circuit, illustrated in view A, is for controlling
two effects have combined to reduce the voltage (Ep)
underfrequency, and the circuit in view B is for
decrease of line frequency is great enough, the
controlling overfrequency; however, both circuits are
resultant decrease of voltage (Ep) to the relay causes it
very similar in design since they work on varying the
to become de-energized. This, in turn, controls the ac
frequency from a preset resonant frequency.
generator output circuit by disconnecting it from the
In both circuits, line voltage is applied through C1
load.
and R1, so that a specific voltage appears across each
In view B of figure 7-32, the same circuit is shown
when line frequency is correct. Keep in mind that as
except that the output terminals are across resistor Ro
frequency goes up or down, the capacitive reactance of
instead of the tuned circuit. This circuit, as illustrated,
C1 varies. Therefore, with a frequency change there is
can be used for overfrequency control. Like the
more or less voltage drop across C1, and this results in
underfrequency circuit, view A, this circuit is resonant
a voltage change across R1.
at 400 hertz, has a specific voltage developed across C1
The voltage of R1 is applied through a parallel
and R1, and the voltage of R1 is applied across the
tuned L-C-R (inductance-capacitance-resistance)
tuned circuit and resistor Ro. The graph in figure 7-32
circuit and a resistor (Ro). Because of the
illustrates the decrease of voltage across Ro when the
characteristics of a resonant parallel circuit, the
tuned circuit is resonant.
impedance of the tuned circuit is maximum and line
When line frequency increases, less voltage drops
current is minimum. Therefore, the current through Ro
across C1, so more voltage drops across R1. Therefore,
is minimum.
more voltage impresses across the tuned circuit and
Since the impedance of the tuned parallel circuit is
resistor Ro. In addition, the tuned circuit goes off
maximum at resonance, its voltage drop (Ep) is
resonance; its impedance decreases; its voltage drop
maximum; the voltage drop (ERo) across Ro is
decreases; line current increases, and thereby increases
minimum. With an off-resonance to either side
the voltage (ERo) drop across Ro. As a result, two
(increased or decreased frequency), the impedance of
effects combine to increase the voltage (ERo) to the
the tuned circuit decreases, and accordingly, the
relay coil with an increase in frequency. When the
voltage (Ep) across it decreases. At the same time, the
increase of line frequency is great enough, the resultant
voltage (ERo) across Ro increases because the current
increase of voltage (ERo) causes the relay to energize.
through the line increases. The rise of Ep is shown by
This, in turn, opens the MEPP's ac generator output
the graph in view A.
circuit, disconnecting it from the load.
The circuit shown is used for underfrequency
Types of AC Generators
control. The parallel tuned circuit is set for resonance
at a frequency of 400 hertz. When the ac generator
Ac generators range in size from the automotive
output frequency is 400 hertz, a specific voltage
alternator to the 90,000 volt-ampere machines used on
develops across C1 and R1. The voltage of R1 applies
t h e N C - 1 0 C . A l l a c g e n e r a t o r s h ave c e r t a i n
across the resistor Ro and the tuned circuit. At this time
characteristics in common, regardless of size, shape, or
the voltage drop across the tuned circuit is maximum
rating.
and can be used to trigger a silicon-controlled rectifier,
The ac output is taken from a set of stationary
bias transistor, or directly control a relay. For purposes
windings (stator).
of discussion, assume voltage Ep is directly applied to
a relay coil, and that at about 400 hertz, Ep is great
The ac generator field (rotor) is a rotating
enough to energize the relay, thereby connecting the ac
magnetic field with a fixed polarity.
generator to the load.
The output voltage is controlled by varying the
When line frequency decreases, more voltage will
strength of the rotating field.
be dropped across C1 (decrease in frequency causes C1
The output frequency is controlled by the speed
reactance to increase) so less voltage is dropped across
of the magnetic field rotation.
R1; therefore, less voltage is impressed across the
tuned circuit and resistor Ro. In addition, the tuned
A prime mover is required to turn the rotor.
circuit goes off resonance with the change in
frequency. Its impedance decreases, and accordingly,
THREE-PHASE AC G E N E R AT O R S . --
the voltage (Ep) decreases (see the graph). As a result,
Current military specifications require that the basic ac
7-24
