Basic Knowledge of Transformers
Transformers are utilized in almost all electronic products. Their principle is simple, but the winding process varies according to different usage scenarios (different purposes). The main functions of transformers include voltage transformation, impedance transformation, isolation, voltage stabilization (magnetic saturation transformers), etc. The commonly used core shapes for transformers are generally E-type and C-type cores.
1. Basic Principle of Transformers
When a sinusoidal AC voltage U1 is applied across the primary coil, an alternating current I1 flows in the wire and generates an alternating magnetic flux ф1, which flows along the core through the primary and secondary coils to form a closed magnetic circuit. A mutual inductance voltage U2 is induced in the secondary coil, and ф1 also induces a self-inductance voltage E1 in the primary coil. The direction of E1 is opposite to that of the applied voltage U1, but the amplitude is similar, thus limiting the magnitude of I1. To maintain the presence of the magnetic flux ф1, a certain amount of electrical energy consumption is required, and the transformer itself also has certain losses. Even when the secondary is not loaded, there is still a certain current in the primary coil, which we refer to as the "no-load current". If the secondary is loaded, the secondary coil generates a current I2, which in turn produces a magnetic flux ф2. The direction of ф2 is opposite to that of ф1, and they cancel each other out, reducing the total magnetic flux in the core. This results in a decrease in the primary self-inductance voltage E1, leading to an increase in I1. It can be seen that the primary current is closely related to the secondary load. When the secondary load current increases, I1 increases, and ф1 also increases. The increased part of ф1 exactly compensates for the magnetic flux cancelled by ф2, maintaining the total magnetic flux in the core unchanged. If the losses of the transformer are not considered, it can be assumed that the power consumed by an ideal transformer secondary load is equivalent to the electrical power obtained by the primary from the power source. Transformers can change the secondary voltage by changing the number of turns of the secondary coil as needed, but they cannot change the power allowed for load consumption.
2. Losses of Transformers
When the primary winding of a transformer is energized, the magnetic flux generated by the coil flows in the core. Because the core itself is also a conductor, an electric potential is induced on a plane perpendicular to the magnetic field lines. This electric potential forms a closed loop on the cross-section of the core and generates a current, resembling a vortex, hence the term "eddy current". This "eddy current" increases the loss of the transformer and causes the transformer core to heat up, leading to an increase in the transformer's temperature rise. The loss generated by the "eddy current" is called "iron loss". Additionally, winding a transformer requires a large amount of copper wire, which has resistance. When current flows through, this resistance consumes a certain amount of power, and this part of the loss often manifests as heat, which we refer to as "copper loss". Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss.
Due to the existence of iron loss and copper loss in transformers, their output power is less than their input power. To describe this, we introduce an efficiency parameter, η=output power/input power.
III. Materials for Transformers
To wind a transformer, we must have a certain understanding of the materials related to transformers. Therefore, I will introduce some knowledge in this regard.
1. Core Materials:
The core materials used in transformers mainly include iron sheets, low silicon iron sheets, and high silicon iron sheets. Adding silicon to steel sheets can reduce their conductivity and increase their resistivity, which can reduce eddy currents and thus decrease losses. We usually refer to steel sheets with added silicon as silicon steel sheets. The quality of silicon steel sheets used in transformers has a significant impact on the transformer's performance. The quality of silicon steel sheets is usually represented by magnetic flux density B. Generally, the B value for black iron sheets is 6000-8000, for low silicon iron sheets is 9000-11000, and for high silicon iron sheets is 12000-16000.
2. Materials commonly used for winding transformers include
enamelled wire, sandbag wire, and silk-covered wire. The requirements for conductive wires are good conductivity, sufficient heat resistance in the insulating paint layer, and a certain degree of corrosion resistance. Generally, Q2 high-strength polyester enamelled wire is preferred.
3. Insulating Materials
In winding transformers, insulating materials are used for the isolation between coil frame layers and between windings. Generally, transformer frame materials can be made of phenolic paperboard, and polyester film or telephone paper can be used for isolation between layers. Yellow wax cloth can be used for isolation between windings.
4. Impregnation material:
After the transformer winding is completed, it goes through another process, which is the impregnation with insulating varnish. This process enhances the mechanical strength of the transformer, improves insulation performance, and prolongs its service life. Generally, cresol varnish can be used as the impregnation material.