Autotransformer

An autotransformer (short autotransformer, also autotransformer ) is unlike other transformers from only one coil for removal of the output voltage (s) has one or more taps. Primary and secondary are thus united in a single coil.

Basics

Because both parts of the autotransformers being electrically connected together, there is no galvanic isolation. For most other transformers, especially in isolating transformers however, there is no conductive connection between input and output circuit.

The closer the values ​​of input and output voltage will lie down together, the more mass and material can be saved by using an autotransformer, since only a portion of the current and voltage must be transformed. This allows different wire sizes in the two winding parts. The Works PBau for which the transformer must be designed mechanically, can be given a known electrical continuity performance PD according to the following formulas:

  • For the following applies:
  • For the following applies:

The ratio u is the quotient of U1 and U2. Is a prerequisite for the use of this cost transformer that no galvanic separation or disconnection from the mains is required.

Example

From the power supply ( U1 = 230 V) is a consumer of the power P2 = 200 W = 200 V are operated with U2.

This results to be transmitted power transformer

The lower part of the winding may be wound from wire considerably thinner than the upper part, because only 26 W/200 V = 0.13 A to flow.

Application

Auto-transformers can be used wherever a line voltage must be provided without galvanic isolation, for example:

  • AC adapter to operate devices with / to different voltage ( eg, 110 to 230 V or vice versa), such as a vacation (travel converter )
  • Saving variable transformers: In these, the output voltage is measured by a over the turns moving grinding or rolling contact. The advantage of (savings ) variable transformers compared to thyristor or triac dimmers is to maintain the sinusoidal shape of the mains voltage and thus the elimination of interference. Also problematic loads with a capacitive, inductive or non-linear behavior can be operated on saving variable transformers.
  • Transformers in machinery and equipment in order to use them in various worldwide voltages can - you can configure on-site by switch or jumper.
  • Autotransformer systems for traction power supply systems, designed as autotransformer with center
  • Ignition coils in heat engines (eg combustion engine): Through its execution of an autotransformer can be a separate ground connection for the primary and secondary winding save
  • Autotransformers were up in the 1950s used in simple tube receivers to keep the construction costs low. Depending on the polarity of the power plug, the chassis was connected directly to the network.

Auto-transformers shall not be used if people with the voltage may come into contact, for example, in open lamps, welding transformers or toy trains, as in certain fault conditions, the current could flow out to the people.

At cruising transformers 230/110 V (for use American devices in Europe), the voltage between the output and ground can be up to 230 volts. For transformers in which the primary and secondary winding are separated from each other, however, no current can flow between the output and earth. When isolating transformer, this separation (isolation) is particularly safe.

Equivalent circuit

For the function and behavior of an auto-transformer to model, and to calculate it, is intended to serve the following equivalent circuit diagram. The equivalent circuit diagram of an auto-transformer differs from the equivalent circuit of a transformer with separate windings. The gear ratio is referred to in the equivalent circuit with ü, where

And

Is.

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