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Transformers are electrical devices that transfer electrical energy between two or more circuits by means of electromagnetic induction. They are used to step up or step down the voltage levels in power systems. There are two main types of transformers - conventional transformers and autotransformers.

A conventional transformer consists of two separate windings, known as the primary winding and the secondary winding. These windings are wound on a magnetic core, which provides a common path for the magnetic flux. The two windings are physically separate and have no electrical connection between them.

In an autotransformer, there is only one winding which acts as both the primary and the secondary winding. The winding is wound on a magnetic core and has a common part called 'common winding'. The primary and secondary sides are electrically connected.

In a conventional transformer, the primary winding is connected to the input voltage source, and the secondary winding is connected to the load. When an alternating current (AC) flows through the primary winding, it creates an alternating magnetic flux in the core, which induces an electromotive force (EMF) in the secondary winding. This induces a secondary current, which is used to supply power to the load.

In an autotransformer, since there is only one winding, the part connected to the input voltage source acts as the primary and the remaining part acts as the secondary. The load is connected across a portion of the winding. The AC current flowing through the primary winding creates a magnetic flux in the core, which in turn induces an EMF in the secondary portion of the winding. The secondary current then flows through the load.

Autotransformers are generally more efficient than conventional transformers because the primary and secondary windings share a common winding, thus reducing the losses due to resistive heating in the windings, and the overall size and weight of the transformer are reduced.

Conventional transformers are widely used in power systems to step-up or step-down voltages in power transmission and distribution. They provide good electrical isolation between the primary and secondary circuits. On the other hand, autotransformers are commonly used for lower voltage applications, such as in voltage regulation and stabilization of power systems, motor starting, and in audio-frequency electrical circuits. In conclusion, the main differences between an autotransformer and a conventional transformer are their construction, working principles, efficiency, and applications. Autotransformers have a single winding that serves as both primary and secondary windings, leading to higher efficiency, while conventional transformers have separate primary and secondary windings, providing better electrical isolation between the input and output.