B737 Electrical System Overview
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Questions and Answers

How many generators are there?

3

Describe the 3 generators.

45 KVA generators, 3 phase, 115 volt, 400 cycle A/C power

When will the generator relay open? (Select all that apply)

  • Over Frequency (correct)
  • Associated Generator Switch moved to off (correct)
  • Fault (correct)
  • Voltage too high (correct)
  • Voltage too low (correct)

    • Which relay actually connects the generator to the generator bus?

    <p>Generator Breaker</p> Signup and view all the answers

    What drives the CSD?

    <p>N2</p> Signup and view all the answers

    What is CSD?

    <p>Constant Speed Drive</p> Signup and view all the answers

    What cools the oil in the CSD?

    <p>Engine Fan Airflow</p> Signup and view all the answers

    Can you connect a disconnected CSD in flight?

    <p>False</p> Signup and view all the answers

    What is the load limit for the engine-driven generator?

    <p>125 amps</p> Signup and view all the answers

    What normally powers TR3?

    <p>AC Main Bus 2</p> Signup and view all the answers

    How many systems can the APU power on the ground? In the air?

    <p>2 on the ground, 1 in the air</p> Signup and view all the answers

    Does AC power from the engine generators automatically apply to the generator buses when the power is of good quality?

    <p>False</p> Signup and view all the answers

    With ground electrical power connected to the aircraft, movement of the GRD PWR switch to ON will _____________.

    <p>apply ground power to both left and right AC electrical systems</p> Signup and view all the answers

    Inflight, with the BUS TRANSFER switch in AUTO, a failure of the #1 engine generator will result in a complete transfer of power from the #2 generator to the #1 AC electrical system.

    <p>False</p> Signup and view all the answers

    What powers TR1?

    <p>Transfer Bus 1</p> Signup and view all the answers

    What powers TR2?

    <p>Transfer Bus 2</p> Signup and view all the answers

    What powers TR3?

    <p>Main Bus 2</p> Signup and view all the answers

    When does the TR3 disconnect relay open? (Select all that apply)

    <p>Auto pilot ILS approach</p> Signup and view all the answers

    How will you know that a TR has failed?

    <p>You might not until you are on approach because they work in parallel.</p> Signup and view all the answers

    How long will the battery supply backup DC power?

    <p>30 minutes</p> Signup and view all the answers

    How many times can you expect to start the APU on the battery using backup DC power?

    <p>1</p> Signup and view all the answers

    What busses and circuits are powered when the battery switch is on?

    <p>Switched Hot Battery Bus - providing power to the hot battery bus, battery bus, and APU start circuit</p> Signup and view all the answers

    Does the ground power charge the battery?

    <p>True</p> Signup and view all the answers

    Other than ground power, what charges the battery?

    <p>AC Main Bus 2</p> Signup and view all the answers

    How many amps should you see on the ammeter right after APU start?

    <p>High charge rate at first, slowly coming down to 16, and pulsing for about 2 minutes until the battery is charged</p> Signup and view all the answers

    In flight, will both standby busses remain powered during a loss of all generators?

    <p>True</p> Signup and view all the answers

    On the ground (classic), will both standby busses remain powered during a loss of all generators?

    <p>False</p> Signup and view all the answers

    With normal power on the aircraft, what powers the Hot Battery Bus?

    <p>The Battery Charger</p> Signup and view all the answers

    How many volts will you see on the voltmeter if the switch is in the BAT position?

    <p>28-30 if the battery is being charged, 24 if it is not being charged</p> Signup and view all the answers

    What does a 0 reading from a TR mean on the metering panel?

    <p>A malfunction</p> Signup and view all the answers

    Will BAT BUS or STBY PWR display on the DC ammeter?

    <p>False</p> Signup and view all the answers

    What does the Gen off bus blue light mean?

    <p>Generator Breaker is open</p> Signup and view all the answers

    What is a TR?

    <p>Transformer Rectifier</p> Signup and view all the answers

    What three things does the blue light tell you?

    <p>APU is up to speed, Power is good, Not being used for anything</p> Signup and view all the answers

    Bus Off light means what?

    <p>Specified Generator bus is not powered</p> Signup and view all the answers

    How long can we operate with the low oil quantity?

    <p>10 hours</p> Signup and view all the answers

    When does the APU door start to open?

    <p>When we place the APU switch to on</p> Signup and view all the answers

    Study Notes

    Electrical System Overview

    • The B737 Classic features a total of 3 generators to provide power.
    • Each generator has a capacity of 45 KVA and produces 3-phase, 115-volt, 400-cycle A/C power.

    Generator Relay Functions

    • Generator relays will open during:
      • Over frequency conditions
      • Insufficient voltage
      • Excessive voltage
      • System faults
      • When the associated generator switch is turned off.

    Connecting Generators

    • The connection between the generator and the generator bus is made via the Generator Breaker.

    Constant Speed Drive (CSD)

    • The CSD maintains a constant speed and is driven by the N2 engine speed.
    • Engine fan airflow is responsible for cooling the oil within the CSD.
    • CSDs cannot be reconnected in-flight; ground maintenance is required for any disconnection.

    Engine Driven Generator Limits

    • The load limit for the engine-driven generator is 125 amps.

    Auxiliary Power Unit (APU) Capabilities

    • The APU can power 2 systems on the ground and 1 system in-flight.
    • AC power from engine generators must be manually selected to ON for it to be applied to the generator buses.

    Ground Power Procedures

    • Connecting ground electrical power and toggling the GRD PWR switch to ON applies ground power to both AC electrical systems.
    • During a failure of the #1 engine generator, power does not automatically transfer to the #1 AC electrical system.

    Transfer Buses

    • TR1 is powered by Transfer Bus 1.
    • TR2 is powered by Transfer Bus 2.
    • TR3 gets its power from Main Bus 2.

    TR Disconnect Relay Operations

    • The TR3 disconnect relay opens during:
      • Flight Director ILS approach
      • Autopilot ILS approach.
    • TRs generally operate in parallel, making failure detection difficult until approach.

    Battery and Power Management

    • The battery provides backup DC power for approximately 30 minutes.
    • The APU can be started once using backup DC power.
    • The switched hot battery bus powers circuits related to the battery bus and APU start circuit when the battery switch is on.
    • Ground power charges the battery automatically; AC Main Bus 2 also charges the battery.

    APU Start Conditions

    • Right after APU start, the ammeter shows a high charge rate that decreases to about 16 amps, pulsing until the battery is fully charged.

    Standby Buses

    • In-flight, both standby buses remain powered during a total generator loss from the hot battery bus.
    • On the ground, if the standby switch is in AUTO, both standby buses won't remain powered during generator failure; selecting BAT powers them.

    Voltage and Meter Readings

    • When the switch is in the BAT position, the voltmeter reads 28-30 volts if charging, and 24 volts if not.
    • A reading of 0 on a TR indicates a malfunction.
    • BAT BUS or STBY PWR does not display on the DC ammeter.

    Indicator Lights

    • The blue "Gen off bus" light indicates the generator breaker is open.
    • The "Bus Off" light indicates a specified generator bus is not powered.
    • A blue light signals that the APU is operational, power quality is good, and it is not powering any systems.

    APU Operational Conditions

    • The APU can operate with low oil for up to 10 hours.
    • The APU door begins to open automatically when the APU switch is turned to ON.

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    Description

    Dive into the essential electrical systems of the B737 Classic, understanding the role of generators, relay functions, and auxiliary power units. This quiz covers generator capacities, operational limits, and maintenance requirements for the constant speed drive (CSD). Test your knowledge on the practical aspects of aircraft electrical systems.

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