A good general view of field-oriented control (FOC) can be found in Texas Instruments’ 1998 white paper, “Field Orientated Control of 3-Phase AC Motors”, which can be downloaded for free. For a more recent treatment, though its references go back at least as far as 1980, readers can check out “A Seamless Whole Speed Range Control of Interior PM Synchronous Machine without Position Transducer” by Filka, Balazovic, and Dobrucky from the IEEE (ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4778532). The paper was presented at the International Power Electronics and Motion Control Conference (PEMC) in 2006. Depending on your IEEE membership, it may cost you $10 to download it.
For those who want a more visually oriented explanation, Microchip’s David Wilson presented an online tutorial at Electronic Design’s “One Powerful Day” block of Web seminars on September 29. The presentation, called “Controlling Permanent Magnet Synchronous Motors,” is archived on the Electronic Design Web site. Readers can view it for free, although access requires registration. To register, start at www.electronicdesign.com, "One Powerful Day", and scroll down to Session 5.