TM 55-1520-240-23-7
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MISCELLANEOUS INSTRUMENTS
(Continued)
8-57
The fuel quantity system also includes a switching box
and an inverter. These are used to provide fuel quantity
indication at the refueling station.
NO BREAK - WORK HARDER
The inverter provides 115 vac to the fuel quantity
indicating system when fuel quantity is checked at the
refueling station. The inverter is located in the cabin
overhead at sta. 220.
NO BREAK - WORK HARDER
The switching box removes the fuel quantity probe
signals from the cockpit indicator and switch and
connects the signals to the refueling station indicator
and switch. This occurs when the refueling station is
activated. The switching box is located at sta. 220.
ROTOR TACHOMETER INDICATING SYSTEM
Description
The rotor tachometer system includes two identical
circuits. Each circuit indirectly measures and indicates
rotor rpm in percentage between 0 and 130. One
hundred percent equals 225 rotor rpm. The system
includes a speed sensor in each ac main generator and
a rotor tachometer indicator in each pilot’s instrument
panel.
NO BREAK - WORK HARDER
Each speed sensor is a simple stator coil within the
main generator PMG (permanent magnet generator)
that provides a signal whose frequency is proportional
to ac generator rpm and rotor rpm. The sensor output is
available regardless of the generator on/off state. Loss
of rpm output can occur only as the result of a failed
sensor coil or a sheared generator shaft.
NO BREAK - WORK HARDER
The sensor signal is processed within each indicator
by converting the signal frequency to a proportional
dc voltage level. The dc lever is then amplified within
a conventional servo loop within the indicator. Rotor
rpm is shown on the dial face using a dial and sub dial
pointer. The short sub dial pointer indicates rotor rpm in
percent from 0 to 60 percent. The long dial pointer then
indicates between 60 and 130 percent.
NO BREAK - WORK HARDER
The rotor tachometers are clamp mounted in pilot’s
instrument panels and powered by a 28 vdc circuit
breaker.
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