AC Generators:
There are two basic commercial AC generator designs: synchronous and induction generators. The easy way to think of induction generators is to simply think of them as motors being driven by an engine rather than producing power. In fact, they work just that way. Induction motors use power by running slightly less than synchronous speed (say 1,760 rpm rather than 1,800 rpm (synchronous)). That same motor would put power into the circuit if it were run at 1,840 rpm. Unfortunately, doing so requires reactive power (VARs). These VARs are not available when customers operate such a generator when the lights go out due to a storm. So, no one uses induction generators for emergency or standby generators. In fact, customers should only consider induction generators as part of motor generator sets for ride through protection.

Therefore, virtually all commercial emergency and standby generators are synchronous machines. They operate at exactly the same frequency as all other generators connected to the same circuits (60 cycles in the US). Making sure the generator is precisely matched to this frequency before interconnecting is the job of the switchgear designed specifically for this purpose.

Where the generator is only going to be used as an emergency or standby generator when the lights go out, there is no need for this paralleling switchgear. The lights go out, the generator is started, and the loads are switched to the generator. These switches are relatively inexpensive and are the most common way small generators are actually operated.