The Generator is electrical generator which produce AC or dc power. The synchronization phenomena is mostly considered for the ac power generators or alternators. Synchronization is to match the frequency, voltage and phase angle of the two or more working generators and for three phase, phase sequence. In practical mostly used and simplest method is lamp testing. In this method one lamp or bulb is inserted between two alternators phases. the lamp must be between the Generator and line phases RYB simultaneously. When the bulb is on, it means generators are not synchronized and you have to change the speed of alternator for synchronization. if bulb is off then your alternator is synchronized.
The other method is sync meter which do the same work by digitally or analog measuring the voltage and frequency of both alternators or alternator and line.
The things always keep in mind for the synchronization are:
RMS voltage,
Frequency,
Phase angle,
and phase sequence.
These all things are matched when your frequency is matched if both of the alternators are, of same capacity or rating.
The rating is different then there may be difficulty in synchronization but can be synchronized. The synchronization of Alternators are always done by change the speed or Alternators for same design frequency and voltage. Synchronization is mostly done on power plants for adding the new upcoming generator in line.