For synchronous motor,
Under excitation - Lagging power factor
Over excitation - Leading power factor
For synchronous generator
Under excitation - Leading power factor
Over excitation - lagging power factor
For lagging power factor = (IR*cosφ + IXsinφ)/V_r
For leading power factor = (IR*cosφ - IXsinφ)/V_r
Zero voltage regulation occurs on leading power factor only.
The neutral point of a generator is usually earthed to facilitate protection of the stator winding and associated system.
Earthing also prevents damaging transient overvoltages in the event of an arcing earth fault or ferro resonance.
For HV generators, impedance is usually inserted in the stator earthing connection to limit the magnitude of earth fault current.