the instructors portion. Save
  this file under another name and delete slides 1
  through 14. The remainder of the presentation
  can be edited and slides removed if you are not
  going to use all the pieces.
• Good luck
• If you have any feed back please feel free to
  contact me at instructor_at_jatc112.org
• Fraternally, Greg McMurphy LU112

15
Eddy Currents

• Objectives
• At the end of this activity participants should
  be able to
• State Lenzs Law
• State why eddy currents cause losses in
  transformers and motors

16
Eddy Currents

• Parts
• 1-a strong magnet
• 2-copper bus
• 3-some aluminum ladder tray rail
• 4-aluminum rigid
• 5-pvc conduit
• 6-shims and rubber bands

17
Eddy Currents

• Gather the parts on the list
• Appoint a recorder for each lab team.
• In your AC theory book or a Hermans text find
  out what Lenzs Law is. Write it out.
• What are eddy currents? Write this answer out.

18
Eddy Currents

• Stand the two pieces of copper bus on the table
  spaced just far enough apart for the magnet to
  pass between them.
• Stand them long axis up so that the magnet will
  have the maximum length of fall possible
• Drop the magnet between them and write down what
  happens.
• Drop the magnet the same distance to the table at
  a location away from the copper bus. What happens?

19
Eddy Currents

• How long does it take for the magnet to fall in
  here versus out in the air?
• What path does it take?

20
Eddy Currents

• Stand the two pieces tray on the table spaced
  just far enough apart for the magnet to pass
  between them.
• Stand them long axis up so that the magnet will
  have the maximum length of fall possible
• Drop the magnet between them and write down what
  happens.
• Drop the magnet the same distance to the table at
  a location away from the tray. What happens?

21
Eddy Currents

• How long does it take for the magnet to fall in
  here versus out in the air?
• What path does it take?

22
Eddy Currents

• Drop the magnet down the aluminum conduit.
• Drop a pencil down the same conduit.
• Drop them each down the pvc conduit.
• Compare the rate of fall in each case.
• Watch the fall of the objects looking down the
  conduits. What happens?

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Eddy Currents
AL VS. PVC What are the differences in rate of
fall and path of the magnet in each and in the
path of the magnet compared to the path of the
pencil dropped in the same conduit?
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Eddy Currents

• STOP FOR DISCUSSION
• WHAT HAPPENED?
• WHY DID IT HAPPEN?
• HOW DOES THIS RELATE TO THE CONSTRUCTION OF
  MOTORS AND TRANSFORMERS?

25
Eddy Currents

• Lenzs Law
• When a magnetic field cuts a conductor a current
  will be induced, the magnetic field caused by the
  current will oppose the motion that caused the
  current.

26
Eddy Currents

• As the magnet falls, the magnetic field around it
  constantly changes position. As the magnet passes
  through a given portion of the metal tube, this
  portion of the tube experiences a changing
  magnetic field, which induces the flow of eddy
  currents in an electrical conductor, such as the
  copper or aluminum tubing. The eddy currents
  create a magnetic field that exerts a force on
  the falling magnet. The force opposes the
  magnet's fall. As a result of this magnetic
  repulsion, the magnet falls much more slowly.

27
Eddy Currents

• Eddy currents are often generated in transformers
  and motors and lead to power losses. To combat
  this, thin, laminated strips of metal are used in
  the construction of power transformers and motors
  rather than making them out of one solid piece of
  metal. The thin strips are separated by
  insulating glue, which confines the eddy currents
  to the strips. This reduces the eddy currents,
  thus reducing the power loss.

28
Eddy Currents

• The original idea for this demonstration came
  from www.exploratorium.edu science snacks. There
  are many more activities there for you to explore.