[efrem]

Can you explain me what mean a Hall offset and Hall location ? What is the back EMF voltage and/or BEMF? And how can I mesure the resistance and the impedance of an AC servo motor ? Does it's possible to determine the number of pole a AC motor ?

Thanks

[Al_The_Man]

Hall effect switches refer to the Hall device often used to commutate BLDC motors, these you will generally find looking like small transistors, three spaced at the commutation angle on the rear of the motor, often now, there are tracks on the encoder that take the place of hall-effect switches.
BEMF is the voltage that is generated by the act of the motor being turned, this is generated wether by an external means or when powering the motor normally, this also acts to counteract the power feeding the motor.
You can find out the number of poles by finding out how many electrical cycles there by connecting the three phase windings to three 10k resistors and conecting the ends of the resistors together to make a star point. A 'scope is preferable, but you may be able to use a meter. Connect one lead to the common (star point) and the other to the end of a winding. count how many times the meter/scope goes positive for one mechanical revolution.
See the Renco encoder site for a picture of lining up hall switches to winding, and you will see the resulting waveforms.
Al.