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2.  Engine Tachometer Generator
A.  The tachometer generator is a mechanism, which transforms the rotational input energy into equivalent electrical energy. It consists of a three-phase stator winding, end shields, and a permanent magnet rotor assembly, all contained in a sealed case. The tachometer generator is used to sense a compressor rotor speed and generate corresponding alternating electrical signals for the tachometer indicator operation. The N1 tachometer generator is located on the front accessory drive, and the N2 tachometer generator is on the aft right side of the accessory drive gearbox. (Fig. 1) Two types of tachometer generators are currently in use. One is a "Bearing" Type 2CM9 Tachometer Generator (approx. 3.62 inches in length) and the other is a "Bearingless" Type 2CM15 Tachometer Generator (approx. 2.12 inches in length) The "bearing" type tachometer generator consists of a three-phase stator winding, end shields, and a permanent magnet rotor mounted on bearings, all contained in a sealed case. It is used in both the N1 and N2 positions. The "bearingless" type tachometer generator is installed in the N1 and N2 positions and consists of a stator housing and a separate permanent magnet rotor assembly. At installation, (done only in the engine shop) the rotor assembly is mechanically attached into the square engine driven spline.
B.  Each tachometer generator is driven by its respective compressor rotor through a reduction gearing. The generator drive shaft turns the rotor assembly inside the stator coils, thus generating alternating electrical signals. The frequency of the generator output signals is a function of the engine compressor rpm.
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3.  Engine Tachometer Indicator
A.  The tachometer indicator shows compressor rotor speeds. It consists of a three-phase synchro motor, a rotating drag assembly, a calibrated clock spring, all hermetically sealed in a case, and a round dial with a subdial. Each engine tachometer indicator shows its respective engine rotor speed as a percentage. On airplanes with pointer readout, the round indicator dial is graduated for readings between zero and 110 percent rpm, while the small subdial is graduated in ten divisions for each 10 percent change in speed. On airplanes with pointer and digital readout, the numeric readout is a 3-wheel counter showing percent RPM in hundreds, tens and units with 1/2% interpolation marks on the right hand wheel. There are four tachometer indicators, two for N1 and two for N2 compressor rotors, mounted in two rows on the center instrument panel.
B.  The alternating electrical signals from the respective tachometer generator are fed into a tachometer indicator which cause the drive shaft of the indicator synchro motor to rotate. The synchro motor drives the magnetic drag assembly or flux coupling which, in turn, restrained by a clock-type spring, moves the indicator pointers to a dial position corresponding to the compressor rotor speed.
4.  Operation
A.  The engine tachometer system operates on self generated electrical power. The airplanes electrical power is required only for the integral lighting of the tachometer indicators (Fig. 2).
B.  Each tachometer generator (N1 and N2) is driven by its respective compressor rotor through reduction gearing. The generator drive shaft turns the rotor assembly inside the stator coils, thus generating alternating electrical signals. These signals are transmitted to the corresponding tachometer indicator synchro motor by a two wire system, while the third phase is completed to ground. The alternating electrical signals cause the drive shaft of the indicator synchro motor to rotate. This rotation, through a flux coupling, tends to rotate the indicator pointer which is restrained by a spring. The speed of the synchro motor determines the degree to which the indicator pointer is rotated, and corresponds to the related compressor rotor speed. On airplanes with pointer readout, the round indicator dial is graduated for readings between zero and 110 percent compressor rpm. The small subdial is graduated in ten divisions for each 10 percent change in speed. On airplanes with pointer and digital readout, the numeric readout display is graduated between 00 and 110 percent compressor RPM. A master indicator(s) may be attached to the test receptacle(s), mounted in the control cabin, to provide a means of checking the system.
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H62568
505  Engine Tachometer System 
Feb 20/86  Figure 1  77-12-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 
　
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