Autotransformer

An auto-transformer (autotransformer short, even autotransformer ) is unlike other transformers of only a coil that for removal of the output voltage (s) has one or more taps. The primary and secondary sides are thus combined in a single coil.

Adjustable autotransformer from 0 V to 240 V.

Circuit diagram of an autotransformer

750 MVA three-phase autotransformer

Circuit diagram of a three-phase autotransformer (regulation of the voltages in the string and medium voltage output 2 UVW)

Basics

Because both parts of the autotransformer are electrically connected to each other, there is no galvanic separation . Most other transformers, especially isolating transformers , have no conductive connection between the input and output circuit.

Autotransformers allow the use of a smaller design with the same power and are characterized by a lower voltage drop. The closer the values ​​of the input and output voltage are to each other, the more mass and material can be saved by using an autotransformer, since only part of the current and voltage has to be transformed. This enables different wire thicknesses in the two winding parts.

The construction output (also type output or size ) P _construction , for which the transformer must be physically designed, can be calculated using the following formulas if the electrical throughput P _D is known:

• The following applies to: ${\displaystyle U_{1}>U_{2}}$ ${\displaystyle P_{\mathrm {Bau} }=P_{D}\cdot \left(1-{\frac {1}{\ddot {u}}}\right)}$
• The following applies to: ${\displaystyle U_{1}<U_{2}}$ ${\displaystyle P_{\mathrm {Bau} }=P_{D}\cdot (1-{\ddot {u}})}$

The transmission ratio ü is the quotient of U ₁ and U ₂ .

The saving effect decreases as the voltage ratio increases, so that the transmission ratio should not exceed 3: 1. Autotransformers can be used all the more advantageously, the closer the input and output voltages are to one another.

In the case of low voltages, for example to operate 24 V halogen lamps on the 230 V power supply, autotransformers are hardly worthwhile.

example

A consumer with a power consumption of P ₂ = 200 W with U ₂ = 200 V is to be operated from the power supply system ( U ₁ = 230 V) .

${\displaystyle {\ddot {u}}={\frac {230\,\mathrm {V} }{200\,\mathrm {V} }}}$

This results in the transformer power to be transmitted

${\displaystyle P_{\mathrm {Bau} }=200\,\mathrm {W} \cdot \left(1-{\frac {200\,\mathrm {V} }{230\,\mathrm {V} }}\right)=26\,\mathrm {W} }$

The lower part of the winding can be made of considerably thinner wire than the upper part, because only 26 W / 200 V = 0.13 A flow here.

application

Autotransformers can be used wherever different voltages have to be provided and galvanic or mains isolation is not required, e.g. B .:

• Mains adapter for operating devices with / on a different mains voltage (e.g. 110 to 230 V or vice versa), e.g. B. when traveling (travel adapter)
• Savings variable transformers : In these, the output voltage is controlled by a moving over the turns grinding tapped or roller contact. The advantage of (savings) variable transformers opposite thyristor or triac - dimmers is that these (sinusoidal) away, and thus no time course of the network voltage electromagnetic interference generated. Problematic loads with capacitive, inductive or non-linear behavior can also be operated on autotransformers.
• Transformers in machines and devices in order to be able to use them with different mains voltages worldwide - they can be configured on site using changeover switches or jumpers.
• Autotransformer systems for traction power supply systems, designed as autotransformers with center tap
• Ignition coils in heat engines (e.g. internal combustion engines ): As they are designed as an autotransformer, a separate ground connection for the primary and secondary windings can be saved
• Autotransformers were used in simple tube receivers until the 1950s in order to keep construction costs low. Depending on the polarity of the mains plug, the chassis was connected directly to the mains.

Autotransformers must not be used if people can come into contact with the voltage, for example in the case of open lamps, welding transformers or toy trains, as the current could flow away through people in certain fault conditions.

With 230/110 V travel transformers (when US devices are used in Europe), the voltage between output and earth can be up to 230 V. In the case of transformers with separate primary and secondary windings, however, no current can flow between the output and earth. With the isolating transformer , this separation (isolation) is particularly safe.

Equivalent circuit diagram

Simplified equivalent circuit of an autotransformer

Complete equivalent circuit of an autotransformer

To model the function and behavior of an autotransformer, as well as to calculate it, the following equivalent circuit diagram should serve. The equivalent circuit diagram of an autotransformer differs from the equivalent circuit diagram of a transformer with separate windings . The gear ratio is designated in the equivalent circuit with ü , where

${\displaystyle I_{2}'={\frac {I_{2}}{\ddot {u}}}}$

and

${\displaystyle U_{2}'=U_{2}\cdot {\ddot {u}}}$

is.

See also

• ignition coil
• Starting transformer

literature

• Peter Bastian, Horst Bumiller, Monika Burgmeier, Walter Eichler, Franz Huber, Jürgen Manderla, Jürgen Schwarz, Otto Spielvogel, Klaus Tkotz, Ulrich Winter, Klaus Ziegler: Electrical engineering . 26th, revised and expanded edition. Europa-Lehrmittel , Haan-Gruiten 2008, ISBN 978-3-8085-3160-0 .
• Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics. Westermann Schulbuchverlag, Braunschweig, 1998, ISBN 3-14-221730-4 .
• Günter Springer: Expertise in electrical engineering. 18th edition, Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9 .