APU Electrical Power Supply PDF

APU ELECTICAL POWER SUPPLY WEEK 3 STARTING — SYSTEM DESCRIPTION The APU starting system starts the APU engine. The starting sequence is initiated by switches on the cockpit overhead panel and is controlled by the APU Electronic Control Unit (ECU) and the APU Generator Control Unit...

APU ELECTICAL POWER SUPPLY WEEK 3 STARTING — SYSTEM DESCRIPTION The APU starting system starts the APU engine. The starting sequence is initiated by switches on the cockpit overhead panel and is controlled by the APU Electronic Control Unit (ECU) and the APU Generator Control Unit (GCU). COMPONENT LOCATOR Generator Control Unit The APU GCU is mounted in the ceiling of the baggage compartment. The GCU controls the APU driven starter-generator unit in both starter and generator mode. Right DC Box The right dc box is located in the aft equipment bay. The right dc box contains relays, contactors, fuses and circuit breakers for controlling APU starting, power generation and power distribution. APU Contactor Box The APU contactor box is located in the aft equipment bay. The APU contactor box contains relays, contactors, fuses and circuit breakers for controlling APU starting, power generation and power distribution. APU MASTER Switch The APU MASTER switch (513P) is located on the cockpit overhead panel. When set to the ON position, the switch opens APU fuel shut-off valve and enables APU operation. APU STARTER Switch The APU STARTER switch (514P) is located on the cockpit overhead panel. The switch provides the capability for the flight crew to start and stop the APU engine. Starter-Generator Unit The APU driven starter-generator unit is mounted on the APU accessory gearbox Field Resistor The field resistor (274P) is mounted in the ceiling of the baggage compartment near the APU GCU. The field resistor is used to provide battery power to the APU driven starter-generator unit field winding during starter mode. Electronic Control Unit (ECU) The ECU is mounted on the right side of the APU enclosure. The ECU controls and monitors the APU engine. OPERATION STARTING THE APU The APU engine requires fuel and ignition in addition to the starting subsystem in order to start APU MASTER Switch Set to ON Position The APU MASTER switch (513P) receives 28 Vdc from the APU CONTR (636P) circuit breaker. When selected to ON, the APU MASTER switch energizes relays (520P, 522P, 629P), supplies electrical power to the APU STARTER switch (514P), to the APU ON input of ECU (602P) and relay (518P). When relay (520P) energizes, it supplies battery bus power from circuit breaker (621P) to the start relay (7000S). Relay (520P) also supplies a ground to the Data Concentrator Unit (DCU), indicating that the APU has been powered up.  When relay (522P) energizes, it supplies electrical power from the APU FIRE SHUTDOWN RELAY (149W) to the APU fuel shut-off valve (33Q), causing it to open. When relay (629P) energizes, it supplies battery bus power from circuit breaker (621P) to the ECU (602P) input +28 Vdc IN. APU STARTER Switch Set to START Position The APU STARTER switch is momentarily selected to the START position to initiate the start cycle. Closed switch (514P), contacts (2, 3) energize relay (518P). Electrical power from closed switch (514P), contacts (5, 6) cause the relay (518P) to latch in the energized condition. When energized, relay (518P) applies the 28 Vdc START / STOP input to ECU (602P). ECU Outputs STARTER Signal The ECU (602P) responds to the 28 Vdc START / STOP signal with the STARTER signal output. The STARTER signal energizes relay (7000S). Closed contacts (A1 and A2) of relay (7000S) supply the START SW signal from relay (520P) through circuit breaker (258P) switch contacts to the APU GCU (250P1, ENGINE STARTING CYCLE pin W). APU GCU Outputs START CONTACTOR Signal When the APU GCU receives the START SW(switch) input signal (250P1, pin W), it issues the START CONTACTOR signal (250P1, pin Z), energizing APU start relay No. 1 (255P). When energized, APU start relay No. 1 (255P) supplies right battery (36P) voltage from circuit breaker (14P) to APU start relay No. 2 (254P) and the START MOTOR ON signal to ECU (602P). The open contacts of APU start relay No. 1 (255P) remove electrical power to the BATTERY LINE CONTACTOR (38P) causing the right battery to be disconnected from the right battery bus. When APU start relay No. 2 energizes, relay (254P), closed contacts (A1, A2) energize the APU start contactor (265P). The APU start contactor (265P) connects the right battery to the armature of the APU starter-generator unit (260P). The APU start contactor (265P) also supplies electrical power to the field winding of the APU starter-generator unit through a power resistor (274P).  With electrical power applied to the armature and field windings of the starter-generator unit, it acts like an electric motor. The starter- generator output torque is applied to the APU engine through a slip clutch. The ECU (602P) controls the application of fuel and ignition as the APU turbine gains speed. APU N Rotor Speed is 50% When the APU N2 speed is 50%, the ECU (602P) drops the STARTER signal. This causes relay (7000S) to open, dropping the START SW signal to the APU GCU (250P). In response to the loss of the START SW signal, the GCU drops the START CONTACTOR signal, causing the APU start relays No. 1 and No. 2 to de-energize. The APU contactor (265P) also opens, disconnecting the battery from the field winding and armature. If enabled, the APU GCU (250P) can then control the starter- generator (260P) to produce electrical power. APU Shut-down APU STARTER Switch Set to STOP Position The APU is shut down by selecting the APU STARTER switch (514P) momentarily to the STOP position. The open switch (514P), contacts 5, 6 removing the latching voltage to relay 518P, causing it to de-energize. Open relay (518P) contacts drop the START / STOP signal to the ECU (602P). The ECU (602P) then performs an orderly shutdown of the APU engine. APU MASTER Switch Set to OFF Position After the APU engine shutdown is complete, the APU MASTER switch (513P) is selected to the OFF position. The open contacts of the APU MASTER switch (513P) remove electrical power to the APU STARTER switch (514P), APU ON input of the ECU (602P) and relays (520P, 522P, 629P). When relay (522P) opens, it applies voltage to close the APU fuel shut-off valve (33Q). POWERPLANT UNIT MOUNTING The APU is mounted inside a fire resistant enclosure. The APU is mounted inside the enclosure through the use of mounts that provide support and vibration isolation for the APU. The forward end of the APU is supported by mounting adapters, mounting brackets and vibration isolators. The aft end of the APU is supported by a mount adapter and an isolator. ELECTRICAL INSTALLATIO N  The functions of the electrical installation system of the APU engine system are: ○ To provide a path for electrical power to operate APU units / components electrically ○ To provide a path for the Electronic Control Unit (ECU) to communicate with the electrical, sensing and operational components of the APU ○ To supply the electrical interface between the aircraft, APU and ECU The electrical path is provided as follows: ○ The ECU connects through the aircraft wiring to the APU / aircraft interface connection on the aft side of the APU enclosure The electrical components are used, along with the ECU, to perform the sensing and control functions necessary to safely and Electrical Components The APU wiring harness has an electrical connector (P1) that connects to the electronic control unit interface connection which is on the aft side panel of the APU enclosure. The APU wiring harness also has electrical connectors and terminal lugs connect to components as follows: ○ Oil level sensor, Low oil pressure switch ○ Fuel solenoid control valve ○ Start counter, Starter / generator ○ Fuel control unit ○ Ignition exciter ○ Speed sensor ○ Oil temperature sensor, Inlet temperature sensor ○ Exhaust gas temperature sensor