Generation - ATPL 021 — Airframe, Systems and Powerplant

The battery is a reserve; the generators are the income. This lesson covers how mechanical rotation becomes electrical power aboard, from the light aircraft's DC generator to the transport's brushless three-phase machines, the constant-speed drives that hold their frequency at 400 Hz, and the converters that reshape power between AC and DC: the transformer, the TRU (transformer rectifier unit) and the static inverter. Everything here runs on the electromagnetism lesson's induction principle: move a magnetic field past a conductor and a voltage appears.

The DC generator

A DC generator is induction put on a shaft: the engine spins windings through a magnetic field, a voltage is induced in them, and a mechanical switch on the shaft (the commutator, a segmented ring the output brushes ride on) flips the connection every half turn so the output current always leaves in the same direction: direct current from a rotating machine. The DC alternator reaches the same result the other way around: it generates AC and rectifies it to DC with diodes (one-way valves for current), which is lighter and kinder to its brushes, and is what most light aircraft actually carry.

Voltage control exists because the engine does not hold still. An induced voltage grows with rotation speed, so a generator geared to an engine that idles at one speed and cruises at another would swing its output all over the scale, boiling the battery at high rpm and abandoning it at low. The regulator holds the output steady the only practical way: by controlling the field current, the small current that creates the machine's magnetic field. Weaken the field as the engine speeds up, strengthen it as the engine slows, and the output sits on 28 volts across the working rpm range.

Reverse-current protection guards the quiet end of that range. Whenever the generator's voltage falls below the battery's (engine at idle, or a failed generator), current would flow the wrong way: from the battery and busbar back into the generator, draining the battery to run the generator as a motor. Between generator and busbar sits a reverse-current device: a relay that senses the direction of flow and opens the connection, or a diode that simply refuses to conduct backwards, exactly the magnetic circuit breaker's logic from the protection half of the electromagnetism lesson, built into every generator circuit.

The starter generator folds two machines into one. A generator and an electric motor are the same machine run in opposite directions: feed it power and it makes torque, feed it torque and it makes power. So on many turbine installations (APUs and turboprops especially) one machine on the engine's gearbox does both jobs: powered from the battery it motors the engine up to starting speed, and once the engine runs, the same machine is driven as the generator. Its purpose is weight and simplicity: one machine, one gearbox drive, no separate starter.

The AC generator

The transport aircraft's AC generator is brushless and three-phase, built as a chain of small machines on one shaft so that no current ever has to cross a rubbing contact. A small permanent-magnet machine on the shaft generates first power for the control unit; the control unit feeds a stationary field winding, inducing current in a rotating exciter winding; that current passes through rectifier diodes mounted on the shaft itself and energises the main rotating field; and the main field, spinning inside the stationary output windings, induces the three output phases. No brushes means nothing to wear, no arcing at altitude, and no maintenance item at the heart of the electrical system. Voltage control works exactly as it did on the DC machine: the generator field current, metered by the control unit, sets the output voltage.

Frequency follows rpm: every full rotation of the field sweeps the output windings through a fixed number of magnetic cycles, so output frequency is directly proportional to the generator's rotation speed (and to its number of magnetic pole pairs). Hold the shaft at constant speed and the output sits at 400 Hz; let the shaft follow the engine and the frequency wanders with the throttle. A generator allowed to do exactly that is called a frequency wild generator: driven straight off the engine with no speed control, its frequency roams with engine rpm, which is acceptable only for consumers that do not care about frequency, above all resistive heating loads such as ice-protection heaters.

Four power sources can drive an aeroplane's AC generator:

Engine — the normal source: each engine's accessory gearbox drives its own generator.
APU — the auxiliary power unit's generator powers the aircraft on the ground and backs up the engine generators in flight.
RAT — the ram air turbine, a small propeller-driven emergency generator that swings out into the airstream when the aircraft has lost its other sources.
Hydraulic — a hydraulic motor can spin a generator, letting one power system carry the other in an emergency.

Constant speed: CSD and IDG

An engine-driven AC generator has a problem the DC machine never had: its frequency follows the shaft, and the field current trick that steadies voltage can do nothing about frequency. The CSD (constant speed drive) solves it mechanically: a hydro-mechanical transmission between the engine's gearbox and the generator that continuously varies its gear ratio, so that whatever the engine does between idle and full thrust, the generator shaft turns at one constant speed and the output holds 400 Hz. The IDG (integrated drive generator) is the modern packaging: the same constant-speed drive and the generator built into one housing on the engine.

The CSD is monitored through its oil, because the drive is a hydraulic machine and its oil is both its blood and its thermometer: the crew's indications are oil temperature (and its rise across the drive) and oil pressure, with high temperature or low pressure announcing a drive eating itself. The drive can be mechanically disconnected in flight: a guarded switch fires a disconnect mechanism that uncouples the drive from the engine gearbox, stopping a failing CSD or IDG before it seizes or shakes itself apart. The decision is one-way: a disconnected drive cannot be reconnected in flight; the generator is lost until maintenance resets the mechanism on the ground, so the disconnect is used when the indications leave no doubt.

The CSD or IDG has its own independent oil system, separate from the engine's: its own reservoir, its own pump, its own cooler, its own quantity. The trap is geographic: the drive lives on the engine, so a CSD or IDG oil leak streaks the same cowling as an engine oil leak and looks identical from the ground. The indications tell them apart: an IDG leak drains the drive's oil (rising drive temperature, falling drive pressure) while the engine's oil quantity sits untouched, and mistaking one for the other misdirects the whole response.

Changing the current's shape

A transport aircraft generates three-phase AC but consumes both AC and DC, so power must be reshaped in both directions. Three converters do it:

Transformer — changes an AC voltage to another AC voltage, up or down, by induction between two windings: no moving parts, nothing but two coils sharing a magnetic core.
TRU (transformer rectifier unit) — a transformer and a bank of rectifier diodes in one box: takes 115-volt AC in and delivers DC out at 28 volts, feeding the DC busbars and charging the battery. Its output is DC.
Static inverter — the TRU's mirror: takes the battery's 28-volt DC and delivers 115-volt, 400 Hz single-phase AC, electronically and with no moving parts (which is what 'static' means). Its output is AC, and its hour comes when the generators are gone and the essential AC instruments must run from the battery.

What the exam asks

The machines' principles and the CSD/IDG behaviours are asked as function-and-consequence questions; the converters as input/output definitions. The overview, following the learning objectives:

Describe the DC generator/alternator principle, voltage control and why it is needed, reverse-current protection, and the starter generator

Describe the brushless three-phase generator; field current controls voltage; frequency vs rpm; the frequency wild generator; the four drive sources (engine, APU, RAT, hydraulic)

Describe the CSD and IDG, the monitored parameters, the consequences of an in-flight disconnect, and the independent oil system's engine-leak lookalike

State the functions and outputs of the transformer, the TRU and the static inverter

The DC generator

The AC generator

Check you understood

Constant speed: CSD and IDG

Changing the current's shape

What the exam asks