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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////

N2 SPEED SENSOR
G-M56-MM-0057(1)-02-B
N2 Speed Sensor (Control Alternator) Figure 3
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CFM56 ENGINES
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B.  The N2 speed sensor is mounted on a bolt pad to the top forward face of the accessory gearbox at the 8 o'clock position. The sensor is spline-mounted to the gearbox shaft by a self-locking nut. The stator and case are completely separable and mount directly to the gearbox housing.
 5.  N2 Tachometer Indicator (Fig. 4)
_______________________
 A.  The N2 tachometer indicator converts signals produced by the N2 speed sensor into both pointer and digital readouts. It includes a starter cutout relay which ensures starter disengagement at maximum starter run speed and prevents re-engagement at engine speeds that would damage starter or engine. The relay, when energized, also inhibits the PMC INOP indication.
 B.  The indicator is similar to N1 indicator consisting of dial face, main indicator pointer, overlimit warning light, N2 numerical readout, power supply, and indicator circuits.
 6.  Operation
_________
 A.  N1 Indication System (Fig. 2)
 (1)  
The N1 indicator is a microprocessor based instrument which performs its various functions under the control of a software program. With the software program, all indicator functions are regularly sampled thereby producing a continuous updating of actual N1% RPM pointer, numeric displays, and output functions.

 (2)  
The N1 indication system is operative when 28v dc power is supplied by the EIS or N1 TACH IND circuit breakers on the P6 panel.

 (3)  
Actual N1 fan speed is measured by speed sensor elements which provide an ac voltage whose frequency is proportional to the fan speed. The sensor incorporates dual sensing elements with one element providing N1 signal for the power management control and the other element providing signal to the N1 tachometer indicator. A magnetic ring mounted on the fan shaft is provided with teeth. The passage of each tooth generates an alternating voltage in the sensor element proportional to actual N1 speed.

 NOTE: The sensor ring has one tooth thicker than the 29 others to
____ generate a signal of greater amplitude used as phase reference for trim balance.
 (4)  AIRPLANES WITHOUT EIS; The N1 tachometer indicator displays actual N1% RPM by pointer indication and Light Emitting Diode (LED) display. The pointer is positioned with a DC motor driven by a closed loop DC servo consisting of a digital to analog converter, output amplifier, sample/hold circuit, and servomotor drive amplifier. The pointer is returned to zero by a spring in event of power or N1 signal failure. The LED display is a four digit dot matrix in the upper half of the dial face controlled by a microprocessor within the indicator to simulate action of a four-drum mechanical connector. The LED dot matrix of LED's is used to form four digits displaying hundreds, tens, unit, and tenths. System failure causes the numeric display to go blank.
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CFM56 ENGINES
737-300/400/500/ /MAINTENANCE MANUAL ////////////////////////

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