Diagram on Eddy Currents (1 Viewer)

[jsttesting]

Hi,

I was wondering if anyone can explain to me how eddy currents work....I know there are a a lot of thread open on this topic, but none of them seem to have a diagram. I'm trying to summarise the following dot point from the syllabus, and need a diagram of on electromagnetic braking.

"Gather secondary information to identify how eddy currents have been utilised in electromagnetic braking"


Thanks..!!!!

 

[KFunk]

Think of a space probe 7 type amusement ride where a compartment with people in it drops from a great height and slows to a halt at the bottom. What you have in these rides is a metal plate of low electrical resistance (eg. copper) behind the compartment. As the compartment slips down the tower it will read a point where a whole lot of permanent magnets have been installed.

As the compartment moves into and through the magnetic field created by these magnets it will experience a change in flux. Eddy currents will then be created in the copper plate which oppose the change in flux which created them. Hence the eddy currents create a magnetic field of an opposite polarity to that which created them which attracts the compartment towards the tower. This attraction between the magnetic fields opposes the down ward motion of the compartment (i.e opposing the change in flux which is cause the eddy currents) and, in effect, slows the compartment down.

I'll see if I can make a diagram for you.

 

[KFunk]

Ok, this diagram will probably be non-existent in about 48 hours due to crappy hosting servers but here it is:

http://img193.exs.cx/img193/9388/breaking8rp.png

I know it's poor quality but hopefully if gives you the gist.

*In it's first position the compartment isn't in a magnetic field.

*It then moves to its second position where it is under the influence of a magnetic flux going into the page. Since it has moved from an area of no magnetic field to an area of magnetic field eddy currents form to oppose the change of flux. In this case anti-clockwise eddy currents form to make a magnetic field out of the page which oppose the magnetic field going into the page.

*When it moves to the third position the top half of the compartment has moved, once more, to an area of flux going into the page so anti-clockwise eddy currents generate a magnetic field out of the page. The bottom half of the compartment has moved from an area of magnetic field to an area of no magnetic field so it apposes the change by briefly making a magnetic field in the same direction as the one it has just left. Hence the eddy currentsare clockwise and make a mag. field into the page.


Now, hopefully I didn't make that too confusing but you should be able to see that the eddy currents that are induced create a magnetic field which apposes the change in flux that the conducter (in which the eddy currents form) experiences. Also note that the change in flux is due to the movement of the compartment through the magnetic field and hence the magnetic fields created by the eddy currents appose the movement of the compartment. This is the absic principle behind EM breaking.

 

[lfc_reds2003]

or letting a magnet drop through a metal tube....

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.

sorry that was very random ...

 

[jsttesting]

Thanks a lot guys! The diagram really helps, and the explanations are awesome....

Thanks once again....

Just one more ques....

KFunk; in your diagram, if you're using the right hand palm rule, which way would you face the thumb (because, your fingers would point into the page in the second section of your diagram....therefore you could have the finger either up or down).

Just wondering how that works....

 

[KFunk]

I don't think you would really use the right hand palm rule here (assuming that we're talking about the same thing) since that applies more to the force a current carrying object or moving charged particle experiences in a magnetic field. I was using the right hand screw rule to work out the direction of the eddy currents though.

 

[jsttesting]

Ahh...I see how it works.....Thanks a lot!!