Transformers are almost used in all electronic products. Their principle is simple, but the winding process of transformers may have different requirements according to different usage scenarios (different purposes). The main functions of transformers include voltage conversion, impedance conversion, isolation, voltage stabilization (magnetic saturation transformers), etc. The commonly used iron core shapes for transformers are generally E-type and C-type.
1、 The basic principle of transformers:
It is a simplified diagram of the principle of a transformer. When a sinusoidal AC voltage U1 is applied to both ends of the primary coil, there is an alternating current I1 in the wire, which generates alternating magnetic flux ф 1. It passes through the primary and secondary coils along the iron core to form a closed magnetic circuit. Induction of mutual inductance potential U2 in the secondary coil, while ф 1 will also induce a self induced potential E1 on the primary coil, whose direction is opposite to the applied voltage U1 but with a similar amplitude, thereby limiting the size of I1. To maintain magnetic flux ф The existence of 1 requires a certain amount of electrical energy consumption, and the transformer itself also has certain losses. Although the secondary is not connected to the load at this time, there is still a certain amount of current in the primary coil, which is called no-load current. If the secondary is connected to a load, the secondary coil generates current I2, which in turn generates magnetic flux ф 2, ф The direction of 2 is related to ф On the contrary, it counteracts each other, reducing the total magnetic flux in the iron core, resulting in a decrease in the primary self induced voltage E1 and an increase in I1, indicating a close relationship between the primary current and the secondary load. When the secondary load current increases, I1 increases, ф 1 also increases, and ф The addition of part 1 perfectly supplements the information provided by ф The portion of magnetic flux offset by 2 is used to maintain the total magnetic flux in the iron core unchanged. If the loss of the transformer is not taken into account, it can be considered that the power consumed by the secondary load of an ideal transformer is the electrical power obtained from the primary power source. Transformers can change the secondary voltage by changing the number of turns of the secondary coil as needed, but cannot change the allowed power consumption of the load.
2、 Transformer losses:
When the primary winding of a transformer is energized, the magnetic flux generated by the coil flows through the iron core. As the iron core itself is a conductor, an electromotive force is induced in a plane perpendicular to the magnetic field line. This electromotive force forms a closed loop on the cross-section of the iron core and generates current, resembling a vortex, so it is called eddy current. This eddy current increases the loss of the transformer and increases the temperature rise of the transformer due to the heating of the iron core. The loss caused by eddy currents is called iron loss. In addition, winding transformers requires a large amount of copper wire, which has resistance. When current flows through these copper wires, the resistance will consume a certain amount of power, and this loss is often converted into heat and consumed. We call this loss copper loss. So the temperature rise of transformers is mainly caused by iron loss and copper loss. Transformers have iron and copper losses, so their output power is always lower than the input power. Therefore, we introduce an efficiency parameter to describe this, η= Output power/input power.
3、 Material of transformer:
1. Iron core materials: The iron core materials used in transformers mainly include iron sheets, low silicon sheets, and high silicon sheets. Adding silicon to the steel sheets can reduce their conductivity, increase their resistivity, reduce eddy currents, and reduce their losses. We usually refer to steel sheets with added silicon as silicon steel sheets. The quality of the silicon steel sheets used in transformers is closely related, and the quality of silicon steel sheets is usually represented by magnetic flux density B. Generally, the B value of black iron sheets is 6000-8000, low silicon sheets are 9000-11000, and high silicon sheets are 12000-16000,
2. The materials commonly used for winding transformers include enameled wire, sand wrapped wire, silk wrapped wire, and the most commonly used enameled wire. The requirements for wires are good conductivity, sufficient heat resistance of the insulation paint layer, and certain corrosion resistance. In general, it is best to use high-strength polyester enameled wire of Q2 model.
3. In winding transformers, insulation materials must be used for the isolation between the coil frame layers and the winding resistance. Generally, the transformer frame material can be made of phenolic cardboard, the layers can be isolated with polyester film or telephone paper, and the winding resistance can be isolated with yellow wax cloth.
4. Impregnation material: After the transformer is wound, the final process is to impregnate insulation paint, which can enhance the mechanical strength of the transformer, improve insulation performance, and extend its service life. Generally, cresol varnish can be used as the impregnation material.
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