Autotransformere

Autotransformere, or autotransformers, are a type of electrical transformer in which the primary and secondary circuits are connected through a common winding. Instead of two separate windings, an autotransformer uses a single continuous winding with a tapping point. The input voltage is applied across a portion of the winding and the output is taken from another portion. Because the windings are shared, there is no galvanic isolation between input and output.

Operation and characteristics: The output voltage is proportional to the turns ratio between the portions of the winding used for input and output. A portion of the power is transferred inductively between windings, while a larger portion flows conductively through the common winding. This arrangement makes autotransformers smaller and cheaper for modest voltage changes, since less copper and core material is needed. However, the lack of isolation means they are unsuitable where isolation is required, and the common winding imposes voltage and current limits for safety and protection.

Applications: Autotransformers are commonly used for voltage regulation in distribution networks, for stepping up or stepping down in low-to-medium voltage ranges, and as starting autotransformers for induction motors to reduce starting current. They are also used in voltage converters and in equipment that requires adjustable output within a limited range. They are selected when the required voltage change is small and galvanic isolation is not necessary.

Limitations and safety: The absence of isolation means touch and shock hazards if accessibility or grounding is not properly managed. Protective devices and correct rating are essential. The kVA rating of an autotransformer can be higher than that of a comparable two-winding transformer for the same windings, but users must account for the shared winding when selecting protection and safe operating limits.