15.2 AC Generator Principles & Construction
Key Takeaways
- A simple AC generator induces a sinusoidal EMF when a loop rotates at constant speed in a uniform magnetic field — Faraday’s law and flux cutting
- Instantaneous EMF follows e = E_max sinθ (or cosθ) with θ = ωt; polarity reverses each half-turn, producing AC at the slip rings
- Revolving-armature machines rotate the output winding and use slip rings; revolving-field machines rotate the field and keep the AC winding on the stator — preferred for high power
- Single-phase alternators provide one AC voltage; two-phase uses 90° displacement; three-phase uses three windings 120° apart electrically
- Aircraft main AC generation is typically three-phase, 115/200 V, 400 Hz from engine-driven or APU-driven generators with a rotating field
15.2 AC Generator Principles & Construction
Quick Answer: Rotate a coil in a magnetic field (or rotate the field past a coil) → induced AC EMF. Waveform is sinusoidal for constant speed in a uniform field. Revolving armature = AC on rotor + slip rings; revolving field = field on rotor, AC on stator (high-power preference). Single-phase one voltage; two-phase 90°; three-phase three voltages 120° apart — aircraft standard.
CAAS SAR-66 Module 3 topic 3.17 AC Generators applies Faraday induction to rotating machines that deliver alternating current without a commutator. Contrast with DC generators (topic 3.12): the commutator made external current unidirectional. Here, slip rings (revolving armature) or stationary stator windings (revolving field) deliver true AC.
Rotation of a Loop in a Magnetic Field
Consider a single-turn rectangular loop spinning at constant angular speed in a uniform field between N and S poles.
Faraday’s law: induced EMF is proportional to the rate of change of flux linkage. Equivalently, each side cuts flux at a rate depending on the angle between velocity and B.
| Rotor position (idealised) | Flux cutting / EMF |
|---|---|
| Coil plane parallel to B (maximum cutting speed of sides) | Maximum EMF magnitude |
| Coil plane perpendicular to B (sides moving parallel to B) | Zero EMF |
| After further 90° | Maximum again but reversed polarity |
| Full 360° | One complete AC cycle |
If θ is the angle from a chosen zero, for constant ω:
e = E_max sinθ = E_max sin(ωt)
(with the zero chosen so the sine form holds; cosine form is the same wave shifted 90°).
| Symbol | Meaning |
|---|---|
| e | Instantaneous induced EMF |
| E_max | Peak EMF |
| θ | Angular position of the coil |
| ω | Angular velocity = 2πn (n in rev/s) or 2πf |
| f | Electrical frequency of the generated wave |
Frequency link. For a machine with p pairs of poles (or P poles → p = P/2):
f = (N × P) / 120
where N is rotor speed in rev/min and f is in Hz. Faster rotation or more poles → higher frequency.
Worked example 1. A 2-pole (P = 2) alternator must produce 400 Hz.
400 = (N × 2) / 120 → N = 400 × 120 / 2 = 24 000 rev/min.
That is why aircraft 400 Hz machines run fast or use multipole designs geared from the engine/CSD/IDG — Module 3 wants the f–N–P relationship, not a specific gearbox schematic.
Worked example 2. Same machine idea at f = 50 Hz, P = 4 poles.
50 = (N × 4) / 120 → N = 50 × 120 / 4 = 1500 rev/min (classic utility synchronous speed for 4-pole 50 Hz).
Waveform from the Rotating Loop
Plot e versus time (or versus θ):
- Starts at zero (chosen origin).
- Rises to +E_max.
- Falls through zero.
- Reaches −E_max.
- Returns to zero — one cycle.
That is the sinusoidal waveform assumed throughout AC theory (topic 3.13). Non-uniform field or non-constant speed distorts the wave; Module 3 treats the ideal sine unless a stem says otherwise.
| Quantity on the wave | Generator meaning |
|---|---|
| Peak E_max | Maximum induced EMF (function of flux, turns, and speed) |
| RMS ≈ 0.707 E_max | Nameplate AC voltage basis |
| Period T = 1/f | Time for one electrical cycle |
| Polarity reversal | Why the output is alternating |
Factors raising E_max (qualitative): stronger field flux, more turns on the coil, higher rotational speed (conductors cut flux faster). Same family of ideas as DC generator EMF, but without commutation to DC.
Revolving Armature vs Revolving Field
Two construction philosophies place either the output winding or the field on the rotor.
Revolving-armature alternator
| Feature | Description |
|---|---|
| What rotates | Armature (AC winding) |
| What is stationary | Field poles (often) |
| Output connection | Slip rings and brushes take AC off the rotor |
| Advantage | Simple to visualise — rotating loop textbook model |
| Limitation | Full generated power and high voltage/current through brushes and slip rings — awkward at high power |
Revolving-field alternator
| Feature | Description |
|---|---|
| What rotates | Field winding (or permanent-magnet field — see §15.3) |
| What is stationary | Armature / stator AC windings |
| Field supply | DC to rotor via slip rings (or brushless exciter) — field current is much smaller than load current |
| Output connection | Stator terminals — no sliding contacts in the main AC power path |
| Advantage | High power and high voltage handled in stationary windings; better cooling and insulation practicality |
| Comparison | Revolving armature | Revolving field |
|---|---|---|
| Main AC winding | Rotor | Stator |
| Sliding contacts carry | Full AC output | Excitation DC only (typical) |
| Preferred for large aircraft / utility | Rare for main generation | Standard |
| Teaching model | Closest to single rotating loop | Field spins; stator sees rotating flux |
Aircraft practice. Main engine and APU generators are almost always revolving-field machines: a DC-excited rotor (often with a brushless exciter machine on the same shaft) produces a rotating magnetic field; three-phase stator windings deliver 115/200 V, 400 Hz class power to the AC buses through generator control units.
Worked concept. A 40 kVA aircraft generator at ~115 V line-to-neutral would require enormous brush gear if that entire load current had to leave a revolving armature. Exciting a field with tens of amps (or less, depending on design) through slip rings—or with a brushless exciter—is far more practical.
Single-Phase Alternators
A single-phase alternator has one working AC winding (or a set connected as one phase). Output: one alternating voltage between the phase leads (plus neutral if provided).
| Trait | Single-phase |
|---|---|
| Voltages | One phase voltage |
| Instantaneous power to a resistive load | Pulses at 2f (goes to zero twice per cycle) |
| Typical use | Smaller loads, lighting, some ground equipment; not the main large-aircraft generation standard |
| Construction | Could be revolving armature or field; still one phase set |
Single-phase is electrically simple but delivers less smooth power than polyphase systems of comparable size.
Two-Phase Alternators
A two-phase machine has two windings displaced by 90° electrical. Ideal voltages are equal in magnitude and frequency, 90° apart.
| Trait | Two-phase |
|---|---|
| Phase displacement | 90° |
| Smoothness | Better instantaneous power than single-phase |
| Modern aircraft main power | Uncommon as the primary distribution standard |
| Exam value | Contrast with 120° three-phase; do not confuse 90° with 120° |
Memory: two-phase → quadrature (90°); three-phase → 120°.
Three-Phase Alternators
A three-phase alternator has three stator (usually) windings spaced 120° electrical apart. Driven by one rotating field, they produce three voltages of equal magnitude and frequency, displaced by 120°.
| Trait | Three-phase |
|---|---|
| Phase displacement | 120° |
| Instantaneous power | Much smoother delivery to balanced loads |
| Conductor economy | Transmit more power for given copper than three separate single-phase systems |
| Aircraft | Standard for main AC generation and much of the distribution |
| Connection options | Star (wye) or delta — detailed in §15.3 |
Phasor picture. Draw three arrows 120° apart, equal length: V_A, V_B, V_C. At any instant the algebraic sum of balanced three-phase voltages is zero—a property used in star/delta analysis.
Worked example 3 — sequence. As the field rotates, phase A peaks, then 120° later phase B, then 120° later phase C (ABC sequence), or the reverse sequence if rotation or labelling reverses. Sequence matters for motor rotation (topic 3.18).
Construction Overview (Revolving-Field Three-Phase)
| Part | Role |
|---|---|
| Stator | Laminated iron core with three-phase windings in slots — main AC output |
| Rotor / field | Poles with field windings (or PM) creating flux |
| Excitation | DC field current sets flux → controls output voltage under GCU regulation |
| Drive | Engine gearbox, CSD, or IDG sets speed → sets frequency (for fixed-pole machines) |
| Cooling | Blast air or oil cooling on many aircraft generators |
| Terminal block | Three-phase (and often neutral) connections to feeders |
Voltage regulation idea (preview). E ∝ Φ × speed. For roughly constant speed, the generator control unit trims field current to hold voltage as load changes. Frequency is primarily a speed problem (CSD/IDG or variable-frequency architectures on some modern types).
Section Synthesis
- Rotating loop → sinusoidal e = E_max sin(ωt); f linked to speed and poles.
- Slip-ring revolving armature teaches the principle; revolving field builds real high-power machines.
- Single-phase = one voltage; two-phase = 90°; three-phase = 120° and aircraft main AC.
- Peak EMF rises with flux, turns, and speed; frequency rises with speed and pole count.
Next (§15.3): how three-phase windings are star- or delta-connected, and how permanent-magnet generators fit aircraft systems (especially backup and excitation roles).
In a simple single-loop AC generator rotating at constant speed in a uniform field, the induced EMF waveform is ideally:
Why are large aircraft AC generators normally built as revolving-field machines rather than revolving-armature machines?
Three-phase alternator voltages are displaced from each other by what electrical angle under balanced design?
A 4-pole alternator runs at 12 000 rev/min. What electrical frequency does it generate?