An autotransformer is an electrical transformer in which there is one winding, a portion of which is common to both the primary and the secondary circuits. In other words, the primary and secondary coils have some or all windings in common.
An autotransformer is commonly used for the voltage conversion of local power line voltage to some other Voltage value needed for a particular piece of electrical equipment. Most often, this conversion is from 125 Volts to 250 Volts, or 250 Volts to 125 Volts.
Unlike an isolation transformer, an autotransformer uses common windings and offer no interference or disturbance isolation.
A given size autotransformer will support a load equal to its rated value whether it is connected in the 125 Volts to 250 Volts configuration, or in the 250 Volts to 125 Volts configuration.
These units are employed in custom designs or when converting industrial/military equipment between various operating voltage systems. Most often, this conversion is from 125 Volts to 250 Volts, or 250 Volts to 125 Volts.
Unlike an isolation transformer, it uses common windings and offer no interference or disturbance isolation. You add any suppression or filtering networks your system requires.
With a single tapped winding, an autotransformer is generally preferred to an isolation transformer, with two separate windings, for numerous reasons. It is much smaller and lighter than an isolation transformer. It also has better voltage stability and greater overload tolerance. It performs in much the same way as the electrical transformer that the electric utility uses to bring power to a building.
An autotransformer is a tapped winding transformer that changes the voltage available locally to the voltage required by a particular load. Thus, a load may operate anywhere around the world, as long as a transformer is available to convert the local voltage to the voltage it requires.
"Variac" is a trademark of General Radio (mid-20th century) for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. The term is often used to describe similar variable autotransformers made by other makers. An autotransformer is an efficient and quiet method for adjusting the voltage to incandescent lamps. While lightweight and compact semiconductor light dimmers have replaced variacs in many applications such as theatrical lighting, these transformers are still used when an undistorted variable voltage sine wave is required.
For more information consult any manufacturer listed in our Electrical Transformer Buyer's Guide.