In some cases, it is necessary to vary the voltage within a small range. This is easiest to do not with two-winding transformers, but with single-winding, called autotransformers... If the transformation ratio differs little from unity, then the difference between the magnitude of the currents in the primary and in the secondary windings will be small.
Autotransformers are classified as special-purpose transformers. Autotransformers differ from transformers in that their low voltage winding is part of the higher voltage winding, that is, the circuits of these windings have not only magnetic, but also galvanic connection. In an autotransformer, the power loss is less than in a two-winding transformer, at the same rated power. This is due to the fact that in a two-winding transformer, all the power is transmitted from the primary circuit to the secondary circuit by electromagnetic means, and in the autotransformer only part of this power is transmitted by electromagnetic means, and the rest is transmitted directly from the primary to the secondary circuit as a result of electrical connection between them, therefore, calculate the autotransformer this power is not required.
The closer the transformation ratio to unity, the less part of the power is transmitted electromagnetically, therefore, the less weight and dimensions of the autotransformer. For example, with a transformation ratio equal to 2, half the power is transmitted electromagnetically to the secondary circuit, and with a transformation ratio equal to 3, already 2/3 of this power. Hence, the advantages of autotransformers are manifested only at low transformation ratioswhen the difference in currents is small. With high transformation ratios, there is practically no benefit from the use of autotransformers.
However, autotransformers offer significant cost benefits. So, for comparison, consider two options with the same power. We offer a 60VA transformer at a price of 170 UAH. , an autotransformer of the same power will cost 105 UAH. Roughly the same proportion will remain at higher capacities.
Structurally, autotransformers practically do not differ from transformers. There are two windings on the magnetic core frame. The conclusions are taken from two windings and a common point. Most of the parts of the autotransformer do not differ structurally from the parts of the transformer.
Thus, it is advisable to use autotransformers for a slight decrease or increase in voltage, when a reduced current is set in the part of the winding, which is common to both autotransformer circuits, which allows it to be made with a thinner wire and save non-ferrous metal. At the same time, the consumption of steel for the manufacture of a magnetic circuit decreases, the cross-section of which is less than that of a transformer. It happens that the voltage does not reach the standard 230V or, on the contrary, exceeds it. Aautotransformer can easily correct it with the step with which it is calculated and executed. The autotransformer, complete with control and switching equipment, does this automatically.
Let's make an autotransformer power from 20VA to 300kVA in single-phase or three-phase design. In a constructive binding with terminal blocks or in a housing. Input, output voltage and number of windings - at your request.
Autotransformer in a 3-phase version has some limitations, because. harmonic currents are not suppressed and leakage currents are generated. A large 3-phase autotransformer may have a hidden winding, not connected to the external one, to suppress these harmonics.
Disadvantage autotransformer it is necessary to insulate both windings for a higher voltage, since the windings are electrically connected. A significant drawback of autotransformers is the galvanic connection between the primary and secondary circuits, which does not allow them to be used as power ones in 6-10 kV networks when the voltage drops to 0.38 kV, since 380 V is supplied to the equipment on which people work.