A transformer is an extended version of an inductor. The flux that is created inside the inductor is used here to induce voltages at another coil, which is termed a secondary coil. If the rate of change of flux can induce a voltage across the primary coil, from which it is created, then it is also possible to induce a voltage across the secondary coil, provided that we can pull the flux to flow through the other coil. The rate of change of flux will induce a voltage of as many turns as we use. If the turn is double the turn in primary then the voltage will also be double. If we increase the number of secondary coils, then the voltage will be induced in all the secondary coils according to the number of turns present in each secondary coil.
We can increase or decrease the secondary voltage level according to our requirements. If the secondary voltage is increased then it is called a step-up transformer and for the decreasing case, it is called a step-down transformer. Each secondary voltage will act as a separate voltage source. Here the other advantage we get from a transformer is that each secondary voltage source is an isolated voltage source. There is no electrical connection between the primary and the secondary. Whatever the voltage level is that the secondary is totally an isolated part.
An autotransformer has only a single winding with two end terminals, plus a third at an intermediate tap point. The primary voltage is applied across two of the terminals, and the secondary voltage is taken from one of these and the third terminal. The primary and secondary circuits, therefore, have a number of windings turn in common. An adjustable autotransformer is made by the secondary connection through a sliding brush, giving a variable turn ratio.
For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. In this case, the magnetic circuits are connected together, the core thus containing a three-phase flow of flux. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ, and Y-Y. If a winding is connected to the earth (grounded), the earth connection point is usually the center point of a Y winding.
A leakage transformer also called a stray-field transformer, has a significantly higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, which is sometimes adjustable with a set screw. This provides a transformer with an inherent current limitation due to the loose coupling between its primary and secondary windings. The output and input currents are low enough to prevent thermal overload under all load conditions – even if the secondary is shorted. Leakage transformers are used for arc welding and high-voltage discharge lamps.
A resonant transformer is a kind of leakage transformer. It uses the leakage inductance of its secondary windings in combination with external capacitors, to create one or more resonant circuits. Resonant transformers such as the Tesla coil can generate very high voltages and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator.
A current transformer is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relaying, where they facilitate the safe measurement of large currents. The current transformer isolates measurement and controls circuitry from the high voltages typically present on the circuit being measured. Voltage transformers (VTs)–also referred to as potential transformers (PTs)–are used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential.
A zigzag transformer is a special-purpose transformer. It has primary windings but no secondary winding. One application is to derive an earth reference point for an ungrounded electrical system. Another is to control harmonic currents.
A pulse transformer is a transformer that is optimized for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. Special high-voltage pulse transformers are also used to generate high-power pulses for radar, particle accelerators, or other high-energy pulsed power applications.
In the same way that transformers are used to create high-voltage power transmission circuits that minimize transmission losses, speaker transformers allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than-normal speaker voltages. This application is common in public address applications. Such circuits are commonly referred to as constant voltage or 70-volt speaker circuits although the audio waveform is obviously a constantly changing voltage.
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 the voltage from 208v to 240v for lighting applications. One major advantage of Buck-boost transformers is their low cost, compact size, and lightweight.
Pad Mounted Transformers are usually single-phase or three-phase and is used where safety is a main concern. Typical Applications is a restaurants, commercial buildings, shopping malls, and institutions.
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 require minimum maintenance to provide many years of reliable trouble-free service. Unlike liquid fill transformers which are cooled with oil or fire-resistant liquid dielectric, dry-type units utilize only environmentally safe, CSA and UL-recognized high-temperature insulation systems. Dry-type transformers provide a safe and reliable power source that does not require fireproof vaults, catch basins, or the venting of toxic gasses. These important safety factors allow the installation of dry-type transformers inside buildings close to the load, which improves overall system regulation and reduces costly secondary line losses.
Dry-type transformers are a rather mature product and technology but, of all the components in a power system, a transformer replacement can be a physically challenging event, with extended delivery of a replacement or repair unit, and expensive transportation costs. These are transformers whose core and coils are not immersed in insulating oil.
Fire-resistant dry type or “cast resin” transformers are well suited for installation in high-rise buildings, hospitals, underground tunnels, schools, steel factories, chemical plants, and places where fire safety is a great concern. Hazard-free to the environment, dry-type transformers have over the years proven to be highly reliable.
“Dry type” simply means it is cooled by normal air ventilation. The dry-type transformer does not require a liquid such as oil silicone or any other liquid to cool the electrical core and coils.
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