Alternator fundamentals (AC generation principle): Most practical alternators generate electricity by which physical arrangement?

Introduction:
Alternators convert mechanical energy to AC electrical energy. Their mechanical and magnetic architectures determine efficiency, reliability, and how power is collected. Understanding which part rotates—field or armature—matters for practical machine design and power delivery to loads.

Given Data / Assumptions:

• Industrial and automotive alternators as common references.
• Desire to produce substantial current at moderate voltage safely and efficiently.

Concept / Approach:

Modern alternators typically use a rotating magnetic field (rotor) and a stationary armature (stator). Keeping the armature windings stationary simplifies routing high currents out of the machine (solid terminals, no brushes for load current) while only the field excitation requires slip rings or brushless excitation systems.

Step-by-Step Reasoning:

Verification / Alternative check:

Automotive alternators and large grid alternators follow this architecture. Brushless exciters remove the need for slip rings even for field excitation, further improving reliability.

Why Other Options Are Wrong:

• a coil rotating within a magnetic field: Historically used in small generators, but most practical power alternators use rotating field with stationary armature.
• a permanent magnet rotating within a varying electromagnetic field: Not the mainstream architecture for high-power alternators.
• none of the above / moving the stator while the rotor is stationary: Contradict established machine design.

Common Pitfalls:

• Equating small hand-cranked generators with large alternator designs; scale changes the optimal architecture.
• Confusing DC generators (commutators) with AC alternators (slip rings/brushless excitation).

Final Answer:

a magnetic field rotating around fixed windings