Auto Transformers
An Autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps.

Auto Transformer Applications
Auto-Transformers are primarily used to either step up or step down single or three phase line voltages to desired levels.

Auto Transformer Construction
Construction is characterized by a single wire coil common to both the primary and secondary circuits, wound around a silicone steel core. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. As the same winding is used for input and output, the flux in the core is partially cancelled, and a smaller core can be used. For voltage ratios not exceeding about 3:1, auto transformer is cheaper, lighter, smaller and more efficient than a true (two-winding) transformer of the same rating.
By exposing part of the winding coils and making the secondary connection through a sliding brush, an auto transformer with a near-continuously variable turns ratio can be obtained, allowing for very small increments of voltage.

Auto Transformers vs. Isolation Transformers
Physically smaller and more economical to purchase, the auto-transformer can be an attractive alternative to an equivalently rated isolation transformer, under the correct conditions.
The principle difference between an auto-transformer and an isolation transformer is the separation of the secondary windings. Because the auto-transformer uses a single coil winding for both the primary input and the secondary output, any electrical noise, voltage spikes, sags or any other undesirable condition will pass through unchecked. Equipment susceptible to damage by poor line conditions will not be protected. And noise and harmonics generated by components on the secondary side will be allowed to transmit onto the main supply line. Because the auto-transformer can transmit line disturbances directly, local building codes may prohibit their use in certain areas. Auto-Transformers also should not be used in closed delta connections as they will introduce into the circuit a phase shift which causes higher power use.
Auto-Transformers are best used in applications where the line voltage needs to be matched to a protected piece of equipment. A good example would be a machine tool which was manufactured in Europe and designed to operate on 400V. The drives and controls within the machine would be protected by either inductors or transformers already installed in the machine tool. An isolation transformer could convert a 480 supply voltage to 400V but the additional isolation would be redundant and expensive. An auto-transformer would perform the voltage conversion for a fraction of the cost of the isolation transformer, in a package which would be approximately two thirds smaller.

Buck Boost Transformers
Buck Boost Transformers are small, single phase, transformers designed to reduce (buck) or raise (boost) line voltage from 5 to 20%.
A buck boost transformer is the ideal solution for changing line voltage by small amounts. The major advantages are their lost 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.
When a buck boost transformer has the primary and secondary windings connected, it becomes an autotransformer. Now only the secondary windings are transforming voltage and current. The majority of the KVA load passes directly from the supply to the load. This is why they can supply a load with a larger KVA rating than the nameplate indicates.
They are ideal for low voltage lighting control applications.
There are two basic types of buck boost transformers, self adjusting (active) or passive designs. The active types monitor incoming voltages and will adjust the outgoing voltage to be within an acceptable range.
Passive transformers are used for larger equipment where the amount is fixed. This is commonly used when someone wants to use a piece of equipment that is made for European power (220VAC to 230VAC) in the United States, which has both 208V and 240V service available.

Custom Transformers
Custom Transformers are designed for a certain performance specifications and size requirements.  The company works with your engineering specification.
Custom transformers are transformers designed per the customer's drawing or specification. There are several reasons why you may need a custom design transformer. The main reason is usually the transformer requires a unique features. Another consideration is it may involve construction processes that require specialized material, equipment or handling. High voltage coils may be one example. Because of corona concerns, coils may have to have "void free' vacuum impregnation with epoxy, silastic, or another material. Also some time you could not find a suitable catalog part or to guard against vendor obsoleteness, perhaps against a vendor going out of business.

When buying a custom transformer make sure the manufacturer can provide you with the following:
· Develop a relationship with the engineering staff and make sure you feel they can delivery a quality product, plus handle your order efficiently and promptly.
· There engineers should be able to design a new part,  modify or duplicate an existing part.
· They have computer-assisted design techniques to develop customized design solutions while keeping cost effectiveness in balance.
· Quotes should be processed in three day to one week, depending on the complexity of the transformer desired. If you ask for a faster response they should try to delivery.
· Prototype development and production usually should be accomplished in a matter of days, depending on availability of materials. There design software should eliminated approximations and guesswork, enabling them to provide prototypes that work right the first time.
· The manufacturer should continually research new concepts and developments in materials technology to provide you with a better and more efficient transformer.
· They should embrace a "follow-through" policy on your transformer from design to production of the complete unit.

Drive Isolation Transformer
Motor drives, such as variable frequency and adjustable speed, are becoming more and more popular every day, They are used to adjust the speed of induction motors by varying the frequency.
Motor drives are popular because of their energy savings and they are easy to control. These motor drives do cause harmonic voltage distortion, electrical noise, and line voltage notching. By putting a drive isolation transformer in front of these motor drive is a simple, cost-effective solution to the problem cause by motor drives.
Inserting a drive isolation transformer in front of the motor drive reduces the harmonic distortion, attenuates electrical noise, and diminishes line voltage notching caused by motor drives. The addition of the inductive reactance from a transformer to the motor drives power source effectively reduces the notching effects it has on the power to other loads. In three phase applications, drive isolation transformers reduce harmonic distortion by eliminating the triple harmonics (when the transformer is connected delta-wye).

High / Medium / Low Voltage Transformer
There are many different types of voltage transformers. A high / medium / low voltage transformer operates with high / medium / low voltages.  
Typically, the High Voltage Transformers are used in power transmission applications, where voltages are high enough to present a safety hazard.
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.

There is no universally accepted definition for a high/medium/low voltage transformer, although some industry standards do specify various minimum voltages. These definitions are generally based on safety considerations or the voltage where arcing will occur.
Due to the voltage and frequency that it must handle, every type of transformers has a significantly different core geometry, winding techniques, and insulation methods that ordinary transformers. For example, factors such as the volts/turn ratings of the secondary wire, insulating material dissipation, and corona level must be carefully considered.

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.
The Industrial control transformers are specifically designed for high inrush applications requiring reliable output voltage.
These transformer are designed to meet industrial applications where electromagnetic devices such as relays, solenoids and other control components are used.  Control transformers (also known as machine tool transformers) provide isolation to control components from power surges and lighting circuits, and provide good power regulation under inrush conditions. The inrush current requirement of control elements can be up to 15 times the transformer's nominal rating. While the inrush current is occurring the control transformer keeps the voltage powering the control element as steady as possible.

Isolation Transformers
Isolation Transformers have primary and secondary windings that are physically separated from each other. Sometimes isolation transformers are referred to as "insulated".
This is because the windings are insulated from each other. In an isolation transformer the output winding will be isolated, or floating from earth ground unless bonded at the time of installation. Secondary neutral to ground bonding virtually eliminates common mode noise, providing an isolated neutral-ground reference for sensitive equipment and an inexpensive alternative to the installation of dedicated circuits and site electrical upgrades.
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. They also block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments. Isolation transformers are different from auto transformers in which the primary and secondary share a common winding.

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.
The majority of medical grade transformers are isolation transformers.
There are generally two types of isolation, one that relies on Safety Ground, and one that relies on Double or Reinforced Insulation.

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.

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.
A transformer mounted on an electrical service pole, usually at the level of the overhead cables but occasionally at ground level. Pole-mounted transformers are the common breadbox transformers used for converting distribution voltage to the voltage used by homes and low-volume commercial installations.
Pole mounted transformers are used in extensive rural area.

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.

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.

Configuration for Single Phase Power
Single-phase transformers are often used to supply power for residential lighting, receptacle, air-conditioning, and heating needs.
A transformer with a 120 volt AC secondary can take care of the lighting and receptacles. But, a transformer with a 240 volt AC secondary could handle all the residential needs mentioned. A 240 volt AC secondary could handle the heavier 240 volt power requirements of air conditioning and heating. The same 240 volt AC secondary could handle the 120 volt AC needs by tapping the secondary in the center.

(under construction)
Figure 1. 120 Vs. 240 Volt Secondary

volt secondary


Single phase transformers can be made even more versatile by having both the primary winding and secondary winding made in two equal parts. The two parts of either winding can then be reconnected in series or parallel configurations. 



(under construction)
Figure 2. Single-Phase Transformer with Secondary Divided Into Sections

divided into sections


Series Configuration / Parallel Configuration
Single-phase transformers usually have their windings divided into two or more sections. When the two secondary windings are connected in series, their voltage is added. When the secondary windings are connected in parallel, their currents are added.

Step-Up Transformers
A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "step up" the voltage applied to it.

How does a step Up transformer or step down transformer works
A transformer is made from two or more coils  of insulated wire wound around a core made of iron. The number of times the wires are wrapped around the core ("turns") is very important and determines how the transformer changes the voltage. If the primary has fewer turns than the secondary, you have a step-up transformer that increase the voltages.  When voltage is applied to one coil (frequently called the Primary or input) it magnetizes the iron core, which induces a voltage in the other coil, (frequently called the secondary or output). The turns ratio of the two sets of windings determines the amount of voltage transformation.

step-up transformer



There are two points to remember:
1) Transformers only work with alternating current. Using direct current will create a magnetic field in the core but it will not be a changing magnetic field and so no voltage will be induced in the secondary coil.

2) Using a step up transformer to increase the voltage does not give you something for nothing. As the voltage goes up, the current goes down by the same proportion. The power equation shows that the overall power remains the same. P=V x I  Power = Voltage x Current
In reality, the power output is always less than the power input because the changing magnetic field in the core creates currents (called eddy currents) which heat the core.  This heat is then lost to the environment, it is wasted energy.

Electricity is first produced at the power plants. Electricity is then sent to step-up transformers where low-voltage electricity is changed to high voltage to facilitate the transfer of power from the power plant to the customer. Voltage must be increased so that the electric current has the "push" it needs to efficiently travel long distances.
From the step-up transformer, transmission lines carry the high voltage electric current long distances through thick wires mounted on tall towers that keep the transmission lines high above the ground. Insulators made of porcelain or polymers are used to prevent the electricity from leaving the transmission lines.
High-voltage transmission lines carry the electric current to substations where the voltage is lowered so it that can be distributed locally on smaller power lines known as distribution lines. These voltages are reduced one last time at smaller pole-top transformers to utilization voltages, to make the power safe to use in our homes.

Step-Down Transformers
A Step-down transformer is one whose secondary voltage is less than its primary voltage. The step down transformer is designed to reduce the voltage from the primary winding to the secondary winding.
This kind of transformer "step down" the voltage applied to it.
There is many uses for step-down transformer and the larger devices are used in electric power systems, and small units in electronic devices.
Industrial and residential power transformers that operate at the line frequency, may be single phase or three-phase, are designed to handle high voltages and currents.  Efficient power transmission requires a step-up transformer at the power-generating station to raise voltages, with a corresponding decrease in current.  Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used for long-distance transmission lines to reduce losses. At the receiving end, step-down transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels.
In electronic equipment, transformers with capacities in the order of 1 kW are largely used ahead of a rectifier, which in turn supplies direct current (DC) to the equipment.  Such electronic power transformers are usually made of stacks of steel alloy sheets, called laminations, on which copper wire coils are wound. Transformers in the 1-to100-W power level are used principally as step-down transformers to couple electronic circuits to loudspeakers in radios, television sets, and high-fidelity equipment.  Known as audio transformers, these devices use only a small fraction of their power rating to deliver program material in the audible ranges, with minimum distortion.  The transformers are judged on their ability to reproduce sound-wave frequencies (from 20 Hz to 25 kHz) with minimal distortion over the full sound power level.

Three Phase Transformers
A three phase transformer is a three-legged iron core.  Each leg has a respective primary and secondary winding.
Most power is dispersed in the form of three-phase AC.  Before we proceed any further you should understand what is meant by three phase power. Basically the power company generators produce electricity by rotating 3 coils or windings through a magnetic field within the generator. These coils or windings are spaced 120 degrees apart. As they rotate through the magnetic field they generate power which is then sent out on three lines as in three-phase power. A 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 we need and maintain the proper phasing or polarity.
Three phase transformers is a common and popular method for electric power transmission. There are many benefits to three phase power:
· first, all three wires can carry the same current,
· secondly, power transfer is constant into a linear and balanced load.
Since three phase power is the most common way in which power is produced, transmitted, an used, an understanding of how three phase transformer connections are made is essential. In this segment we will discuss different types of three phase transformers connections, and present examples of how values of voltage and current for these connections are computed.

Three Phase Transformer Construction
A three phase transformer is constructed by winding three single phase transformers on a single core.

Three Phase Transformer Connections.
There are only 4 possible transformer combinations:
· Delta to Delta - use: industrial application
· Delta to Wye - use: most common, commercial and industrial
· Wye to Delta - use: high voltage transmissions
· Wye to Wye -  use: rare, causes harmonics and balancing problems
Three phase transformers are connected in delta or wye configurations. A wye-delta transformer has its primary winding connected in a wye and its secondary winding connected in a delta. A delta-wye transformer has its primary winding connected in delta and its secondary winding connected in a wye.

Delta Connections
A delta system is a good short-distance distribution system. Applications of these transformers are usually found in  neighborhood and small commercial loads close to the supplying substation. Only one voltage is available between any two wires in a delta system. The delta system can be illustrated by a simple triangle. A wire from each point of the triangle would represent a three-phase, three-wire delta system, the voltage would be the same between any two wires.

Wye Connections
In a wye system the voltage between any two wires will always give the same amount of voltage on a three phase systems. However the voltage between any one of the phase conductors and the neutral will be less than the power conductors. This is due to the square root of three phase power. In a wye system, the voltage between any two power conductors will always be 1.732 (which is the square root of 3) times the voltage between the neutral and any one of the power phase conductors. The phase-to-ground voltage can be found by dividing the phase-to-phase voltage by 1.732.

Single-phase transformers in a  three phase bank
When you have a three phase transformer and it is not available in the proper size or turns ratio a three phase conversion is needed.  Single phase transformers can be connected to form a three phase bank. When three single phase transformers are used to make a three phase transformer bank, their primary and secondary windings are connected in a wye or delta connection.

Open Delta Connection
The open delta transformer connection can be made with only two transformers instead of three. Usually the delta transformer are used in small business where the three phase transformer would be excessive.  It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers.
The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are used. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current.

Toroidal Transformers
Toroidal transformers are devices that transfer electrical energy from one electric circuit to another, without changing the frequency, by the principles of electromagnetic induction.
The energy transfer usually takes place with a change of voltage.  Toroidal transformers typically have copper wire wrapped around a cylindrical core so the magnetic flux, which occurs within the coil, doesn't leak out, the coil efficiency is good, and the magnetic flux has little influence on other components
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.