Autotransformers of power supply at a frequency of 50Hz

The difference between an autotransformer (AT) and a transformer is that the AT has only one winding, which is both a primary and a secondary circuit. This is an AT with one tap. It is possible to manufacture AT with several taps, both for changing U1 and for changing Un. The AT design is exactly the same.

The principle of operation of the AT practically does not differ from the principle of operation of the transformer. Consider the work of AT. We connect it to an alternating current network with a voltage of U1 - windings 1 + 2 or 1. In this case, some kind of EMF E1 is induced in the AT winding, which means that in that part of the winding to which the load is connected, EMF En is induced, and a current 1n flows. Through the common part of the AT turns during each half-period, two currents I1 and 1n are induced and flow, which have an opposite direction. Therefore, the AT winding can be made with a wire of a smaller diameter than that of a simple transformer of the same power.

  For the lowering and increasing AT, the formulas are respectively true:

   nPN = (1 + 2) / 2> 1 (U1 / Un),
nPV = 1 / (1 + 2) <1 (U1 / Un),
where nПН and nПВ; AT transformation ratios.

  Unlike a transformer, an AT has both an electrical and a magnetic connection between the primary and secondary circuits. Thus, electrical energy is transmitted to the consumer, unlike a transformer, not only magnetically, but also electrically.

  The power of the step-down and step-up AT, respectively, can be calculated by the formulas:
  Rn = UnIn / nPN + UnIn (nPN; 1) / nPN = Rel + Rem,
  Rn = UnInnPV + UnIn (1; nPV) = Rel + Rem,

  where Rel, Rem - respectively, the electrical and electromagnetic components of the power coming from the primary circuit to the secondary.
  The overall dimensions of the AT magnetic circuit and the power losses are determined by the electromagnetic component of the power (the electrical component is not taken into account).

For decreasing and increasing AT, respectively:
 Rem = UnIn (1; 1 / nPN),
 Rem = UnIn (1; nPV).

  Rem AT at the required output power is the smaller, the closer the transformation ratio is to unity and the smaller the cross-sectional area of the AT magnetic circuit (usually the transformation ratio is chosen in the range 1.2 ... 2).
It can be concluded that with a sufficiently small transformation ratio, it is possible to reduce both the mass and dimensions of the AT, and the consumption of copper for the AT winding, which significantly reduces the cost of AT in comparison with a transformer of the same power. In addition, the efficiency of an AT with a low transformation ratio is higher than the efficiency of a transformer. With a large transformation ratio, the use of AT is unprofitable. In addition, the presence of electrical connection between the low and high voltage parts requires specialized protection measures when powering the electronic equipment.
  In practice, radio amateurs often make AT according to the schemes shown in Fig. 2, a, b. This design allows, using different switches, to obtain different voltages to power a particular equipment. The more taps, the more and more smoothly the resulting voltage can be regulated. 
   Attention! The equipment powered by the AT must not be grounded: a short circuit may occur, since a phase of this voltage is present at one of the ends of the supply voltage. Such equipment should be housed in an insulated enclosure.
   The calculation of a low-power AT almost does not differ from the calculation of a transformer of the same power given in previous articles. A small example of calculating an autotransformer.
   1. Choose the typical size of the AT core with the consumed electromagnetic power:
    a) for lowering AT (nPN> 1):
   Rem = Pn (1; 1 / nPN) = Pn (1 / (1 + 2));
   b) for increasing AT (nPV <1):
   Rem = Pn (1; nPV) = Pn (2 / (1 + 2)).
If AT is with taps, then Ram should be calculated for all transformation ratios.
  2. Calculate the rated current in the primary circuit with an active load:
   I1 = Рн / U1АТcos.
In practice, ATcos is 0.9 ... 0.95.
  3. When calculating the diameter of the wire of the AT windings, the following must be taken into account: for the lowering AT, the current in the winding 1 is I1, and the current in the winding 2 is Itotal = In; I1; for by;
outgoing AT current in winding 2; In, and the winding current is 1; Itot = I1;
  4. The voltage drop across the AT windings, when determining the number of turns of AT parts, can be ignored.
  The industry produces general-purpose ATs for currents up to 32 A, which are connected to a single-phase alternating current network with a frequency of 50 Hz and a voltage of 127 and 220 V, as well as a three-phase network with a frequency of 50 Hz and a voltage of 220 and 380V. The industry also produces special ATs for connection to a network with a frequency of 400 Hz. It is necessary to carefully look at the marking of industrial vehicles.
  Example of industrial vehicle labeling:
rated load current 10 A;
number of regulated circuits 1;
Efficiency 95%;
idle current 0, 3 A.
In AT designations:
A ; autotransformer;
O ; single phase;
T ; three-phase;
WITH ; dry with natural air cooling;
M; with oil filling;
H; with voltage regulation under load;
NS ; with an electromechanical drive;
D; with two adjustable chains.
Further, after the letters, indicate the nominal load current in amperes, the nominal primary voltage (U1) in volts, the year of development, the symbol of the climatic version, the category of placement, the place of installation (built-in, stationary, etc.), etc. The radio amateur, as a rule, is only interested in the ratings indicated in the example.

   O.G. Rashitov, Kiev                                                                                                                                                                                                 

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