Red Seal Electrician Practice
TransformersQuestion 1144 of 1506

A large power transformer on a 60 Hz system makes a steady hum at 120 Hz. What is the main cause?

  1. Magnetostriction: the core length changes as it magnetizes
  2. Arcing at the tap switch contacts inside the tank
  3. Vibration of the supply cables on their supports
  4. The cooling fans running at twice the line speed
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Correct answer: Magnetostriction: the core length changes as it magnetizes

The core changes length slightly whenever it is magnetized, in either direction. That happens at each flux peak, so the hum is at twice the supply frequency.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 10.8: Practical Considerations - Transformers

Practice transformers

Questions last reviewed: 18 October 2026