Autotransformer

What is an Autotransformer?

Transformers (isolating transformer, autotransformer) are of high importance in industrial power supply Many devices require a separate power supply with nominal voltage of the device. On the other hand there are power supply networks with voltages such as 24 V, 220 V and 380 V. The transformer allows connection of devices which nominal voltage does not match the required network voltage.

Transformers are available in different designs. In addition to the Isolating transformer the autotransformer has been established. An autotransformer contains one coil instead of two coils. The output voltage is taken in on different parts of the coil (winding, tap). Multiple taps allow high output voltages. Inside of the autotransformer there is no galvanic isolation. The different areas created by the taps are called serial coil or parallel coil. The diameter of the wire may differ in the areas of the autotransformer. The part responsible for low power can be equipped with a small diameter conductor.

Autotransformer Advantages

An autotransformer is smaller than an isolating transformer with similar power. Less assembly space is required. By using an autotransformer less material is needed. The further the values of input and output voltage are apart the lower the savings are. Autotransformers are best suited for small voltage adjustments. An autotransformer has a smaller voltage drop than a two coil transformer with similar power.

Where are Autotransformers used?

If galvanic isolation is not required an autotransformer may be used. Due to safety reasons a transformer without galvanic isolation can only be used in certain applications. Autotransformers are used in power supply of electric trains. In addition autotransformers are used as ignition coil in internal combustion engines.

A different voltage can be matched to the nominal voltage by use of an autotransformer. This is common when an electric component was manufactured in a foreign country. Machines and devices should be equipped with an autotransformer so they can be used worldwide.

For industrial applications multi phase autotransformers are available. An example is a three phase autotransformer for use in the control cabinet with 3x380/400/420 V AC input and 3x230 V AC or 1x220 V AC output.

Power Current in the Autotransformer

An autotransformer may be used to lower voltage (down transformation) or to increase voltage (up transformation). If an autotransformer is used to lower the voltage the current from the primary circuitis split into a part which flows through the load in the secondary circuit and one part which flows through the parallel coil. When lowering the voltage the entire current flows through the serial coil in the transformer. When increasing the voltage the nominal voltage of the secondary circuit flows through the serial coil. In both cases the lower voltage flows through the serial coil. It may be equipped with a wire with a smaller diameter. A substantial amount of material may be saved. A solid tap limits the voltage in the secondary circuit of the autotransformer.

Important values for an autotransformer are the input voltage, input power, output voltage and the output power. Currents during switching on and during a short should be included.