A nice debate about slip control vs. field-oriented control on AC motors in terms of energy-efficiency, economy:
http://industrial.embedded-computing.com/articles/the-versus-field-oriented-control/
Also, a very good document about terminologies, classification, and control methods for motor, a truly motor control compendium:
http://focus.ti.com/download/trng/docs/c2000/TI_MotorControlCompendium_2010.pdf
