outlets are connected phase to neutral. They supply
power generating systems for servicing aircraft have
single-phase power for such items as test equipment
outputs of 120 and 208 volts. These voltages are
obtained by designing the generator to produce 120
volts per winding and connecting them in a wye
An advantage of a delta-connected system is that if
system, as shown in view A of figure 7-33. The voltage
one winding of the power source becomes inoperative,
between the neutral wire and any one phase is 120
it can be disconnected. The system still operates, but at
volts. The phase-to-phase voltages are 208 volts. When
a reduced capacity. When power is distributed in this
a load is connected phase to phase, two windings are in
manner, it is known as an open-delta operation. If one
series across it. The vector sum of the two generated
winding in a wye-connected system fails, it is not
voltages is 1.73 times the voltage of a single phase.
possible to operate the system.
The line-to-line voltage found in a three-phase,
B RU S H L E S S AC G E N E R AT O R S . -- T h e
wye-connected system is the vector sum of the
theory of the brushless generator is not new. But, the
voltages generated by two separate phase windings.
lack of a small rectifier that could withstand the
Because a phase difference of 120 degrees exists
electric current and rotational stresses produced in the
between the two generated voltages, they reach their
generation of ac caused a delay between the drawing
peak amplitudes at different times; consequently, they
board and the production of a brushless generator.
must be added vectorially, not directly. The power is
However, with the development of a silicon diode that
proportional to the product of current and voltage. The
is small, rugged, and has sufficient current capabilities,
higher voltage requires less current for the delivery of
the development of a practical brushless generator was
an equal amount of power. Increasing voltage instead
possible. The use of brushless generators in MEPPs
of current allows the use of smaller current-carrying
now greatly increases the time between scheduled
c o n d u c t o r s i n t h e d i s t r i bu t i o n s y s t e m a n d t h e
generator. This makes the system lighter in weight, less
Brushless Generator Theory.--A brushless
costly, and more adaptable for supplying power to a
generator is shown in schematic form in figure 7-34. It
wide variety of loads.
consists of three main sections: the permanent magnet
W Y E , D E LTA , A N D O P E N - D E LTA
generator (PMG), the exciter-and-rectifier assembly,
SYSTEMS.--Most three-phase distribution systems
and the main ac generator. Each of these sections has
use the wye or delta connection. Compare the two
rotating and stationary parts. The only connections
circuits in views A and B of figure 7-33. The wye
between the rotating and stationary parts are working
system has a distinct advantage over the delta system.
air gaps in each of the three sections (no brushes).
The wye system has two voltages available. The lower
The speed of operation of the brushless generator
voltage (120 volts) is extracted from the
is limited to a narrow range (plus or minus 5 percent).
phase-to-neutral connection. The higher voltage (208
The newer electrical ac generators are driven by
volts) is available from the phase-to-phase connection.
constant-speed drive units or constant-speed engines.
In the four-wire, grounded neutral system, the
In most of the newer MEPPs, the generator's operating
neutral wire is connected to the frame. This constitutes
speed is held to within 1 or 2 percent of the generator's
a ground. The three-phase wires are then connected to
nominal speed. Brushless generators are driven at a
the three-phase power receptacle. The convenience
speed to produce a 400-Hz output.
b Ebc - 120V
Ebc - 208V
Eca - 208V
Eca - 120V
Eab - 208V
Eab - 120V
Enb - 120V
Enc - 120V
Ena - 120V
Figure 7-33.--Wye-delta voltage relationships.