Editing Autotransformer (section)

==Operation==

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 portion of the winding shared by both the primary and secondary is the common section. The portion of the winding not shared by both the primary and secondary is the series section. The primary voltage is applied across two of the terminals. The secondary voltage is taken from two terminals, one terminal of which is usually in common with a primary voltage terminal.<ref>{{cite book |last=Pansini |first=Anthony J. |title=Electrical Transformers and Power Equipment |edition=3rd |date=1999 |publisher=Fairmont Press |isbn=9780881733112 |pages=89–91}}</ref>

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 output current flows directly from the input to the output (through the series section), and only part is transferred inductively (through the common section), allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding.<ref>{{Cite web |url=http://victoruae.com/auto_transformer_detail.html |title=Auto Transformer |publisher=Victor Industrial & Trading Ltd |access-date=2013-09-19 |archive-url=https://archive.today/20130920190856/http://victoruae.com/auto_transformer_detail.html |archive-date=2013-09-20 |url-status=usurped }}</ref> However the voltage and current ratio of autotransformers can be formulated the same as other two-winding transformers:<ref name="Sen64" />

<math display="block">\frac{V_1}{V_2} = \frac{N_1}{N_2} = a </math>

where <math>0 < V_2 < V_1</math>.

The ampere-turns provided by the series section of the winding:

<math display="block">F_S = (N_1 - N_2)I_1 = \left(1-\frac{1}{a}\right)N_1I_1</math>

The ampere-turns provided by the common section of the winding:

<math display="block">F_C = N_2(I_2 - I_1) = \frac{N_1}{a}(I_2-I_1)</math>

For ampere-turn balance, <math>F_S = F_C</math>:

<math display="block">\left(1-\frac{1}{a}\right)N_1 I_1 = \frac{N_1}{a}(I_2-I_1)</math>

Therefore:

<math display="block">\frac{I_1}{I_2} = \frac{1}{a} = \frac{N_2}{N_1}</math>

One end of the winding is usually connected in common to both the [[voltage source]] and the [[External electric load|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. For a step-up transformer, the subscripts in the above equations are reversed where, in this situation, <math>N_2</math> and <math>V_2</math> are greater than <math>N_1</math> and <math>V_1</math>, respectively.

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 of the winding and the common terminal end of the winding of the autotransformer will result in the output load voltage being 50% of the primary voltage.  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 wire|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 nearly full input voltage being applied to the output).  These are important safety considerations when deciding to use an autotransformer in a given application.<ref name=SH11>{{cite book |first1=Donald G. |last1=Fink |author1-link=Donald G. Fink |first2=H. Wayne |last2=Beaty |title=Standard Handbook for Electrical Engineers |edition=Eleventh |publisher=McGraw-Hill |location=New York |date=1978 |isbn=0-07-020974-X |at=pp. 10-44, 10-45, 17-39 <!--HYPHENATED PAGES; do not use endash-->}}</ref>

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 3:1; beyond that range, a two-winding transformer is usually more economical.<ref name=SH11/>

In [[three phase]] power transmission applications, autotransformers have the limitations of not suppressing [[Harmonics (electrical power)|harmonic]] currents and as acting as another source of [[Residual-current device|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.<ref name=SH11/>

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. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.

Like multiple-winding transformers, autotransformers use time-varying [[magnetic field]]s to transfer power. They require [[alternating current]]s to operate properly and will not function on [[direct current]]. Because the primary and secondary windings are electrically connected, an autotransformer will allow current to flow between windings and therefore does not provide AC or DC isolation.