Single-phase transformer. Purpose, device and main characteristics

There is an old college joke. To the teacher's question "how does a single-phase transformer work" the student responds with a hoot: "Uh-uh!". This sound really does take place, it is caused by the fact that when the induction field is induced, a magnetic striction effect arises that causes the magnetic-field plates to vibrate.

A single-phase transformer is designed to create an alternating voltage of the desired value for a load that does not require a three-phase power supply.

Any transformer consists of two mainnodes: a core and coils, there are at least two of them. The principle of operation is simple. As a result of the passage of electric current through the conductor in the primary winding, the electromotive force (EMF) is induced on the secondary winding. The core consists of plates of a ferromagnet, that is, a material that enhances the magnetic field (electrical steel of special grades).

The emf is determined by the formula:

E = 4.44 x F x f x ω

Where:

Ф is the amplitude of the magnetic flux;

f is the frequency of the current;

ω is the number of turns in the winding.

The permissible load power that "pulls"single-phase transformer, is determined by the cross-section of the wire with which the coils are wound, and the quality of the magnetic circuit, in particular the magnetic permeability of the ferromagnet μ. The dimensions of the core and the number of turns are the subject of calculation, which often becomes the topic of course work in technical universities.

In any case, the more powerful single-phasevoltage transformers, the more impressive their dimensions. On their case, most often there is a label with a list of the main parameters (allowable current input and output voltages). However, this does not always happen.

In practice, many repairers often encounterwith the need to replace the burned single-phase voltage transformer. In order to verify the suitability, it is necessary to study the characteristics of the device intended for replacement.

The first thing to do is determine the input winding. In the case of step-down transformers, it has the greatest resistance.

Then, having included it in a network, it is possible to measureoutput voltage in idling mode. The ratio of the input and output EMF is the coefficient of transformation K. It is also equal to the fraction N in / N out, that is, the number of turns in the windings.

After that it is possible as a loadconnect a powerful variable resistance (rheostat) and remove the current-voltage characteristic by determining the value of the rated current. As the load increases, the output voltage gradually decreases.

Transformers are not only power, but alsomeasuring. In those cases where it is necessary to determine a significant value of the current in the circuit, an ammeter is used. It turns on in series, and must have a low resistance in combination with a large cross-section of the wire in the magnetic deflection system. Such a device would be too massive and expensive, therefore single-phase current transformers are used, taking proportionally reduced values, and supplying them to conventional serial ammeters. Calculate the amperage is simple, it remains only to apply the multipliers indicated on the case.