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Alternators are single-phase or polyphase. Variations
include three-phase alternators used as
single-phase units by insulating and not using one
phase lead. Since the lead is unused, it is not
brought out to a terminal. The power rating is
reduced from that of the three-phase unit as limited
by the amount of current carried by a coil. An alternator
designed only for single-phase operation usually
does not have coils in all of the armature slots
because end coils contribute little to the output voltage
and increase the coil impedance in the same
proportion as any other coil.
Single-phase alternators are usually used in smaller systems (limited to 25kW or less) and produce AC power at utilization voltages. Terminal voltage is usually 120 volts. The electric load is connected across the terminals with protective fuses. One voltmeter and one ammeter measure the output in volts and amperes, respectively The two-wire alternator has two power terminals,
one for each end of the armature coil.
Two-wire, single-phase alternator The three-wire, single-phase alternator has
three power terminals; one from each end of the
armature coil and one from the midpoint . Terminal voltage is usually 120 volts
from the midpoint to either end of the armature coil and 240 volts between the two ends. The load is
connected between the two outside wires or between
either outside wire and neutral, depending upon the
voltage required by the load. Assuming alternator
voltage to be 120/240 volts, load 1,0 and load 2,0
would consist of 120-volt lamps and 120-volt single phase
power equipment. Load 1,2 would consist of
240-volt power equipment. Two voltmeters and two ammeters (or equivalent) are required to determine
the load in kilovolt amperes (kVA).
Three-wire, single-phase alternator Polyphase alternators are two, three, or six
phases. Two-phase power is used in only a few localities.
Six-phase is primarily used for operation of
rotary converters or large rectifiers. Three-phase
alternators are the most widely used for power production.
Polyphase alternators have capacities from
3 kW to 250,000 kW and voltage from 110 V to
13,800 V. Two general types of three-phases alternator
windings are the delta winding used in three wire,
three-phase alternators, and the star or wye
winding used in four-wire, three-phase types.
Three-wire, three-phase alternators have three sets of single-phase windings spaced 120 electrical degrees
apart around the armature. One electrical
degree is equivalent to one degree of arc in a two pole
machine, 0.50 degree of arc in a four-pole machine,
0.33 degree of arc in a six-pole machine, and
so on. The three single-phase windings are connected
in series to form the delta connection, and
the terminals are connected to the junction point of
each pair of armature coils (see illustration below - Three-wire, three-phase alternator). The total
current in a delta-connected circuit is always equal
to the vector sum of currents in two-phase windings.
The instantaneous current flows out to the
load through two windings and returns from the
load through the third winding. Since the coils are similar physically and electrically, equal voltages
are generated and applied to the terminals. Due to
spacing of the coils about the armature, the maximum
voltage between the pairs of terminals does
not occur simultaneously.
Three-wire, three-phase alternator The characteristics of
three- wire, three-phase (or delta) alternators are:
- The amount of current through the alternator terminals is the algebraic sum of current through the alternator coils.
- The currents are not equal in magnitude or time.
- Connection between coils can be made either inside or outside the generator.
- In a 60-Hertz machine, each coil experiences
maximum instantaneous voltage, first positive and
then negative, 120 times each second. Disregarding
voltage direction, the maximum instantaneous voltages
occur on successive coils 0.003 seconds apart.
Due to time differences between the voltages and
resulting currents, the amount of current through
the alternator terminals and the amount through
the alternator coils are not equal in magnitude or
time. The current through the alternator is 73 percent
greater than through the coils. Coil and terminal
voltages are the same magnitude. Three voltmeters
and three ammeters (or equivalent) are
required to measure the load on the alternator. The
average value of the three currents times the average
value of the three voltages plus 73 percent gives
a close approximation of the alternator load in kilovolt-amperes. Two single-phase or one two element polyphase kilowatt-hour meter is required
to measure the alternator output in kilowatt-hours.
The four-wire, three-phase alternator has three sets of armature coils spaced 120
electrical degrees apart about the armature, the
same as the three-wire, three-phase alternator. One
end of each of the three coils is connected to a
common terminal (neutral). The other end of each
coil is connected to separate terminals (phase terminals).
Thus, the four-wire alternator has four
terminals which connect to the three-phase conductors
and the neutral of the power-plant bus.
When each end of each coil is brought out to separate
terminals, the connections between coils are
made outside of the alternator, enabling installation
of a more comprehensive protective relaying system.
Four-wire, three-phase alternator
The four-wire, three-phase alternator can be
connected to a transformer instead of the power plant
bus by using a wye-wye transformation. Irregular
(double or triple) harmonics, which may be
produced, can be suppressed by using a core-type
transformer. A third or tertiary winding with a delta
connection may also be used as a suppressor. A
wye-delta transformer may be used if the power
plant bus is three wire and the alternator is four
wire wye connected.
Four-wire three-phase, dual voltage and
frequency alternators are also used. These are supplied
in sizes from 15 to 1500 kW, 127-220 volts,
three-phase, 60 Hertz, or 230-400 volts, three phase,
50 Hertz. Dual stator coils are used on each
phase. Coil ends are brought out to a terminal board
for making connections. Voltage and frequency combinations
are shown in below (Dual voltage and Frequency). 
Dual voltage and Frequency
Most parts of the world have standardized
on either 50 or 60 Hertz alternating current power. Sixty Hertz power is commonly used in the
United States. Fifty Hertz power is used in many
countries outside the United States. The ratio be-teen the 60-50 Hertz frequencies is 6:5. Electrical
energy received at one frequency can be converted
to a different frequency by using a frequency
changer. If a large power requirement exists, it may
be more economical to use a special alternator to
produce power at the desired frequency.
The applicable
equation is: 
Where:
V = generated voltage
K = constant value number (speed)
 = phase/phase angle
N = number of turns
f = line frequency The generated voltage is proportional to the
strength of the magnetic field, phase, and number of
turns in series between terminals and the speed.
References: “Joint Departments of the Army and the
Navy, Operation Maintenance and Repair
of Auxiliary Generators, 26 August 1996”
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