An Auto-Transformer is an electrical transformer with only one winding.It is also known as auto- step down transformer. In auto-transformer, portions of the same windings act as both the primary and the secondary sides of the transformer. An ordinary transformer has seprate primary and secondary windings which are not electrically connected.
Auto-transformer are used to step up or steo down voltage in the 110-120 V range and voltage in the 220-230-240 V range. This allows US electrical equipment to fed from the higher voltage used in Europe and anywhere. Auto-transformer can also be used to supply 230V applicance from 100 to 120V supply within the US. In all cases the supply and the auto transformer must be correctly rated to supply the required power.
OPERATION
An auto transformer has a single windings with two end terminal at intermediat tap point. or it is a transformer in which the primary and the secondary coil have part of, or all of their turns in common. The primary voltage is applied across two of the terminal, and the secondary voltage take from two terminal,almost always having one terminal in common with the primary voltage.
The voltage current ratio of auto transformer can be formulated the same as other two windings trnsformer. V1/V2 = N1/N2 =a
The ampere turn provide by the upper half ; Fu= (N1-N2) I1=(1-1/a) N1I1 The ampere turn provide lower half : F1= N2(I2-I1)=N1
LIMITATIONS:
A failure of isolation of the windings of an auto transformer can result in full input voltage applied to the output.Also,break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load condition may result in near full input voltage being applied to the output). In three phase power tranmission application, autotransformer have the limitation of not suppressing harmonic current and as acting as another source of ground fault currents.
APPLICATION:
Power transmission and distribution Auto transformer are frequently used in power application to interconnect system operating at different voltage classes, for example 132 kv tp 66 kv for tranmission. Another application in industry is to adapt machinery built (for example) 480 v supplies to operate on a 600 v supply.
Audio system
In audio application, tapped auto transformer are used to adapt speaker to constant voltage audio distribution system, and for impedence matching such as between a low impedence microphone and a high-impedence amplifier input.
Railways
In UK railways application, it is common to power the train at 25kv AC. To increase the distance between electricity Grid fedder point , they can be arrenged to supply a 25-0-25kv feed with the third wire (opposite phase) out of reach of the train's overhead collector pantograph.
