An aircraft alternator is the electromechanical device that generates the electrical energy needed to charge the battery and power every light, radio, and instrument once the engine is running. Older airplanes used a generator rather than an alternator to perform the same function, yet the modern alternator is lighter, more dependable, and produces less wear because it runs at greatly reduced speeds while still delivering full output at low engine rpm. Inside the unit a spinning rotor induces alternating current in the stator windings; that alternating current is converted to direct current through a series of diodes called rectifiers, and a voltage regulator continuously adjusts the field circuit so the bus voltage stays at the standard 14.2 volts from idle to full throttle.
What does an aircraft alternator do?
An alternator produces alternating current and this electrical energy powers electrical components and charges the battery. Alternating current is converted to direct current through diodes so that direct current maintains a fully charged battery. Modern aircraft have more demands and thus alternators largely replaced generators. Generators performed the same function, yet generators do not produce rated amperage output at low engine rpm. Output voltage is proportional to rpm, so alternators deliver full amperage from low idle to maximum rpm. A loop maintains equilibrium in the electrical system. Maintaining the battery in good condition ensures that the battery will perform reliably when needed most. The unit creates the only onboard source of electricity once the engine is running. Without its current, radios, lights, fuel pumps, flight instruments, and navigation aids cease, and battery alone lasts minutes.
An aircraft alternator yields alternating current that is converted to direct current, and this direct current powers everything from advanced avionics to subsystems. It prevents the battery from being exhausted and keeps cockpit devices alive, so the pilot is never left with decreasing assets.
How does an aircraft alternator work?
An aircraft alternator works on the principle of electromagnetic induction where a varying magnetic field induces current on coil windings. The rotor, connected to a mechanical drive shaft, spins inside the stator and consists of a centrally located electromagnet energised from the battery via brushes and slip rings. The spinning electromagnet creates the magnetic field that induces alternating current in the stator windings, which have three separate conductor windings to produce three-phase output.
The drive belt conducts rotation from the engine pulley to the alternator pulley. If the belt breaks, alternator failure follows, so the drive coupling also acts as a clutch with an elastomer section that slips in the event of alternator lockup. The typical output from an aircraft AC alternator is 115 V AC at 400 Hz, delivered through a constant speed drive that converts accessory gearbox rotation to a predetermined constant rotation rate. The CSD and generator are housed together in an Integrated Drive Generator unit so that the CSD output is 3-phase 115 V AC at 400 Hz.
A rectifier uses diodes to convert the alternating current to a 28 V DC supply for onboard systems. The strength of the magnetic field is automatically varied by excitement power from the voltage regulator, which mounts on the firewall in the engine compartment and controls the system voltage by controlling the field circuit. A permanent-magnet alternator outputs voltage proportional to rpm, while larger jets use variable-frequency starter generators or multiple alternators. Many light aircraft have only one, yet the SR-20 carries dual alternators for redundancy.
What is the difference between an alternator and a battery in aviation?
The difference between an alternator and a battery in aviation is that the alternator is the active producer of electrical energy, while the battery is a passive reservoir. During normal flight the alternator generates electricity, continuously powering lights and avionics. It is lighter and more efficient than generators. The battery provides stored power for starting the engine, then sits ready as a standby source. Upstream of the battery is the alternator: once the engine is running the alternator recharges the battery so it remains fully charged for the next start or for an emergency.
If the alternator fails you can continue using stored energy from the battery. In that event the battery provides backup electrical power to the airplane, distributing it through the battery bus around the airplane. Because the battery is like a large, rechargeable torch, its supply is limited - enough for twenty to thirty minutes - so the pilot must shed non-critical items and land as soon as practical. Neither unit is directly connected to the electrical systems. Both feed the main bus bar, yet their functions are complementary: the alternator is the primary source while the battery is the safety reserve.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
