--- title: "Why AC prevailed over DC in early 20th century" subject: "Power System Analysis I" chapter: 1 key: "semester_4/power_system_analysis_i:ac-prevailed-over-dc" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/ac-prevailed-over-dc site: BCT Engineering --- # Why AC prevailed over DC in early 20th century Power System Analysis I · Chapter 1: General Background - Asked in: 2072 Asoj, 4 marks; 2071 Bhadra, 5 marks (asked 2 times) - Video: [Why Alternating Current is used in power transmission system?](https://www.youtube.com/watch?v=YtKsTDa4BhY) — HK CircuitEdu - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/ac-prevailed-over-dc - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-1 ## Question Why did AC power generation, transmission and distribution prevail over the DC system in the early 20th century? ## Short answer AC voltage can be stepped up/down efficiently with static transformers, enabling high-voltage transmission with low I^2 R loss over long distances. AC generators have no commutator, induction motors are self-starting, and AC arcs extinguish at current zero. ## Answer 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. | Feature | DC (early 1900s) | AC | |---|---|---| | Voltage change | Not possible statically | Transformer | | Transmission distance | 1–2 km | Tens–hundreds of km | | Generator | Commutator, limited voltage | Simple alternator | | Motor | Commutator motor | Induction motor | DC returned later as HVDC, only after mercury-arc and thyristor converters became available. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What was the main device giving AC an edge over early DC? A. Commutator B. Mercury-arc rectifier C. Static transformer for efficient voltage change D. Rotating converter Answer: C. Transformers (98-99 % efficient) step AC voltage up and down; DC had no equivalent static device. (Key point: Transformer) 2. Why could AC supply customers far away while early DC could not? A. AC has no resistance B. DC had high skin effect C. AC used thicker wires D. AC was sent at high voltage and low current, so I^2R loss was low Answer: D. Low-voltage DC limited supply to 1-2 km; high-voltage AC cut current and losses. (Key point: High-voltage transmission) 3. Why is AC easier to interrupt than DC? A. AC current passes through zero every half cycle B. AC current is smaller C. AC has no arc D. DC has no voltage Answer: A. The natural current zero helps extinguish the arc; DC has none. (Key point: Circuit interruption) 4. Which feature made Tesla's induction motor ideal for industry? A. Needs a commutator for starting B. Robust, cheap, self-starting with no commutator C. Runs only on DC D. Needs brushes Answer: B. It has no commutator and starts by itself, unlike DC commutator motors. (Key point: Induction motor) 5. When did DC return for transmission? A. In the 1880s with Edison B. With the induction motor C. As HVDC after mercury-arc and thyristor converters D. After transformers were invented Answer: C. HVDC became possible only when converters were available. (Key point: Later return of DC) ## Related in Power System Analysis I - [Advantages and limitations of DC transmission](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Adc-transmission-advantages-limitations) — Power System Analysis I, chapter 1 - [Why generation and distribution voltages are lower](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ageneration-distribution-lower-voltage) — Power System Analysis I, chapter 1 - [Skin effect with AC and DC current](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-effect-ac-dc) — Power System Analysis I, chapter 4 - [Typical power system with voltage levels](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atypical-power-system-voltage-levels) — Power System Analysis I, chapter 1 - [Interconnected power system and advantages](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-system-advantages) — Power System Analysis I, chapter 1 - [Maximum power over 20 + j50 Ω line](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Amax-power-20-j50-33kv) — Power System Analysis I, chapter 6 ## Related in other subjects - [AC vs DC transmission comparison](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aac-vs-dc-transmission) — Transmission and Distribution System Design, chapter 1 - [Issues of EHV-AC power transmission](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aehv-ac-transmission-issues) — High Voltage Engineering, chapter 1 - [Dielectric power loss under AC and DC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Adielectric-power-loss-ac-dc) — High Voltage Engineering, chapter 6 - [Advantages and disadvantages of HVDC over AC](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-advantages-disadvantages) — High Voltage Engineering, chapter 1, asked 3×