Key issues in the design process of power transformer

Is the transformer proportional to the current?

The basic issues of transformer design are magnetic flux and current density. The current of the transformer is proportional to the capacity, and the size of the current density (i.e. the thickness of the wire) is considered according to the heat generated by the conductor. For magnetic flux, the basic relationship in electromagnetism is u=4.44fwΦ, where u is voltage, f is frequency, 50Hz here, w is the number of turns of the coil; Φ is magnetic flux. Since the magnetic flux density B of silicon steel sheets is limited by the material, it can generally only be designed to 1.4-1.8 Tesla, and Φ=BS, so to increase Φ, generally only the cross-sectional area of ​​the core can be increased. The core of the transformer is generally a three-phase column type. The cross-sectional area of ​​the core can be determined according to the above formula, and the size of the core window should be based on the principle of putting the coil in. The larger the capacity of the transformer, the thicker the wire, and the larger the core window needs to be.

In the design of the transformer, the amount of copper and iron can be considered in a balanced way. Because once the capacity of the transformer is determined, the current is determined, and the thickness of the wire is determined. If the number of turns W is increased, the magnetic flux Φ can be smaller, and the cross-sectional area of ​​the core can be slightly smaller, but to wind these turns in, the window of the core must be larger; on the contrary, if the number of turns W is reduced, the magnetic flux Φ must be larger, the cross-sectional area of ​​the core must be larger, but the window of the core can be smaller.

What does the capacity of the transformer have to do with?

From the analysis of the above problem, it can be seen that the choice of iron core is related to voltage, while the choice of wire is related to current, that is, the thickness of the wire is directly related to the heat generation. In other words, the capacity of the transformer is only related to the heat generation. For a well-designed transformer, if it works in an environment with poor heat dissipation, if it is 1000KVA, if the heat dissipation capacity is enhanced, it may work at 1250KVA. In addition, the nominal capacity of the transformer is also related to the allowable temperature rise. For example, if a 1000KVA transformer allows a temperature rise of 100K, if it can be allowed to work at 120K under special circumstances, its capacity is more than 1000KVA. It can also be seen from this that if the heat dissipation conditions of the transformer are improved, its nominal capacity can be increased. Conversely, for the same capacity inverter, the volume of the transformer cabinet can be reduced.

Therefore, in some bidding processes, competitors deliberately specify a larger transformer capacity to give users the illusion that they have a larger design margin. This is actually meaningless. The key lies in the size of the transformer and the heat dissipation method, which is also very important.

Why does a current source inverter require a larger transformer capacity?

The design of transformers generally only considers the rated capacity, not the rated power, because the current is only related to the rated capacity. For voltage source inverters, since their input power factor is close to 1, the rated capacity is almost equal to the rated power. This is not the case with current source inverters. The power factor of the input side transformer is at most equal to the power factor of the load asynchronous motor. Therefore, for the same load motor, its rated capacity is larger than that of the transformer of the voltage source inverter.