Hello everybody,

I'm a newbie, and this is my first approach to motor control. In particular, I should control a PMSM motor (about 1 hp) using the sensorless FOC system based on a BEMF observer.
I'm in trouble about the initial rotor position and the strategies used for this purpose.
Can anyone explain or indicate documents about how, in practice, to do this initial alignment ?

Look at Allegro A4960 Sensorless BLDC Motor Driver using back-EMF for feedback.

The problem is not only alignment: control below a certain minimum speed is not possible based on B-EMF.
What I think is the standard startup is to ignore the estimated position and speed, and keep the speed regulator offline.
A constant magnitude current vector has to be kept steady in a certain position for some time (alignment) and then will be rotated at the reference speed (ramped up from zero).
In the meanwhile, B-EMF algorithm has been already started, and above a certain speed FOC torque control and speed feedback can be closed.

If you'll state whether it is an SMPM (surface mount) or IPM (internal), the answers will be more focused on your motor. there are different strategies for these two types.