Losses of A Transformer During Operation

Iron Loss

When the primary winding of the transformer is energized, the magnetic flux generated by the coil flows in the iron core. Since the iron core itself is a conductor, an electromotive force will be induced in the plane perpendicular to the magnetic field lines. This electromotive force forms a closed - loop circuit on the cross - section of the iron core and generates an electric current. Because its shape is like a vortex, it is called an "eddy current". The "eddy current" increases the loss of the transformer, causes the iron core of the transformer to heat up, and further raises the temperature of the transformer. The loss caused by the "eddy current" is called "iron loss". In addition, iron loss also includes hysteresis loss. When an alternating current passes through the transformer, the direction and magnitude of the magnetic field lines passing through the silicon steel sheets of the transformer change accordingly, causing the internal molecules of the silicon steel sheets to rub against each other, releasing heat energy and thus consuming a part of the electrical energy. This is the hysteresis loss.

Copper Loss

A large amount of copper wire is needed to wind the transformer. These copper wires have resistance. When current flows through them, the resistance will consume a certain amount of power, and this part of the loss is often dissipated in the form of heat. We call this kind of loss "copper loss". Copper loss is mainly the loss generated when the resistance of the primary and secondary windings of the transformer passes current. It is proportional to the square of the current, and its magnitude depends on the size of the transformer load and the power factor.

Since the transformer has iron loss and copper loss, its output power is always less than the input power. Therefore, we introduce an efficiency parameter to describe this situation, that is, η = output power / input power.

Introduction to Materials for Winding Transformers

To wind a transformer, we must have a certain understanding of the materials related to the transformer. The following is an introduction to this knowledge:

1. Core Material

The core material used in transformers is usually iron sheets with silicon added. This can reduce the conductivity of the steel sheets and increase the resistivity, thereby reducing the eddy current and the loss. We usually call the steel sheets with silicon added silicon steel sheets. The quality of the transformer is closely related to the quality of the silicon steel sheets used. The quality of silicon steel sheets is usually represented by the magnetic flux density B. Generally, the B value of black iron sheets is 6000 - 8000, that of low - silicon sheets is 9000 - 11000, and that of high - silicon sheets is 12000 - 16000. To reduce the eddy - current loss caused by the change of magnetic flux, the iron core of the transformer should be made of silicon steel sheets with a thickness of 0.35 - 0.5mm stacked together, and the sheets are separated by insulating paint.

2. Commonly Used Conductive Materials for Winding Transformers

The commonly used materials for winding transformers include enameled wire, yarn - wrapped wire, silk - wrapped wire, and paper - wrapped wire. Among them, enameled wire is the most commonly used. The requirements for the wire are good electrical conductivity, sufficient heat - resistant performance of the insulating paint layer, and a certain corrosion - resistant ability. In general, it is recommended to use high - strength polyester enameled wire of the QZ type.

3. Insulating Materials

In the process of winding a transformer, insulating materials are required for the isolation between the coil frames and between the windings. Generally, the frame material of the transformer can be made of phenolic cardboard, epoxy board, or cardboard. Polyester film, telephone paper, 6520 composite paper, etc. can be used for the inter - layer isolation, and yellow wax cloth or imide film can be used for the isolation between windings.

4. Impregnating Materials

After the transformer is wound, there is a final process, which is to impregnate it with insulating paint. It can enhance the mechanical strength of the transformer, improve the insulation performance, and extend the service life. Generally, cresol varnish can be used as the impregnating material, and 1032 insulating paint, resin paint, etc. can also be selected.