Why AC prevailed over DC in early 20th century

In the "war of currents" (1880s–1890s), Edison's DC system lost to the AC system of Tesla and Westinghouse. AC prevailed because of the following reasons:

1. Transformer: AC voltage can be stepped up and down easily and efficiently (98–99 %) with a static transformer. DC had no such device; converting DC voltage needed rotating machines.
2. High-voltage transmission: Using transformers, AC could be sent at high voltage and low current, giving low $I^2R$ loss and allowing power to travel tens of km. DC at low (110 V) voltage could only supply customers within about 1–2 km of the station, needing many small plants.
3. Simpler, cheaper generators: AC alternators have no commutator, so they can be built for higher voltage and larger ratings, with less maintenance.
4. Induction motor: Tesla's polyphase induction motor (1888) was robust, cheap, self-starting and had no commutator—ideal for industry.
5. Easy interruption: AC current passes through zero every half cycle, so circuit breakers can interrupt it easily; DC arcs are hard to extinguish.
6. Large central stations: High-voltage AC allowed big, efficient plants at remote sites (e.g. Niagara Falls hydro plant, 1896, sending power to Buffalo ~40 km away).
7. Polyphase systems: 3-phase AC gave constant power and lower conductor cost.

DC returned later as HVDC, only after mercury-arc and thyristor converters became available.