A medium‑voltage transformer can hum gently during normal operation. The characteristic sound arises mainly from vibrations of the magnetic core, through which the alternating magnetic flux flows. A steady hum usually does not indicate a fault. Concern should be raised by a sudden change in noise level, metallic buzzing, crackling, knocking, excessive vibration or a simultaneous increase in device temperature. In such cases, voltage, load, power quality, the condition of mountings, ventilation and the transformer's foundation should be checked.

An MV transformer rarely operates in complete silence. Even a modern device, made of high‑quality materials and correctly installed, can emit a characteristic low sound. For some, it will be a barely audible murmur. In another room, with hard walls, a large metal enclosure and inadequate ventilation, the same sound can become the dominant acoustic element of the entire station.

This is where the problem begins. The hum itself does not necessarily indicate a fault, but its character can tell a great deal about the transformer's operating conditions. Sometimes the sound is a natural consequence of core operation. Other times, it signals overloading, incorrect voltage, a loose structural element, enclosure resonance or an improperly designed room.

Why does a medium‑voltage transformer hum? How to recognise a sound typical of normal operation? When is immediate action needed? And can noise be reduced without risking overheating of the device?

In this article, we look at the problem from both a technical and practical perspective. We will start with the source of the sound itself, then move on to the most common causes of excessive noise, diagnostic methods and solutions worth considering already at the transformer station design stage.

Why does an MV transformer hum?

The most typical hum of a transformer comes from its magnetic core. The core performs an extremely important job: it guides the magnetic flux and enables the correct transformation of voltage between the medium‑voltage and low‑voltage sides.

It is made of thin, mutually insulated ferromagnetic laminations. Their task is to limit losses caused by eddy currents. When alternating current flows through the transformer windings, a changing magnetic flux is created in the core. Under the influence of this flux, the core material slightly changes its dimensions.

This phenomenon is called magnetostriction.

The deformations are very small, often invisible to the naked eye, but they repeat many times per second. At the grid frequency of 50 Hz, vibrations are generated that can be perceived as a low, steady sound. In practice, the audible frequency is often associated with the 100 Hz component, because the core reacts to the change in the magnetic field in every half‑cycle of the alternating voltage.

It can be said that the transformer "sings" under the influence of the magnetic field. It is not, however, a random song. Its volume and character depend on many factors: the type of laminations used, the precision of core construction, the way it is assembled, mechanical stresses, voltage level and the construction of the tank or enclosure.

If the core has been correctly designed and the transformer operates under conditions consistent with the documentation, the sound should be stable and predictable. It may be audible, but it usually has no sudden changes or additional alarming noises.

Is transformer humming normal?

Yes – a gentle and steady hum from the transformer is a normal phenomenon.

It is worth clarifying, however, what "normal" means. It does not refer to a specific sound level that would be identical for every unit. A large oil‑immersed transformer, a cast‑resin transformer operating in an enclosure and a small unit installed in a well‑soundproofed room can sound completely different.

The most important thing is whether the sound corresponds to the parameters foreseen for the given device and whether it remains stable over time.

A transformer that has emitted a low, steady tone since the day of commissioning, and whose temperature, load and electrical parameters remain correct, most often gives no cause for concern. The sound may be more audible during increased load, especially if the load current or cooling conditions change.

A transformer that operated quietly for many months and then started to hum noticeably louder should be treated differently. Even more attention is required for sounds that were not present before: metallic buzzing, irregular crackling, rhythmic knocking, friction noises or vibration of enclosure elements.

In such situations, it is worth asking a simple question: does the transformer sound the same as before?

This is one of the most important questions in practical diagnostics. A change in sound does not yet indicate a specific cause, but it is a signal that the device or its surroundings should be checked.

When should transformer noise be a concern?

What matters most is not the mere presence of sound, but its change. A transformer can operate for years, emitting a gentle hum that remains at a similar level. If it suddenly starts to resonate, buzz or vibrate, this should be treated as diagnostic information.

A sound that appears only at a specific load should also be concerning. For example, a transformer operating quietly at night starts to hum intensely after the start‑up of large motors, variable frequency drives or other loads with variable power consumption. Such a symptom may indicate a problem with power quality, harmonics or load conditions.

Concern should also be raised by noise combined with other symptoms. If, along with an increase in volume, the temperature of the windings or oil rises, there is a smell of overheated insulation, alarm fans are operating, or the monitoring system registers abnormalities, further operation without checking the cause may be risky.

Attention should also be paid to where the sound is coming from. If the transformer itself is operating stably but one of the covers, enclosure doors or ventilation grilles is resonating loudly, the source of the problem may be mechanical. A small amount of play can turn a quiet hum into a very irritating metallic buzz.

In the medium‑voltage environment, no sound that appears suddenly should be underestimated. The final assessment always rests with qualified personnel, but a rapid report of a change can help detect a problem before it leads to a more serious failure.

What causes louder transformer humming?

One of the most common causes of louder operation is excessive supply voltage. Every transformer has been designed to operate at specific parameters. If the medium‑voltage voltage is higher than the rated value, the core may operate with a higher magnetic flux.

Under such conditions, the magnetising current increases, no‑load losses rise, and core vibrations can become more pronounced. The transformer then starts to hum more intensely and, at the same time, may operate at a higher temperature.

A similar effect can appear in the case of an incorrect tap‑changer setting. Taps allow the transformer ratio to be adjusted to grid conditions, but their position must correspond to the actual voltage parameters and the device documentation. Changing the tap setting is not an activity that should be performed intuitively. It requires de‑energising the device, securing the work area and applying the correct procedure.

Another cause can be voltage waveform distortion. Modern industrial plants use many power electronic devices. Variable frequency drives, rectifiers, switch‑mode power supplies, UPS systems and photovoltaic installations can affect the power quality in the grid.

Higher harmonics increase losses in the transformer and can cause additional vibrations. The device then starts to react to a voltage waveform that deviates from a perfect sine wave. This is precisely why, in the case of unusual noise, a power quality analysis should be performed, rather than limiting the diagnosis to a simple rated voltage measurement.

Can a loose core cause noise?

Yes. A loose core packet is one of the possible causes of characteristic metallic buzzing.

The transformer core must maintain adequate rigidity. The individual elements are precisely assembled and secured to limit mutual movement and the transmission of vibrations. If the clamping pressure changes over time, fastening elements loosen, or mechanical wear appears, vibrations can intensify.

The sound then often ceases to be a soft, steady hum. A harder, more metallic tone appears, sometimes resembling the buzzing of thin sheet metal. Vibrations may be felt on the enclosure or structural elements, though this is not always the rule.

Core diagnostics require particular care. An MV transformer must not be opened or inspected internally while energised. Disconnection, isolation and earthing of the device in accordance with applicable procedures are necessary. Visual inspection and any adjustment work should be carried out by persons with appropriate qualifications and experience.

It is worth remembering that not every metallic sound comes from the core. A loose cover, grille, bracket, guard or supporting structure element can behave very similarly. Therefore, diagnosis should be carried out methodically, starting with the simplest possible causes.

Can the enclosure amplify the transformer's sound?

The enclosure can act like a resonance box. The transformer emits a certain level of vibration, but the way the sound propagates through the room depends on the construction of the entire station.

A large metal surface, a thin cover or an insufficiently rigid wall can start to vibrate under the influence of the device's operation. As a result, small vibrations are amplified and become much more audible.

Sometimes, a small amount of play in a hinge, an undertightened screw or an element touching the enclosure is enough to produce an irritating resonance. The sound may then be audible mainly in a specific part of the room, for example near one of the walls or close to a cable trench.

A similar effect occurs in empty, hard‑surfaced rooms. Concrete walls, ceilings and floors reflect sound waves instead of absorbing them. The transformer may not be working louder, but the sound is perceived as more intense.

For this reason, the noise level should be assessed together with the measurement conditions. The same transformer sounds different in an open container station, different in a large hall, and different again in a small technical room located near office spaces.

Does the installation location affect transformer noise?

The installation location has a very large impact. The transformer generates vibrations that can propagate both through the air and through the building structure.

If the device is placed on an improperly prepared foundation, vibrations can transfer to the walls, ceiling, cable trenches and adjacent structural elements. People in neighbouring rooms may then hear a low sound or feel slight vibrations, even if the noise level in the transformer compartment itself does not seem particularly high.

Levelling is also important. An uneven surface can cause uneven load distribution and additional stresses in the device structure. For oil‑immersed transformers, attention must be paid to correct tank support. For cast‑resin transformers, structural stability, cable routing and proper enclosure fixing are important.

At the design stage, it is worth planning how to reduce vibration transmission. Appropriate vibration isolation solutions can be used, but they must be matched to the device weight and operating conditions. Randomly placing damping material under the transformer is not a technical solution and can worsen the device's stability.

Why can fans cause noise?

In dry‑type transformers and in installations with forced cooling, part of the noise can come from the fans.

This cause is easiest to recognise by observing when the sound appears. If the transformer operates quietly until the fans start, and then a hum, vibration or rhythmic buzzing appears, the cooling system should be checked.

A fan can be noisy due to a worn bearing, an unbalanced impeller, dirty blades or incorrect mounting. The sound can also be amplified by grilles, ventilation ducts and enclosure elements.

In the case of air ducts, turbulence can be a problem. If the airflow is poorly directed, changes direction at a sharp bend or encounters an obstacle, additional aerodynamic noise is generated. The room can then be noisy even though the transformer itself is operating correctly.

Ventilation grilles should not be covered, nor should airflow be restricted to reduce sound. The transformer needs adequate cooling, and any interference with the ventilation can lead to a rise in winding temperature and protection operation.

How to check exactly where the noise is coming from?

The first step is observation. It is worth determining whether the sound occurs constantly or changes with load. Check whether its level increases after the start‑up of specific machines, fans or reactive power compensation systems.

Locating the source is also helpful. The sound should be assessed at the transformer, at the enclosure, at the doors, at the ventilation grilles and in places where the structure contacts the building. In practice, it often turns out that the loudest point is not directly at the transformer.

For a professional assessment, sound level measurement and spectrum analysis can be used. The spectrum allows checking whether the dominant components are related to core operation, fans, mechanical vibrations or auxiliary devices.

When vibration transmission through the structure is suspected, vibration measurements can be useful. Thermography, in turn, can check whether the noise is accompanied by overheating of connections, terminals, cables or apparatus components.

The decibel measurement alone will not answer all questions. Two devices can have a similar noise level but completely different causes and different operational risks. Therefore, the result should be interpreted together with temperature, load, voltage, power quality and the transformer's operating history.

How to correctly measure transformer noise?

The measurement should be carried out under specified and repeatable conditions. The microphone distance from the device, measurement height, background noise level, ambient temperature and transformer load all matter.

If one measurement was taken at minimum load from a distance of two metres, and another at full load from a distance of one metre, a direct comparison of the results may be misleading.

When accepting a new unit, it is worth recording the conditions under which the measurement was made. Such a result becomes a reference point for future inspections. If, after a year or two, the noise level increases significantly, this will be important information for the maintenance team and the service.

The measurement should also include the acoustic background. In a small room, sound reflects off the walls, and other equipment may be operating nearby. Without considering these factors, the result will not always reflect the transformer's actual emission.

How to reduce transformer noise?

The best method of reducing noise is to eliminate its cause. If the source of the problem is excessive voltage, the grid parameters must be analysed. If a loose cover is to blame, the play should be removed. If a fan is causing the noise, its technical condition and mounting method should be checked.

Vibration isolation can limit the transmission of vibrations to the foundation and building structure. It should, however, be matched to the transformer weight and the loads occurring during transport, installation and operation. Incorrectly selected elements can worsen the device's stability.

In technical rooms, acoustic solutions can be applied to walls and ventilation ducts. Their task is to absorb or limit sound reflections. The materials must, however, comply with fire, environmental and operational requirements.

For transformers operating inside buildings, the required noise level should be specified at the purchase stage. A later attempt to "soundproof" a finished installation can be much more expensive and complicated than choosing a device with parameters matched to the facility.

It must also be remembered that sound‑absorbing material will not solve the problem of an overheating transformer, a loose core or incorrect voltage. Soundproofing can reduce the sound level perceived in the room, but it should not be used to hide technical symptoms.

Is an oil‑immersed transformer quieter than a cast‑resin one?

There is no single answer that would be true for all models.

An oil‑immersed transformer and a cast‑resin transformer have different constructions, different cooling methods and different installation conditions. In an oil‑immersed transformer, the sound can be amplified by the tank. In a cast‑resin unit, the enclosure, the way the windings are fixed and fan operation can play a greater role.

The noise level is determined by the specific design solutions, manufacturing quality and installation conditions. Therefore, when choosing a transformer, the data for the specific model should be analysed, rather than relying on the general belief that one type will always be quieter than the other.

In indoor facilities, other features may also be important. A cast‑resin transformer may be preferred due to its limited fire risk and the possibility of installation close to consumers, while an oil‑immersed transformer may better suit the requirements of large outdoor stations and industrial installations. Each case requires individual selection.

How to prevent noise problems?

Most can be done before the transformer is delivered. The designer and investor should determine where the device will operate, who will be nearby, and what acoustic requirements apply in the facility.

A station for a production plant is designed differently than one for an office building, hospital, shopping centre, hotel or data centre. In each of these places, the transformer has the same basic function, but the consequences of noise will be different.

It is worth checking the declared sound power level, measurement conditions, cooling method and foundation requirements. The location of ventilation ducts and cable routes should also be analysed. Sound can travel far from the device itself.

A well‑executed design also considers service access. If the transformer is enclosed in a way that makes inspection difficult, any subsequent diagnostics will be more time‑consuming. It is then easy to overlook a loose element, a deteriorating fan or a change in temperature.

What to do when the transformer suddenly starts working loudly?

If the transformer suddenly starts humming loudly, the first step should be to note the circumstances. It is worth checking when the sound appeared, at what load it occurs and whether other symptoms are present.

The device should not be opened or its elements touched without proper authorisation. Medium‑voltage devices require appropriate safety procedures. Inspection, measurements and any service work should be carried out by qualified personnel.

Until the cause is clarified, attention should be paid to temperature, protection indications, load and fan operation. If a smell of burning, oil leak, signs of overheating, strong vibrations, crackling or alarms appear, the situation should be treated as urgent.

In some cases, the cause will turn out to be a minor resonance of a cover. In others – a problem with voltage, power quality or the mechanical condition of the core. Without measurements, it is not worth guessing.

Transformer noise and the acoustics of the entire MV station

The transformer does not operate in isolation from the rest of the infrastructure. The station room also contains switchgear, protection apparatus, ventilation, cables, busbars and structural elements.

Each of these components can emit vibrations or amplify sound. In switchgear, the source of noise can include, among other things, electromagnetic vibrations, loose elements, drives, ventilation and resonating panels.

Therefore, the station's acoustics should be treated as a whole. A properly designed structure, rigid frames, correct mountings, appropriate partitions and vibration transmission control can improve both working comfort and equipment durability.

More on how MV switchgear acoustics affect safety, diagnostics and infrastructure durability can be found in the article

"The silence that protects: how MV switchgear acoustics affect safety and durability".

A quiet transformer starts with proper selection

The noise level should be one of the parameters analysed before purchasing a transformer. It is not worth leaving this issue to the very end, when the device is already ordered and the room is ready.

When choosing, voltage, power, connection group, short‑circuit impedance, cooling method, dimensions, weight, installation conditions and the declared noise level should be considered. Actual operating conditions are also important: the number of hours under load, the character of the loads, the presence of harmonics and the possibility of future installation expansion.

The Energeks range includes oil‑immersed and cast‑resin MV transformers intended for use in power engineering, industry, infrastructure and installations requiring reliable power supply. The full range can be seen on the Energeks transformers page.

If the project requires fast delivery, transformers available off‑the‑shelf in the Energeks shop are also worth checking. Before selecting a specific model, all parameters should of course be confirmed against the project documentation and station operating conditions.

A sound not worth ignoring

The humming of an MV transformer is a natural consequence of magnetic core operation. The sound itself does not yet mean a fault. Much more important is its stability, character and relationship with the device's other parameters.

A steady tone that has accompanied the transformer's operation from the start is usually within normal operating conditions. A sudden metallic buzz, an increase in noise level, knocking, crackling, vibrations or noise appearing together with elevated temperature require checking.

Sometimes the problem is in the core. Sometimes in the voltage, power quality, fan, foundation or a loose cover. Therefore, correct diagnostics start with observation and end with measurements performed by appropriately trained personnel.

A well‑chosen transformer, a correctly designed station and regular parameter monitoring allow the noise to be kept at a predictable level. And predictability in power engineering is of enormous value – it means greater safety, easier maintenance and a lower risk of unplanned downtime.

Thank you for reading. If you are planning an investment, station modernisation or transformer replacement, we invite you to contact us to discuss a solution matched to real operating conditions. We are also happy to talk about cooperation and partnership opportunities for energy, industrial and infrastructure projects.

We encourage you to check the Energeks transformer range, and for urgent projects, also the transformers available off‑the‑shelf.

We also warmly invite you to our Energeks LinkedIn page, where we publish information about transformers, projects, solutions for the power industry and practical issues related to the operation of electrical infrastructure.

References:

1. IEC 60076-10:2016 – Power transformers: determination of sound levels

2. IEC 60076-1:2011 – Power transformers: general

3. Why Transformers Hum: Magnetostriction, Electromagnetic Forces and Cooling Sources