Definition of autotransformers

The autotransformer is used when there is no need for electrical isolation from the supply network (i.e. between the input and output windings). In this case, the input and output windings are electrically connected. A distinction is made here between feed-through power and power rating, whereby the feed-through power is always greater than the power rating. The lower the difference between the input and output voltage, the lower the power rating.

The corresponding standard for autotransformers is EN 61558-2-13.

Definition and basic properties

An autotransformer – often referred to as an autotransformer or in the plural as an autotransformer – is a special type of transformer (also known colloquially as a transformer) in which the primary and secondary sides are not completely separated from each other. Instead of two separate windings, one common winding is used, which is used for both the supply and the output voltage.

This design means that part of the electrical energy is transmitted directly from the input to the output without the detour via a complete magnetic coupling. This results in design advantages, particularly in terms of size, weight and material usage. At the same time, efficiency is increased as there are fewer losses in the iron core and windings. This makes the autotransformer particularly interesting for applications in which high power is to be transmitted with comparatively small voltage differences.

Functionality

The functionality of an autotransformer is based on a common winding with several taps. Depending on the points at which the voltage is tapped, the output voltage can be higher or lower than the input voltage. The individual turns within the winding determine the voltage ratio.

Part of the power is transmitted inductively, while another part flows directly through the winding electrically. This combination of direct and inductive transmission is characteristic and distinguishes them from classic transformers with galvanic isolation.

This special mode of operation enables a higher degree of efficiency to be achieved as there are fewer energy losses. At the same time, the design enables flexible adaptation to different voltage requirements. Particularly with small voltage differences, the proportion of directly transmitted power is greater, which further increases efficiency.

Throughput and construction work

A key feature of Spartrafos is the distinction between throughput power and construction power.

The total power transmitted corresponds to the total power actually transmitted between the input and output. The power rating, on the other hand, describes the power for which the transformer must be designed, i.e. the power that is actually transmitted via the magnetic coupling.

Since part of the energy is transmitted directly, the power rating is always lower than the throughput power. This results in less material being used and enables more compact designs. This advantage is particularly evident with small voltage differences, as the power rating is then greatly reduced. This allows costs to be saved and high performance values to be achieved at the same time. In practice, this correlation is a decisive factor in the design and dimensioning of autotransformers.

Advantages of autotransformers

Autotransformers offer a number of advantages over conventional transformers:

Lower material requirements (less copper and iron)
More compact design
Higher efficiency
Lower losses
Cost efficiency for suitable applications

These advantages are particularly attractive for applications where galvanic isolation is not required. They also provide an economical solution when adapting mains voltages, for example from 230 volts to other voltage levels. The reduced space requirement can also be a decisive advantage in many technical systems.

Disadvantages and restrictions

Despite its advantages, such a transformer also has some limitations. The most important disadvantage is the lack of galvanic isolation. This means that there is a direct electrical connection between the input and output side.

This can pose a safety risk in certain applications, especially if protective measures are required for people or sensitive equipment. Therefore, autotransformers must not be used everywhere, but only where safety requirements permit.

Another disadvantage is that faults or overvoltages can be transmitted directly from the input to the output. Faults in the network also have a direct effect on the output side. Appropriate protective measures must therefore be carefully designed, especially on electrical connections and every relevant contact.

Typical applications

Auto transformers are used in many areas where voltage adjustment is required but no electrical isolation is necessary. Typical applications are

Voltage adjustment in industrial systems
Starting electric motors (e.g. as autotransformer starters)
Connection of grids with different voltage levels
Controllable power supply units
Test facilities and test systems

A typical example is the adjustment of mains voltages in international applications. Additional information on voltage and current is also recorded and evaluated in test environments in order to ensure the function of the systems.

Difference to safety transformers

In contrast to safety transformers, autotransformers have no galvanic isolation between the primary and secondary sides. While safety transformers were specially developed to protect people and ensure safe electrical insulation, autotransformers focus on efficiency and material savings.

The design of the secondary winding also plays an important role here, particularly with regard to voltage and load capacity.

This results in different areas of application. Safety transformers are primarily used in safety-critical applications, while autotransformers are used more in technical and industrial areas where other protective measures apply.

Standards and regulations

The relevant standard for autotransformers is EN 61558-2-13, which specifies the requirements for design, safety and operation.

Compliance with this standard is important to ensure safe and standard-compliant use. Among other things, it defines limit values, test methods and insulation requirements. Manufacturers must strictly adhere to these specifications in order to ensure the operational safety and reliability of the devices.

These requirements are often described and explained in detail in the technical documentation or in an accompanying article.

Calculation formula and meaning

The type output is calculated using the following formula:

Type power = (1 – undervoltage / overvoltage) * rated power

This formula shows that the required power depends heavily on the ratio between the undervoltage and the overvoltage. The smaller the difference between these two voltages, the lower the required construction power.

This is one of the main reasons why autotransformers are used particularly efficiently for small voltage adjustments. In practice, this enables a very economical design, especially for applications with almost identical voltage levels.

Summary

In summary, it can be said that the autotransformer is an efficient and economical solution for voltage adjustment when no direct electrical connection is required.

Thanks to its special design, it enables a reduction in material, costs and losses. At the same time, its use requires a careful assessment of safety requirements, as it is not electrically isolated from each other.

This makes the autotransformer an important component for efficient energy transmission and for adapting different voltage levels, particularly in industrial applications and technical systems.

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