Aircraft run on 400 hertz power because it makes transformers lighter
Planes and much military gear use electricity alternating 400 times a second, far faster than household mains. The reason is weight. A transformer handling a given amount of power shrinks as frequency rises, which is also why a modern phone charger can be tiny while a grid transformer weighs hundreds of tons.
A transformer moves electrical energy between circuits that never touch. Alternating current in one coil makes the magnetic field in a shared iron core swell and collapse, and that changing field induces a voltage in any other coil wrapped around the same core, exactly as Faraday's law of induction, found in 1831, predicts. Wind more turns on the output coil and voltage rises; fewer turns and it falls, with current shifting the opposite way. Since the first constant-voltage design appeared in 1885, such devices have made long-distance alternating current power practical. They also isolate circuits electrically and link stages in signal equipment.
The idealised version is lossless and perfectly coupled, and real commercial units come surprisingly close. Still, some energy escapes. The core warms through hysteresis, the lag in its magnetisation, and through swirling eddy currents. The copper windings have resistance. Some magnetic flux leaks out and threads only one coil, which does not waste power directly but weakens voltage regulation under heavy load, so designers normally keep leakage very low.
Sometimes leakage is wanted. Deliberately leaky transformers, built with air gaps or magnetic shunts, limit short-circuit current, making them suitable for arc welders, neon signs and sodium or mercury vapour lamps. Air gaps also stop audio transformers from saturating when direct current flows in their windings.
Frequency is the designer's big lever. Raise it and a given core can pass more power before saturating and needs fewer turns, though core losses and the skin effect grow too. Push it very high and the core shrinks dramatically, which is how switched-mode power supplies, made possible by fast power semiconductors, get away with small transformers. At the other extreme, trains crossing borders need transformers that cope with supplies from 50 hertz down to 16.7 hertz at up to 25 kilovolts.
Source: Transformer
