TM 55-4920-328-13
servo motor positions the slidewire in the potentiometer to
equalize the two signals input to the chopper. Equalizing
the signals balances the bridge and acts to stop servo
motor rotation. Input ac power is rectified and filtered to
drive the servo amplifier. Regulated voltage input to the
bridge is derived from a Zener circuit. The servo motor is
referenced directly from the 115v ac, 400 Hz power
source.
b. Gas producer meter M2 and power turbine meter
M3. The GAS PRODUCER meter (4, figure 1-24) and
POWER TURBINE meter (5) are identical in design and
function; therefore, only one will be described. Each
meter is a hermetically-sealed tachometer indicator
consisting of a synchronous motor, magnetic coupling,
and an indicating mechanism. The full scale range of the
instrument is 0 to 110 percent rpm. The double-needle
unit utilizes two dials. The minor (inside) dial is graduated
in percent of rpm from 0 to 50 percent in 1-percent
increments. The major (outside) dial is graduated in I-
percent increments from 50 to 110 percent rpm. The
synchronous motor rotates at a speed proportional to the
frequency of the signal input from the engine tachometer
generator. Output torque proportional to synchronous
motor speed drives the indicating mechanism through the
magnetic coupling. The indicating mechanism is spring-
loaded against the torque through an angle which is
proportional to the applied torque.
c. Oil temp meter M4. The OIL TEMP meter (6,
figure 1-24) is an instrument that displays oil temperature
in an engine under test.
The scale of the indicator is graduated from 0 °F to 300F
in 10-degree increments. The indicator acts as a
resistance thermometer when connected to an external
temperature resistance bulb installed in the engine
lubricating oil environment. 28v dc power is required for
indicator excitation. The indicator movement contains
two slide-wire potentiometers which are adjusted for
calibration. The indicator operates on an unbalanced
bridge principle with the external temperature resistance
bulb forming one arm of the bridge circuit. The indicator
armature consists of two coils which rotate in an air gap of
a permanent magnet. The large coil deflects in the same
manner as a galvanometer; the small coil is connected in
series with one arm of the bridge and acts to oppose the
motion of the larger coil. Refer to table 1-1 for leading
particulars of the indicator.
3. Engine safety unit 3A4. The engine safety
unit (279, figure 7-17) is a 19-inch wide, rack-mounted
assembly
which
monitors
the
following
engine
parameters: EGT, NI, NII, chip detector, and aborted
start. The unit contains two speed channel circuit cards to
sense input signals of a frequency proportional to the
output shaft speed of the engine. Input signals generated
by a pulse generator are converted to an analog voltage
proportional to shaft speed. When the analog voltage
reaches a pre-set trip level, a relay is energized, providing
a contact closure for alarms. The unit is equipped with an
internal power supply which is energized from the system
28v dc utility power supply assembly. The internal power
supply consists of a series of chokes, voltage regulation
transistors, and a printed circuit card to regulate input
power to +10v dc and provide a zero circuit card which
receives input
Change 5 1-44
