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Top 20 things To Know Before Selecting A Power Transformer

1. What is a Transformer and how does it work?

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A transformer is an electrical device designed to transfer alternating current or voltage from one electric circuit to another by means of electromagnetic induction.  It can be designed to "step-up" or Step-down" voltages. Because transformer has no moving parts and is a completely static solid state device, under normal operating conditions,  are usually insure a long and trouble-free life. The simplest type of transformer consists of two or more coils of insulated wire wound on a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage between the primary and secondary depends on the turns ratio of the two coils. Transformers are not limited to one output coil. They can provide a whole range of voltage sources from the one input coil by using separate windings on the secondary side. This makes transformers very useful for supplying complete systems with all of their voltage requirements from one source.

The amount of electricity being produced depends on the number of times the coil is wound around the core. The more times it's wound around, the higher the voltage. So if the input coil has more loops and the output coil has a couple of loops, the input coil will be a higher voltage than the output coil. If the coils have an equal number of loops, their input and output will be equal. If the output coil has more voltage, it's called a "step-up transformer." If the input coil has more voltage, it's called a "step-down transformer."

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.

2. How to selection the right transformer

  • Determine primary voltage and frequency.
  • Determine secondary voltage required.
  • Determine the capacity required in volt-amperes

This is done by multiplying the load current (amperes) by the load voltage for single phase. For example, if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40 equals 9600 VA.A 10 KVA (10,000 volt-amperes) transformer is required. Always select the transformer larger than the actual load. This is done for safety purposes an allows for expansion. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of 3) then divide by 1000.

Suppose you've got the job of designing the power distribution for an entire building, you'll have to select the service transformer and the distribution transformers.

First you must determine the largest load you can expect at any given time. Example, you don't normally run electric heat and air conditioning at the same time. so your calculations would include only the larger of the two loads. You might not operate all equipment simultaneously either, however you would run some loads at the same time. Determine what those loads are-based on operational requirements, then determine the kVA each will draw.  Once you done the individual calculations and added them up to arrive at the maximum load, you should add in  factors for reasonable unplanned load growth (20% is common) and for planned addition load.

But don't stop there. The following includes some additional factors you'll have to consider when choosing a transformer.

  • Pole-mounted, pad-mounted, indoor or outdoor
  • primary voltage: delta or wye (usually dictated by the utility)
  • Secondary voltage (480V for small facilities) or delta-wye
  • 3,4,or 5 wire configuration (4 is the most common)
  • power factor and efficiency
  • Impedance
  • Temperature rise
  • Case style (consider footprint, height, access, cooling)
  • Location, space requirement
  • Seismic requirement
  • Accessories (including taps) and instrumentation
  • Dry-type or liquid-filled
  • Basic impulse level, which is the withstand rating in kV

Based on these criteria, you can work with a manufacturer to select the correct transformer  for your service entrance.

Once you've selected the service entrance transformer, you can supply the feeder circuits. You want to distribute at the highest voltage possible to minimize losses and power quality problems. In a large facility, you probably 14.4kV onsite, through many facilities distribute at 24kV. To select the transformer for these feeders, follow the same steps.

Distribution Transformers

The next layer of distribution is usually 4160V, then 480V, you normally begin to power equipment at these levels. Don't assume loads start at 480V. Some large production equipment requires a 4160V supply. Reactor coolant pumps, chiller pumps, and other high-inertia loads also run on 4160V. A transformer's function is straightforward: to step down the voltage and provide isolation between primary and secondary. When supplying a particular piece of equipment or equipment grouping, however, transformer selection gets more complicated. Motors that run at 4160V, for example, tend to be large, expensive, and mission-critical. Usually, you'll put such a motor on its own transformer and panel with few, if any, other loads. Use the earlier load calculations to size these individual load transformers, and carefully note the load characteristics — this is crucial information to supply to the manufacturer.

Low-Voltage Distribution

Before you size your low-voltage distribution transformers, decide how you're going to lay out your branch circuits. For example, you might use one transformer to supply a motor control center (MCC), another for lighting loads, another for convenience receptacles, and so on.  You could also provide an individual transformer for infrastructure equipment (plant air, elevators, electric doors), or individual transformers for individual equipment. A good planner will also provide a transformer for emergency equipment (fire control, security, emergency egress lighting) that resides on a UPS and may even have an alternate transformer supplying the switchgear or panels through a transfer switch.

In each case, size your distribution transformer based on the maximum load you can expect — just as you did for the service entrance. Also, a piece of large equipment — such as a robotic machine that automates sequential stamping, cutting, and welding operations — may require its own 4160V supply (integral OEM transformers step down to other voltages) while also requiring a 480V supply to corollary equipment. That corollary equipment should not run off the same 4160V supply as the main machine, so you may have two 4160V transformers with very different operating requirements.

What about distributed vs. centralized and size vs. quantity? By using several small transformers, you can reduce loss of function in the event of a transformer failure. Match transformers to load type and use as few transformers as possible (which means maximizing the load per transformer) to effectively reduce voltage drop. However, blindly following either of these approaches ignores the real issues: The small-transformer approach is expensive and inefficient — the money spent on transformers would go much further for maintenance.

Also look at transformers that can provide some flexibility in specialized way. Using  a Autotransformers where primary and secondary winding are magnetically isolated from each other, but electrically connected. Advantages are lower cost, smaller size, less weight, high efficiency, and better voltage regulation. A buck boost transformer allows you to raise or lower voltage to particular equipment. If you have 230V equipment but 208V/120V distribution, a buck boost transformer can raise the voltage to that equipment from 208V to 228.8V, which is close enough for proper operation.

When choosing a transformer, your job is defining the load size and the nature of the load. Often, industry experience makes transformer selection easy once you've done this. But sometimes an application does not lend itself to straightforward decisions. In such cases, you'll need to consult the appropriate standards and references, and work with your manufacturer. The factors outlined here will help you make that a successful collaboration.

3. What voltage power output do you need?

Voltage in the world different in the United States and Canada usually run on 110/120 and most of the rest of the world runs on 220/240 and industrial plant may use 440  to 480. Most utilities will provide a customer with one service or electrical system. You may wonder why anyone would need a transformer. Let say a new industrial plant comes in town. They have  many motors in use at their company so they requested a 480Y/277 volts three-phase system. This takes care of their motor load at 480 volts and their office and plant lighting loads at 277 volts. However to operate their office machinery and incandescent lighting they require 120 volts. They also  have some small horsepower motors they want to operate at 208 volts. Since the utility will only provide them with 480Y/277 volt three-phase system, they require a transformer to provide the rest of there needs. A buck boost transformer is the ideal solution for changing line voltage by small amounts. The major advantages are their low cost, compact size and light weight. They are also more efficient and cost less than equivalent isolation transformers. When connected as an autotransformer, they can handle loads up to 20 times the name plate rating.

4.Single Phase or three phase transformer?

Single-phase power distribution is used especially in rural areas, where the cost of a three-phase distribution network is high and motor loads are small and uncommon.

In North America, individual residences and small commercial buildings with services up to about 100 kV·A (400 amperes at 240 volts) will usually have three-wire single-phase distribution, often with only one customer per distribution transformer. Larger consumers such as large buildings, shopping centers, factories, office blocks, and multiple-unit apartment blocks will have three-phase service. In densely-populated areas of cities, network power distribution is used with many customers and many supply transformers connected to provide hundreds or thousands of kV·A load concentrated over a few hundred square meters.

A single-phase supply connected to a pure single-phase induction motor does not produce a revolving magnetic field, and so practical single-phase motors always have some means of producing a revolving field to generate starting torque. Aside from certain traction power applications, single-phase induction motors greater than 10 or 20 kW are very uncommon.

In Some application a  phase converter is used. A phase converter is simply a rotating machine that converts single-phase utility power into 3-phase electricity to operate 3-phase equipment. Phase converters are typically applied where utility 3-phase is unavailable or too expensive to install. A properly sized and selected converter will operate any load just as well as utility 3-phase and will provide years of trouble-free service.

A rotary phase converter is actually a rotating transformer. Through transformer action a phase converter splits off and phase shifts a portion of the single-phase supply from the utility creating true 3-phase power. When energized, the rotary phase converter uses the single-phase 2-line supply from the utility and creates a manufactured third line of power. The 3 lines (or phases) look identical to utility 3-phase with all three lines shifted 120°. The output of a rotary phase converter is true 3-phase. Each of the three output voltages will be shifted 120 electrical degrees. If the converter is properly sized, these voltages will remain in a balanced state over the entire range of connected loads.

5.What are the different  frequencies of power and there uses?

   The incoming electric transformers voltage is an important factor. The three common frequencies available are 50 Hz, 60Hz and 400 Hz. European power is typically 50 Hz while North American power is usually 60hz. The 400 Hz is reserved for high-powered applications such as aerospace technologies. It is also important to consider the secondary power specifications when evaluating transformers. Other specifications to keep in mind when selecting an electric transformer are: the maximum ratings of the following: secondary current and voltage rating, power and output rating. Power transformers have various configurations according to phase and connections. The most common phases are single-phase and three-phase. Both the size and expense of electric transformers increases in proportion to the number of primary windings.

 A frequency converter is used in some application. A  frequency converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, that is incidental to its principal purpose. Aside from the obvious application of converting bulk amounts of power from one distribution standard to another, frequency changers are also used to control the speed and the torque of the AC motors. In this application, the most typical frequency converter topology is the three-phase two-level voltage source inverter. The phase voltages are controlled using the power semiconductor switches and pulse width modulation (PWM). Semiconductor switching devices and anti-parallel connected freewheeling diodes form a bridge, which can connect each motor phase to the positive or negative dc-link potential. The PWM changes the connections of the phases between the positive and the negative dc-link potentials so that the fundamental wave voltage has the desired frequency and amplitude. The motor reacts primarily to the fundamental voltage and filters out the effects of the harmonic voltages.

Another application is in the aerospace and airline industries. Often airplanes use 400 Hz power so 50 Hz or 60 Hz to 400 Hz frequency converter is needed for use in the ground power unit used to power the airplane while it is on the ground.

Frequency changers are typically used to control the speed of pumps and fans. In many applications significant energy savings are achieved. The most demanding application areas are found on the industrial processing lines, where the control accuracy requirements can be very high.

6.Will Efficiency and Heat be a Consideration?

An ideal transformer would have no losses, and would therefore be 100 efficient. In practice energy is dissipated due both to the resistance of the windings (known as copper loss), and to magnetic effects primarily attributable to the core (known as iron loss). Transformers are in general highly efficient, and large power transformers (around 100 MVA and larger) may attain an efficiency as high as 99.75%. Small transformers such as a plug- in used to power small consumer electronics may be less than 85% efficient.

The looses arise from:

  • Winding resistance: Current flowing through the windings causes resistive heating of the conductors.
  • Eddy currents: Induced currents circulate in the core and cause it resistive heating.
  • Stray losses: Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive object such as the transformers support structure, and be converted to heat. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also from magnetostriction vibration of the core.
  • Hysteresis losses: Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis in the magnetic core. The level of hysteretic is affected by the core material.
  • Mechanical losses: The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.
  • Magnetostriction: The flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to friction heating in susceptible ferromagnetic cores. 

 Efficiency gains can be achieved by using materials with lower resistively or greater diameters. For example, transformer coils made with low resistively conductors, such as copper, can have considerably lower load losses than those made with other material.


All transformers must have some circulation of coolant to remove the waste heat produced by losses. Small transformers up to a few kilowatts in size usually are adequately cooled by air circulation. Larger "dry" type transformers may have cooling fans. Some dry transformers are enclosed in pressurized tanks and are cooled by nitrogen or sulfur hexafluoride gas.

The windings of high-power or high-voltage transformers are immersed in transformer oil - a highly-refined mineral oil that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability of PCB and its environmental accumulation, it is no longer permitted in new equipment. Today, nontoxic, stable silicone-based oils or fluorinated hydrocarbons may be used, where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil.

The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Oil-filled transformers undergo prolonged drying processes, using vapor-phase heat transfer, electrical self-heating, the application of a vacuum, or combinations of these, to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load.

Oil-filled power transformers may be equipped with Buchholz relays - safety devices sensing gas buildup inside the transformer (a side effect of an electric arc inside the windings) and switching off the transformer.

Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

7.Will the transformer be inside or outside?

For harsh environments, whether indoor or outdoor, it's critical that a transformer's core/coil, leads, and accessories be adequately protected.

In the United States, almost all liquid-filled transformers are of sealed-type construction, automatically providing protection for the internal components. External connections can be made with "dead front" connectors that shield the leads. For high corrosive conditions, stainless steel tanks can be employed.

Dry-type transformers are available for either indoor or outdoor installation. Cooling ducts in the windings allow heat to be dissipated into the air. Dry-types can operate indoors under almost all ambient conditions found in commercial buildings and light manufacturing facilities.

For outdoor operations, a dry-type transformer's enclosure will usually have louvers for ventilation. But, these transformers can be affected by hostile environments (dirt, moisture, corrosive fumes, conductive dust, etc.) because the windings are exposed to the air. However, a dry-type can be built using a sealed tank to provide protection from harmful environments. These units operate in their own atmosphere of nonflammable dielectric gas.

Other approaches to building dry-type transformers for harsh environments include cast coil units, cast resin units, and vacuum pressure encapsulated (VPE) units, sometimes using a silicone varnish. Unless the dry-type units are completely sealed, the core/coil and lead assemblies should be periodically cleaned, even in non-harsh environments, to prevent dust and other contaminant buildup over time.

Locating a transformer indoors, on the rooftop, or adjacent to a building in order to minimize the distance between the unit and the principal load results in reducing energy loss and voltage reduction. It also reduces the cost of secondary cable.

On the other hand, such placements of high-voltage equipment require closer consideration of electrical and fire safety issues. These conflicting goals can be satisfied by using transformers permitted by Code and insurance companies.

When liquid-filled transformers are preferred, less-flammable liquids are widely recognized for indoor and close building proximity installations. Wet-type transformers using less-flammable, or high fire point liquids, have been recognized by the NEC since 1978 for indoor installation without the need for vault protection unless the voltage exceeds 35kV. Based on this type of transformer's excellent fire safety record, Code and insurance restrictions have become minimal. Conventional mineral oil units are allowed indoors, but only if they are installed in a special 3-hr-rated vault (with a few exceptions) per the construction requirements of NEC Article 450, Part C. There's a requirement for liquid containment when wet-type transformers are used, regardless of the type fluid employed.

When dry-type units are preferred, they have fewer code restrictions. Obviously, these types of transformers do not need liquid containment. Per the requirements listed in NEC Sec. 450-21, there are minimum clearances that you must observe, and units over 112.5kVA require installation in a transformer room of fire-resistant construction, unless they are covered by one of two listed exceptions. As with liquid units, dry transformers exceeding 35kV must also be located in a 3-hr-rated vault.

A liquid-filled transformer may experience leakage around gaskets and fittings; however, if the installation was carried out correctly, this should not be a problem. Major maintenance procedures may require inspection of internal components, meaning that the coolant will have to be drained. Coils in liquid-type units are much easier to repair than coils in dry-type transformers. Cast coils are not repairable; they must be replaced.

8. Can transformers be operated at voltages other than nameplate voltages?

Transformers can be operated. in some cases at voltages below the nameplate rated voltage. Transformer should not be operated in excess of it nameplate rating, unless taps are provided for this purpose. Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions, and still deliver full rated output voltages at the secondary terminals. Standard tap arrangements are at two-and-one-half and five percent of the rated primary voltage for both high and low voltage conditions.

9. What is the difference between Insulating, Isolating, and Shielded Winding transformers?

Insulating and isolating transformers are the same. they  are used to describe the isolation of the primary and secondary windings, or insulation between the two. A shielded transformer is designed with a metallic shield between the primary and secondary windings to attenuate transient noise. The shielded transformer is used in applications such as computers, process controllers and many other microprocessor controlled devices. All two, three and four winding transformers are of the insulating or isolating types.

10.Why should Dry-Type Transformers never be over-loaded

When you  overloading  a transformer excessive  temperature can cause overheating which result in rapid deterioration of the insulation and cause complete failure of the transformer coils.

11. What is meant by impedance in transformers?

Impedance is the current limiting characteristic of a transformer and is expressed in percentage. It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.

The impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedances in the circuit. When a signal source, such as our composite video output, sees a very low-impedance circuit, it produces a larger than intended current; when it sees a very high-impedance circuit, it produces a smaller than intended current. These mismatched impedances redistribute the power in the circuit so that less of it is delivered to the load than the circuit was designed for--because the nature of the circuit is that it can't simply readjust the voltage to deliver the same power regardless of the rate of current flow. Imagine,  riding in your car down the Interstate in first gear, flooring the gas pedal and going just as fast as you can. It's obvious, as you watch the cars zip past, that no matter how much horsepower you have under the hood, most of that horsepower isn't getting delivered to the road; instead, a lot of it is burning up in the engine as excess heat, and if you keep this driving up for long, you'll damage your engine. The same thing happens in an impedance mismatch between a source and load; power isn't being transferred properly because the source circuit wasn't designed to drive the kind of load it's connected to. In some electronic applications this will burn out equipment.

12. Why are Small Distribution Transformers not used for Industrial Control Application?

Industrial control equipment demands a momentary overload capacity of three to eight times normal capacity. This is customary in solenoid or magnetic contractor applications where inrush current can be three to eight times as high as normal sealed or holding currents but still maintain normal voltage at this momentary overloaded condition. Distribution transformers are designed for good regulation up to 100 percent loading, but their output voltage will drop rapidly on momentary overloads of this type making them unsuitable for high inrush applications.

Industrial control transformers are designed especially for maintaining a high degree of regulation even at eight times normal load. This results in a larger and generally more expensive transformer.

13. Can Single Phase Transformers be used for Three Phase Applications?

Yes, but the transformer output will be single-phase. Simply connect any two wires from a 3- or 4-wire source to the transformer's two primary leads. Three single-phase transformers can be used for three-phase applications. They can be used in delta-connected primary and wye or delta-connected secondary. To avoid an unstable secondary voltage, NEVER connect wye primary to delta secondary.

14.How do I know when the temperature rise is too high?

Thermometers are the best way to determine the temperature. Touch is a poor indicator of proper operating temperature for transformers.  Properly designed transformers can reach 50°C (122°F) above ambient temperature. In an ambient temperature of 20°C (60°F), the total temperature can reach 70°C (190°F), which is too hot to touch.

15.Can transformers be used in parallel?

Yes, it is very common for transformers to be placed in parallel service. To provide maximum efficiency, voltage and impedance values must match closely. A failure to match will cause unbalanced loading for the transformers and may lead to overheating or premature failure.

16.Can I achieve specific sound levels in a transformer?

Before selecting a transformer assure yourself that the sound levels represented have been measured in accordance with the NEMA standards. If your requirement is lower than that available from the manufacturer’s standard product, request a specific sound level on your bid. 

17. Is one insulation system better than other?

It depends on the application and the cost benefit to be realized. Higher temperature class insulation systems cost more and larger transformers are more expensive to build. Therefore, expensive insulation systems are more likely to be found in the larger KVA units.

  • Small fractional KVA transformers use insulation class 130 degrees C
  • Compound filled transformers use insulation class 180 degrees C
  • Larger ventilated transformers are designed to use 220 degrees C insulation

18. What is Exciting Current?

Exciting current, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA. The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and referred to as KVAR.

19. Can Transformers develop Three Phase power from a Single Phase source?

NO. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase.

20. Can air cooled transformers be applied to motor loads?

This is an excellent application for air cooled transformers. Even through the inrush or starting current is five to seven times normal running current, the resultant lower voltage caused by this momentary overloading is actually beneficial in that a cushioning effect on motor starting is the result.

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 HomeContact Power Transformer Co.

Power Transformer Types

Step Up and Step Down Transformers Step Up and Step Down Transformers to Power transformers to step-up ( raise) or step-down (lower) the electrical voltage.
Isolation Transformers Isolation Transformers allows signal or power to be taken from one device and fed into another without electrically connecting the two.
Toroidal Transformers Toroidal Transformers are devices that transfer electrical energy from one electric circuit to another, without changing the frequency, by electromagnetic induction.
Custom Transformers
Custom Transformers are designed to meet certain performance specifications and size requirement that you require. There is a wide range of custom transformer types.
Buck Boost Transformers
Buck Boost Transformers is a ideal solution for changing line voltage by small amounts. Often used to buck (lower), or boost (raise) the voltage from 208v to 240v for lighting applications.
Pole Mounted Transformers
Pole Mounted Transformers are mounted to poles for overhead electrical lines. Used in various applications. Are available in single phase or three phase transformers.
Medium Voltage Transformers
Medium Voltage Transformers are used with a medium range of voltages. They come in a full range from liquid-filled, convention dry type as well as cast coil.
Pad Mounted Transformers Pad Mounted Transformers are a excellent choice for commercial and industrial such as manufacturing facilities, refineries, office buildings, schools, hospitals, restaurants, and retail stores. They come in various sizes and can be used underground as well.
High Voltage Transformers High Voltage Transformers typically these voltage transformers are used in power transmission applications. High voltage transformers are also used in microwave.

 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 TransformersFederal 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 TransformersHammond 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.

Additional Useful Resources:
Transformer Selection Guide • Custom Transformers • Dry-Type Transformers • Auto Transformers • Control Transformers • Step-Up Transformers • Step-Down Transformers • Harmonic Cancellation • Isolation Transformers • K-Factor Rated Transformers • 3-Phase Transformers • European Voltage Transformers • Drive Isolation Transformers • High Voltage Transformers • Epoxy Encapsulated NEMA 4 Transformers • Weatherproof NEMA 3R Transformers • Transformers • AC Transformer • Voltage Transformer • Buy Transformer • New Transformer • Output Transformer • Transformer KVA • Converter Transformer • Line Transformer • Toroidal Transformer • Oil Filled Transformers • Voltage Regulator • Automatic Voltage Regulator • DC Power Supply • PDU • Rack PDU • Power Conditioner • Power Line Conditioner • Load Center • Switchgear • Voltage Converter • Transformer Wiring • Transformer Circuit • Variable Transformer • Pole Transformer • Transformer Pad • VA Transformer • WYE Transformer • Potential Transformer • Transformer Protection • Variac • Transformer Rating • Sunbelt Transformer • Pacific Transformer • Jefferson Transformer • Electric Transformer • Power Transformer • Transformer Sizing • Transformer Rectifier • Center Tap Transformer • Power Distribution • Industrial Transformer • Replacement Transformer • Insulation Transformer • Micron Transformers • Westinghouse Transformer • Power Supply Transformer • Instrument Transformer • Pulse Transformer • Substation Transformer • Furnace Transformer • Pad Mounted Transformers • Transformer Manufacturer • Distribution Transformers • GE Transformers • Step Up Transformers • Step Down Transformers • Buck Boost Transformers • High Voltage Transformers • Isolation Transformer • Single Phase Transformer • Hammond Transformers • Buck Boost Transformers • Diesel Generators • Diesel Power Generators • Diesel Generator Sets • Power Generators • Onan Diesel Generators • Kipor Diesel Generator • Kubota Diesel Generator • Cummins Diesel Generator • Yanmar Diesel Generator • Variable Frequency Drive • Soft Start Drives • Motor Control • Baldor Motors • Weg Motors • Cold Ironing • Motor Generator