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.

Heat

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.