What do you mean by Auto-transformer and What are its functional usages?

Auto transformer - transformer in which a single winding acts as primary and secondary.

Few applications are :

• power applications to interconnect systems operating at different voltage classes.
• In audio applications, it is used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding and as well as secondary winding.

Usages:-

1) It boost up the voltage and keep it stable.

2) Some transformer in the market can change 220v to 110v  and vice-versa.

An autotransformer (sometimes called autostep down transformer)[1] is an electrical transformer with only one winding.

An autotransformer (sometimes called autostep down transformer)[1] is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone and not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not electrically connected.

Autotransformer is a traditional device that was used in an electrical system to add some variation to the output of the system.

- In lighting, you can change the supply of the lamp to change its brightness.

- In Motors, you can smoothly ramp up the supply of the motor to protect it at starting.

- In Voltage Stabilizers, an auto transformer is integrated with a servo control to maintain the voltage at a certain point.

Auto transformer applications are decreasing nowadays and replaced with more advanced and higher quality devices like semiconductors.

- Lighting Brightness is now controlled by an Electronic Dimmer.

- Motors now are supplied by VFDs or Softstarters that use Semiconductors to ramp up voltage.

- Some stabilizers stopped using the servo control and started integrating Semiconductors also.

Auto Transformer is still useful, but its applications are decreasing and replaced with better devices.