Why a synchronous motor is a constant speed motor?

Synchronous motor work on the principle of force developed due to the magnetic attraction established between the rotating magnetic field and the main pole feed.
Since the speed of rotating magnetic field is directly proportional to frequency the motor operates at constant speed.

Th synchronous speed motor's is in MAGNETIC LOCKED  state with the field RMF because of poles of rotor created by external excitation power source,and it rotates with the speed of RMF so it is a constant speed motor and its' speed can not be easily changed?