A transformer hums mainly because of a phenomenon called magnetostriction in its core

 WHY DOES A TRANSFORMER HUM?

A transformer hums mainly because of a phenomenon called magnetostriction in its core, along with mechanical vibrations in the windings and structure. The humming sound is a normal operating characteristic of most power and distribution transformers.

1) MAGNETOSTRICTION – MAIN REASON

The transformer core is made of laminated silicon steel, which is a ferromagnetic material. When AC voltage is applied to the primary winding, it produces an alternating magnetic flux in the core. Due to magnetostriction, the core material slightly changes its dimensions whenever it is magnetized.

As the magnetic flux increases, the core expands slightly. As the flux decreases or reverses, the core contracts. Since AC changes direction every cycle, this expansion and contraction happens twice per cycle.

For example:

- At 50 Hz supply, vibration occurs at 100 Hz.

- At 60 Hz supply, vibration occurs at 120 Hz.

These rapid dimensional changes create vibrations, which produce the familiar humming sound.

2) WINDING VIBRATIONS

When load current flows through the primary and secondary windings, magnetic forces act between conductors. These forces cause mechanical stresses in the coils. If windings are not tightly clamped or properly impregnated, they may vibrate and increase the noise level.

3) LOOSE CORE LAMINATIONS

Transformer cores are built from thin laminated sheets to reduce eddy current losses. If these laminations are not tightly pressed together, small gaps allow them to vibrate individually. This increases the sound intensity.

4) TANK AND STRUCTURAL RESONANCE

The vibrations from the core and windings are transmitted to the transformer tank and mounting structure. The tank may act like a sounding board and amplify the noise.

5) EFFECT OF VOLTAGE AND LOAD

Transformer hum is present even at no-load because it depends mainly on voltage (magnetic flux), not on load current. Overvoltage increases magnetic flux density, pushing the core toward saturation, which increases vibration and noise. Heavy load can add additional noise due to winding forces.

SUMMARY

Transformer hum is primarily caused by magnetostriction in the core due to alternating magnetic flux. The core expands and contracts twice per cycle, producing vibrations at double the supply frequency. Additional noise may come from winding vibration, loose laminations, and structural resonance. Normal humming is expected, but excessive noise may indicate overvoltage or mechanical looseness.