Power Transformer Noise Reduction: Causes and Control

In the case of transformer noise, many projects wait until the sound environment in the machine room or factory exceeds the standard and are complained before returning to the issue. In fact, noise control, like thermal design and insulation design, is one of the constraints that transformers must incorporate from the electromagnetic design stage. The noise of a transformer is composed of the superposition of multiple sources. Noise reduction is never something that can be solved by replacing a rubber pad. It must be handled section by section along the line of sound source, propagation path and installation environment.

Noise comes from the core and accessories

The noise from the transformer comes first from the core. The core material will undergo slight size changes in an alternating magnetic field, that is, magnetostriction, and the core laminations will vibrate and radiate sound outward. The closer the core magnetization is to saturation and the higher the working magnetic flux density, the more obvious this vibration will be; when the voltage-to-frequency ratio exceeds the design value, the core will enter an overexcitation state, and the excitation current, temperature rise, vibration and noise will all rise synchronously.

In addition to the core body, the cooling attachment is another sound source that is easily overlooked. The forced air cooling fan of the dry transformer, the oil pump or fan of the liquid immersion product, will itself sound. Harmonics in the load will increase the vibration of the magnetic core and structural parts. Loosening of the magnetic core or clamps, and resonance between the equipment and the building structure may further amplify the original noise. Therefore, the hearing perception of the same transformer will vary greatly in different scenes and under different loads, and it is difficult to directly judge the internal state based on the sound size alone.

First, do a good job in the core

Since the magnetic core is the most important sound source, the first landing point of noise reduction is placed on the magnetic core design. The magnetic core is usually stacked with cold-rolled oriented electrical steel or other low-loss magnetic materials. The material grade, single-piece thickness, working magnetic flux density and joint structure together determine the level of no-load loss and noise. Selecting more suitable materials, appropriately reducing the design magnetic flux density, and controlling the thickness of the monolithic piece can all reduce the vibration caused by magnetostriction from the source.

Core seams are another key detail. The stepped seam structure allows the magnetic flux to be distributed more evenly at the joint, thereby reducing joint loss and no-load noise. In addition to suppressing eddy currents, the insulating layer between the laminations also helps to maintain a stable electrical and mechanical state between the laminations. The trade-offs in these designs are often constrained by losses, costs, and volume, and cannot be amplified separately from the electromagnetic and thermal designs of the entire machine.

Noise hazards left behind during the manufacturing process

No matter how well the design is set, deviations in the manufacturing process will bring the noise back. If burrs are left in electrical steel sheets during shearing or slitting, or short circuits occur between laminations, excessive stress is introduced during assembly, and the quality of the joints does not meet standards, it will be directly reflected in no-load loss and noise. Therefore, the core manufacturing process has clear requirements for shearing, stacking, seam control and clamping.

Compression of windings and structural parts is equally important. The winding compression and structural reinforcement were originally intended to limit transportation vibration and axial and radial displacement during short circuits. If they were not done firmly, an additional noise caused by loosening would occur during operation. Dry products also need to be equipped with fans, temperature control and necessary protective housings during final assembly. The installation quality of these accessories will also affect the vibration performance of the entire machine.

Don’t miss the fan and cooling attachment

When reducing noise, only focus on the transformer body, which often misses the contribution of cooling accessories. Forced air cooling products must ensure that the fan’s power supply, direction, interlocking, alarm and backup strategies are in line with the design. Once the fan is shut down, the equipment cannot continue to be used according to the forced cooling capacity and needs to be derated according to the nameplate or manufacturer data. On the other hand, the choice of cooling scheme itself also participates in the noise level: the noise characteristics of natural air cooling and forced air cooling are different, and the practice of increasing capacity with a fan must be established only when the temperature, airflow and accessory availability conditions are met.

This means that noise and heat dissipation are a pair of metrics that need to be weighed simultaneously. Weakening cooling for noise reduction will cause temperature rise and life problems; adding more fans for capacity increase will raise the sound level again. The truly usable solution is to press the noise of the body and accessories together to within the target while meeting the temperature rise limit.

Vibration isolation and sound transmission control at the installation site

At the installation site, the control focus shifted to vibration isolation and sound transmission paths. Here, we must first distinguish several different levels of problems: vibration of the transformer body, noise generated by magnetostriction, solid sound propagating through the structure, and air sound propagating directly in the air, each of which requires different treatment methods. Adding just one rubber pad won’t solve the entire noise problem.

If you really want to choose a vibration damping pad or vibration isolator, you need to first give the total mass of the equipment, the loads at each fulcrum, the position of the center of gravity, the excitation frequency, the target vibration isolation efficiency, the allowable displacement and the environmental durability, and then determine the static deflection, natural frequency and damping based on this. Choosing too soft may bring displacement and stability problems; vibration isolation measures cannot weaken seismic anchoring, short-circuit mechanical strength, ventilation clearance, grounding and maintenance safety. Seismic constraints and normal vibration isolation need to be designed in conjunction.

There is also a class of details that are often overlooked, which are vibration bypasses formed by busbars, cable tubes, ground conductors, air ducts and housing connections. They may transmit vibrations directly to the building, bypassing the vibration isolation elements, using designed flexible connections where necessary while maintaining electrical continuity. If the vibration isolation pad is unevenly compressed, the fulcrum load is incorrectly calculated, or it is crossed by a rigid pipeline, the effect will be greatly reduced, and even the equipment will be tilted and locally stressed.

How to measure noise reduction effect

The quality of noise reduction will ultimately be confirmed by measurement, and the measurement caliber of sound level directly affects the conclusion. Sound level testing requires distinguishing between the transformer body and cooling accessories such as fans and pumps, and stipulates whether to use sound pressure or sound intensity methods, measurement distance and background noise correction methods. Sound level data also need to explain the weighting method, operating status and cooling accessory status. Factory measurements and field values often vary due to background noise and installation environment.

So when you get a sound level number, the first thing to look at is whether it is measured under natural cooling or forced cooling, no load or load, at what distance, and whether there is any background correction. Otherwise, the numbers from different sources cannot be compared at all. For situations such as computer rooms where there is a clear target value for noise, the same evaluation method should be used to check it, rather than relying on an unconditional silent description. After unifying the measurement diameters, noise can truly transform from a vague perception into an engineering indicator that can be designed, accepted, and reproduced.