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

Note:  Normal AC voltmeter indications for the APU generator with the AC buses loaded are: 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. Standby power and the battery bus will display
only DC voltage.

Normal indication is 26 +/- 4 volts.
The TEST position is used by maintenance.

Electrical -System Description
737 Operations Manual
Electrical Power Controls and Monitoring Schematic
-0+  2 
LOW OIL 
-50 DC +50  C S 4 0  PRESSURE 
380  FR Q  420  HIGH OIL 
TEMP 
DISCONNECT 
DRIVE TEMP 
20  120 110 AC  ~T  RISE 
DC V LTS 0 40  0  VO TS 30 
IN 
DRIVE CAN BE 
RECONNECTED 
ONLY ON 
GRD  10 20RISE 
BAT  TR 1 TR 2  GEN 1 APU GEN GEN 2  0  30 120 80IN 
40  160 
BUS BAT  TR 3  PWR GRD  INV  OIL TEMP GEN DRIVE 
STBY PWR  TEST  STBY PWR  TEST  C 
FAN 
AIR 

GEN
GEN 2
DRIVE
TRANSFER BUS OFF
VOLT
BUS
REG
OFF
GEN OFF BUS 100
50 150 OFF
FROM
AC
GENERATOR 1
0 AMP RES 200
ON GEN 2
GENERATOR BUS 1 GENERATOR BUS 2
BUS TRANS
A F
O
U F
T O
115V AC TRANSFER 115V AC TRANSFER BUS 1 BUS 2
TR2
DC BUS 2
CONDITION:  ENGINE DRIVEN GENERATORS ON AC BUSSES
Copyright . The Boeing Company. See title page for details.
6.20.8  June 7, 2002

Electrical -System Description 737 Operations Manual
DC Power System
28 volt DC power is supplied by three TR units, which are energized from the AC transfer busses and main bus 2. The battery provides 28V DC power to loads required to be operative when no other source is available.
Transformer Rectifier Units
The TRs convert 115 volt AC to 28 volt DC, and are identified as TR1, TR2, and TR3.
TR1 and TR2 receive AC power from transfer bus 1 and transfer bus 2, respectively. TR3 receives AC power from main bus 2.
Under normal conditions, DC bus 1 and DC bus 2 are connected in parallel via the TR3 disconnect relay. In this condition, TR1 and TR2 are each powering DC bus 1 and DC bus 2. TR3 powers the battery bus and serves as a backup power source for TR1 and TR2.
The TR3 disconnect 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.

Battery Power
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.
DC busses powered from the battery following a loss of both generators are:
.
battery bus

.  
DC standby bus

.  
hot battery bus

.  
switched hot battery bus

The switched hot battery bus is powered whenever the battery switch is ON.
The hot battery bus is always connected to the battery. There is no switch in this circuit. The battery must be above minimum voltage to operate units supplied by this bus.
Electrical -System Description 737 Operations Manual

Battery Charger
The purpose of the battery charger is to restore and maintain the battery at full electrical power. The battery charger is powered through AC ground service bus with provisions for automatic switching to main bus 2 when the ground service bus is unpowered.
DC Power Receptacle
An auxiliary 28V DC power receptacle is provided near the battery in the electronic compartment. A placard located adjacent to the receptacle gives complete instruction for connecting external DC power. With external DC power connected, the battery is paralleled with the DC external power source and the external power source will power all circuits normally supplied by the battery. In the event that the airplane battery is depleted, the APU can be started using DC external power.
Copyright . The Boeing Company. See title page for details.
　
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