Three Phase -Alternator Flashcards
■ There are two basic types of three-phase alternators:
the revolving-armature
type and the revolving-field type.
The rotating-armature type is the least used because
of its limited voltage
and power rating.
The rotor of the revolving-field-type alternator contains
electromagnets.
DC must be supplied to the field before the alternator can
produce an output
voltage.
The output frequency of an alternator is determined by the number of
of stator
poles and the speed of rotation.
Three factors that determine the output voltage of an alternator are
a. the length of the conductor of the armature or stator winding.
b. the strength of the magnetic field of the rotor.
c. the speed of the rotor.
The output voltage is controlled by the amount of
DC excitation current.
Before two alternators can be connected in parallel, the output voltage of the
two machines should be the same,
the phase rotation of the machines must
be the same, and the output voltages of the two machines must be in phase.
Three lamps connected between the two alternators can be used to
test for phase rotation.
A synchroscope can be used to determine phase rotation and
and difference of
frequency between two alternators.
.
Two devices used to prevent a high voltage being induced in the rotor
when the DC excitation current is stopped are
a field-discharge resistor and
a diode.
Many large alternators use a brushless exciter to
supply DC to the rotor
winding.
