Synchronous electric motors have two possible excitation factors which are the direct current in the field winding and that of the three-phase alternating current source in the armature. If your field current is exactly enough to produce the required magnetomotive force, requiring no magnetizing current or reactive power, the electric motor runs at unit power factor. If your field current is less than what is said to be necessary when the motor is underexposed, insufficiency in magnetomotive force needs to be restored by the armature, the motor runs at a delayed power factor.
https://www.mrosupply.com/seals/oil-seals/5075282_472439_timken/ If your field current exceeds what is said, when the motor is over-excited, the surplus magnetomotive force must be properly balanced in the armature and an advanced current component is present, the motor will run at an advanced power factor. The synchronous motor when operating in the overexcited and empty condition is called the synchronous capacitor and performs the same function as a capacitor bank, being more economical than static capacitors.