TM 55-4920-328-13servo motor positions the slidewire in the potentiometer toequalize the two signals input to the chopper. Equalizingthe signals balances the bridge and acts to stop servomotor rotation. Input ac power is rectified and filtered todrive the servo amplifier. Regulated voltage input to thebridge is derived from a Zener circuit. The servo motor isreferenced directly from the 115v ac, 400 Hz powersource.b.Gas producer meter M2 and power turbine meterM3. The GAS PRODUCER meter (4, figure 1-24) andPOWER TURBINE meter (5) are identical in design andfunction; therefore, only one will be described. Eachmeter is a hermetically-sealed tachometer indicatorconsisting of a synchronous motor, magnetic coupling,and an indicating mechanism. The full scale range of theinstrument is 0 to 110 percent rpm. The double-needleunit utilizes two dials. The minor (inside) dial is graduatedin percent of rpm from 0 to 50 percent in 1-percentincrements. The major (outside) dial is graduated in I-percent increments from 50 to 110 percent rpm. Thesynchronous motor rotates at a speed proportional to thefrequency of the signal input from the engine tachometergenerator. Output torque proportional to synchronousmotor speed drives the indicating mechanism through themagnetic coupling. The indicating mechanism is spring-loaded against the torque through an angle which isproportional to the applied torque.c.Oil temp meter M4. The OIL TEMP meter (6,figure 1-24) is an instrument that displays oil temperaturein an engine under test.The scale of the indicator is graduated from 0 °F to 300Fin 10-degree increments. The indicator acts as aresistance thermometer when connected to an externaltemperature resistance bulb installed in the enginelubricating oil environment. 28v dc power is required forindicator excitation. The indicator movement containstwo slide-wire potentiometers which are adjusted forcalibration. The indicator operates on an unbalancedbridge principle with the external temperature resistancebulb forming one arm of the bridge circuit. The indicatorarmature consists of two coils which rotate in an air gap ofa permanent magnet. The large coil deflects in the samemanner as a galvanometer; the small coil is connected inseries with one arm of the bridge and acts to oppose themotion of the larger coil. Refer to table 1-1 for leadingparticulars of the indicator.3.Engine safety unit 3A4. The engine safetyunit (279, figure 7-17) is a 19-inch wide, rack-mountedassembly which monitors the following engineparameters: EGT, NI, NII, chip detector, and abortedstart. The unit contains two speed channel circuit cards tosense input signals of a frequency proportional to theoutput shaft speed of the engine. Input signals generatedby a pulse generator are converted to an analog voltageproportional to shaft speed. When the analog voltagereaches a pre-set trip level, a relay is energized, providinga contact closure for alarms. The unit is equipped with aninternal power supply which is energized from the system28v dc utility power supply assembly. The internal powersupply consists of a series of chokes, voltage regulationtransistors, and a printed circuit card to regulate inputpower to +10v dc and provide a zero circuit card whichreceives inputChange 5 1-44
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