Most of the transformers used outside the United States and Canada are designed to operate at ___ Hz.

• A. 40
• B. 50
• C. 60
• D. 70

Answer: (B)

Transformers are designed around the frequency of the power system because the magnetic flux in the core depends on the voltage and the operating frequency. The turns-per-volt is chosen so that, at the system frequency, the core flux stays within its design limits. Since most countries use a 50 Hz power grid, most transformers sold outside the United States and Canada are built for 50 Hz. In contrast, North America uses 60 Hz, so those transformers are designed for 60 Hz. The other frequencies listed aren’t the global standard, so 50 Hz is the correct choice.