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The thermocouple harness senses the temperature between the two turbine sections (Ng and Np) at T4.5, the TGT harness provides a direct reading to the ECU for TGT limiting function, the signal is relayed direct to the CDU via the SDC for cockpit TGT indication.
The Np sensor and Torque and O/S sensor contain a permanent magnet and wire coil that senses pulses induced by teeth on the power turbine shaft, these pulses are used to generate a power turbine speed signal that's used for Np governing and for cockpit indication of % RPM 1 or 2.
The torque and O/S sensor references torque by actually measuring power turbine shaft twist. This is accomplished by pinning a reference shaft to the front end of the power turbine shaft and allowing it to rotate relative to the drive shaft. Nr loads the power turbine on the output end while Ng drives the power turbine rotors; this causes the power turbine shaft to twist. This shaft twist causes the teeth on the power turbine shaft to vary in position relative to the reference shaft teeth. A torque circuit within the ECU processes the electronic pulses. At intermediate power (1,543 SHP), the output torque is 403 ft/lb and the twist of the shaft is 7.4o. A torque signal is relayed from each engine to the other engine for the load share function.
Np reference for the Np governor is adjustable via the INCR/DECR switch on the collective sticks, either switch simultaneously controls both ECU’s; individual adjustment of each engine is not allowed. Pilot authority to adjust the Np reference is limited to 96-100% rpm 1 or 2, with the pilot side switch having authority should both switches be actuated at the same time.
A torque (Q1/Q2) signal is exchanged between both ECUs' for the load share function.
e. Operation of Electrical Control Unit Circuits
(1)
ECU cockpit signal generation, in addition to the speed governing functions, the ECU conditions and forwards the Np signal to the cockpit. An isolation transformer receives Np speed pulses from the Np sensor. These pulses are converted into a power turbine speed signal that is forwarded to the PDU through the Signal Data Converter (SDC). During an alternator failure the cockpit Np signal, % RPM 1 or 2, is lost because the isolation transformer needs alternator power to function.
(2)
ECU cockpit signal generation, in addition to the speed governing functions, the ECU conditions and forwards the TRQ signal to the cockpit. A torque circuit receives pulses from the torque and O/S sensor. The timing differential between the pulses is converted into a torque signal to the PDU through the SDC. The torque circuit also forwards the torque information to the load share comparator to both
ECU's. During an alternator failure, the cockpit TRQ signal is lost because the torque circuit needs alternator power to function.
(3)
The TGT limiting system protects engine hot section components from stress or damage by limiting the temperature at T4.5 to approximately 843 oC. When TGT limiting is active it reduces fuel flow at the HMU by actuating the torque motor. When the TGT limiter is active it overrides the Np governor and load sharing circuits. The TGT limiter can only reduce fuel flow. TGT limiting is disabled when the system is in ECU lockout. When the system is in ECU lockout care must be taken to keep the TGT within operational ranges.
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