http://www.pdn.ac.lk/eng/old/mechanical/menu/class/downloads/notes/flywheel_ppt.pdf
http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1192&context=icec
http://www.electrical4u.com/what-is-braking-types-of-braking-regenerative-plugging-dynamic-braking/