Single-phase transformer. Purpose, device and main characteristics

There is an old college joke. On the teacher's question "how does a single-phase transformer" 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 right size for a load that does not require a three-phase power supply.

Any transformer consists of two main nodes: 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 magnitude of 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 a single-phase transformer will "pull" 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-phase voltage 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 face the need to replace a 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, plugging it into the network, you can measure the output 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, you can connect a powerful variable resistance (rheostat) as a load 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 also measuring. In those cases where it is necessary to determine a significant amount of 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, so single-phase current transformers are used that take off proportionally reduced values and feed them to conventional serial ammeters. Calculate the amperage is not difficult, it remains only to apply the multipliers indicated on the case.