In the Boeing 737, the primary electrical power is generated by two engine-integrated drive generators (IDGs), which provide three-phase, 115-volt, 400 Hz alternating current (AC).

  • Normal operation: Each IDG powers its respective bus system.
  • Single IDG operation: An IDG can also supply essential and non-essential loads from the opposite side’s bus system if one IDG is inoperative.

Additional power sources:

  • Transformer rectifier (TR) units and the main battery/battery charger provide DC power.
  • The main and auxiliary batteries supply backup power for the AC and DC standby systems.
  • The APU generator can power both AC transfer buses on the ground or in flight.

Key principles of the 737 electrical system:

  1. AC power sources are not paralleled.
  2. Connecting a power source to a transfer bus automatically disconnects the existing source.

The electrical power system is divided into three main categories:

  • AC Power System
  • DC Power System
  • Standby Power System

AC POWER SOURCES

The 3 phase, 115/200 volts (nominal) at 400 Hz.

  • LH IDG (IDG 1) (75 KVA) — ENG 1
  • RH IDG (IDG 2) (75 KVA) — ENG 2
  • APU starter-generator (90 KVA below 32,000 ft, and goes down to 66 KVA at 41,000 ft)
  • External power (90 KVA)

The AC power system design prevents two sources to the same load at the same time. However, one AC power source can supply power to both transfer buses through the bus tie breakers (BTBs).

  • AC transfer Bus 1 powered by IDG 1
  • AC transfer Bus 2 powered by IDG 2
  • The APU can power either or both buses through the BTBs.

Static Inverter

  • The static inverter supplies single phase, 115V AC output to the AC standby bus.
  • The static inverter converts 24 volt DC power from the battery to 115V AC power.
  • The AC standby bus usually gets power from the AC transfer bus 1.
  • The static inverter remote control circuit breaker (RCCB) (EE) connects battery power to the static inverter.

DC POWER SOURCES

  • Three transformer rectifier units (TRUs) change 115V AC to 28V DC.
    • TRU 1 < AC transfer Bus 1 > DC Bus 1
    • TRU 2 < AC transfer Bus 2 > DC Bus 2
    • TRU 3 < AC Bus 2 (AC Bus 1) > BAT Bus
  • Main battery > BAT Bus < Main battery charger < DC Bus 2 / GND SVCE BUS 2
  • Auxiliary battery > BAT Bus < Auxiliary battery charger < DC Bus 1 / GND SVCE BUS 1
  • HOT BATTERY BUS

Batteries

  • Nominal 24V DC: range is 22–30 volts.
  • 20 cell nickel-cadmium battery with a 48 amp-hour capacity.
  • The battery supplies power for APU starting and is a standby power source if all other power supplies do not operate.
  • Some A/C – Two batteries – Main and Auxiliary.
  • The auxiliary battery helps the main battery supply standby power only.
  • The battery charger has these two basic modes of operation:
    • Battery charge mode (constant current, then constant voltage)
    • Transformer rectifier mode (constant voltage)
  • With full charge, the batteries supply a minimum of 60 minutes (30 minutes if one battery) of standby AC and DC power.
  • The ground crew call horn will sound a continuous tone 2 minutes after a battery discharge condition is detected. A battery discharge condition is indicated when the airplane is on the ground with the battery switch selected on and the battery charger is powered off.

PROTECTION

  • Left generator control unit (GCU 1) – LH IDG
    • Monitor power quality and Control generator control breaker (GCB 1).
  • Right generator control unit (GCU 2) – RH IDG
    • Monitor power quality and Control generator control breaker (GCB 2).
  • APU generator control unit (AGCU)
    • Monitor power quality and Control auxiliary power breaker (APB).
  • Bus power control unit (BPCU)
    • BPCU works with the GCU1,GCU2, and AGCU.
    • Monitor external power quality and Control external power contactor (EPC).
    • Control bus tie-breakers (BTB1 & BTB2) position.
  • Standby power control unit (SPCU) – P6 panel
    • Control distribution of DC power.
  • Start converter unit (SCU)
    • The APU GCU and the starter converter unit (SCU) work together to control and protect the APU starter-generator.

CONTROL PANELS

These modules on the P5 panel give manual control of the electrical power system:

ELECTRICAL METERS, BATTERY, AND GALLEY POWER MODULE

  • BAT DISCHARGE Light: The amber BAT DISCHARGE light gives indication of battery discharge. The BAT DISCHARGE light temporarily comes on during an APU start with battery power.
  • TR UNIT Light: The amber TR UNIT light gives indication of transformer rectifier unit (TRU) failure. The TR UNIT light comes on for any one of these reasons:
    • Any TRU fails on the ground, or
    • TRU 1 fails in flight, or
    • TRU 2 and TRU 3 fail in flight.
  • ELEC Light: The ELEC light gives indication that the DC system or standby power system has a failure. You use BITE to find the failure. The amber ELEC light comes on only when the airplane is on the ground.
  • MAINT Switch: You use the maintenance switch during BITE. The BITE operates only on the ground. The maintenance switch is a momentary push-button switch.
  • Battery Switch: The main battery energizes these buses and components when you put the battery switch to the ON position:
    • Switched hot battery bus
    • Battery bus
    • Static inverter
    • AC standby bus
    • DC standby bus
    • P5-13 alphanumeric display

The battery switch is a two-position switch. The cover, when down, is a guard for the switch in the ON position. You must lift the cover before you move the switch to the OFF position.

  • DC Meters Selector: Selects DC source for DC voltmeter and DC ammeter indications. TEST – used by maintenance for BITE.
  • AC Meters Selector: Selects AC source for BITE for AC voltmeter, AC ammeter and frequency meter indications. TEST – used by maintenance for BITE.
  • CAB/UTIL Switch:
    • OFF – removes electrical power, (for BITE), from galley and cabin equipment systems including:
      • all 115V AC galley busses
      • left & right recirculation fans
      • fwd and aft door area heaters
      • drain mast heaters
      • lavatory water heaters
      • logo lights
      • potable water compressor
      • 115V AC shaver outlets when installed
      • LED cabin lighting
    • ON – supplies power to galley and cabin electrical systems.
  • IFE/PASS SEAT Switch:
    • OFF – removes electrical power from installed components of the passenger seats, in-flight entertainment systems, and other power systems including:
      • 115V AC audio entertainment equipment
      • 115V AC video entertainment equipment
      • cabin telephone equipment
      • FAX machine
      • 28V DC video equipment and passenger seat electronic outlets.
    • ON – supplies electrical power to the installed components of the passenger seats, in-flight entertainment systems, and other power systems.

GENERATOR DRIVE AND STBY PWR MODULE

  • DRIVE Light: The amber DRIVE light comes on when the IDG oil pressure is less than the operation limit or when there is an underfrequency condition while the engine operates.
  • Generator Drive Disconnect Switch: The generator drive disconnect switch operates the disconnect mechanism for its (IDG) when the related engine start lever is in the idle position. This removes engine accessory gearbox power from the IDG. There is one switch for each IDG. Each switch is spring-loaded to the NORMAL position. The DISCONNECT position is a momentary position. The guard holds the switch in the NORMAL position. You must lift the guard to move the switch.
  • STANDBY PWR OFF Light: The amber STANDBY PWR OFF light comes on when any of these buses do not have power:
    • AC standby bus
    • DC standby bus
    • Battery bus
  • Standby Power Switch: The standby power switch gives you manually control of the AC and DC standby power bus sources. The switch is a three-position switch. The switch is usually in the AUTO position. The guard holds the switch in the AUTO position.

AC SYSTEMS GENERATOR AND APU MODULE

  • GROUND POWER AVAILABLE Light: The GRD POWER AVAILABLE light is on when external AC power is connected and the quality is good. The light is bright blue when on.
  • Ground Power Switch: You use the ground power switch to control external power to the AC transfer buses.
    • Three position switch, spring-loaded to neutral.
  • TRANSFER BUS OFF Light: The amber TRANSFER BUS OFF light comes on when the AC transfer bus does not have power. There is one light for each AC transfer bus.
  • SOURCE OFF Light: The amber SOURCE OFF light supplies indication that an AC transfer bus is not energized by the selected source. There is one light for each transfer bus.
    • The left SOURCE OFF light is related to these sources:
      • IDG 1 (GEN 1 switch)
      • APU (left APU GEN switch)
      • External power (ground power switch)
    • The right SOURCE OFF light is related to these sources:
      • IDG 2 (GEN 2 switch)
      • APU (right APU GEN switch)
      • External power (ground power switch)
  • The SOURCE OFF light does not indicate that the AC transfer bus is de-energized. In flight, the left SOURCE OFF light comes on when GCB 1 trips. However, the bus transfer function lets IDG 2 power AC transfer bus 1. These are the examples:
    • IDG1 fails
    • The bus transfer function closes the BTBs
    • Both AC transfer buses get power from IDG2
  • GEN OFF BUS Light: The blue GEN OFF BUS light comes on when the engine generator control breaker (GCB) is open. This shows that the IDG is not a power source in use. The light goes off when the GCB closes.
  • APU GEN OFF BUS Light: The blue APU GEN OFF BUS light shows that the APU is running, but its generator is not a power source in use. The light is on when the APU is running and the auxiliary power breakers (APBs) is open. The light goes off when the APB closes or you shut down the APU.
  • Engine Generator Switches: The engine generator switches give manual control for IDG power source selection. Each switch is a three-position switch and is spring-loaded to the center (neutral) position. The ON and OFF positions are momentary positions. When you put a generator switch momentarily to the ON position, you make that IDG the power source for its AC transfer bus. If IDG power quality is good, the electrical power system first removes the present power source (open its generator breaker). Then, the generator breaker for the IDG closes and the IDG now supplies power.
  • APU Generator Switches: The APU engine generator switches give manual control for APU generator power source selection. There are two switches because there are two bus tie breakers (BTBs). The switches operate like the engine generator switches.
  • Bus Transfer Switch: The bus transfer switch gives you manual control of the BTBs, the DC BUS TIE RELAY, and the TRU 3 TRANSFER RELAY. The switch has two positions (AUTO and OFF). The switch is usually in the AUTO position. A cover is a guard for the switch in the AUTO position. In the AUTO position, the BTBs, the DC BUS TIE RELAY, and the TRU 3 TRANSFER RELAY work automatically as necessary. In the OFF position, the DC BUS TIE RELAY and the TRU 3 TRANSFER RELAY cannot energize. The operation of the BTBs when the switch is in the OFF position depends on the electrical power source conditions before you move the switch.
    • In the AUTO position, the BTBs and the DC bus tie relay operate automatically as necessary.
    • In the OFF position, the BTBs open and isolate the AC transfer buses from each other if one IDG is supplying power to both AC transfer buses. The DC bus tie relay will also open. This isolates DC bus 1 and DC bus 2 from each other. The position also resets the BTB trip circuits.

Ground Service

  • For ground servicing, a ground service switch is on the forward attendant’s panel.
  • The switch provides ground power directly to the AC ground service buses for utility outlets, cabin lighting and the battery charger without powering all airplane electrical buses (AC transfer buses).
  • The ground service switch is a momentary push button and is overridden when both AC transfer buses are powered. — Illuminated (white).

External Power Receptacle

  • EXTERNAL POWER CONN Light Illuminated (amber)
  • NOT IN USE Light Illuminated (white)
  • PARKING BRAKE Light Illuminated (red)
  • NWW (Nose Wheel) Light Switch

BITE

These electrical power components have Built-In Test Equipment (BITE):

  • Electrical meters, battery, cabin utility, and in-flight entertainment power module – on panel.
    • Meter Selector must be in TEST position to use BITE.
  • Generator control units (GCUs) – front face.
  • Bus power control unit (BPCU) – front face.

IDG

The IDG takes the variable input speed and changes it into a constant speed of 24,000 RPM for its internal AC generator.

Manual IDG Disconnect

You use the generator drive disconnect switch to do a manual disconnect of the IDG. When you put the switch to the disconnect position, power goes to a solenoid on the IDG if the engine start lever is in the idle position. The solenoid energizes and disconnects the CSD from the input drive shaft. You can reset the manual disconnect at the IDG. The engine must not be turning when you do the reset.

A manual disconnect is usually done when the DRIVE light comes on. The DRIVE light shows that the IDG oil pressure is less than the minimum operating limit or there is an underfrequency condition. A switch cover and the start lever in the idle position prevent an accidental disconnect. The IDG input shaft also has a shear section that breaks when torque is too high. This prevents the engine accessory gearbox from being damaged because of an IDG failure.

Thermal IDG Disconnect

The IDG also has a thermal mechanism that automatically disconnects the CSD section of the IDG if the IDG oil temperature gets too hot. Thermal disconnect occurs at 184°C. You must remove and disassemble the IDG to reset a thermal disconnect.

IDG Oil System

Inside the IDG are pumps that move the oil through the IDG and out through two external air/oil heat exchangers.

There is a spring actuated vent valve on the IDG to keep constant internal pressure in the housing. Before you service the IDG, push the vent valve to relieve pressure built up in the case. The case pressure relief valve prevents IDG case rupture if there is a high pressure in the IDG case. The valve starts to operate at 30 to 40 psi and is fully open at 60 psi.

Oil from the IDG goes through the left and right air/oil heat exchangers. Air from the engine fan goes through the air/oil heat exchangers to decrease the temperature of the IDG oil. The air/oil heat exchangers have an internal bypass which opens if the air/oil heat exchanger becomes clogged. The amber DRIVE light comes on if the IDG oil pressure decreases to less than the minimum operating limit of 165 psi. The DRIVE light also comes on for an under frequency fault. The GCU controls this light.

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