Part 4: Understanding Power Transformer Noise

What is noise?

Sound is essentially the vibration or movement of air or another medium, which is perceived by the ears of humans or animals. As noise, often described as an unwelcome sound, can disrupt communication, work, or relaxation. Measured in decibels (dB), its effects vary based on the sound's frequency and intensity.

Noise is a subjective experience, varying from person to person. For instance, the sound at a heavy metal concert might reach 130 dB and be enjoyed by a crowd of fans, yet to others, it is merely loud noise. Conversely, if the same concert-goers were next to a siren at the same or even a lower volume, they would likely cover their ears and complain about the noise. The key distinction between sound and noise lies in the perception of the listener. As we age, our hearing and our perceptions of sound and noise evolve.

As our lives become increasingly filled with electrical equipment and appliances, we must accept that there will always be someone who finds the noise bothersome. If we look back to the mid-1900s, running appliances were significantly louder than they are today. Advances in design and materials have led to much quieter devices.

Power transformers have always produced a distinctive sound, which some find soothing while others consider it an annoying noise. Since their introduction into the manufacturing sector, engineers have continuously sought ways to reduce the noise generated by transformers, as they have become increasingly integral to our daily lives.

Over the years, various remedies have been developed to address the issue of objectionable noise, but no single solution exists due to the numerous factors that can contribute to and amplify the noise. For instance, older amplifiers often had wooden chassis, which helped reduce transformer vibration noise. In contrast, modern amplifiers typically use aluminum or steel chassis, which can amplify the buzz caused by transformer vibrations. Additionally, the increasing demand on voltage lines has significantly taxed power supplies. New power supplies that utilize advanced techniques, such as regulators, have also had an impact.

All power transformer hum.

The hum originates from magnetostriction, a phenomenon where the metal core of a transformer, typically composed of thin sheets known as "core laminations," vibrates due to magnetic forces. This vibration results in the characteristic humming sound.

Transformer designs often incorporate interleaved core laminations to minimize losses and manage temperature rise. Magnetostriction, a property of magnetic materials, causes them to alter their shape or dimensions during magnetization. As the lamination magnetizes, it expands, and when demagnetized, it contracts. With alternating voltage and current, each lamination sheet undergoes expansion and contraction twice in a full frequency cycle at each peak. These expansions and contractions occur at different points and times across the core laminations, creating micro movements that result in vibrations between each sheet. This erratic movement generates a hum as magnetic flux flows through the laminations.

The core laminations of the transformer experience alternating flux, causing vibrations. In the United States, power lines operate at a frequency of 60 cycles per second (60 Hz), resulting in the transformer vibrating at 120 cycles per second, which is twice the power line frequency. In contrast, countries like those in Europe use a supply frequency of 50 cycles per second (50 Hz), leading to transformer vibrations at 100 cycles per second.

Magnetic flux is influenced by the applied voltage and current, which in turn affects the amount of magnetostriction produced. When designing, it is crucial to understand the core material and its flux density, as well as to design power transformers around the supplied voltage. Changes in the supplied voltage and current will result in corresponding changes in the magnetic flux.

Transformer cores are inherently asymmetrical, resulting in a hum, noise, and vibration that is not a pure tone but rather a complex sound with multiple harmonics. These harmonics are directly related to the power line frequency. By analyzing the noise, it is possible to determine whether it originates from the transformer or other sources.

Transformer hum is a design issue, right?

Yes and no. Designing a power transformer involves adhering to specific requirements such as voltage, current, and size. The core's flux density, provided by the manufacturer, plays a crucial role in calculating the number of turns in the windings. For example, M6 29 gauge grain-oriented silicon steel lamination is a popular choice due to its high flux density, high permeability, and low losses. This material offers a flux density range of 15,000 to 18,000 gauss, or 1.5 to 1.8 Tesla. The flux range is fixed, meaning the magnetostriction effect remains constant regardless of changes in magnetization.

Hum vs. Buzz

Sound is inherently subjective but distinguishing between a hum and a buzz can be helpful. A hum is characterized as a low, steady, continuous sound, while a buzz is described as a low, continuous humming or murmuring sound. Essentially, a buzz can be considered a type of hum, but not all hums are buzzes. Another way to describe a buzz is as a low, vibrating, humming sound, often produced by machinery, indicating a more mechanical nature. In the context of transformers, a hum results from the magnetostriction effect on the steel core, whereas a buzz is a secondary sound caused by the vibration of the steel core. This mechanical noise can originate from the windings in the transformer, the vibration of the transformer on a chassis, or the resonance of the chassis or other components.

Is loaded transformer is louder?

Some believe the transformer noise is increased by putting a load on the transformer. If a power transformer is running at its designed voltages and currents, as it heats up the hum will actually lesson from the initial energizing. Some may hear a noise that is contributed by the winding as the transformer is loaded. These are a mechanical noise that is caused by the forces existing between the wires a in the transformer vibrating at twice the frequency of the power line. This noise is normally drowned out by the hum of the core. It is a very low hum. If the line voltage changes, this can cause an increase is the hum. Again, this is due to the change in flux and the change of the voltage from the design. This voltage change is also reflected into the secondary voltages.

The noise generated by a loaded system can vary in intensity depending on the design and components used. For instance, a power supply built with low-quality components that are out of tolerance can create an imbalance in the design. This imbalance can exert unusual stress on the transformer, resulting in a louder noise.

Some people believe that transformer noise increases when a load is applied. However, if a power transformer operates at its designed voltages and currents, the hum will actually decrease as it heats up from the initial energizing. The noise attributed to the windings when the transformer is loaded is a mechanical noise caused by the forces between the wires vibrating at twice the frequency of the power line. This noise is usually drowned out by the core's hum, which is a very low hum. Changes in line voltage can cause an increase in the hum due to the change in flux and voltage from the design, which is also reflected in the secondary voltages. The loaded noise can become louder depending on the design and components used in loading. For example, a power supply design using low-quality components that have a high tolerance can create an unbalanced load effect on the transformer, producing a louder noise.

The issue less thought of.

When designing a new product or building from an existing design, it is crucial to consider the power line voltage. A power transformer uses the power line voltage and frequency to determine the core material and size, which can be challenging when designing for international markets. For instance, the United States uses a standard 120V, 60Hz, while the European Union uses 230V, 50Hz, and other regions may use 240V, 50Hz, or 100V, 60Hz. Over the past few decades, the increased use of digital equipment, computers, appliances, and HVAC units has strained aging power grids and lines. In the US, the voltage on a standard outlet can vary by 5 volts or more above or below 120V, depending on the time of day and usage. Additionally, the rise in digital equipment and switching power supplies has led to "dirty" power lines with DC backwash voltages and phase issues. When designing or building electrical equipment, it is essential to account for these factors. For example, a power transformer with a 120V primary and a 600V secondary will produce 610V if the power line supplies 122V. If the transformer has a loading factor, the secondary voltage could be even higher. This slight increase in voltage can also increase the transformer flux. Dirty DC voltages on the power line can contribute to noise, potentially causing load issues and performance problems with other components, especially those of lower quality or higher tolerance.

What is acceptable noise?

Let's go over what we know:

• Sound is a movement of air.
• The transformer core produces vibration.
• The amount of hum produced is fixed by the design and core material.
• Adjustments to the transformer design to reduce the hum have little effect.
• Dirty power lines and varying voltages may cause issues.
• Loading of a transformer with low quality or high tolerance components can increase the noise.
• The core vibrations may produce noise from mounting and resonating other components or parts.

In the United States, the National Electrical Manufacturers Association (NEMA) has established a standard known as ST 20, which specifies the acceptable sound levels for transformers. This standard is determined based on the transformer's rated volt-amperes, ensuring that the noise produced by these devices remains within acceptable limits.

The chart below give an example for typical sound levels.

EDCOR specializes in manufacturing dry-type transformers with a capacity of less than 500 VA. According to NEMA standards, the acceptable noise level for these transformers is 40dB. However, some users still find this noise level to be unsatisfactory.

Now the big question.
How do I get rid of this noise?

Due to the laws of physics, transformers inherently produce some level of noise, which is unavoidable. Therefore, our focus should be on identifying and mitigating other sources of noise to ensure optimal performance and minimal disruption.

The first step in selecting a power transformer is to choose one that best suits your application or design. It's crucial to ensure that your design does not draw more current than the transformer's rating, as this can lead to overheating and core saturation. Power transformers operate most efficiently at their rated power, and running them underloaded can result in increased noise. Therefore, it's important to match the transformer's capacity with your specific needs to ensure optimal performance and longevity.

Varnished or encapsulated?

When choosing a transformer type, you have the option of varnished or encapsulated (potted). Each has its advantages and disadvantages. Some manufacturers opt for encapsulating their transformers, a process that is quite expensive but can help reduce noise depending on the encapsulating material. However, if the material is too rigid, it will still transmit vibrations. Additionally, encapsulation limits the essential heat dissipation required by a power transformer to release heat from the core. This restriction can lead to overheating and eventual failure.

Varnishing is a cost-effective method that still allows for heat dissipation. The transformer is immersed in an electrical varnish and then dried either by air or in an oven. Oven drying is faster, but it results in a yellow, cracked, and crusty appearance.

Vacuum pressure impregnation (VPI) is a varnishing process where a transformer is placed in a vacuum vessel filled with varnish. The vacuum removes air from the transformer, allowing the varnish to penetrate deeply into the coil windings. This method can help reduce coil vibrations. Although VPI is more expensive due to the time and specialized equipment required, it offers a slight improvement in performance. Other processes exist, but they are primarily used for transformers with a capacity over 1KVA.

Mounting

If a transformer is producing an unacceptable noise, it's essential to identify the type of noise and its source. Nowadays, metal chassis are commonly used in products or builds, making the most frequent issue the vibration buzzing of the transformer mounted directly to the metal chassis. To resolve this, you can simply install a rubber mounting pad between the transformer and the metal chassis to absorb the vibration.

Additionally, using sealing washers on the mounting screws that secure the transformer to the chassis can further help. A sealing washer is a metal washer with a rubber washer attached to one side. If you use sealing washers, ensure that a grounding wire is attached to a mounting screw of the transformer to ground it to the chassis. These mounting pads and sealing washers can effectively address resonating issues as well.

If the noise is caused by transformer vibrations resonating through the chassis or other components, you can attempt to realign the core material by gently tapping the transformer's core with a hammer. Be careful to tap just enough to adjust the core without leaving marks. This method is less effective for older transformers or those with dry varnish, as the core alignment is harder to change.

Choosing high-quality components with low tolerances and designing a well-balanced circuit are crucial for minimizing noise. Careful circuit design and component selection play a significant role in achieving optimal noise reduction.

Power lines issues is not an easy problem to solve. There are though, equipment like power conditioners  and uninterrupted power supplies that are used a lot in the audio industry. Conditions help clean up the power line while uninterrupted power supplies help maintain a constant voltages during peaks and brown outs.

Power line issues can be challenging to address, but using equipment like power conditioners and uninterrupted power supplies, commonly found in the audio industry, can be beneficial. Power conditioners help to clean up the power line, ensuring a stable and clean electrical supply. Uninterrupted power supplies maintain consistent voltage levels during peaks and brownouts, providing reliable power and protecting sensitive equipment.
 
Ensure all hardware connections are secure, as any vibration can lead to excessive noise from loose parts.

Last note.

EDCOR employs power conditioners and Variac variable transformers for testing equipment. Using a variable transformer is highly recommended for testing purposes, as it allows you to gradually increase the voltage while monitoring voltages and current. This method helps identify and prevent damage to components caused by shorts, miswiring, and faulty parts.

https://variac.com

https://www.amazon.com/s?k=variable+transformer

Previous: Part 2: Designing a Single-Phase Power Transformer