Electrical -System Description
AC Power Schematic

Copyright . The Boeing Company. See title page for details.
July 28, 2000  6.20.7
Electrical -System Description 737 Flight Crew Operations Manual

Electrical Power Controls and Monitoring Generator Drive
The IDGs contain the generator and drive in a common housing, and are lubricated and cooled by a self–contained oil system. An integral electro–mechanical disconnect device provides for complete mechanical isolation of the IDG.
The generator drive (DRIVE) amber caution light is illuminated when low oil is sensed in the IDG. IDG low oil pressure is caused by one of the following:
.
IDG failure

.
engine shutdown

.
IDG automatic disconnect due to high oil temperature

.
IDG disconnected through generator drive DISCONNECT switch. A generator drive disconnect switch is installed. This switch disconnects the generator from the engine in the event of a generator drive malfunction.

Reactivation of the generator may be accomplished only on the ground by maintenance personnel.
AC Voltmeter, Ammeter and Frequency Meter
AC voltage and frequency may be read on the AC voltmeter and frequency meter for standby power, ground power, generator No. 1, APU generator, generator No. 2 and the static inverter. Frequency is indicated only when the generator is electrically excited. The voltage regulator automatically controls the generator output voltage.
Current readings for the two engine IDGs and the APU generator may be read on the AC ammeter.
The TEST position is used by maintenance and connects the voltage and frequency meter to the power systems test module for selection of additional reading points.
Normal indications are:
.
AC voltmeter – 115 +/- 5volts

.
Frequency meter – 400 CPS +/- 10 CPS.

Note:  Normal AC voltmeter indication for the APU generator with the AC busses loaded: 110 - 125 volts.
DC Voltmeter and Ammeter
DC voltage and amperage may be read on the DC voltmeter and ammeter for the battery and each of the three TRs. The standby power and battery bus displays only DC voltage.
Copyright . The Boeing Company. See title page for details.
6.20.8   September 30, 2002

Electrical -System Description
737 Flight Crew Operations Manual
Normal indication is 26 +/-4 volts. During primary charge cycle operation battery
voltage can be as high as 30 +/- 3 volts.
The TEST position is used by maintenance.

Electrical Power Controls and Monitoring Schematic
YA801 -YA803
BUS OFF TRANSFER 
OFF SOURCE 
BUS GEN OFF 

Copyright . The Boeing Company. See title page for details.
September 30, 2002  6.20.9

Electrical -System Description 737 Flight Crew Operations Manual

YA613 - YA722, YA809 - YK104
BUS OFF TRANSFER 
OFF SOURCE 
BUS GEN OFF 

Copyright . The Boeing Company. See title page for details.
6.20.10  September 26, 2003

Electrical -System Description 737 Flight Crew Operations Manual
DC Power System
28 volt DC power is supplied by three TR units, which are energized from the AC transfer busses. The battery provides DC power to loads required to be operative when no other source is available.
On the ground, an amber ELEC light comes on to indicate that a fault exists in DC power system or standby power system. The ELEC light is inhibited in flight.
Transformer Rectifier Units
The TRs convert 115 volt AC to 28 volt DC, and are identified as TR1, TR2, and TR3.
TR1 receives AC power from transfer bus 1. TR2 receives AC power from transfer bus 2. TR3 normally receives AC power from transfer bus 2 and has a backup source of AC power from transfer bus 1. Any two TRs are capable of supplying the total connected load.
Under normal conditions, DC bus 1, DC bus 2, and the DC standby bus are connected via the cross bus tie relay. In this condition, TR1 and TR2 are each powering DC bus 1, DC bus 2, and the DC standby bus. TR3 powers the battery bus and serves as a backup power source for TR1 and TR2.
The cross bus tie relay automatically opens, isolating DC bus 1 from DC bus 2, under the following conditions:
.  
At glide slope capture during a flight director or autopilot ILS approach.
This isolates the DC busses during approach to prevent a single failure
from affecting both navigation receivers and flight control computers

.  
Bus transfer switch positioned to OFF.

In–flight, an amber TR UNIT light illuminates if TR1, or TR2 and TR3 has failed. On the ground, any TR fault causes the light to illuminate.
Battery Power
Single Battery YA801 - YA803
A 24 volt nickel–cadmium battery is located in the electronics compartment. The battery can supply part of the DC system. Battery charging is automatically controlled. A fully charged battery has sufficient capacity to provide standby power for a minimum of 30 minutes. Battery voltage range is 22–30 volts.
　
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