An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps
where electrical connections are made. An autotransformer can be
smaller, lighter and cheaper than a standard dual-winding transformer
however the autotransformer does not provide electrical isolation.
Autotransformers 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.
One end of the winding is usually connected in common to both the voltage source and the electrical load.
The other end of the source and load are connected to taps along the
winding. Different taps on the winding correspond to different voltages,
measured from the common end. In a step-down transformer the source is
usually connected across the entire winding while the load is connected
by a tap across only a portion of the winding. In a step-up transformer,
conversely, the load is attached across the full winding while the
source is connected to a tap across a portion of the winding.
As in a two-winding transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting the load between the middle and bottom of the autotransformer will reduce the voltage by 50%. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected.
Autotransformers 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.
Operation
An autotransformer 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
As in a two-winding transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting the load between the middle and bottom of the autotransformer will reduce the voltage by 50%. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected.
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