A servo amplifier actually fires DC voltage into the three phases of a servo motor. At the time a phase is fired, it is completely dependent upon rotor position in reference to the stator windings to be fired. In other words, you need commutation to run a servo motor, which the servo amplifier provides. Most servo motors will have an encoder that is coupled to the rotor and aligned to give rotor position in reference to stator windings. We actually tried to run a Yaskawa servo open loop on a VFD, and no matter how we tuned the drive, the end result was a very inefficient motor that drew high amps and had no torque. Check out the article called Your Guide to Types of Servo Motors; the article goes through the different types of servo motors and how they differ from each other.