Aircraft Generator vs. Alternator: Construction and Operation Differences

Every aircraft electrical system depends on at least one device that converts the mechanical energy of a rotating engine into usable electrical power. For decades, the direct-current (DC) generator was the standard solution. Starting in the mid-twentieth century, the alternator—particularly the alternating-current (AC) alternator with solid-state rectification—gradually replaced the generator in most light aircraft because it offers superior output at low engine speeds and fewer maintenance demands. Understanding the construction and operational differences between these two machines is not only a core topic on the FAA AMT Airframe knowledge test but is also essential for anyone who must inspect, troubleshoot, or sign off aircraft electrical work.

This article walks through how each device is built, how it produces electricity, how its output is regulated and controlled, and where each design still appears in the fleet today. All facts are grounded in the FAA Aviation Maintenance Handbook — Airframe (FAA-H-8083-31) and the General handbook (FAA-H-8083-30).

The DC Generator: Construction and Operation

A conventional aircraft DC generator is a rotating-armature machine. Its major components are the field assembly (stationary electromagnets surrounding the interior of the case), the armature (a wound rotor that spins inside the field), the commutator (a segmented copper ring mounted at one end of the armature shaft), and carbon brushes that ride on the commutator to carry current to the external circuit.

When the armature conductors rotate through the magnetic field produced by the field windings, a voltage is induced in those conductors by electromagnetic induction. Because the conductors alternately pass north and south magnetic poles, the voltage induced in any single conductor is actually alternating. The commutator's job is to convert that internal AC into usable DC at the output terminals: as each armature coil passes through the point of maximum voltage, the commutator segments connected to that coil are in contact with the brushes, so current always flows in the same direction in the external circuit. The result is a pulsating but essentially unidirectional (DC) output.

The field current—the current flowing through the stationary field windings—determines how strong the magnetic field is and therefore how much voltage the generator produces. A voltage regulator adjusts field current to keep output voltage stable as engine speed and electrical load change. Most aircraft generators also incorporate a reverse-current cutout relay (or current-voltage regulator unit) that disconnects the generator from the bus whenever generator voltage falls below battery voltage, preventing the battery from motoring the generator backward at low RPM.

The Alternator: Construction and Operation

The aircraft alternator reverses the arrangement. It uses a rotating field (the rotor carries the field winding) and a stationary armature (called the stator, which carries the output winding). Slip rings—smooth, continuous rings rather than segmented commutator bars—carry the relatively small field current to the rotating field winding. Because field current is small and the rings are smooth, brush wear and arcing are dramatically reduced compared with a generator's commutator-brush arrangement.

The stator windings are arranged in three sets spaced 120° apart, producing three-phase AC output. This three-phase AC is then converted to DC by a rectifier assembly consisting of six or more silicon diodes (two per phase, arranged in a full-wave bridge configuration). Silicon diodes have no moving parts, require no maintenance, and fail in predictable ways. The diode bridge produces a smooth, ripple-free DC output that is fed to the aircraft bus. Because silicon diodes naturally block reverse current flow, alternators do not need a separate reverse-current cutout relay.

Voltage is regulated by varying the field current through the rotor. A solid-state voltage regulator senses bus voltage and adjusts field current—and therefore rotor magnetic field strength—to keep output voltage within limits (typically 13.5–14.5 V in a 14-volt system, or 27.5–28.5 V in a 28-volt system).

Why the Design Differences Matter in Practice

The rotating-armature versus rotating-field distinction creates several practical consequences that technicians must understand.

• Output at low RPM: Because the stator of an alternator can have many more turns of wire than a generator armature (weight and centrifugal stress are not concerns for a stationary winding), alternators reach their rated output voltage at much lower shaft speeds—sometimes as low as 1,000–1,500 alternator RPM. A generator may not reach rated voltage until engine RPM is well into the cruise range. This is why an alternator can charge a battery during taxi and low-power operations while many older generator-equipped aircraft required high RPM to generate useful charging current.
• Size and weight for a given output: Alternators are generally lighter and more compact than generators of equivalent rated output, partly because the stationary stator can be wound with heavier wire to handle high current without rotational stress concerns.
• Brush and commutator wear: Generator brushes carry the full armature output current across the commutator, producing significant arcing, heat, and wear. Technicians must inspect commutators for grooving, burning, and mica buildup between segments, and replace brushes at regular intervals. Alternator slip rings carry only low-level field current; brush wear is minimal, and slip rings rarely need machining.
• Failure characteristics: A generator that loses its field excitation simply stops producing output—a partial or gradual failure is possible. An alternator field failure also stops output, but the solid-state regulator and diode pack introduce additional failure modes: an open diode causes reduced, rough output; a shorted diode can destroy the regulator or cause AC ripple on the DC bus. A failed alternator diode is detected with an AC voltmeter or oscilloscope across the bus—a healthy alternator shows near-zero AC ripple, while a failed diode shows measurable AC component.
• Flashing the field: DC generators can lose residual magnetism (especially after storage), requiring the technician to briefly apply external DC to the field windings to restore residual flux before the generator will self-excite. Alternators do not self-excite in this way; they depend entirely on field current supplied by the regulator and battery, so field flashing is not a procedure associated with alternators.

Key Numbers and Rules

• Standard light aircraft bus voltages: 14 V (nominally 12-V battery systems) or 28 V (nominally 24-V battery systems).
• Regulated alternator output is typically held to within ±0.5 V of the target bus voltage by the solid-state regulator.
• Most aircraft alternators produce three-phase AC internally, rectified to DC before reaching the bus—not single-phase.
• A full-wave three-phase bridge rectifier uses a minimum of six diodes.
• Generator brush length minimums are specified in the manufacturer's overhaul manual; brushes worn to less than the minimum serviceable length must be replaced.
• Commutator runout and minimum diameter after machining are also manufacturer-specified limits that must not be exceeded during reconditioning.
• An overvoltage relay (crowbar circuit) protects the bus by disconnecting the alternator if output voltage rises above approximately 16 V (14-V systems) or 32 V (28-V systems).
• Alternators must never be operated on open circuit (no load) at high field excitation—extremely high voltages can destroy the diode pack. Always connect a load or battery before exciting the field.

Where Each Type Still Appears

Pure DC generators are still found on many older certificated aircraft—pre-1960s designs, some agricultural aircraft, and certain military-surplus platforms. Because these aircraft are type-certificated with generator systems, replacement in kind is required unless an STC authorizes an alternator conversion. Many technicians encounter generator systems during annual inspections on vintage aircraft, making hands-on familiarity essential.

Alternators dominate modern general aviation light aircraft (Cessna 172, Piper Cherokee, Cirrus SR series, and similar). High-performance turbine aircraft often use AC alternators (called AC generators or integrated drive generators) that deliver three-phase AC directly to AC buses without rectification, but that topic extends beyond basic airframe electrical systems.

Common Test Traps

• Confusing which part rotates: The FAA knowledge test frequently asks which component rotates in a generator versus an alternator. Remember: generator = armature rotates; alternator = field rotates.
• Reverse-current relay: The test may ask which device requires a reverse-current cutout relay. The answer is the DC generator—not the alternator, because the diodes block reverse current automatically.
• Field flashing: Field flashing restores residual magnetism in a DC generator. This procedure does not apply to alternators, and selecting it as an alternator maintenance step is a common distractor.
• Output type: Some students assume alternators produce DC directly. They produce three-phase AC internally; DC output comes from the internal rectifier diodes. The bus receives DC, but the machine itself is an AC generator.
• Open-circuit alternator damage: Operating an alternator at high excitation with no load or battery connected can spike voltage high enough to burn out the diode pack. This is why a battery must always be on the bus when an alternator is brought online.