Motor driving question

[elecLear78]

Please excuse me i do not much of motor and how to drive. But i am hearing two ways to drive and getting really confused. One method is to know the position of hall sensors and based on that switch the correct legs. The other method is give 3 phase stator voltage then the effective magnetic flux will be in some direction and rotor will try to align to this flux. Change the stator voltage again the rotor will align and hence will keep rotating. Now my doubt why in the second case hall information is not required. Sorry for not providing enough information as i am only in study phase and don't know much about the motor driving.

 

[MaxHeadRoom78]

I assume you are talking about BLDC motor drive.
With 3 Hall effect sensors the commutation point is known precisely, the other method is to exercise the motor very briefly and take a sample of stator current in order to locate the stator winding's WRT the P.M. fields.
Some use a quadrature encoder to continue reading the commutation point once the latter method is achieved.
For details on how this is achieved, see Manufacturers App notes from such as PicMicro etc.
Max.

 

[dr pepper]

No argument to Max's discussion, however there is at least one other way, detect zero cross voltage on the phase that isnt driven, this gives you an idea of the electrical position of the rotor (for more than 2 poles this isnt the mechanical position), then you can fire the coils to give decent torque.
This is a fairly difficult thing to get right, if its one of your first projects you'll need to do some research.
If you have position feedback things are much simpler.