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Acceleration Limiter Valve Orifice Check 502
49-31-101Page 602 Figure 601 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Jun 20/87
APU IGNITION AND STARTING SYSTEM - DESCRIPTION AND OPERATION
WARNING THE CURRENT INVOLVED IN THE IGNITION UNIT IS OF A VERY HIGH VOLTAGE AND CAN BE FATAL. BE SURE THAT POWER IS REMOVED FROM THE SYSTEM FOR A MINIMUM OF 3 MINUTES BEFORE MAKING ANY DISCONNECTIONS. AFTER DISCONNECTING THE HIGH TENSION LEAD, ENSURE COMPLETE DISCHARGE OF CAPACITORS BY IMMEDIATELY SHORTING IGNITION UNIT TERMINAL TO GROUND.
1. General
A. The APU ignition and starting system provides the means of rotating the APU engine to starting speed and for igniting the fuel-air mixture in the combustion chamber. The system consists of a starter motor, start relay, ignition unit, igniter plug and a high voltage lead. (See figure 1.)
B. During an APU start, the starter motor cranks the APU engine. The ignition unit converts 28 volt dc power into a high voltage current which is conducted to the igniter plug by the high voltage lead. The resultant high energy spark starts combustion of the fuel-air mixture in the combustion chamber. Engagement and disengagement of the starter motor and the application of ignition are controlled automatically. (See figure l.)
2. Starter Motor
A. The starter motor provides initial power for rotating the APU engine compressor, turbine, and engine-driven accessories to a speed high enough to obtain good airflow for combustion. The starter motor is mounted on the accessory drive section. (See figure 1.) The starter motor consists of a series-wound electric motor, a spring-loaded friction clutch, and a pawl-engagement mechanism. When the starter motor is energized and its speed increases, centrifugal force throws the pawls into engagement with the ratchet on the accessory drive shaft. The shock of initial engagement of the pawls is absorbed by the spring-loaded friction clutch. The friction clutch also limits the starting torque, which in turn limits the current drain on the battery during initial phase of the starting cycle. When the starter motor is de-energized, the engine speed will exceed starter motor speed. The ratchet on the accessory drive shaft will override the pawls until the starter motor speed slows to where the centrifugal force is small enough to allow the springs to retract the pawls.
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Aug 15/67 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 49-41-0 Page 1
3. Start Relay
A. A heavy duty start relay R5 is provided to carry the heavy electrical load occurring during starter motor operation. The relay is located in the electrical and electronic compartment. (Fig. 1)
4. Ignition Unit
A. The ignition unit provides the high voltage current required to produce the spark at the igniter plug. The unit consists of a transformer, vibrator, rectifier, booster coil, and a series of capacitors, all enclosed in a hermetically sealed metal container mounted on the turbine plenum at approximately six o'clock position. (Fig. 1)
B. When the ignition unit is energized, the vibrator is supplied with 28-volt dc power. The vibrator converts the input current to a pulsating output current. This pulsating output current is transmitted to the transformer, where the voltage is stepped up, then passed through the rectifier unit into the storage capacitors. When the storage capacitors are fully charged, the capacitor air gap will break down. This allows the capacitors to discharge a surge of high intensity current into primary winding of the booster coil. The secondary winding of the booster coil produces a surge current, which is transmitted to the center electrode of the igniter plug by a high voltage lead.
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