Auto Wound Transformers
An auto transformer has a single winding with two end terminals, and one or more terminals at intermediate tap points. The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding however, a transformer with separate windings isolates the primary from the secondary, which is safer when using mains voltages.
Auto transformers are often used to step up or down between voltages in the 110-117-120 volt range and voltages in the 220-230-240 volt range, e.g., to output either 110 or 120V (with taps) from 230V input, allowing equipment from a 100 or 120V region to be used in a 230V region.
Auto transformers are very simply one tapped winding, they offer no isolation between the primary and secondary windings therefore do not have the isolation properties of a Double wound transformer with safety isolating screen. Auto transformers are however popular in many sectors machine builders with additional protection within the circuit favour the auto transformer due to its size and cost advantages.
By effectively reducing the equivalent frame size you ultimately reduce the cost of the transformer, to calculate the equivalent frame size (VA) of an auto transformer our engineers have provided the following equation.
The Auto transformer is advantageous when the high and low voltage spread is closest. The closer the voltages the smaller the equivalent frame size of the transformer will be.