There is an old institute joke. To the teacher’s question “how does a single-phase transformer work”, the student responds with a buzz: “Oooo!”. Such a sound does occur, it is due to the fact that when the induction field is induced, a magnetic-striction effect arises, causing the plates of the magnetic circuit to vibrate.
A single-phase transformer is designed to create an alternating voltage of the required value for a load that does not need a three-phase power supply.
Any transformer consists of two main components: 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, an electromotive force (EMF) is induced on the secondary . The core consists of ferromagnet plates, that is, a material that enhances the magnetic field (electrical steel of special grades).
The value of the EMF is determined by the formula:
E = 4.44 x F x f x ω
Where:
F is the amplitude of the magnetic flux;
f is the current frequency;
ω is the number of turns in the winding.
The permissible load power that a single-phase transformer "pulls" is set by the cross-section of the wire that the coils are wound and the quality factor 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 term paper in technical universities.
In any case, the more powerful the single-phase voltage transformers, the more impressive their size. On their case, most often there is a label listing the main parameters (allowable current input and output voltages). However, this is not always the case.
In practice, many repairmen often face the need to replace a burnt single-phase voltage transformer. In order to verify suitability, the characteristics of the replacement device should be examined.
The first thing to do is determine the input winding. With step-down transformers, it has the greatest resistance.
Then, by plugging it into the network, you can measure the output voltage in idle mode. The ratio of the input and output EMF is the transformation coefficient K. It is also equal to the fraction N I / N N, that is, the number of turns in the windings.
After that, it is possible to connect a powerful alternating 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 drops.
Transformers are not only power, but also measuring. In cases where it is necessary to determine a significant amount of current in the circuit, an ammeter is used. It is connected in series, and should 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, they use single-phase current transformers that take proportionally reduced values and feed them to ordinary serial ammeters. It is not difficult to calculate the amperage, it remains only to apply the factors indicated on the case.