The value and the position of the stator flux phasor must be known. They could be determined by direct measuring, but it would be applicable only to special motors, that have incorporated, from manufacturing, flux transducers (Hall effect, measuring windings).
In order to avoid this expensive solution and to be able to apply the control method to any existing motor, it is preferred the estimation of the stator flux, based on the measured values of the stator voltage and current, using the expression
,
where 
and 
are the stator voltage and current phasors, respectively
,
,
expressed in the stationary frame, fixed to the stator.
The stator flux is practically obtained by the composition of the two terms
,
,
resulting:
,
,
.It results that, by measuring only the phases voltages and currents, it is possible to reconstruct the value and the position of the stator flux phasor and the instantaneous value of the developed electromagnetic torque.
In practice, taking into account that the motor is supplied by a tri-phased voltage source inverter, it is necessary to measure only the phases currents and the DC voltage U, the phases voltages resulting upon the inverter's topology.
