Understanding Transformer Noise

Noise is defined as unwanted sound. Transformer "humming" has been known to soothe people but generally it is thought to be a nuisance. The causes and reduction of transformer noise has come to prominence again. The reason for this transformers are being placed closer to the populace like high rise office buildings, apartments, shopping malls and in their gardens. 

When placing a transformer it is becoming necessary to locate these units carefully and some planning. The best rule is plan ahead. Finding out you have a noise problem after the placement of this unit is costly, time consuming and frustrating. It is absolutely necessary to consider each case and apply the general rules of acoustic technology and be familiar with what causes transformer noise.

Transformer's Hum  

Transformer noise is caused by a phenomenon called magnetostriction.  In very simple terms this means that if a piece of magnetic sheet steel is magnetized it will extend itself. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of  magnetization.  This extension and contraction is not uniform, consequently the extension and contraction varies all over the sheet.

A transformer core is made  from sheets of special steel. The reason it is construction in this matter is to reduce losses and to reduce the heating effect. When the extensions and contractions described above are taking place erratically all over a sheet, and each one is behaving erratically, you can then get a picture of a moving, writhing construction when it is excited. These extensions are small dimensionally and cannot be seen by the naked eye. They are sufficient to cause a vibration, and as a result noise.

The act of magnetization by applying a voltage to a transformer produces a flux, or magnetic lines of force in the core. The degree of flux will determine the amount of magnetostriction and hence, the noise level. The obvious answer is reduce the amount of flux to reduce the nose. But it is not that simple, transformer voltages are fixed by there system requirements, and the amount of magnetization, by the ratio of these voltages to the number of turns in the winding. The decision on what ratio of voltage to turns will be  mainly economic. It means that the amount of flux at the normal voltage is invariably fixed, thus setting the noise and vibration level. Also increasing or decreasing magnetization does not increase or decrease the magnetostriction by the same amount. In technical terms the relationship is not linear. Therefore, we can not reduce the noise level at the source. 

We now know that the transformer hum is caused by the extension and contraction of the core laminations when magnetized. We can expect this extension and contraction to take place twice during a normal voltage or current cycle. What this means is the transformer is vibrating at twice the frequency of the supply, for example, for a 60 cycles per second supply frequency, the noise or vibration is moving at 120 cycles per second. This is called the fundamental noise frequency. Since the core is not symmetrical and the magnetic effects do not behave in a simple way, the resultant noise is not pure in tone. This is the noise or vibration produced is not only composed of a 20c/s frequency, we find the noise is made up of frequencies of odd multiples of the fundamental known as 1st, 3rd, 5th and 7th harmonic. What this means is we get noise frequencies of 120 (1st), 360 (3rd), 600 (5th) 840 (7th) cycles per second. They are not equally important and we find that the first and third harmonics predominate and produce most of the transformer sound. With this knowledge we can measure the amount of noise at these frequencies and determine whether amongst a number of other noises, we really are picking up a transformer noise.

One of the questions usually asked  – “what proportion of the transformer noise is contributed by the windings and does the noise increase as the load increases?”. There are  mechanical forces existing between individual conductors in a winding when the transformer is excited. These forces will produce a vibration and a noise, but only one which is pure in tone, i.e. at twice the exciting frequency –120 cps. This, however, is swamped by the fundamental and harmonics produced by the core.
The difference between no load and full load, at constant flux density is usually no greater than 1 or 2dB. An exception to this is when special flux shields are placed inside a transformer tank to reduce stray flux effects.
We know now that the noise from a transformer is caused by mechanical movement of the individual lamination of the core under magnetization. The pulsation will cause not only air disturbances, thus producing noise, but also physical vibration of the core structure and everything attached to it. The vibration will have similar frequencies to those measured in the noise analysis.
Reducing  these mechanical pressure pulsations is vital to noise and vibration control and consequently, isolating the core and coils of a transformer, either in the tank or through a tank, or just as the core and coils, is important. Baffling transformer noise and forgetting to isolate the vibrations will only lead to a disappointing result and is something which should not be done. Remember noise is usually air borne. Vibration is ground borne. They are very much connected.

Cutting Air Born Noise

Putting the transformer in a room in which the walls and floors are massive enough to reduce the noise on the other side. Walls can be of brick, steel, and concrete. Noise is usually reduced as it tries to pass through a massive wall.

Putting the transformer inside an enclosure which uses a limp wall technique. This is done by using two thin plates separated by a rubbery material. The noise hits the inner sheet and some energy is used up inside the viscous material. the outer sheet should not vibrate

Build a screen wall around the unit. It will reduce the noise to those near the wall, but the noise will get over the screen and fall elsewhere. Screens have been made from wood, concrete, brick and dense bushes. 

Now that you know the basic on what cause transformer noise below is some useful pointer in helping reduce noise levels:

• First assess what level is tolerable. Your manufacturer can help you assess these and also assist you on the corrections.
• Isolation of the transformer from the ground is vital. Installation must ensure that nobody tightens down shipping bolts but removes them. Connecting cables must be as flexible as possible. Ventilation ducts must be placed in positions where these are effective thermally with affecting the acoustic performance.
• Transformer noise is difficult to change at the source. Flux density reduction is the main thrust, but this means increased cost.
• Transformer core constructions help. Reputable manufacturers will use good joints, flat steel, consistent thickness, good core supports, few bolts.
• Noise reduction by distance is the simplest form. If you can do this with out to much cost excellent. 
• Noise reduction by screens, bushes and etc. is the next simplest . Remember the shadow effect means the noise could be heard outside the shadow of that screen.
• Full enclosure is usually the only option for a troublesome transformer. Full enclosure can be made of any material with a high mass/weight ratio.
• Pay close attention to access doors and removable covers on enclosures. Tight fits are essential
• Watch the dimensions of rooms in which units are mounted. Damp them if necessary, suitable for transformer frequencies. choose damping materials compatible with transformer frequencies.
• Carry out sound surveys before and after installations. Remember to do a frequency analysis so that transformer noise can be differentiated.
• Remember transformers need cooling air in rooms. Be careful when you position air ducts, ventilators and grilles.
• Make sure the vibration isolators are correctly mounted and will accommodate transformer frequencies.
• Consider very carefully where transformers will be mounted. Resilient structures such as wooden mezzanines might be harmful.

It is impossible to give all the pointers and suggestions that might help you in producing a noise free site. Your manufacturer should give you all the assistance you need. The main point is to plan ahead with your transformer noise problem.

Great Power Transformer Articles

1. Top 20 Things to Know before Selecting a Power Transformer  - This is a great article to help educate and inform you on how to select the right transformer before you buy. It will help you make an informed decision and covers questions including, what voltage power output do you need?, will it be single phase or three phase?, etc. We answer the 20 most asked questions so you can be well informed and choose the right transformer for the job. Choosing the right transformer can be a daunting task for the inexperienced. This section takes the first step toward becoming a confident, knowledgeable consumer. This article addresses the process of choosing these transformers at its most fundamental level.

2. How Does Transformer Manufacturer Company Brand Affect Power Transformer Quality? - For manufacturers of large power transformers, product design and features seem fairly standard. But different manufacturers offer unique features. there are several standards such as ASTM D 3487 and IEEE Standard C57.12.90. Quality transformers can have a significant impact on cost. Did you know that some transformers brands improved materials, design and quality can save you 30%, or more, in energy cost? Understanding the differences can play a key role in making an informed selection.

3. How Does Winding Metal Type Change a Transformers Properties? - gain knowledge on how the winding and types of metal used can change transformer properties. The conducting material used for the winding depends upon the application. Small power and signal transformers are often wound with solid copper wire. Larger power transformers may be wound with copper wire, or aluminum and may include rectangular conducts. When copper wiring is used it will increase the efficiency of the transformer and will generally generate a lot less heat. Read more to learn about the many other efficiencies gained by choosing a transformer with a metal composition that is fit for your needs

4. Understanding Power Transformer "K-Factor Rating" - A great information article on what "K-Factor Rating" is and the effects they have on transformer choice. The K-Factor rating assigned to a transformer and marked on the transformer case in accordance with the listing of Underwriter Laboratories. It is an index of the transformer's ability to supply harmonic content in its load current while remaining within its operating temperature limits.

Read Addition Power Transformer Articles - Additional informal helpful articles about power transformers.

 Power Transformer Information:

 Power Transformer Manufacturer

• ACME Transformers - With Acme Electric being in business over 80 years, they have always believed in offering there customers superior service, quality and technical expertise in the transformer market.
• AMVECO Transformers - AMVECO designs and manufactures toroids transformers, current transformers, and auto transformers. Most AMVECO products are custom designed utilizing their state-of-art proprietary CAD programs.  The AMVECO engineers can quickly generate designs in a matter of hours, if needed.
• Federal Pacific Transformers -  Federal Pacific is a division of Electro- Mechanical Corporation, a privately held, American owned company founded in 1958. Federal pacific offers dry-type transformers from .050 KVA through 10,000 KVA single and three phase, up to 34.5 KV, 150 KV BIL with UL approval through 15 KV.
• Marcus Transformer - Ever since they opened their doors for business a half a century ago, they have been a leader in innovative transformer design. As a family-owned company they are proud of the reputation they have earned for making quality-built transformers that deliver exceptional performance and savings.
• Hammond Transformers -  Hammond Manufacturing was founded in 1917 in Guelph, Ontario, Canada. In the last 3 decades it has expanded to the US and the international markets offering many types of power transformers. 
• TEMCo Transformers - TEMCo Transformer, a family-owned business which has been manufacturing and distributing electrical products since 1968. They focus on transformers that significantly reduce power consumption over 30 percent compared to competitive makes.
• GE Transformers - GE has been a key player in the energy industry for more than a century.  Since the installation of their first steam turbine in 1901. They have become number one provider of high-technology power generation and distribution equipment.
• Jefferson Electric Transformers - Jefferson Electric has been a pioneer and innovator of magnetic products since 1915. Jefferson broad line of dry-type transformers are backed by quality assurance systems so stringent that each and every unit gets thoroughly tested before it goes out there door.
• More power transformer brands - Check out more companies by clinking this link.

 Power Transformer Types

• Distribution Transformers - Distribution transformers are generally used in electrical power distribution and transmission power. This class of transformer has the highest power, or volt-ampere ratings. and the highest continuous  voltage rating.
• Substation Transformers - Substation Transformers are large devices which usually weigh tens of thousands of pounds.   They are filled with tens of thousands of gallons of heat transfer fluid.  Although they are typically 99.8% efficient in the transforming of electricity from one voltage to another, processing hundreds of Mega Volts-Amps of electricity force the liberation of hundreds of BTUs per second.
• Medical Grade Isolation
   Transformer - Medical Grade Transformers generally refer to the transformers used in medical devices as well as hospital, biomedical and patient care equipment. There are a number of strict safety rules, guidelines and laws governing the design, construction and the test of these transformers.
• Drive Isolation Transformer - They are used to isolate a drive from a main power line to prevent the transmission of harmonics that the drives produce back into the power line.  They stop drive harmonics from disrupting computers and other sensitive equipment.
• Toroidal Transformers - Toroidal Transformers are more efficient than the cheaper laminated EI types of similar power level. Some of the advantages are smaller size, lower weight, less mechanical hum, (making them superior in audio amplifier), low-off-load loss.

 Power Transformer Types

• Step-Up Transformers - A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "steps up" the voltage applied to it. -
• Step-Down Transformers - A Step-Down Transformer is  designed to reduce voltage from primary to secondary.  They can range from sizes from .05 KVA to 500 KVA
• Isolation Transformers - An Isolation Transformer is a device that transfers energy from the alternating current (AC) supply to an electrical or electronic load.  It isolates the windings to prevent transmitting certain types of harmonics.
• Buck Boost Transformers - Buck Boost Transformers make small adjustments to the incoming voltage. They are often used to change voltage from 208v to 240v for lighting applications.  Major advantages of Buck boost transformers include; low cost, compact size and light weight. 
• High Voltage Transformer - There are many different types of voltage transformers. A High Voltage Transformer operates with high voltages. Typically, these voltage transformers are used in power transmission applications, where voltages are high enough to present a safety hazard.
• Medium Voltage Transformers - A Medium Voltage Transformer can be connected directly to a primary distribution circuit and generally has the most load diversity. These voltage transformers have installation practices that are generally in accordance with application recommendations from the Institute of Electrical and Electronic Engineers (IEEE).
• Low Voltage Transformers - A Low Voltage Transformer is an electrical device that transforms 120 volts (line voltage) into 12 volts or 24 volts (low voltage). Some uses for low voltage transformer are in landscaping lighting.
• Single Phase Transformers - In electrical engineering, single-phase electric power refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
• Three Phase Transformers - Three Phase Transformers must have 3 coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage needed while maintaining the proper phasing or polarity.
• Custom Transformers - Custom Transformers are designed for a certain performance specifications and size requirements.  The company works with your engineering specification. 
• Industrial Control Transformers - Industrial Control Transformers are used to convert the available supply voltage to the required voltage to supply industrial control circuits and motor control loads.
• Pad Mounted Transformers - Pad Mounted Transformers are usually single phase, or three phase, and used where safety is a main concern. Typical applications; restaurant, commercial building, shopping mall, institutional. 
• Pole Mounted Transformers - Pole Mounted Transformers are used for distribution in areas with overhead primary lines. Outside a typical house one can see one of these devices mounted on the top of an electrical pole.
• Oil Filled Transformers - Oil Filled Transformers are transformers that use insulating oil as insulating materials.  The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period.
• Dry Type Transformers - Dry-Type Transformers are available for voltages up through 34.5 kV (although the most common upper limit is 15) and KVA ratings up through 10,000 (with 5000 as the usual limit). Dry-type use air as a coolant, lowering health and environmentally concerns.
• Auto Transformers - An Autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built for 480 V supplies to operate on the local 600 V supply.
• More power transformer types - Read further about additional transformer types and their uses.

 Power Transformer Term Definitions

• Electrical Transformers - Electrical Transformers are devices used to raise or lower the voltage of alternating current. For instance, power is transported over long distance in high voltage power lines and then transformers lower the voltage so that the power can be used by a business or household.
• Isolating Transformers - An Isolating Transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits.  An Isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. 
• Transmission Power Lines - A Transmission Line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic or acoustic waves as well as electric power transmission. Components of transmission lines include wires, coaxial cables,  dielectric slabs, option fibers, electric power lines, and waveguides.
• Transformer Voltage - The measure of the amount of force on a unit charge because of the surrounding charge.
• Transformer Phase - Most transformer are either single phase or three phase.
• Transformer Frequency - The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.
• Transformer K Factor - Some transformers are now being offered with a k-factor rating. This measure the transformer's ability to withstand the heating effects of non-sinusoidal harmonic currents produced by much of today's electronic equipment and certain electrical equipment.
• Primary Voltage - The coil winding that is directly connected to the input power.
• Secondary Voltage - The coil winding  supplying the output voltage.
• Harmonic Cancellation - Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetic technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st.
• Weatherproof - Enclosed transformers come with a weatherproof standard set by NEMA.
• Epoxy Encapsulated - A process in which a transformer or one of its components is completely sealed with epoxy or a similar material. This process is normally preferred when a unit might encounter harsh environmental conditions.
• More power transformer terms - Such as inductor, ground fault, core saturation, current transformer, faraday shield, etc.

Related Transformer Products

• Voltage Regulators - A Voltage Regulator is an electrical regulator designed to automatically maintain a constant voltage level.  It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
• AC Line Reactor - AC Line Reactors is a three phase transformer used in conjunction with AC variable frequency and DC motor drive. They are a bi-directional protective filtering device.
• Line Power Conditioners - Power or Line Conditioners regulate, filter, and suppress noise in AC power for sensitive computer and other solid state equipment.
• DC Power Supplies - Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company to a well-regulated lower voltage DC for electronic devices.
• Rotary Phase Converters - Rotary Phase Converters are commonly used in home or small commercial or industrial settings. Rotary phase converters convert single-phase power into three-phase power. This is a very cost-effective way to power three-phase electric motors and other three phase equipment.
• Frequency Converters - A Frequency Changer or Frequency Converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency.
• Voltage Converters - A Voltage Converter changes the voltage of an electrical power source and is usually combined with other components to create a power supply.
• Magnetic Motor Starters - Magnetic Motor Starters are essentially heavy duty relays mounted in boxes, often equipped with heater/thermal overloads matched to the motor they start.
• Motor Starting Auto Transformers - An Auto Transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter.

For an additional resource the Best of Industry Web Directory : Electrical Power Transformer Directory section is quite useful.