Found where I read it. See bottom of page 26 and 27.
Daniel
Daniel, Whoever wrote that article is basing his information on very old technology, there has not been a wound rotor AC servo motor used for many years, for a motor to be made synchronous to the applied frequency, it requires either a permanent magnet on the rotor or energized coils of some kind.
I think the author was mixing induction motor technology with his explanation, an ordinary AC induction motor can never be a synchronous motor, without some other tricks.
Apart from dismantling the motor to prove an AC servo motor has magnets on the rotor, is to back feed the motor shaft by some means and 'scope the three phase stator leads, you will see a generated three phase, the magnitude of which is dependant on rpm.
Generation will not occur with a solid iron rotor, only by energizing rotor coils, IF it has them.
Nosmo.