What Is An Autotransformer?

What Is An Autotransformer?
One winding for both primary and the secondary circuits

What is an autotransformer? Auto transformers have only one winding, a portion of which is common to both the primary voltage and the secondary side voltage circuits. In other words, the primary and secondary windings have some or all windings in common. Autotransformers are usually employed for the higher voltage conversion of local distribution supply voltage to some other secondary voltage on the secondary side that is needed for a particular electrical device. Most often, this conversion is from 125 Volts to 250 Volts, or 250 Volts to 125 Volts. Unlike an isolation transformer, variable autotransformers use a single winding and offers no interference or disturbance. Any 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.


Sometimes, it's not necessary to provide dc isolation between the primary winding and the sec­ondary winding of a transformer. Then an autotransformer can be used. This has a single, tapped winding. Its schematic symbol is shown in Fig. 18-8A for an air core, and Fig. 18-8B for a ferromagnetic core.

An autotransformer can be either a step-down or a step-up device. In Fig. 18-8, the autotransformer at A is step-down, and the one at B is step-up.

An autotransformer can have an air core, or it can be wound on any of the afore­mentioned types of ferromagnetic cores. You'll sometimes see this type of transformer in a radio-frequency receiver or transmitter. It works quite well in impedance-matching applications, and also in solenoidal loopsticks.

Variable autotransformers are occasionally, but not often, used at audio frequencies and in 60-Hz utility wiring. In utility circuits, autotransformers can step down by a large factor, but they aren't used to step up by more than a few percent.

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 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 primary supply voltage regulator stability and greater short circuited overload tolerance. It performs in much the same way as the electrical transformer that the electric utility uses a voltage regulator to reduce voltage drop in a building electrical system.

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 drop to the voltage it requires.

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.

Read Our Other Utility Transformer Pages: Padmount Transformer ExplainedPole Mounted Transformers, Electrical Power Transformers, Power Transformers Explained, Transformer Manufacturers, Step Up Transformer, High Voltage Transformers ExplainedThree Phase Transformers Explained