The output frequency of an alternator depends on the number of poles and the rotational speed. The speed corresponding to a particular frequency is called the synchronous speed for that frequency. This chart gives some examples:
Reference 1: RPM for 50Hz, RPM for 60Hz and RPM for 400Hz.
Reference 2: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 and 40.
Source: http://en.wikipedia.org/wiki/Alternator#Synchronous_speeds
Reference 2: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 and 40.
Source: http://en.wikipedia.org/wiki/Alternator#Synchronous_speeds
