A transformer getting louder: what the hum reveals

The normal hum: where does it come from?

Every transformer emits a low, steady hum, around 100 Hz (twice the 50 Hz grid frequency) and its harmonics. This noise comes from magnetostriction: under the magnetic field, the core laminations expand and contract very slightly, dozens of times a second. It is inherent to operation, present from the moment the unit is energised, and broadly constant. A silent transformer does not exist.

Why the noise rises

A transformer that has become louder almost always tells one of these stories:

• Higher load: the hum intensifies with current. If the load has grown (extension, new equipment), the noise follows, often with heating, see our guide on summer overheating.
• Harmonics: non-linear loads (variable-speed drives, switch-mode power supplies, IT, LED) distort the current and amplify the magnetic noise, while heating the unit. A transformer not designed for this (K-factor) suffers.
• Gradual loosening: over thermal cycles, core clamping, tank panels or radiators can loosen and start to vibrate. The noise becomes more "mechanical", sometimes a rattle.
• Resonance: the room, a cover or a support resonates with the transformer's frequency and amplifies a noise that was until then discreet.
• Cooling: on a fan-cooled unit (ONAF / AF), worn fan bearings add a mechanical noise distinct from the hum.

The noise that should make you act now

One category of noise is not just an acoustic-comfort matter: crackling, frying or snapping sounds, especially inside an oil-immersed unit, can betray partial discharge or a developing arc. This is a potentially serious signal, all the more so if it comes with one of the following:

• Abnormal heating or repeated trips (DGPT2, protections).
• A drop in oil level, traces, a smell.
• A detected gas release (Buchholz relay).

What you can make objective

Before concluding, document it: log when the noise appears (at what load, at what time of day), check accessible external fixings and the state of the fans, and, on an oil unit, consider an oil / dissolved-gas analysis (DGA) that "listens" inside the unit without opening it. These turn an impression into a diagnosis.

When the noise is part of a bigger picture

A transformer that becomes loud and hot and trips and has twenty years of service is not posing a noise question, but an end-of-life one. At that point, patching it up piece by piece often costs more than replacing it with a new, better-sized, low-loss unit. That is the subject of our guide on replacing an old transformer and of our replacement / retrofit offer.

This article gives general guidance and does not replace a diagnosis by a qualified professional on your installation. Faced with an abnormal noise together with heat, smoke or a smell, isolate the unit and call for help without delay.