TM 5-685/NAVFAC MO-912
Figure 7.92 Three-wire, three-phase alternator.
similar physically and electrically, equal voltages
(d) The fou r-wire, three-phase alternator (see fig
are generated and applied to the terminals. Due to
4-8) has three sets of armature coils spaced 120
spacing of the coils about the armature, the maxi-
electrical degrees apart about the armature, the
mum voltage between the pairs of terminals does
same as the three-wire, three-phase alternator. One
not occur simultaneously. The characteristics of
end of each of the three coils is connected to a
three- wire, three-phase (or delta) alternators are:
common terminal (neutral). The other end of each
(1) The amount of current through the alterna-
coil is connected to separate terminals (phase ter-
tor terminals is the algebraic sum of current
minals). Thus, the four-wire alternator has four
through the alternator coils.
terminals which connect to the three-phase con-
(2) The currents are not equal in magnitude or
ductors and the neutral of the power-plant bus.
When each end of each coil is brought out to sepa-
(3) Connection between coils can be made ei-
rate terminals, the connections between coils are
ther inside or outside the generator.
made outside of the alternator, enabling installation
(c) In a 60-Hertz machine, each coil experiences
of a more comprehensive protective relaying sys-
maximum instantaneous voltage, first positive and
then negative, 120 times each second. Disregarding
(e) The four-wire, three-phase alternator can be
voltage direction, the maximum instantaneous volt-
connected to a transformer instead of the power-
ages occur on successive coils 0.003 seconds apart.
plant bus by using a wye-wye transformation. Ir-
Due to time differences between the voltages and
regular (double or triple) harmonics, which may be
resulting currents, the amount of current through
produced, can be suppressed by using a core-type
the alternator terminals and the amount through
transformer. A third or tertiary winding with a delta
the alternator coils are not equal in magnitude or
connection may also be used as a suppressor. A
time. The current through the alternator is 73 per-
wye-delta transformer may be used if the power
cent greater than through the coils. Coil and termi-
plant bus is three wire and the alternator is four
nal voltages are the same magnitude. Three voltme-
wire wye connected.
ters and three ammeters (or equivalent) are
(f) Four-wire three-phase, dual voltage and
required to measure the load on the alternator. The
frequency alternators are also used. These are sup-
average value of the three currents times the aver-
plied in sizes from 15 to 1500 kW, 127-220 volts,
age value of the three voltages plus 73 percent gives
three-phase, 60 Hertz, or 230-400 volts, three-
a close approximation of the alternator load in
phase, 50 Hertz. Dual stator coils are used on each
kilovolt-amperes. Two single-phase or one two-
phase. Coil ends are brought out to a terminal board
element polyphase kilowatt-hour meter is required
for making connections. Voltage and frequency com-
to measure the alternator output in kilowatt-hours.
binations are shown in figure 4-9.