Hey DCB! It doesn't tell you absolute position. To do field oriented control, you need 2 of the currents in the 3 phases, and the rotor flux angle.
In a PMSM motor, the rotor flux angle is the same as the rotor angle. Unfortunately, in a AC induction motor, there are no permanent magnets, and the rotors magnetic field is induced by the current in the stator. The rotor angle lags behind the rotor flux angle.
The rotor flux angle is truly horrible to compute.
You need the following variables. Well, you are trying to solve for θ.
Imr = Magnetizing current (as calculated from measured values)
fs = Flux speed (as calculated from measured values)
T = Sample (loop) time (parameter in program)
n = Rotor speed (measured with the shaft encoder)
Tr = Lr/Rr = Rotor time constant (must be obtained from the motor manufacturer)
θ = Rotor flux position
ωb = Electrical nominal flux speed (from motor name plate)
Ppr = Number of pole pairs (from motor name plate)
See Pg. 4 for computing the rotor flux angle:
http://ww1.microchip.com/downloads/e...tes/00908B.pdf