7.6 AC Generators & Motors

Why This Matters for the Exam

Modern transport-category aircraft use a three-phase AC electrical system operating at 115V / 200V AC at 400 Hz. Generators (alternators) driven by the aircraft engines are the primary source of this electrical energy. The power generated must drive various AC motors, such as fuel boost pumps, hydraulic pumps, and cooling fans.

For the EASA Part-66 Module 3 exam, a candidate must understand the construction and operation of single-phase and polyphase AC alternators, the mathematical differences between Star (Wye) and Delta wiring configurations, and the operating characteristics of AC induction and synchronous motors. A solid grasp of slip and synchronous speed calculations is frequently tested.

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AC Generators (Alternators)

An AC generator, or alternator, converts mechanical energy from the engine into electrical energy.

Construction: Rotating Field vs. Stationary Armature

Unlike small DC generators, aircraft alternators are constructed with a rotating field (rotor) and a stationary armature (stator).

• Rotor: The rotating part. It contains electromagnet coils supplied with a relatively small DC excitation current through slip rings and brushes (or brushless exciter systems). This creates a rotating magnetic field.
• Stator: The stationary part. It contains the main power windings. As the rotor turns, its magnetic field cuts across the stator windings, inducing high-voltage, high-current AC.
• Advantage: Tapping high AC power from stationary stator terminals eliminates the need for heavy, high-current slip rings and brushes, which would suffer from rapid wear, arcing, and electrical noise at high altitudes. Slip rings are only required to carry the small DC excitation current.

Three-Phase Alternators

A three-phase alternator has three independent stator windings placed 120 electrical degrees apart around the stator. As the rotor rotates, it induces three alternating voltages that are identical in magnitude and frequency but shifted in phase by 120 degrees. This provides a continuous, balanced delivery of power.

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Star and Delta Configurations

The three windings of a three-phase alternator (or three-phase load) can be connected in two ways:

1. Star (Wye) Connection

In a Star connection, one end of each of the three windings is connected to a common junction called the neutral point. The other ends of the windings are connected to the three line terminals (A, B, C).

• Voltage Relationships: There are two voltages in a Star system:
  • Phase Voltage (Vph): The voltage measured between any single line and the neutral point (nominally 115V AC on aircraft).
  • Line Voltage (VL): The voltage measured between any two of the three line wires (e.g., A to B). VL = sqrt(3) x Vph = 1.732 x 115V = 200V
• Current Relationship: The current flowing through the line is equal to the current in the phase winding: IL = Iph
• Aircraft Standard: Typical aircraft distribution systems are 115V/200V Star systems. The neutral point is connected to the aircraft fuselage (ground structure), allowing single-phase 115V loads to run between any phase line and the ground, while three-phase loads (like heavy pumps) run line-to-line at 200V.

2. Delta (Δ) Connection

In a Delta connection, the three windings are connected end-to-end in a closed loop, forming a triangle. The three line terminals are connected to the three corners of the triangle. There is no neutral point.

• Voltage Relationship: The line voltage is equal to the phase voltage: VL = Vph
• Current Relationship: The line current is the vector sum of the phase currents: IL = sqrt(3) x Iph = 1.732 x Iph
• Application: Used for high-power, balanced loads where a neutral wire is not required, such as windshield de-icing heating elements, galley ovens, and three-phase motors.

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AC Motors

AC motors convert electrical energy back into mechanical rotation. They are categorized into synchronous and induction motors.

1. Synchronous Motors

In a synchronous motor, the rotor rotates at the exact speed of the stator's rotating magnetic field.

• Operation: The stator is supplied with three-phase AC, creating a rotating magnetic field. The rotor is magnetized by a separate DC source (or uses permanent magnets) and locks into step with the rotating magnetic field.
• Synchronous Speed (Ns) Formula: Ns = 120 x f / P Where:
  • Ns = Synchronous speed in revolutions per minute (RPM)
  • f = Frequency in Hertz (Hz) (typically 400 Hz on aircraft)
  • P = Number of poles in the stator windings
• Characteristics: Synchronous motors run at a constant speed regardless of the load (up to a pull-out torque limit). They are used in precision timing devices, clocks, and tachometer systems.

2. Induction (Asynchronous) Motors

Induction motors are the most common type of AC motor on aircraft due to their simplicity, ruggedness, and low maintenance. They are often called squirrel-cage induction motors.

• Operation: Three-phase AC supplied to the stator creates a rotating magnetic field. This field cuts across the short-circuited rotor conductors (the squirrel cage), inducing an electromotive force (Faraday's Law) and a resulting current. This current generates its own magnetic field, which reacts with the stator field to produce torque.
• Slip (S): For current to be induced in the rotor, there must be relative motion between the rotating stator field (Ns) and the actual rotor speed (Nr). Therefore, the rotor must always turn slower than the synchronous speed. This difference in speed is called slip: S = ((Ns - Nr) / Ns) x 100%
  • Note: If the rotor were to turn at synchronous speed (Nr = Ns), there would be no relative motion, no flux cutting, no induced rotor current, and thus zero electromagnetic torque. The motor would slow down until relative motion was re-established. Slip typically ranges from 2% to 6% under normal operating loads.

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Worked Exam Calculation Scenarios

Scenario 1: Star System Voltage

An aircraft alternator winding is connected in a Star configuration and generates a phase voltage of 115V AC.

1. Calculate the line-to-line voltage available to power a three-phase hydraulic pump motor: VL = sqrt(3) x Vph VL = 1.732 x 115V = 199.18V AC This is designated as the standard 200V AC line-to-line voltage on aircraft.

Scenario 2: Synchronous Motor Speed

A synchronous motor driving an aircraft instrument cooling fan has 6 poles and is powered by the 400 Hz electrical system.

1. Calculate the speed of the fan: Ns = 120 x f / P Ns = 120 x 400 / 6 = 48000 / 6 = 8000 RPM The synchronous motor runs at exactly 8000 RPM.

Scenario 3: Induction Motor Slip

A 4-pole three-phase induction motor is connected to the 400 Hz aircraft supply. Under full load, a tachometer measures the actual rotor speed at 11,400 RPM.

1. Calculate the synchronous speed (Ns): Ns = 120 x f / P Ns = 120 x 400 / 4 = 48000 / 4 = 12000 RPM

2. Calculate the percentage slip (S): S = ((Ns - Nr) / Ns) x 100% S = ((12000 - 11400) / 12000) x 100% S = (600 / 12000) x 100% = 5% The motor operates with a slip of 5%.