AC Generators, CSD and IDG

An alternator is an AC generator: a machine in which a magnetic field and a set of conductors move relative to each other, so that an alternating electromotive force (EMF) is induced in the conductors. On transport aeroplanes, engine-driven generators of this kind are the primary source of electrical power in flight. The battery is kept for starting and as an emergency reserve, and the APU generator serves mainly on the ground and as a backup in flight.

Most transport aeroplanes generate three-phase AC at 115 V between each phase and neutral, 200 V between phases and a constant 400 Hz, and obtain their 28 V DC from it through transformer rectifier units. Holding that frequency while engine speed moves between idle and take-off thrust is the task of the constant speed drive (CSD). Built into one package with the generator it becomes the integrated drive generator (IDG) fitted to the engines of the Airbus A320 family, the Boeing 737 NG and the Embraer E-Jets. How generators are regulated, protected and connected is covered in generator control, protection and paralleling, and how their power reaches the loads in electrical power distribution.

Why aircraft generate AC

Alternators replaced DC generators on large aeroplanes for several reasons. The whole output of a DC generator must pass through a commutator and brushes, which wear, spark and behave worse in the thin air at altitude; an alternator delivers its output from fixed terminals. AC machines give more power for their weight, AC voltage is easily stepped up or down by transformers, DC is easily obtained from AC through transformer rectifiers, and three-phase AC motors are simpler than DC motors. A higher system voltage also means less current for the same power, and so lighter cables.

The frequency of 400 Hz, rather than the 50 or 60 Hz of mains supplies, is chosen because transformers, motors and inductors can be made much smaller and lighter at a higher frequency. The penalty, greater losses when power travels a long way, does not matter over the short cable runs inside an aeroplane. AC electrical theory covers RMS values, phase angle and power factor.

Alternator principles

The frequency of an alternator depends only on its speed and its number of magnetic poles:

f = (number of poles ÷ 2) × (rpm ÷ 60)

An eight-pole machine turning at 6,000 rpm therefore produces (8 ÷ 2) × (6,000 ÷ 60) = 400 Hz. Any change of speed changes the frequency, which is why a constant-speed drive is needed wherever a constant frequency is wanted. The output voltage depends on the strength of the magnetic field, and a regulator holds it steady by varying the field current as load and speed change (electromagnetism and induction explains why).

Aircraft alternators have three output windings spaced 120° apart and joined at one end, a star connection. The common point, the neutral, is earthed to the airframe and carries any out-of-balance current when the phases are unevenly loaded. Each phase gives 115 V to neutral, and the voltage between two lines is √3 × 115, about 200 V; in a star connection the line current equals the phase current.

An alternator is rated in kilovolt-amperes (kVA), apparent power, not in kilowatts. Its windings heat with the full current whatever the phase angle of the load, so kVA measures what the machine can carry; the true power in kW depends on the power factor of the loads.

Rotating-field and brushless alternators

In a rotating-armature alternator the output windings turn inside a fixed field and the output leaves through slip rings, a design used only for small ratings. Practical aircraft machines are rotating-field alternators: the DC field winding is on the rotor and the output windings are on the stator. The full output current then flows from fixed terminals, and only the much smaller field current has to reach the rotor.

In a brushed machine that field current passes through slip rings, continuous conducting rings on the shaft, against which spring-loaded carbon brushes held in a brush block bear. Unlike a commutator, slip rings do not reverse the connection as the shaft turns; they simply carry current to and from a rotating part. The brushes still wear and need inspection. The same slip ring and brush block arrangement carries current to the heating elements of electrically de-iced propellers.

The brushless alternator removes even these. A small exciter, a second alternator on the same shaft, generates AC in windings on the rotor. Diodes mounted on the rotor, the rotating rectifier, convert it to DC and feed it straight to the main field winding. With no brushes or slip rings there are no rubbing contacts to wear or arc, which improves reliability, reduces maintenance and suits operation at high altitude. The control unit regulates the output by varying the current in the exciter's field winding, which is fixed to the stator.

A permanent magnet alternator (PMA) takes its field from permanent magnets, so it needs no excitation current and produces output whenever it turns. That makes it a self-contained supply. Each A320 engine's FADEC has its own PMA, driven by the accessory gearbox, which keeps engine control powered independently of the aircraft network once the engine is running; if it fails, the FADEC switches to aircraft 28 V DC. On the E190-E2 the PMA takes over from aircraft power above 50 % N2. A light aeroplane's alternator, by contrast, has a wound field that must be excited with current from the battery, which is why it stays dead when the battery is flat.

Frequency-wild generation

A frequency-wild alternator is driven straight from the engine gearbox, with no constant speed drive, so its frequency rises and falls with engine speed. That suits resistive loads, such as the heater mats of electrical ice protection, whose current does not depend on frequency. Frequency-wild machines cannot be paralleled, since their frequencies differ, and where constant-frequency power is needed their output is rectified to DC and, if necessary, inverted back to AC.

The constant speed drive

A constant speed drive (CSD) is a hydraulic transmission between the engine accessory gearbox and the generator. Engine speed varies from idle to take-off thrust; the CSD varies its ratio continuously so that the generator turns at constant speed and its output stays at 400 Hz. On the Boeing 737 the IDG keeps generator speed constant throughout the normal operating range of the engine. On the A320 the accessory gearbox, and with it the IDG, is driven by the high-pressure spool (N2).

The drive runs in its own oil, which lubricates and cools it, and that oil is in turn cooled by fuel. On the A320 some high-pressure fuel flows through the IDG heat exchanger and returns, through a FADEC-controlled fuel return valve, to the outer wing tank; on the 737 the IDG oil passes through a fuel/oil heat exchanger on the engine. A constant frequency matters for more than tidiness: only machines at the same constant frequency can be paralleled, and the speed of AC motors follows the supply frequency.

The characteristic CSD failures show as low oil pressure and high oil temperature. On the A320 the IDG FAULT light comes on when the oil outlet temperature exceeds 180 °C or the oil pressure is low; the low-pressure part is inhibited below 14 % N2 so that it does not trigger during engine start. The ECAM ELEC page shows the temperature pulsing as an advisory above 147 °C, and the IDG disconnects itself automatically at 200 °C.

The integrated drive generator

An integrated drive generator (IDG) combines the CSD and the alternator in a single unit mounted on the engine accessory gearbox, delivering three-phase 115/200 V at 400 Hz. Each generator has its own generator control unit, which regulates its output and connects it to its bus. Switching a generator off electrically does not stop the drive: on the A320, selecting the GEN pushbutton OFF de-energises the generator field and opens its line contactor, but does not affect the IDG.

The first-generation E190 has 40 kVA IDGs, and a static inverter feeding its AC standby bus.

Drive disconnect

A drive that leaks, overheats or otherwise fails can damage itself and threaten the engine, so an IDG has a disconnect: an electrically triggered mechanical device that separates the drive from the engine gearbox so that the unit stops turning. It can be operated in flight, but reconnection is a ground task.

• Airbus A320. A guarded IDG pushbutton on the overhead ELEC panel. It must not be held for more than 3 s, or the disconnection mechanism may be damaged, and it is used only with the engine running or windmilling; otherwise the IDG is damaged at the next engine start. The FAULT light goes out once the IDG is disconnected, and only maintenance can reconnect it.
• Boeing 737 NG. A guarded generator drive DISCONNECT switch, effective only with electrical power available and the engine start lever at IDLE. The amber DRIVE light shows IDG low oil pressure, caused by an IDG failure, engine shutdown, automatic disconnection for high oil temperature or use of the switch. The IDG cannot be reconnected in the air.
• Embraer E190-E2. The IDG knob is held at DISC for at least 1 s but less than 3 s. The amber IDG light at an oil temperature of 168 ± 5 °C calls for a manual disconnect; the IDG disconnects itself at 185.6 °C or above, or if its shaft fails. Only maintenance can reset it, on the ground.

Variable speed constant frequency

A variable speed constant frequency (VSCF) system does electronically what the CSD does hydraulically. The generator is driven directly and runs frequency-wild; its output passes through an electronic converter, a rectifier followed by an inverter, housed with the control functions in a generator converter control unit (GCCU), which delivers constant 115/200 V 400 Hz three-phase power. Doing without the hydraulic drive improves reliability and reduces maintenance, and the electronics can be placed away from the heat of the engine.

Other AC sources

The APU generator usually needs no CSD: an APU such as the E190-E2's is a constant-speed gas turbine that governs its own speed, so its generator already runs at constant frequency. On the A320 it produces the same output as an engine generator and can replace either or both of them at any time, in flight or on the ground.

When all main generation is lost, the emergency source depends on the type. On the A320 the ram air turbine (RAT) extends automatically if AC BUS 1 and AC BUS 2 are both lost above 100 kt. It pressurises the blue hydraulic system, whose hydraulic motor drives the emergency generator. For the 8 s or so before that generator comes on line, the batteries supply the essential network, with the static inverter providing AC above 50 kt. The E190-E2's RAT deploys automatically when no AC bus is powered.

Alternator failure is handled automatically on a transport aeroplane: the generator control unit trips the faulty machine off its bus, and the bus tie logic restores the supply from the APU generator or the other engine generator. In a light aeroplane with a single alternator, a failure shows as a discharge on the centre-zero ammeter or a zero reading on the loadmeter, usually with a low-voltage light, and the battery then carries the whole load until it is exhausted.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.