autotransformers

An autotransformer is a transformer in which a single winding acts as both primary and secondary, with a common portion shared by the input and output and a tapped point to set the voltage difference. Unlike a two-winding transformer, the primary and secondary are not electrically isolated.

Construction and operation: An autotransformer consists of one continuous winding on a magnetic core, with a tap at a fraction of the total turns. The input is applied across the entire winding; the load is connected between the tap and one end. The portion of the winding that is common to both circuits carries most of the current, while the series portion provides the voltage difference between input and output.

Ratings and performance: The output voltage is determined by the tap position. Because part of the winding is common, autotransformers use less copper and core material than an equivalent two-winding transformer, and can handle higher apparent power for the same core. The portion of power transferred inductively corresponds to the series winding, while the remainder is conducted magnetically through the common portion.

Advantages and limitations: Benefits include smaller size, lighter weight, lower cost, and good efficiency for moderate voltage changes or high kVA requirements. Limitations include lack of isolation between input and output, which poses shock and fault protection considerations, and stricter insulation/clearance requirements to accommodate the line-to-tap voltages.

Applications: Autotransformers are used for voltage regulation in distribution networks, motor starting and current reduction, and adjustable voltage sources in laboratories and testing equipment. They are not suitable when electrical isolation between circuits is required, and care must be taken to ensure proper protection and grounding.