Power Transformer Noise: Causes, Measurement and Control

Power transformers emit a low, continuous humming sound when in operation, a phenomenon common to almost all dry and liquid-immersed transformers. This sound mainly comes from the vibration of the iron core and windings under the action of an alternating magnetic field, and does not in itself indicate a malfunction in the equipment. But when the sound suddenly gets louder, sharper, or mixed with irregular noises, it’s worth taking seriously. Only by clarifying the sources, measurement methods, and control methods of noise can we distinguish which sounds fall within the normal range, which need to be checked, and how to control the sound level within an acceptable range during the design and installation stages.

Where does the noise come from

Transformer noise is caused by the superposition of several factors, the most important of which is the magnetostriction of the iron core. The iron core usually uses low-loss magnetic materials such as cold-rolled oriented electrical steel. Under the action of alternating magnetic flux, the size of the silicon steel sheet will periodically expand and contract with the strength of the magnetic field, forming mechanical vibration, which is then transmitted outward through the iron core and clamps, and finally becomes audible air sound. The material grade, silicon steel sheet thickness, working magnetic flux density and joint structure will directly affect the no-load loss and noise level. This is why the sound levels of transformers of the same capacity may differ significantly.

Structural resonance cannot be ignored either. Iron cores, clamps, housings, and even mounting foundations all have their own natural frequencies, and vibrations are significantly amplified when the excitation frequency approaches these natural frequencies. The fan installed on forced air-cooled products is another independent noise source, and the fan operation itself produces airflow and mechanical sounds. When harmonic currents exist on the load side, the harmonics will also generate additional periodic excitation in the core and windings, further raising the noise. The installation environment will also change the final sound level heard. The same equipment will sound different when placed outside an open room or in an enclosed room.

The core is often the key

In most cases, the iron core is the key component that determines the amount of noise. Burrs during the manufacturing process, short circuits between silicon steel sheets, mechanical stress during stacking, and joint quality can all increase no-load loss and noise. The stepped joint structure can make the magnetic flux distribution at the core joint smoother, thereby reducing joint loss and no-load noise. This is why, in projects with high low-noise requirements, the core selection and stacking processes are emphasized separately.

How to measure noise

When evaluating the noise of a transformer, one cannot look at just one isolated number. Sound level data must specify what quantity was measured, which weighting method was used, at what distance, the operating status of the equipment, and whether the cooling accessories were put into use. During the test, the transformer body and cooling accessories such as fans and pumps must be handled separately, and the methods, distances and background corrections for sound pressure or sound intensity must be specified for each. Measurements taken in the factory and those heard on-site often differ due to background noise and the installation environment, making it easy to misjudge when comparing two numbers directly.

For projects like data centers that are sensitive to the sound environment, sound level verification must further specify whether it is natural cooling or forced cooling, no-load or loaded, the measurement distance, and the background noise level before it can be compared with the noise target in the computer room. Any unconditional promise of silence is not verifiable.

Take abnormal noise seriously

Noise is smooth and monotonous during normal operation. If there are significant changes in the sound, you should pay attention. Abnormal noise may come from over-excitation, harmonics, loose magnetic core or structure, fan or pump failure, or it may be caused by external resonance. The size of the sound alone cannot determine whether there is an internal failure. Voltage and frequency together determine magnetic flux density. When the ratio of voltage to frequency exceeds the design value, the excitation current, core loss, noise and overheating risk will rise together. This state requires verification of operating conditions as soon as possible.

Reduce noise

Reducing transformer noise usually starts from both the design and installation levels. The vibration isolation design must first distinguish between problems such as transformer body vibration, magnetostrictive noise, structural sound transmission and airborne sound. Adding a rubber pad alone cannot solve all the noise. The selection of vibration damping pads or isolators requires the total mass of the equipment, the load at each fulcrum, the center of gravity position, the excitation frequency, the target vibration isolation efficiency, the allowable displacement and the environmental durability. The static deflection, natural frequency and damping of the vibration isolation elements must also be coordinated with the excitation of the transformer and the building structure. Choosing too soft may lead to displacement or stability problems.

It should also be noted that busbars, cables, grounding conductors, air ducts, and enclosure connections can all become bypass channels for vibration, transmitting the vibration of the main body to the building structure. If necessary, designed flexible connections should be used to maintain electrical continuity. Vibration isolation measures cannot weaken seismic anchoring, short-circuit mechanical strength, ventilation clearance, grounding or maintenance safety. Earthquake constraints and normal vibration isolation requirements need to be designed in a collaborative manner.

Although dry transformers poured with resin do not have an oil tank, their sound level is still affected by core magnetostriction, structural resonance, fans, load harmonics and installation environment. It cannot be presumed that they must have low noise just because the oil tank is eliminated in the structure. What truly determines the sound level is the core material and process, cooling method, load conditions, and installation method.

By treating noise as an engineering indicator that needs to be managed, clarifying its source, measuring it in a standardized manner, and optimizing it one by one for magnetic cores, fans, vibration isolation, and installation, the transformer can meet power supply needs while controlling its impact on the surrounding environment within an acceptable range.