Introduction

Generally a transformer is an electrical device or machine which inductively transfers the electrical power operating at a particular current and voltage of one circuit to the other circuit which is operating at different current and voltage level. Most of the transformers are manufactured such that their characteristics must match for special application requirements such as constant current, constant voltage, higher impedance, etc.

The most common types of transformers found in electrical transmission systems, industries and electronic applications include power transformers, instrument transformers, tap changing transformers, auto transformers, RF transformer, audio transformers, etc. They all are different in size, ratings and shape with one another but the basic principle of operation of all these are same. The various types of transformers are discussed in this article so let us have a look on them.

Power Transformer

Some of the power transformers are used in generating stations, substations and power transmission lines for either stepping down or stepping up the voltage. By using step-up power transformer, the voltage level in the transmission line is increased, thereby low current flows through the line. Therefore, I2R losses in the transmission lines are reduced. The step-down power transformers are used to supply loads in industries at its rated voltages.

Some of the power transformers also supply the power to the electronic circuits. The power transformer can be single or three phase units based on the application it is employed for. With respect to the unique features of tap changing transformer, auto transformer and distribution transformers are generally belongs to the family of power transformers. Some power transformers are discussed below.

Laminated Core Type Transformer

These are most commonly used transformers and are available from milliwatts to megawatts range. These types of transformers are used in electric power transmission and also in appliances to supply the low voltage. This transformer consists of a laminated core to reduce the eddy currents. A core of thin steel or CRGO or CRNGO ‘E’ and ‘I’ laminations are used for low and high power transformers which can be single or three phase transformers. These laminations are clamped together with bolts. Both primary and secondary windings are wound on a former and are placed around the central limb of the core. These transformers use split bobbin to provide high insulation between the windings for small appliances. Between the primary and secondary, shields may be used to reduce the electromagnetic interference.

Toroidal Core Transformers

This type of transformer offers many advantages over laminated core transformer that it provides quite and efficient operation with reduced stray or external magnetic fields. Due to less weight and small size, these are easily designed for any application operating either low or high voltage. A highly efficient donut shaped core is used which is made from grain oriented silicon iron and is cut to form a ribbon of steel. This core is further wrapped by copper windings like a very tight clock spring. Compared to the EI laminated core transformer, toroidal core transformers are more expensive. However, for a given rating a smaller and lighter will be the toroidal transformer compared with EI laminated type transformer. Also, it provides less leakage of magnetic field and higher efficiency. These are available from a few tens of VA’s to thousands of VA’s. Mostly, they come with center single hole mounting by a bolt with washers and rubber pads.

Auto Transformer

Auto transformers are different from standard two or three winding transformer as it contains only a single winding that acts as both primary and secondary. In this, the part of this single winding is common for both primary and secondary and hence these are electrically connected (two windings are electrically isolated in case of traditional transformer). So this transformer works on conduction as well as induction. In this, a laminated core is wound by a single winding and part of this winding is divided into primary and secondary.

These are classified into step-up and step-down auto transformer. In step-down auto transformer, full winding acts as primary and the part of it act as secondary and hence voltage induced in the secondary is low as compared with primary. On other hand, reverse will be the case for a step-up transformer. Three phase power transformers are used in power distribution systems which can be either star or delta connected auto transformers. But mostly star connected auto transformers are used for high power applications.

Variable auto transformers comes with number tappings on single winding and secondary connection with a sliding carbon brush. Therefore, by sliding the carbon brush variable voltage is produced in the secondary which is equal to the turns ratio between the whole winding and the tapping.

Auto transformers are used as stators for safe start of various electrical machines like synchronous motors, induction motors, etc. And these are also used as furnace transformers and boosters.

Poly Phase Transformer

This type of transformer is commonly employed for three phase electric power systems such as power grids and transmission lines that they transfer large amounts of high voltages. These are most economical due to the widespread use of three phase AC generation, transmission, distribution and utilization systems. This type of transformer consists of three windings which are wound around three legged core and immersed in a tank. These primary and secondary windings can be connected in different combination of the connections such as star-star, star-delta, delta-delta and delta-star. These can be a step-up or step-down three phase transformers depend on the application or load. Due to the common core for all the windings, the lesser will be the leakage magnetic flux and hence efficiency of the transformer is high.

Oil Cooled Transformers

Oil cooled transformer are large power transformers used in various units ranges from large generating station or substation units to power distribution units. These transformers are filled with standard transformer oil (or mineral oil) to provide the cooling as well as insulation to the windings and core. In oil cooled transformers, core and coils are submerged or immersed in the liquid or oil. As compared to the air cooled transformers, oil provides better insulation and acts as a better conductor of heat. These types include

• Oil Immersed Self Cooled Transformers

In this type heat generated by the core and windings is passed to the oil by conduction process. When the heat of the oil rises due to the core and winding temperature, oil starts moving inside the tank and reaches to its walls where this heat is taken away naturally by ambient air. This oil continues to circulate and hence dissipates heat in the atmosphere.

• Oil Immersed Forced Air Cooled Transformers

In this type of cooling method, heat dissipation is improved by directing the forced air on the transformer outer surface using fans. These fans operate automatically when the temperature reaches to a certain level.

• Oil Immersed Water Cooled Transformers.

In this type, heat is extracted or dissipated by means of forced water pumped through the coils immersed in the oil just below the top of the tank. This water is further cooled in heat exchangers, or spray ponds, or cooling towers.

• Oil Immersed Forced Oil Cooled Transformers.

In this type, heat is extracted by circulating the forced oil using the pump. The oil is pumped upward to the windings and then back by the way of external radiators such that the heat is dissipated by the forced air from fans in the external radiators. This type of cooling is used for very high capacity transformers and in such cases air blast cooling radiator is used.

RF Transformers

RF transformers are used in a variety of electronic circuits for several reasons such as impedance matching to transfer maximum power, DC isolation between the circuits, voltage and current step-up or step-down, interfacing between unbalanced and balanced circuits, etc. These transformers come as connector packages, surface mount packages and other different configurations. The steel laminations are not used for RF transformers. The working frequencies of this transformer are ranging from 30 KHz to 30 MHz and most often by the addition of capacitor to one winding helps to tune their windings for a particular frequency.

These can be air core, ferrite core, balun type transformers. Air core RF transformers used in printed circuit boards such that a few turns of a wire are soldered onto it. Ferrite core transformers are used in superheterodyne radio receivers which are mostly tuned type transformers. Balun transformers are used to connect the unbalanced and balanced circuits such as balanced amplifiers (common mode rejection applications).

Audio Transformer

Audio transformers are specially designed transformers used to carry the audio signal in audio circuits. The working frequencies are ranging from 20 Hz to 20 KHz for this type of transformer. These are used for multi functionalities such as stepping up or stepping down the signal voltage, converting a circuit from balanced to unbalanced and vice-versa, decrease or increase of impedance of a circuit, blocking DC component of current and allowing AC signal, and to provide electrical galvanic isolation from one audio device to the other. These types of transformers include microphone input, line input, moving-coil phono input, line output, inter-stage and power output, microphone output, splitter, impedance conversion, direct box, Hum eliminators, loudspeaker toroidal AF transformers, etc.