Auto Transformer – Working Principle, Features, and Types

An Auto Transformer is an electrical transformer that uses a single continuous winding for both the primary and secondary circuits. Unlike conventional transformers with separate windings, an auto transformer shares part of the same coil, which makes it more efficient, compact, and economical. It is widely used where electrical isolation is not required.

Working Principle of Auto Transformer

The working principle of an auto transformer is based on electromagnetic induction. When AC voltage is applied to the winding, a magnetic flux is produced in the core, inducing voltage across different sections of the same winding.
Power transfer occurs in two ways:

• Inductively, through the magnetic field

• Conductively, through direct electrical connection

Because of this combined power transfer, losses are reduced and efficiency is improved.

Key Features of Auto Transformer

• Single winding for both input and output

• Less copper usage, reducing cost

• Higher efficiency than two-winding transformers

• Smaller size and lighter weight

• Better voltage regulation

• Lower leakage reactance and losses

Types of Auto Transformer

Step-Up Auto Transformer

A step-up auto transformer increases voltage when the secondary turns are more than the primary turns. It is used where low voltage needs to be converted into higher voltage.

Step-Down Auto Transformer

A step-down auto transformer reduces voltage when primary turns exceed secondary turns. It is commonly used in motor starters and voltage control systems.

Conclusion

An auto transformer is an efficient solution for voltage transformation where compact size, lower cost, and better performance are required. Its simple construction and high efficiency make it suitable for many industrial and electrical applications.