Electrical power systems and basic electrics
Scope
This domain covers DC and AC generation, distribution, protection, batteries and emergency power. Expect bus architecture, generator control, battery characteristics and source paralleling.
DC systems
- Typical 28 V DC. Batteries are lead-acid or NiCd; NiCd can thermally run away and needs temperature monitoring.
- A reverse-current cut-out prevents the battery discharging through the generator; an overvoltage relay trips the generator.
- Bus bars include main, essential, emergency and battery buses. Contactors and bus ties connect them; circuit breakers protect wiring.
- A transformer-rectifier unit (TRU) converts AC to DC; a static inverter converts DC to AC for essential AC loads.
- Ground power is connected through an external power contactor with polarity and voltage checks.
AC systems
- Typical 115 V AC, 400 Hz, three-phase. An integrated drive generator (IDG) or constant-speed drive keeps frequency constant; the IDG can be disconnected once in flight and cannot normally be reconnected.
- Paralleling requires equal voltage, frequency and phase; load sharing is by voltage regulator and governor.
Emergency power
- A RAT, standby generator, battery or APU generator powers essential buses after loss of main power.
Worked example
A 24 V, 40 Ah battery supplies a 20 A essential load. Ideal endurance = 40/20 = 2 h. At high discharge rate and low temperature, usable capacity falls, so expect less. NiCd batteries also suffer voltage depression if overcharged.
Traps
A generator trip does not mean the battery is flat. Do not parallel AC sources without synchronising. A CSD disconnect is one-shot. Circuit breakers protect cables, not equipment.