Alternators are lighter, smaller and more efficient than generators, yet they furnish higher output at lower engine rpm. With fewer moving parts running at greatly reduced speeds, they incur less wear and tear while demanding less maintenance. Generators remain heavier, require more attention, and cannot match the low-speed performance of today's piston-engine installations. While generators retain specific advantages, the industry standard now favors alternators for their dependability and ability to power the entire electrical system even when the engine is idling.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between an alternator and a generator in aviation?
The difference between an alternator and a generator is that the alternator has no commutator. Inside a generator the magnetic field is stationary while the armature winding spins. The alternator reverses this geometry so that the magnetic field rotates inside a fixed stator. Because the generator must pass its full electrical output across a carbon-brush/commutator/copper-segment junction, it uses carbon brushes that have a shorter life and it is larger and heavier. The commutator limits the device to a low rpm range, whereas the alternator can turn at higher, more efficient speeds.
A generator creates a portion of its output from residual magnetism and can therefore continue to produce power and even charge a dead battery if the aircraft battery is lost. An alternator will never charge a dead battery because it needs external excitation. Generators are less efficient, their amperage ratings are lower, and they require periodic polarization after installation whereas alternators require none. Generators are not sensitive to errant electrical spikes or reversed polarity, making them mechanically rugged, while alternators demand cleaner electrical protection yet deliver lighter, higher-output service for modern aircraft systems.
How does an alternator differ from a magneto in an aircraft?
An alternator differs from a magneto in that an alternator uses an electromagnet whereas a magneto uses permanent magnets. A magneto is a special type of engine-driven alternating current generator that uses permanent magnets as the source of energy. In a magneto, magnets rotate around stationary coils to generate electricity. A magneto combines a generator and ignition coil into one compact unit. A magneto does not contain a commutator and does not need a battery and is a self-powered ignition system used for ignition in aircraft engines.
An alternator uses an electromagnet. The field circuit energizes the electromagnet of the alternator rotor. As the rotor spins, varying north-south lines of magnetic force induce alternating current in the conductor. The stator in the alternator spins within the windings which are fixed. The voltage regulator controls system voltage by controlling the field circuit that energizes the electromagnet of the alternator rotor. The alternating current is converted to direct current through diodes (rectifiers).
Which performs better: airplane alternator or generator ?
An alternator performs better than a generator. Both machines try to produce steady bus-level power, yet the alternator delivers 6-10% more current from the first taxi roll to full-throttle cruise because its three-phase stator windings ride inside a varying magnetic field produced by a rotor electromagnet that is energised the moment the master switch closes. The generator relies on a fixed physical magnet and whatever residual magnetism is left after shutdown. Until the engine reaches a modest cruise RPM, the carbon brush commutator cannot carry full output across the copper segment junction, so a deeply-discharged battery receives little help.
Heat, weight and endurance separate the two. The ES-602D, a modern 60 A direct-drive alternator, sheds its heat through the front case and keeps the electromagnet in tight balance, giving lower heat and simpler V-belt load. The generator must move copper coils and a brush block at crankshaft speed, inviting vibration that loosens brackets and manufactures fatigue cracks. Because the brushes carry the total output, every flight lays down a layer of carbon dust that mixes with engine oil and turns the commutator into an abrasive lap. Once commutator segments go out of round, the resulting dielectric arcing and dirt throws a spray of low-radio-frequency hash that arrives as static in the headset and speckled snow on the HSI. Alternator brush life is longer: only two tiny units ride on slip rings whose current is a few hundred milliamperes for the rotor field, not the entire 40-60 A bus load.
The alternator's AC wave is born inside the machine and must be rectified to DC power through a rectifier bridge, but the diodes sit quietly on a heat sink, free of commutator arcing, so the ship's radios stay clean. In service logs, generators often come off for commutator under-cutting long before the alternator next to them needs only a dust wipe and a new belt. For tighter rpm range, cleaner waveform, lighter bracket load and faster low-speed charging, the alternator is the better choice.
What is the impact of the loss of an alternator or generator in an aircraft equipped with all electronic flight displays?
In an aircraft whose panels are entirely glass, the loss of the generator or alternator is a compounded threat because almost all piston-airplane glass-cockpit systems have no backup to the attitude/heading reference system or to the air data computer. Once the generator or alternator goes offline the electrical source is battery alone, and the ALT OUT warning illuminates on the primary flight display when the alternator side of the split master is Off; the warning extinguishes only when the alternator side is back On. Because modern all-electric glass-panel aircraft rely on those batteries for survival, robust system monitoring and failure-warning provisions are incorporated into the electrical system, yet pilots have very little time available from the battery after an alternator fails. The first symptoms are decreasing voltage and a discharge indication in the ammeter, after which the radio and transponder always fail first because those components draw more amps than most other systems. No transponder is present when electrical power is lost, and the lack of transponder information leads to loss of separation or airspace infringement.
The effects of failed components include reduced navigation capabilities and communication problems that prevent air traffic control from providing better assistance. If the primary flight display fails, its information will automatically appear on the multifunction display in reversionary mode, but that does nothing for a total power loss. The pilots still have an airspeed indicator, altimeter, vertical speed indicator, and wet compass, yet no flaps, no gear, and no turn-coordinator are present when electrical power is lost. In day-VFR conditions an electrical failure does not necessarily constitute an emergency, because the pilot still has a running engine, fuel, eyes, and the basic pitot-static instruments. Defenses include multiple primary generators, a secondary (APU) generator, and on turbine equipment a tertiary (RAT) generator, all of which reduce the potential for loss of all electrical generation capability.
What are the advantages of alternators and generators on an aircraft?
Advantages of alternators include that they reduce overall weight because they are lighter and more compact for the power produced. They deliver full rated output at low engine rpm, so they can charge the battery even on the ground and still operate the entire electrical system at slower engine speeds. Their lower operating temperature and better thermal performance lengthen the time between overhauls and cut vibration.
Advantages of generators are that they can produce electrical power even if the battery is dead, giving the aircraft an independent source of energy. A voltage regulator automatically varies the strength of the magnetic field to keep the output within safe limits, while residual magnetism lets the unit begin generating without external excitation. On some installations the same machine acts as a starter-generator, spinning the turbine for engine start and then switching to generation duty once the engine is running.
