In an autotransformer, the common portion of the single winding acts as part of both the “primary” and “secondary” windings. The remaining. Electrical Tutorial about the Autotransformer and the Variable Auto 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 autotransformer is that it. An autotransformer is an electrical transformer with only one winding. The “auto” ( Greek for and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits. Other advantages of autotransformers include lower leakage reactance, lower.

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An autotransformer is an electrical transformer with only one winding. The ” auto ” Greek for “self” prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not electrically connected.

The winding has at least three taps where electrical connections are made. Since part of the winding does “double duty”, autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits.

Other advantages disadvaantages autotransformers disadvantayes lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass.

An example of an application of an autotransformer is one style of traveller’s voltage converter, that allows volt devices to be used on volt supply circuits, or the reverse.

What is an Auto Transformer? – Working, Advantage, Disadvantage & Uses – Circuit Globe

An autotransformer with multiple taps may be applied to adjust voltage at the end of a long distribution circuit to correct for excess voltage drop; when automatically controlled, this is one example of a voltage regulator. An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points.

It is a transformer in which the primary and secondary coils have part of their turns in common. The primary voltage is applied across two adantages the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. 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.

One end of the winding is usually connected in common to both the voltage source and the electrical load.

The other end auttransformer 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. 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.

If one of the center-taps is used for the ground, then the autotransformer can be used as a balun to convert a balanced line connected to the two end taps to an unbalanced line the side with the ground. An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either.

A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a 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 conditions may result in near full input voltage being applied to the output.

These are important safety considerations when deciding to use an autotransformer in a given application. Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3: In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of ground fault currents.

A large three-phase autotransformer may have a “buried” delta winding, not connected to the outside of the tank, to absorb some harmonic currents. In practice, losses mean that both standard transformers and autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if it is used to step up.

Like multiple-winding transformers, autotransformers use time-varying magnetic fields to transfer power. They require alternating currents to operate properly and will not function on direct current. On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulatorsso that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.

A special form of autotransformer called a zig zag is used to provide grounding on three-phase systems that otherwise have no connection to ground. A zig-zag transformer provides a path for current that is common to all three phases so-called zero sequence current.

In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input. In railway applications, it is common to power the trains at 25 kV AC.


To increase the distance between electricity Grid feeder points, they can be arranged to supply a split-phase disadvabtages feed with the third wire opposite phase out of reach of the train’s overhead collector pantograph.

Autotransformer Advantages and Disadvantages

The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. This system increases usable transmission distance, reduces induced interference into external equipment and reduces cost.

Ahtotransformer variant is occasionally seen where the supply conductor is at a different voltage to the autotranwformer wire with the autotransformer ratio modified to suit. Autotransformers can be used as a method of soft starting induction motors.

By exposing part of the winding coils and making the secondary connection through a sliding brusha continuously variable turns ratio can be obtained, allowing for very smooth control of output voltage. The output voltage is not limited to the discrete voltages represented by actual number of turns. The disadvzntages can be smoothly varied between turns as the brush has a relatively high resistance compared with a metal contact and the actual wnd voltage an a function of the relative area of brush in contact with adjacent windings.

Typically the primary connection connects to only a part advantates the winding allowing the output voltage to be varied smoothly from zero to above the input voltage and thus allowing the device to be used for testing electrical equipment at the limits of its specified voltage range. The output voltage adjustment can be manual or automatic.

The manual type is applicable only for relatively low voltage and is known as a variable AC transformer often referred to by xisadvantages trademark name Variac. These are often used in repair shops for testing devices under different voltages or to simulate abnormal line voltages. The type with automatic voltage adjustment can be used as automatic voltage regulatorto maintain a steady voltage at the customers’ service during a wide range of line and load conditions.

Another application is a lighting dimmer that doesn’t produce the EMI typical of most thyristor dimmers. From toVariac was a U. InInstrument Service Equipment applied for and obtained the Variac trademark for the same type of product.

From Wikipedia, the free encyclopedia. Electrical Transformers and Power Equipment. Archived from the original on This industry standard text describes the various European electrification principles. No comparable publications seem to exist for American railways, probably due to the paucity of electrified installations there.

Electrical Disadvanhages – I. Retrieved from ” https: All articles disadvanrages unsourced statements Articles with unsourced statements from January Views Read Edit View history. In other projects Wikimedia Commons. This page was last edited on 19 Decemberat By using this site, you agree to the Terms of Use and Privacy Policy.