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Transformers for converting sets

Functions of transformers:

- Transformation of magnitude of voltage;

- A galvanic outcome (the power circuit are separated from a circuit of a load);

- Transformation of phase number of a voltages system;

- Shift of phases of primary and secondary voltages.

The operating conditions of a transformer for converter differ from the operating conditions of transformers for linear load and standard frequency in 50 Hz.

The nonlinear loads are a reason of nonlinear currents through windings of the transformer and nonlinear magnetic fluxes in a core. The heightened frequency causes additional losses in the core and windings.

Constructive differences:

- Owing to heavier emergency operation it is necessary to augment inductance of windings for limitation of emergency currents, and also safely to fix windings and core for the reason that dangerous electrodynamic forces arise at emergencies;

- For series of schemes a total power of secondary windings S₂ is greater than a total power of primary windings S₁;

- The dispersed magnetic fluxes essentially influence to character of electromagnetic processes

in the converter. They determine inductive reactance of dispersion Х_s, which, in basic, is determined by inductance L_a). The character of distribution of dispersed magnetic fluxes depends on a core shape, type and windings disposition each from other, their construction, etc.

The resistance of windings is determined r_a.

For low power converters r_a >> Х_a and dispersed fluxes is neglected at such convertersdesigning.

For high-power converters there are Х_а >> r_a (as a rule, ), and r_a is neglected, but commutation stage of rectifiers influences to a marked degree to operation and converters designing.

For converters of average power it is necessary to take into account as r_a as Х_а.

There are two types of transformers for rectifiers and dependent invertors.

1) The transformer for the single-cycle schemes (half-wave, three-phase zero, full-wave three phase with LR), where exists direct -current component of a secondary winding, and for compensation of a forced magnetization flux it is necessary to arrange two secondary windings on a same rod (for example, "«zigzag" for a zero scheme). For an identical disposition of secondary windings in relation to primary they execute as rigid each other winded spirals, that the dispersed fluxes between secondary windings would be minimum ones.

2) The transformer for the two-stroke schemes differs from usual only by higher values Х_s (single-phase and three-phase bridge-schemes). At compiling equivalent circuits of resistance of a primary winding reduce to secondary, because parameters of a load I_d and U_d are data for calculation of converter.

where r '₁ -active resistance reduced to a secondary winding from a primary winding,

r₂ - active resistance of a secondary winding.

Х_а = Х ' _1S+X_2S,

where Х ' _1S - dispersed inductance of a primary winding reduced to a secondary winding,

X_2S - dispersed inductance of a secondary winding.

The parameters of the equivalent schemes of the transformer define by practical consideration with the help of experiences short circuit and open-circuit conditions.

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