To convert voltage in electrical engineering, transformers or autotransformers are used. Due to the similarity of the names of these two devices, they are often confused or equated to the same. However, this is not the case, although the principle of operation is similar, but the design and their scope of application are fundamentally different. Therefore, let's look at the differences between a transformer and an autotransformer in order to understand what the difference is.

Content:

Definitions
Operating principle
The main differences

Definitions

A transformer is an electromagnetic device that transmits energy through a magnetic field. It consists of two or more windings (sometimes called coils) on a steel, iron or ferrite core, depending on the number of phases, input and output voltages. Its main feature is that the primary circuit and the secondary circuit are not electrically connected to each other, that is, the windings do not have electrical contacts. This is called galvanic isolation. And this connection of the coils is called inductive.

Below you see a conventional graphic designation of a two and three-winding transformer on the electrical schematic diagram:

They are step-up, step-down and isolation (the input voltage is equal to the output voltage). Moreover, if you apply power to the secondary winding of the step-down transformer, you will receive an increased voltage on the primary windings, the same rule works for the step-up transformer.

An autotransformer is one of the variants of a transformer with one winding wound on a core, in principle similar to the previous case. In it, unlike normal trance, the primary and secondary circuits are electrically connected to each other. This means that it does not provide galvanic isolation. You can see the conventional graphic designation of the autotransformer below:

Autotransformers are available with fixed output voltage and adjustable. The latter are known to many under the name LATR (laboratory autotransformer). They can also be both downward and upward. In an adjustable LATR, the secondary circuit is connected to a contact sliding along the coil.

Important! Due to the lack of galvanic isolation, autotransformers, by definition, cannot be isolating, unlike conventional ones!

Another difference is the number of autotransformer windings - usually it is equal to the number of phases. Accordingly, single-winding products are used to power single-phase devices, and three-winding products are used for three-phase devices.

Operating principle

Briefly and in simple terms, we will consider how each version works.

The transformer has at least two windings - primary and secondary (or several). If the primary is connected to the network (or other source of alternating current), then the current in the primary winding creates a magnetic flux through the core, which, penetrating the secondary turns, induces into them EMF. The principle of operation is based on the phenomena of electromagnetic induction, in particular Faraday's law. When current flows in the secondary winding (into the load), the current in the primary winding also changes due to mutual induction. The voltage difference between the primary and secondary windings is determined by the ratio of their turns (transformation ratio).

Uп / Ud = n1 / n2

n1, n2 - the number of turns on the primary and secondary.

Speaking of an autotransformer, it has one winding, if there are several phases, the same number of windings. When an alternating current flows through it, the magnetic flux that occurs inside it induces an EMF in the same winding. Its value is directly proportional to the number of turns. The load (secondary circuit) is connected to the tap from the turns. On the step-up autotransformer, power is supplied not to the ends of the winding, but to one of the ends and a tap from the turns, in contrast to the transformer. What was shown in the diagram above.

The main differences

To make it easier for you to understand what is the difference between a conventional transformer and an autotransformer, we have collected their main differences in a table:

Transformer	Autotransformer
Efficiency	The efficiency of an autotransformer is higher than that of a conventional one, especially with a small difference between the input and output voltage.
Number of windings	Minimum 2 or more depending on the number of phases	1 or more, equal to the number of phases
Galvanic isolation	There is	No
Risk of electric shock when supplying electrical household appliances	With an output voltage of less than 36 Volts - not high	High
Safety for powered appliances	High	Low, if there is a break in the coil on the turns after tapping to the load, the entire supply voltage will fall on it
Price	High, consumption of copper and steel for cores is large, especially for three-phase transformers	Low, due to the fact that there is only 1 winding for each phase, the consumption of copper and steel is lower

Scope of application

Transformers are used everywhere - from power plants and substations designed for tens and hundreds of thousands of volts to powering small household appliances. Although power supplies have recently been used, they are also based on a generator and a transformer on a ferrite core.

Autotransformers are used in household mains voltage stabilizers. LATRs are often used in laboratories for testing or repairing electronic devices. Nevertheless, they have found their application in high-voltage networks, as well as for the electrification of railways.

For example, on railway such products are used in 2x25 networks (two of 25 kilovolts). As in the diagram above, in sparsely populated areas, a 50 kV line is laid, and 25 kV is supplied to the electric train via an overhead wire from a step-down autotransformer. This reduces the number of traction substations and line losses.

Now you know what is the fundamental difference between a transformer and an autotransformer. To consolidate the material, we recommend watching a useful video on the topic:

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