737 MAX Flight Crew Operations Manual
Electrical -
System Description
6.20.8
MN-FLT-OH-201
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 buses 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
Dual Battery
Two 24 volt nickel–cadmium batteries, main and auxiliary, are located in the
electronics compartment. The batteries can supply part of the DC system. The
auxiliary battery operates in parallel with the main battery when the battery is
powering the standby system. At all other times, the auxiliary battery is isolated
from the power distribution system. Battery charging is automatically controlled.
Two fully charged batteries have sufficient capacity to provide standby power for
a minimum of 60 minutes. Battery voltage range is 22–30 volts.
DC buses 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. An amber BAT DISCHARGE light comes on when excessive battery
discharge is detected.
March 1, 2021
