AC GENERATOR OPERATION
Paralleling AC Generators
Most electrical power grids and distribution systems have more than one AC generator operating
at one time. Normally, two or more generators are operated in parallel in order to increase the
available power. Three conditions must be met prior to paralleling (or synchronizing) AC
Their terminal voltages must be equal. If the voltages of the two AC generators are not
equal, one of the AC generators could be picked up as a reactive load to the other AC
generator. This causes high currents to be exchanged between the two machines, possibly
causing generator or distribution system damage.
Their frequencies must be equal. A mismatch in frequencies of the two AC generators
will cause the generator with the lower frequency to be picked up as a load on the other
generator (a condition referred to as "motoring"). This can cause an overload in the
generators and the distribution system.
Their output voltages must be in phase. A mismatch in the phases will cause large
opposing voltages to be developed. The worst case mismatch would be 180° out of
phase, resulting in an opposing voltage between the two generators of twice the output
voltage. This high voltage can cause damage to the generators and distribution system
due to high currents.
During paralleling operations, voltages of the two generators that are to be paralleled are
indicated through the use of voltmeters. Frequency matching is accomplished through the use
of output frequency meters. Phase matching is accomplished through the use of a synchroscope,
a device that senses the two frequencies and gives an indication of phase differences and a
relative comparison of frequency differences.
Types of AC Generators
Figure 5 Stationary Field, Rotating Armature
As previously discussed, there are two types of
AC generators: the stationary field, rotating
armature; and the rotating field, stationary
Small AC generators usually have a stationary
field and a rotating armature (Figure 5). One
important disadvantage to this arrangement is that
the slip-ring and brush assembly is in series with
the load circuits and, because of worn or dirty
components, may interrupt the flow of current.