An alternator is a part of the aircraft's electrical system. As a synchronous generator, it converts mechanical energy to alternating-current electrical energy, providing power to lights, avionics, and maintaining battery charge during flight. The machine is designed to endure vibration, extreme temperatures, and altitude changes typically encountered in aviation service. The charging circuit comprises the alternator itself, a voltage regulator or alternator controller, the alternator switch, and, contributing to a lesser degree, the ammeter and the overvoltage relay. Principal internal parts include the rotor, stator, rectifier, voltage regulator, brush-holder assembly, slip rings, bearings, and brushes. As externally regulated units, aircraft alternators are controlled by a dedicated voltage regulator or alternator controller that governs output to guarantee stable system voltage.
What is an alternator on an aircraft?
An alternator (or synchronous generator) is an electrical generator which converts mechanical energy to electrical energy in the form of alternating current. It generates electrical energy to power electrical components.
An alternator is the piece of equipment that serves as the root provider for an aircraft's total electrical needs. It is a small electromechanical device turned by the engine through a drive belt connected to the crankshaft. Aircraft alternators are designed to generate electrical power for aircraft systems, converting the alternating current created in the conductor into direct current through a series of diodes so that direct current charges the aircraft battery and powers lights, avionics, and auxiliary power outlets.
Aircraft alternator output is the electrical energy that powers electrical components. The electrical energy is in the form of alternating current inside the machine, but the rotating rectifier assembly mounted on the rotor rectifies this output so that the aircraft receives stable direct current. The voltage regulator, usually mounted on the firewall in the engine compartment, varies the excitement current flow to maintain a charging-system voltage between 13.8 and 14.2 volts in a 12-volt system, or between 27.1 and 28.4 volts in a 24-volt system. An aircraft 28-volt system using a 24-volt battery therefore charges the battery at 27.5 to 27.75 volts.
In terms of DC, an aircraft alternator is a rotating machine that generates electricity in direct-current form for the electrical system charging circuit that consists of the alternator, the switch, and the overvoltage relay. In AC terms, it is still an alternator whose internal alternating current is converted to direct current before it leaves the device.
How big is a plane alternator?
Plane alternators weigh as little as 6.9 lbs (3.13 kg) and are up to four pounds (1.81 kg) lighter than competitors. The FS1-14B alternator weighs 6.0 pounds (2.72 kg), making it lightweight and compact. The AL12-EI60 kit, with brackets and hardware, totals 12.8 lbs (5.8 kg), while its 70 amp version adds only 2 pounds (0.9 kg). Lightweight design is achieved through compact housings and internally regulated components, so the alternator fits standard engine pads without extra structure.
What are the parts of an aircraft alternator?
The parts of an aircraft alternator are listed below.
- Aircraft alternator has three stator windings
- Aircraft alternator includes a brush holder assembly with spring and brushes
- Alternator diodes allowing current flow in one direction are components within the aircraft alternator
- Aircraft alternator includes a slip ring end housing assembly with bearing
- Alternator's rotor backed by bearings is a part of the aircraft alternator
- Alternator's drive hub is part of the aircraft alternator and drive hub assembly
- Alternator output terminal identified by a red insulator is part of the aircraft alternator
- Alternator drive hub retained by a bi-metal thrust washer is integral to the aircraft alternator
- Aircraft alternator contains a stator
- Aircraft alternator has a rectifier assembly
- Aircraft alternator has a drive end housing with bearing
- Alternator's overvoltage relay is part of the aircraft alternator charging circuit
- Alternator field wire connecting to F1 terminal is associated with the aircraft alternator
The rotor is a spinning electromagnet supported by bearings at either end. It spins inside the conductor called the stator. The stator contains three separate conductor windings, and its output is rectified by six diodes in the rectifier assembly before the direct current leaves the B+ terminal. Two carbon-graphite brushes, each with a flexible braided copper lead and coil spring, ride on separate slip rings to feed the small field current into the rotor. The brush-holder housing keeps these assemblies in place. The drive hub, mounted on the alternator shaft and retained by a bi-metal thrust washer, a castellated nut, and a cotter key, is driven by a face gear on the crankshaft. A modern elastomeric drive coupling, which superseded the spring-type hub in 1996, slips if the alternator locks up, protecting the engine. A ground wire bonds the alternator frame to the aircraft structure, while the B+ output wire feeds the bus bar through the alternator circuit breaker, distributing power to radios, avionics, lighting, and instruments.
What is the difference between an alternator and a generator in aviation?
The difference between an alternator and a generator is that alternators have no commutator and instead rely on a rectifier bridge to change the alternating current produced in the stationary armature windings into direct current. In a conventional generator, the magnetic field is stationary and the armature spins, whereas in an alternator the magnetic field is rotated by an engine-driven shaft (rotor) while the coil windings (stator) remain fixed.
Aircraft generators are typically larger and heavier than alternators and require more space to fit in. Generators are heavy, have lower amperage ratings, and demand more frequent maintenance due to carbon-brush commutator segments that wear quickly and create electrical noise. Alternators are lighter, produce more power for their size and weight, and deliver a more consistent output across a wider rpm range. Alternators can generate useful current at lower engine speeds, while generators have a low rpm range and must spin faster to reach full output.
Both devices ultimately produce the electricity needed by the aircraft, but alternators do so more efficiently because they use only the required amount of energy and run at greatly reduced speeds. Generators produce voltage at all times and consume all the energy they create, making them less efficient. The magnetic field strength in either unit is automatically varied by excitement power from the voltage regulator, yet the alternator's brush life is longer and its upkeep is simpler.
Operational differences appear during battery-dead scenarios: a generator can self-excite through residual magnetism and therefore charge a dead battery, while an alternator needs battery power to wake up its magnetic field and normally will not begin charging a stone-cold dead battery. Because a generator can also function as a motor, it is often used as a starter-generator on turbine airplanes, whereas alternators are regarded as more dependable for normal charging duty after the engine is running.
What is involved in an aviation alternator overhaul?
An aviation alternator overhaul begins with removal of the unit from the aircraft, followed by complete disassembly. The rotor, stator, diodes, rectifiers, brush holder, bearings, seals, slip rings, and drive coupling are inspected against the OEM manual. Any part that is worn, cracked, discoloured, or beyond limits is replaced. New bearings and oil or grease seals are installed even if the old ones appear serviceable, and the drive coupling is either replaced or overhauled at the same time. The shop cleans the slip rings, tests diode continuity, verifies designed power capacity on a bench rig, and reassembles the alternator with lock-tabbed through-bolts torqued to specification.
If the alternator has suffered sudden stoppage in an accident, a qualified mechanic must perform a magnetic-particle inspection (ASTM E 1444) of the drive-shaft threaded end and rotor shaft before the unit is permitted to return to service.
Manufacturers recommend that alternators be opened for a comprehensive internal inspection every 500 hours, and that they be sent for full overhaul at engine TBO - typically 2,000 hours - although many owners elect to overhaul or replace the alternator twice between engine overhauls.
A repair of the alternator addresses only the failed parts - brushes, a diode, or a bearing - while a rebuild follows the same process as an overhaul but reuses additional parts. An overhauled unit is therefore generally regarded as equivalent to new, especially when many internal parts are not reused, and is returned with the inspected coupling installed and ready for flight.
An aviation alternator overhaul begins with the complete demolition of the generator so that each part can be removed. First examination showed graphite particulate aggregation around the brushes and faint marking on the slip hoops. PI scoured indicators of undue wear, scanned for fracturing, and applied precise measuring devices to every tolerance. Old brushes were replaced and bearings replaced with current qualified components; the rotating mechanism was substituted for a qualified unit. Slip rings were re-machined, superficial faces revived to a fine finish, while alloy components were cleansed in an authorized medium to eliminate oil and contaminants. After classification of each part, I guaranteed correct placement, utilized proper torsion during reassembly, and gave regard to the circumstance of every fastener. The procedure transitioned to reassembly, the generator was put together, and a final check confirmed that the status of brushes and status of slip rings met overhaul limits.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
