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by DBreg2007

Working of an auto transformer

Working of an auto transformer
There are times when one requires the voltage to be higher or lower. Many people might think that getting a variable voltage with a fixed AC supply is difficult. But in reality, a fixed AC voltage can be converted into a variable AC voltage using an auto transformer.
An auto transformer is a transformer in which the primary and secondary windings are electrically connected so that a part of the winding is common to both the windings.In this article, we will discuss the construction and working principle of an auto transformer.
An auto transformer comprises of a single copper wire. The wire is common to both the primary and secondary circuit. The copper wire is wound around a silicon steel core. Three taps are provided over the windings which provide three levels of output voltage. The primary and secondary windings are connected electrically and coupled magnetically. This property makes auto transformers cheaper, small and more efficient for voltage ratings less than three ordinary transformers. Also, an auto transformer has lower reactance, lower losses, smaller excitation voltage and better regulation compared to its two winding counterpart.
The main working principle of an auto transformer is to step up or step down voltage. They comprise of a single winding. The primary voltage is applied across the two ends of the winding. The primary and the secondary share the same neutral point. The secondary voltage is obtained across any one of the tapping and the neutral point.
The energy transfer mainly takes place through the process of conduction. Only small part of the energy is transferred inductively. The voltage per turn is same in the primary and secondary wire. The voltage can be varied by simply varying the number of turns. One terminal is connected to one of the tapping while the other is connected to the neutral. An auto transformer is nothing but a conventional two winding transformer connected in a special way.
In an auto transformer, the input and output power are almost equal. It has a lot of advantages compared to conventional transformers. It facilitates smooth variation of voltage, more efficient than a conventional transformer, requires less conductive material, smaller and less expensive, less copper loss and has superior voltage regulation capacity compared to a two winding transformer.
The main limitation of the auto -transformer is that the primary and secondary are not electrically isolated. Any undesirable condition at the primary will affect the equipment connected to the secondary.
It is mainly used in testing laboratories as an auto starter for induction machines.
Power transformers
As the name implies, power transformers transform voltage. Their main job is to hold low voltage and high voltage. That is high current circuit and low current circuit respectively. It works on Faraday’s principle.
The skeleton of the transformer is made of laminated metal sheets. It is carved into either a shell type or core type. The sheets are wound and then connected using conductors to form three 1-phase or one 3-phase transformer. Three 1-phase transformers have each bank isolated from the other and thereby offer continuity of service when one bank fails. A single 3-phase transformer, whether core or shell type; will not operate even with one bank out of service. This 3-phase transformer, however, is cheaper to manufacture, has a smaller footprint, and operates relatively with higher efficiency.
The metal portion of the power transformer is immersed in fire retardant insulation oil inside a tank. The conservator on top of the tank allows the expanding oil to spill into it. The load tap changer on the side of the tank allows change in the number of turns on the high voltage. That is low current winding for voltage regulation. The bushings on top of the tank allow for conductors to safely enter and exit the tank.
The transformer can be operated beyond its normal rating. Power transformers are fitted with fans that cool the transformer core to a point below the specified temperature. But prolonged overloading is not recommended as it will deteriorate the winding insulation.
The primary and secondary windings on the transformer, ofcourse insulated from each other, rely on induction principle alone to generate electro motive force, with the flux path isolated to the laminated sheets of metal.
To enable conduction of currents, the windings are wound either as a delta or star, on each side. The use of these connections delta-star, star-delta, star-star, or delta-delta make a huge impact on the design of power system. So the choice of connection is critical.
A star-star connected transformer is rarely applied in the power system. However to incorporate the design advantage of a star winding and those of delta winding, a third winding – a delta tertiary is built into the two winding star-star transformer.
Power transformers have number of applications. It can be used to connect a:
* Capacitor bank – for voltage or power factor correction
* Reactors – for limiting ground fault currents
* Resistors – for limiting ground fault currents
* Station service transformer – AC power for equipment inside the substation
* Distribution system – to power a town or an industrial customer