Generators and alternators: Science and limits

Why generators beat any heat engine on efficiency

What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.

A generator turns rotation into electricity by moving a magnetic field past coils of copper wire, which induces a voltage (Faraday's law). Large power-station generators convert up to about 99% of the shaft power they receive, so they are among the most efficient machines ever built.

The conversion, step by step

Turbine or engine turns the rotor
Mechanical (shaft) → Rotating magnetic field · small losses to bearing friction and windage (air or hydrogen drag)¹Induction in the stator
Changing magnetic field → Electrical (AC voltage and current) · up to 98.9% (air-cooled, up to 400 MVA); up to 99% (hydrogen and water-cooled, up to 2,250 MVA)¹²Step-up to the grid
Electrical at about 18-27 kV → Transmission voltage · handled by the transformer tile¹

The science

Moving a conductor through a magnetic field, or changing the field through a coil, pushes electrons along the wire: the induced voltage equals the rate of change of magnetic flux through the coil (Faraday's law), and its direction opposes the change (Lenz's law), which is why the turbine must keep pushing to deliver power. In a power-station alternator the rotor is an electromagnet fed with DC from an exciter, and it spins inside a fixed stator whose three sets of windings deliver three-phase AC. The frequency is fixed by speed and number of poles: a two-pole machine must spin at 3,000 rpm for 50 Hz or 3,600 rpm for 60 Hz, and a four-pole machine (common in nuclear plants) at 1,500 or 1,800 rpm (calculated: speed = 120 x frequency / poles). A dynamo makes DC instead by using a commutator to flip the connections each half-turn. Wind turbines often use permanent-magnet or doubly fed induction generators so they can run at varying speed behind power electronics.

Limits set by science

Limit	Value	Why it exists	Best today
No Carnot limit	theoretical efficiency close to 100%¹	Electromagnetic conversion is not a heat engine; losses are only electrical resistance in copper (I squared R), eddy currents and hysteresis in the steel core, friction and windage.	Up to 99% for the largest hydrogen- and water-cooled units.
Cooling limit	air cooling to about 400 MVA; hydrogen and water cooling beyond¹²	Losses become heat, and the copper insulation must stay below its temperature rating. Hydrogen is about one fourteenth as dense as air (calculated from molar masses of 2 and 29) and conducts heat well, so it cuts drag and carries heat away; water inside hollow stator bars cools the biggest machines.	Arabelle's 4-pole GIGATOP reaches 2,250 MVA, described as the most powerful generator in the world.
Mechanical limit on rotor size	two-pole rotors spin at 3,000-3,600 rpm (calculated)¹	Centrifugal stress on the rotor forging limits its diameter; very large nuclear units therefore use four poles and half the speed.	2-pole units up to 1,400 MVA; 4-pole up to 2,250 MVA.

Sources

GIGATOP generators Arabelle Solutions, 2025
Air-cooled generators (GEN-A) GE Vernova, 2025