An autotransformer is a type of transformer that has a single winding, unlike conventional transformers that have separate primary and secondary windings. It works on the principle of electromagnetic induction but uses part of the winding as both the primary and secondary, making it more compact and efficient.

Key Features of an Autotransformer:

Uses a common winding for both input (primary) and output (secondary).

Provides a variable output voltage using taps or a sliding contact.

More compact and efficient than conventional transformers.

Has lower losses and better voltage regulation.

Types of Autotransformers:

1. Step-up Autotransformer – Increases voltage.

2. Step-down Autotransformer – Decreases voltage.

3. Variable Autotransformer (Variac) – Provides adjustable output voltage.

Advantages:

Requires less copper and core material.

More efficient due to reduced losses.

Smaller size and lower cost.

Better voltage regulation.

Disadvantages:

No electrical isolation between primary and secondary (risk of electrical shock).

Limited use in high-voltage applications due to insulation concerns.

Faults can transfer from input to output more easily.

Applications:

Voltage regulation in power transmission.

Starting of induction and synchronous motors.

Laboratory testing equipment (Variac).

Railway electrification systems.