If you want to build something like a Magnabend or ElectraBrake your best bet is to go to the following link:-
Magnabend Electro-Magnetic Sheet Metal Bending Machine
This site is maintained by the gentleman that invented the Magnabend
He has generously put up all the information you would possibly need to build an electromagnetic brake similar to the Magnabend. There is lots to read and the information seems to be very complete.
Edit: Sorry all I did not notice the original post date. Anyway hopefully the link will help someone