Why Do Transformers Hum?

A Simple Explanation of a Common Electrical Sound

Transformers are everywhere in our power systems – from massive utility substations to small adapters at home. But if you’ve ever stood near one, you’ve likely heard a familiar low-pitched buzzing or humming sound. This “hum” isn’t just noise; it has a scientific explanation rooted in the way transformers work.

The hum from transformers is primarily due to a phenomenon called magnetostriction. This refers to the physical expansion and contraction of the transformer's core material when it’s magnetized. As alternating current (AC) flows through the transformer, it constantly changes direction, causing the magnetic field to fluctuate.

These fluctuations make the iron core of the transformer slightly expand and contract. Even though the movements are tiny – not visible to the eye – they happen rapidly, 100 times per second in a 50 Hz power system. This rapid vibration is what generates the audible humming sound.

Interestingly, the sound frequency is not 50 Hz but typically twice that – around 100 Hz. That’s because magnetostriction depends on the square of the magnetic field, which makes the core vibrate twice for every full cycle of AC current.

Another contributing factor is loose components or laminations inside the transformer. The core is built from thin laminated steel sheets stacked together to reduce energy losses. If these laminations aren’t tightly clamped, they can vibrate slightly, adding to the hum.

The transformer’s casing can also act like a sounding board, amplifying the vibrations. Just like how a guitar body makes string vibrations louder, the metal housing of a transformer can make the hum more noticeable.

Transformer hum is usually more prominent in larger transformers, especially those outdoors or in substations. These handle higher voltages and larger magnetic fields, leading to more core movement and louder sound.

It’s important to note that the hum is not a sign of malfunction. A quiet, consistent hum is considered normal. However, if the sound suddenly increases or becomes erratic, it could indicate a mechanical or electrical issue.

Engineers do try to minimize the hum through better design. They use materials with lower magnetostriction, tightly clamp the core, and use sound-dampening enclosures.

In some sensitive locations like hospitals or offices, special low-noise transformers are used to reduce acoustic disturbance. Noise regulations may also guide transformer installation and shielding.

So, the next time you hear that steady buzz from a transformer, you’ll know it’s the sound of magnetic energy at work – a quiet hum from the heart of our power system.