24-31-00
ALL ú ú 08 Page 11 ú Oct 18/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(16)
During APU starting, the APU battery charger is disabled.
(17)
The battery chargers are energized from the 115-volt ac ground service bus through 3-phase thermal circuit breakers.
(18)
The battery charger has three distinct modes of operation, the constant current charge mode, the constant voltage charge mode, and the T-R mode.
(19)
The battery charger design uses a switching mode high frequency (20 kHz) power inversion technique as described below.
(20)
A full wave rectifier circuit converts the three-phase, 400-Hz, 115-volt power input into nominal 270-volt dc power. EMI filtering components consisting of capacitors and inductors are included in the charger input circuit to attenuate audio and radio frequency currents and voltages.
(21)
A half-wave bridge inverter transforms the 270-volt dc power into a 20 kHz pulse-width-modulated quasi-square wave.
(22)
A power rectifier assembly rectifies the half-wave bridge inverter output and provides a 40-kHz train of pulse-width modulated pulses at nominal 28 volts dc.
(23)
Output filtering L-C components is used for EMI emission suppression. An integrated circuit is used in the logic part of the inverter drive circuit.
(24)
A "soft start" circuit is included in the charger basic control loop. When the "soft start" circuit is not in its inhibit mode, the regulator loop controls the converter (input ac power to 270-volt dc power), via the pulse-width modulator and drive circuit, allowing the input current to build up gradually.
(25)
The battery charger is designed for continuous operation with both natural convection and forced air cooling. Blow-through cooling air flow is provided.
(26)
Both the APU and main battery are charged at a constant current to a 12 percent overcharged condition based on time to reach a temperature compensated voltage inflection point.
(27)
The battery charger is capable of recharging a completely discharged battery to a 100 percent state of charge within 75 minutes. The battery charger has two charging modes:
(a)
Constant Current Charging Mode
1) The battery terminal voltage is sensed and if the voltage level falls below 23 |0.1 volt, a constant current charge of 38 amps is initiated. As charging progresses, the battery voltage will rise and will reach a temperature compensated inflection voltage level (which for normal ambient temperature is about 31 volts dc). The inflection voltage level is automatically adjusted for battery temperature as measured by the temperature sensor inside the battery. The time, from start of charge to reach this level, is "memorized" by the charger. The charger then continues to charge the battery for a selected percentage of the time required to reach this temperature-compensated voltage inflection point. After elapse of the overcharge period, the constant current charging cycle is terminated and a constant voltage charge mode is initiated.
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24-31-00
ALL ú ú 04 Page 12 ú Oct 18/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
2) The battery charger will also start a charge cycle independently of the battery terminal voltage whenever power to the charger is initially applied or power to the charger is interrupted for longer than 0.5 seconds.
3) If the battery is at a very low temperature, the voltage temperature-compensated inflection point is shifted upwards to a maximum of 36 volts dc.
(b) Constant Voltage Charge Mode
1) Upon completion of the overcharge cycle the charger will assume a constant voltage mode of operation, holding the voltage level at 27.75 volts. In this mode the charger is also current limited at 38 amps. The purpose of this floating mode is to assure that small load demands on the battery will not deplete its capacity.
2) The model 4-254-03 battery charger operates in a hysteresis mode when it is in the constant voltage mode and at low current (less than approximately 3A). The charger will shutdown and and restart in a cyclic operation. The battery voltage will change in a sawtooth fashion by up to 100 mVp-p. The period of the off-on operation varies with the condition of the battery, but is approximately 10 seconds.
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