Alternator/Generator and Voltage Regulator

On-board electrical generation

While the engine runs, the electrical generation system supplies all power for avionics, lighting, pitot heat, etc. and recharges the battery. On engine stop the battery takes over alone.

Generator vs alternator

DC generator — older system

• Produces DC directly.
• Uses a commutator and carbon brushes.
• Delivers adequate voltage only from about 1 200 RPM — at lower RPM the battery discharges.
• Lower efficiency, more wear parts.
• Found on older Cessna/Piper types.

Alternator (AC generator with rectifier) — current standard

• Produces AC first, then rectified to DC via a diode bridge.
• Uses slip rings instead of brushes → longer life.
• Delivers adequate voltage even at low engine RPM (idle ~700 RPM) — battery charges in ground idle.
• Higher efficiency and power density.
• Standard in modern light aircraft (C172, PA-28, DA40).

Alternator construction

Main parts:

• Rotor with electromagnet (field winding), driven by V-belt from the engine.
• Stator with three-phase windings in which the rotor's changing magnetic flux induces an AC voltage (Faraday).
• Diode bridge (6 diodes) — converts 3-phase AC to pulsating DC.
• Voltage regulator — controls the rotor field current; adjusts output so the bus stays at 14 V (or 28 V).

Voltage regulator

Task: keep bus voltage constant at target, regardless of:

• Engine RPM,
• Load (how many loads are switched on),
• Temperature.

Operation: measures bus voltage; if too low → more field current to rotor → stronger magnetic field → more induction. If too high → less field current.

Typical target voltage:

• 14 V bus: 13.8–14.2 V.
• 28 V bus: 27.6–28.4 V.

Distribution of generated power

From the alternator a main feed goes to the:

• Main bus — feeds nearly all consumers (avionics bus, lights, pitot heat).
• Battery — through the master solenoid (main relay); battery charges when alternator voltage > battery voltage.

On-board loads

Typical loads in a light aircraft:

• High: starter (100–300 A, brief), pitot heat (5–15 A), landing light (5–10 A), heated seats.
• Medium: radio (TX 2–3 A, RX 0.3 A), transponder (0.5 A), GPS/EFIS display (1–3 A), position lights.
• Low: instrument lighting, anti-collision lights (LED modern).

Alternator failure

Symptoms:

• Red "ALT" / "LOW VOLTAGE" warning on the panel.
• Ammeter shows discharge (negative).
• Bus voltage slowly drops.

Action (standard procedure):

1. ALT switch off and on (reset);
2. If no effect: switch off non-essential loads (landing light, pitot heat if dispensable, heating, secondary avionics);
3. Battery for essential functions only (radio, one transponder, possibly one display);
4. Land at the nearest suitable aerodrome;
5. In IMC with electrical mandatory instruments: highest priority — exit to VMC or aerodrome immediately.

Battery endurance: at reduced load approx. 30–45 minutes, depending on battery state.