Study Notes: Auto‐Transformer | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

Auto-transformer
This single winding is “tapped” at various points along its length to provide a percentage of the primary voltage supply across its secondary load. Then the auto-transformer has the usual magnetic core but only has one winding, which is common to both the primary and secondary circuits.
Therefore in an auto-transformer, the primary and secondary windings are linked together both electrically and magnetically. The main advantage of this type of transformer design is that it can be made a lot cheaper for the same VA rating, but the biggest disadvantage of an auto-transformer is that it does not have the primary/secondary winding isolation of a conventional double wound transformer.
The section of winding designated as the primary part of the winding is connected to the AC power source with the secondary being part of this primary winding. An auto-transformer can also be used to step the supply voltage up or down by reversing the connections. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding, then the secondary voltage is “stepped-down” as shown
This single winding is “tapped” at various points along its length to provide a percentage of the primary voltage supply across its secondary load. Then the auto-transformer has the usual magnetic core but only has one winding, which is common to both the primary and secondary circuits.
Therefore in an auto-transformer the primary and secondary windings are linked together both electrically and magnetically. The main advantage of this type of transformer design is that it can be made a lot cheaper for the same VA rating, but the biggest disadvantage of an auto-transformer is that it does not have the primary/secondary winding isolation of a conventional double wound transformer.
The section of winding designated as the primary part of the winding is connected to the AC power source with the secondary being part of this primary winding. An auto-transformer can also be used to step the supply voltage up or down by reversing the connections. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding, then the secondary voltage is “stepped-down” as shown.

The voltages developed in the windings are dependent on the flux linkages. The windings are wound on the same magnetic core so they link the same flux.

• V1 / N1 = V2 / N2.

So whenever voltage V1 exist across primary winding, then voltage V2 will be induced across the secondary winding irrespective of changes in connections.
Similarly the magnetic circuit demands that mmf should be balanced. It implies the primary side ampere turn should equal the secondary side ampere turn.

• I1 . N1 = I2 . N2

Here,

• Auto-transformers are also used for voltage regulation in distribution networks, for starting of induction motors and as lighting dimmers. Auto-transformers are also used in electric traction.
• One main disadvantage about auto-transformer is that the primary and secondary are electrically connected. So the electrical disturbance i.e. high voltage transients from one side can be easily transmitted to the other side.
• The other disadvantage is that the impedance of the auto-transformer is considerably low, so the short circuit current will be more.
• More over an open circuit in common winding results in full primary side voltage across the load which is harmful.
  

Disadvantages of an Auto-transformer

• The main disadvantage of an auto-transformer is that it does not have the primary to secondary winding isolation of a conventional double wound transformer. Then an auto-transformer can not safely be used for stepping down higher voltages to much lower voltages suitable for smaller loads.
• If the secondary side winding becomes open-circuited, current stops flowing through the primary winding stopping the transformer action resulting in the full primary voltage being applied to the secondary terminals.
• If the secondary circuit suffers a short-circuit condition, the resulting primary current would be much larger than an equivalent double wound transformer due to the increased flux linkage damaging the auto-transformer.
• Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary winding automatically Earth’s the primary as there is no isolation between the two windings. Double wound transformers are sometimes used to isolate equipment from earth.

The auto-transformer has many uses and applications including the starting of induction motors, used to regulate the voltage of transmission lines, and can be used to transform voltages when the primary to secondary ratio is close to unity.
An auto-transformer can also be made from conventional two-winding transformers by connecting the primary and secondary windings together in series and depending upon how the connection is made, the secondary voltage may add to, or subtract from, the primary voltage.