AC and Transformers

In the 1880s, Edison's DC system could deliver power about half a mile from the generator. Westinghouse's AC system, using transformers, could deliver it hundreds of miles. AC won the War of Currents for one reason: transformers can step voltage up and down cheaply and efficiently, and transformers only work with AC.

A transformer requires a changing magnetic field to transfer energy between two coils. AC provides that change automatically (the current reverses direction 120 times per second at 60 Hz). DC produces a steady magnetic field that induces no voltage in a secondary coil. This is why every grid in the world runs on AC for transmission and distribution: the ability to step voltage up (reducing current and losses for long-distance transmission) and back down (for safe end-use) depends entirely on the transformer, which depends entirely on AC.

The voltage transformation follows a simple ratio: V_out / V_in = N_out / N_in (turns ratio). Power is conserved: higher voltage means proportionally lower current. This is the key insight. Losses in transmission lines scale with I²R. Stepping voltage up by 10x cuts current by 10x and cuts losses by 100x.

Worked Example

A power plant generates at 20 kV. A transformer steps this up to 400 kV for transmission.

• Find the turns ratio. 400/20 = 20. The secondary coil has 20x more turns.
• At 500 MW output, calculate current at each voltage. At 20 kV: I = 500,000/20 = 25,000 A. At 400 kV: I = 500,000/400 = 1,250 A.

By what factor do resistive losses decrease after stepping up voltage?

Losses scale with I². (25,000/1,250)² = 20² = 400. A 400-fold reduction in transmission losses, all because a transformer could step voltage up. This is the physical reason AC dominates grid transmission.

The transformer-AC partnership is the foundational technology of the modern grid.

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