Aircraft AC Power Systems, Alternators, and Inverters

Aircraft AC Power Systems: Overview

Modern transport-category and many general aviation aircraft use alternating current (AC) as a primary or secondary power source. AC power is well suited to aircraft because transformers can step voltage up or down efficiently, and AC motors are lighter and simpler than equivalent DC motors. Most large aircraft operate 115 V AC at 400 Hz rather than the 60 Hz used in household current. The higher frequency allows smaller, lighter transformers, inductors, and motors — a critical advantage in aviation.

How Aircraft Alternators Work

An alternator converts mechanical energy (shaft rotation) into AC electrical energy through electromagnetic induction. As a rotor carrying a magnetic field spins inside a set of stator windings, the changing magnetic flux induces a sinusoidal voltage in each winding. The output frequency depends directly on rotor speed and the number of magnetic pole pairs:

Frequency (Hz) = (RPM × Number of pole pairs) / 60

On aircraft driven by a constant-speed drive (CSD) or integrated drive generator (IDG), the gearbox maintains rotor speed so that output frequency stays at 400 Hz regardless of engine speed changes. Brushless alternator designs are common in aviation because they eliminate the wear and arcing associated with brush-type machines, improving reliability and reducing maintenance.

A three-phase alternator produces three sinusoidal voltages spaced 120° apart. Aircraft systems typically distribute this power in a wye (Y) configuration, where the line voltage is √3 times the phase voltage. At 115 V line-to-neutral, the line-to-line voltage is approximately 200 V.

Voltage Regulation

An unregulated alternator output voltage varies with load and speed. A voltage regulator senses output voltage and adjusts the current supplied to the rotor field winding (excitation current) to hold output voltage at a set value — typically 115 V AC (±2 V on many systems). Solid-state regulators respond quickly and precisely. If output voltage climbs above safe limits, an over-voltage relay (OVR) trips offline, disconnecting the generator bus and protecting avionics from damage. Over-voltage protection is a required design feature under FAA regulations.

Split-Bus Architecture

Large aircraft separate AC power into independent buses — often labeled AC Bus 1, AC Bus 2, and an AC Essential (or Emergency) Bus. Each bus is normally fed by its own generator. This split-bus arrangement prevents a single generator failure from cutting power to the entire aircraft. Bus tie breakers (BTBs) can connect buses together when one generator is lost, and automatic load shedding sheds non-essential loads to protect essential avionics and flight instruments.

Static Inverters

A static inverter converts DC input (typically 28 V DC from the battery or DC bus) into AC output using solid-state switching circuits — no moving parts. Transistors or MOSFETs switch the DC on and off at high speed; filtering and transformers shape the output into a stable sine wave at the required voltage and frequency (often 115 V, 400 Hz). Static inverters are quiet, lightweight, and reliable, and are commonly used to power AC instruments and avionics when engine-driven generators are not available (e.g., ground operations on battery alone, or in emergencies).

Rotary Inverters

A rotary inverter is an older design that uses a DC motor driving an AC generator on the same shaft. DC power spins the motor, which turns the generator, producing AC output. Because it has rotating parts, a rotary inverter is heavier, noisier, and requires more maintenance than a static inverter. You may still encounter rotary inverters in older general aviation aircraft and during FAA written exam questions.

AC Circuit Quantities: Impedance, Current, and Power Factor

In a DC circuit, opposition to current is resistance (R). In an AC circuit, inductors and capacitors also oppose current through reactance. The total opposition is impedance (Z), measured in ohms:

• Inductive reactance: X_L = 2πfL (f in Hz, L in henries)
• Capacitive reactance: X_C = 1 / (2πfC) (C in farads)
• Impedance (series R-L-C): Z = √(R² + (X_L − X_C)²)
• Current: I = V / Z (Ohm's law for AC)

Power factor (PF) describes how effectively a load converts apparent power (VA) into real, useful power (watts):

PF = cos θ = R / Z, where θ is the phase angle between voltage and current.

A purely resistive load has PF = 1.0 (unity). Inductive loads (motors, solenoids) have a lagging power factor less than 1.0, meaning some current flows without doing useful work. Aircraft power systems are designed to keep power factor close to unity to minimize generator and wiring size.

Apparent power (VA) = V × I; Real power (W) = V × I × PF

Safety in the Lab

• Wear safety glasses at all times when working with the bench alternator simulator and oscilloscope — arcing or component failure can eject debris.
• Treat all AC test points as energized until confirmed de-energized with a meter. Aircraft 115 V AC at 400 Hz is lethal.
• Apply lockout/tagout (LOTO) procedures before connecting or disconnecting leads on the simulator when it is not in your immediate control.
• Use insulated test probes rated for the voltages present. Inspect probe insulation for cracks or damage before each use.
• Keep the work area dry and do not place drinks near electrical test equipment.