Basic knowledge of transformers

Basic knowledge of transformers

Understand transformers
Transformers play the role of power conversion and communication in power transmission and distribution systems. For example, power plants need to transmit power in a high-voltage manner (345KV, 161KV, 69KV, etc.) during power transmission and distribution. When approaching the user end, the voltage is gradually reduced to 110V. , these procedures must be accomplished by transformers; many electrical appliances used daily also have transformers inside them, or are equipped with external transformers to convert the 110V AC power supply into an appropriate voltage value, which can then be rectified and filtered circuit to obtain a low-voltage DC working power supply; some products have switched to switching power supply (switch power) to replace the traditional coil-type transformer. This type of transformer has the advantages of high efficiency, no magnetic leakage, and low noise, and is gradually used in applications such as computers. Wait for the device. This article will introduce the basic principles of general small traditional coil-type transformers, and explain common issues related to use and purchase.
1. Voltage Principle
Coil-type transformer was first produced by the Hungarian Ganz Company in 1885. It has been more than a century since. It has been widely used for a long time and has been improved in many ways. Different types have been designed, but its principle is still the same. of. This transformer uses the principle of electric energy and magnetic energy conversion induction to wind two sets of coils on a common "core", as shown in Figure 1. The one connected to the power end is called the "primary coil" or "primary coil" )", the one connected to the load end is called "secondary coil" or "secondary coil".
When the primary coil is connected to an AC power source, the current passing through the coil will produce a magnetic flux change in the core, and the secondary coil at the other end will generate another alternating current of the same frequency due to the induced electromotive force (emf). Equation (1) "Faraday's Law" explains the relationship between induced electromotive force, magnetic flux and the number of coil turns, where ε is the induced electromotive force, N is the number of coil turns, and Φ is the magnetic flux. Under ideal circumstances, the magnetic flux BBΦ of each turn of the coil is the same, so dtdBΦ is also the same, so the induced voltage is proportional to the number of coil turns (formula (2)). If the number of secondary side coils (N2) is greater than the primary side coil Number (N1), then the transformer is a "step-up transformer", otherwise it is a "step-down transformer".

Figure 1. Schematic diagram of transformer principle
2. Selection of iron core
There are also different forms of iron core design. The basic requirement of the iron core material is that it must have a large "relative permeability (Km)", that is, a low magnetic resistance. Common core materials include heat-treated and annealed soft iron and silicon steel sheets. These materials usually have a magnetic permeability nearly ten thousand times higher than that of air, and can produce more magnetic lines of force than other materials. For example, if an air-core coil can produce a magnetic line of force, it means that the same coil is on a silicon steel sheet. It can produce about 10,000 magnetic lines.
The structure of the iron core can be classified into "outer iron type" and "inner iron type" based on appearance, as shown in Figure 2. Figures 3 and 4 illustrate the current common core forms. Figure 3(d) shows the "EI type core". Although the EI type core has high magnetic leakage and low efficiency, it is economical, convenient and easy to manufacture, and is currently the most common core form. Figure 3(e) shows the "C-shaped core (cut core)". The right side of Figure 4 shows the "R-core core" improved from the C-shaped core. The cross-section of the core is similar to a circle to facilitate winding of coils. , and the circular cross-section can avoid the problem that the magnetic field lines of the general square core cross-section are easy to concentrate on the four right-angled ends, causing uneven distribution of the magnetic field lines, which may cause local saturation of the magnetic force. Figures 3(a), 3(b), and 3(c) show the "toroidal core". Transformers using toroidal cores are the transformers with the best electrical characteristics at present, with high efficiency, low magnetic leakage and transient response. It’s fast, but the production process is cumbersome and the cost is high.

Figure 4. The design principles of core structures of current main transformer types
are to save costs and reduce joints of silicon steel sheets. Silicon steel sheet cores are processed by punching out the window, and most of the remaining material punched out cannot be reused, so proper design can reduce the remaining material.

Materials are an important consideration in manufacturing costs. In addition, as mentioned earlier, the magnetic permeability of silicon steel sheets is much greater than that of air. Therefore, if there are too many seams in the core structure, the electromagnetic conversion efficiency will be reduced. The most common EI type core has many different seam shapes (Figure 5). In addition to reducing seams, staggering the seams of each silicon steel sheet can also enhance the magnetic permeability (Figure 6).

3. Transformer losses
In theory, when the conversion efficiency of the transformer is 100%, the input power of the primary coil side is the same as the output power of the secondary coil side. However, in fact, it is impossible for all the magnetic lines of force generated by the excitation to be confined to the core. Coupled with other internal losses, the conversion efficiency is bound to decrease. Generally, the losses of transformers can be divided into two categories: "iron losses" and "copper losses". The former has nothing to do with the load, so it is also called "no load loss", while the latter is related to the size of the load, called "load loss" , respectively explained as follows.
(1) No load loss
Due to the internal resistance of the coil, the excitation current of the primary coil will cause internal loss (P=I2R), but this current is extremely small and can usually be ignored. Furthermore, different materials of the core silicon steel sheets and changes in voltage frequency will cause "hysteresis loss". The higher the voltage frequency, the greater the hysteresis loss. The widely used method to reduce the hysteresis effect is to use silicon steel sheet cores containing about 3% silicon. On the other hand, materials with high magnetic permeability usually have low magnetic flux saturation density, but it can be achieved by adding silicon. Elements increase the magnetic flux saturation density.

Another very important transformer energy loss is "Eddy Current loss". Since the iron core itself is also a conductor, when the iron core passes through the magnetic field lines, a current loop will be formed on the section perpendicular to the magnetic field lines in the iron core, as shown in the figure. As shown in 7(a), eddy current will also cause internal loss of P=I2R and turn into heat energy. The phenomenon of eddy current cannot be completely solved, but to reduce the influence of eddy current, the current method is as shown in Figure 7(b). The core is stacked with mutually insulated sheets (refer to Figure 6). Each sheet is only about 0.2 mm to 0.35mm, the thinner the thickness of the silicon steel sheet, the smaller the eddy current. In addition, adding silicon elements to the core as mentioned in the above description can not only increase the magnetic flux saturation density, but also reduce the conductivity of the core. That is, reducing eddy currents.

Figure 7. Eddy current and chip stacked core
(2) Load loss
Load loss includes two parts: "resistance loss" and "drift loss". Resistance loss is caused by the resistance of the coil itself. The greater the current at the load end, the greater the resistance loss. The resistance loss of the winding accounts for nearly 50% of the copper loss of the transformer. Therefore, the main consideration to reduce copper loss is to increase the cross-sectional area of ​​the conductor and Reduce the number of coil turns. Carefully selecting better copper materials and using copper wires with thicker diameters can also reduce resistance. In addition, if you use a core material with extremely high magnetic permeability or use a seamless wound core, you can reduce the number of coil turns and reduce copper losses. purpose.
Wandering loss is mainly caused by magnetic leakage. All induced magnetic force lines cannot be confined in the core. Therefore, magnetic leakage will generate eddy currents in the winding conductors, causing losses. The way to reduce wandering loss is to install a shielding mask. This In addition to reducing the impact of magnetic leakage on the coil, it can also reduce the interference of the transformer on external electronic parts or circuits. Shielding masks are commonly made of copper and aluminum. Among them, copper has a better shielding effect, but is more expensive.
Transformer noise is another common problem, known as "hum". The humming sound is caused by the alternating magnetic force changes that produce slight vibrations on the silicon steel sheet. This vibration frequency is the same as the AC voltage. If the power supply contains harmonic components, it will cause noise amplification. Several methods to solve the hum of the transformer are as follows:
(1) Reduce the magnetic flux density
of the core. Reducing the magnetic flux density of the core can reduce the hum of the transformer. However, this method requires increasing the cross-sectional area of ​​the core, which greatly increases the weight of the transformer and increases the cost. And the problem of reduced applicability.
(2) Use silicon steel sheets with small magnetic distortion.
Use silicon steel sheets with high magnetic permeability and high directionality, such as HI-B type or ZDKH type silicon steel sheets.
(3) Improve core joints and assembly.
In addition to reducing joints, transformer noise can also be improved by changing the core stacking method to ladder overlap. In addition, during the core assembly process, care should be taken not to cause local stress in the core, and the binding force of each part should be uniform.
(4) Adhesive vacuum impregnation.
This method is the best noise reduction method for small transformers. Put the entire assembled transformer into a varnish tank for vacuum impregnation. For R-core and toroidal transformers, this is the best method. Noise can be reduced to a very low level. A more rigorous approach is to perform a vacuum impregnation after the core assembly is completed, and then perform another vacuum impregnation after the waterlogging assembly is completed.

A transformer is a device that converts AC voltage, current and impedance. When AC current flows through the primary coil, AC magnetic flux is generated in the iron core (or magnetic core), causing voltage (or current) to be induced in the secondary coil. A transformer consists of an iron core (or magnetic core) and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary coil, and the remaining windings are called secondary coils.

1. Classification
According to the cooling method: dry-type (self-cooling) transformer, oil-immersed (self-cooling) transformer, fluoride (evaporative cooling) transformer.
Classification according to moisture-proof method: open transformer, potted transformer, sealed transformer.
Classified by core or coil structure: core-type transformers (insert core, C-type iron core, ferrite core), shell-type transformers (insert core, C-type iron core, ferrite core), Toroidal transformer, metal foil transformer.
Classification according to the number of power supply phases: single-phase transformer, three-phase transformer, multi-phase transformer.
Classified by use: power transformer, voltage regulating transformer, audio transformer, medium frequency transformer, high frequency transformer, pulse transformer.

2. Characteristic parameters of power transformer
1. Operating frequency

The core loss of the transformer has a great relationship with the frequency, so it should be designed and used according to the frequency of use. This frequency is called the operating frequency.

2. Rated power

Under the specified frequency and voltage, the transformer can work for a long time without exceeding the output power of the specified temperature rise.
3. The rated voltage
refers to the voltage allowed to be applied to the coil of the transformer, which shall not exceed the specified value during operation.
4. Voltage ratio
refers to the ratio between the primary voltage and the secondary voltage of the transformer. There is a difference between no-load voltage ratio and load voltage ratio.
5.
When the secondary of the no-load current transformer is open circuit, there is still a certain current in the primary. This part of the current is called no-load current. No-load current consists of magnetizing current (generating magnetic flux) and iron loss current (caused by core losses). For a 50Hz power transformer, the no-load current is basically equal to the magnetizing current.
6. No-load loss
refers to the power loss measured on the primary side when the secondary side of the transformer is open-circuited. The main loss is the core loss, followed by the loss (copper loss) caused by the no-load current on the primary coil copper resistance. This part of the loss is very small.
7. Efficiency
refers to the percentage of the ratio of secondary power P2 to primary power P1. Generally, the greater the power rating of the transformer, the higher the efficiency.
8. Insulation resistance
indicates the insulation performance between the coils of the transformer and between each coil and the iron core. The insulation resistance is related to the performance of the insulating material used, temperature and humidity.

3. Characteristic parameters of audio transformers and high-frequency transformers
1. Frequency response
refers to the characteristics of the secondary output voltage of the transformer changing with the operating frequency.
2. Passband:
If the output voltage of the transformer at the intermediate frequency is U0, the frequency range when the output voltage (the input voltage remains unchanged) drops to 0.707U0 is called the passband B of the transformer.
3. Primary and secondary impedance ratio.
The primary and secondary impedances of the transformer are connected to appropriate impedances Ro and Ri, so that the primary and secondary impedances of the transformer match. Then the ratio of Ro and Ri is called the primary and secondary impedance ratio. In the case of impedance matching

4. Losses of the transformer
When the primary winding of the transformer is energized, the magnetic flux generated by the coil flows in the iron core. Because the iron core itself is also a conductor, an electric potential will be induced on a plane perpendicular to the magnetic lines of force. This electric potential forms a closure on the cross section of the iron core. The circuit creates an electric current, which looks like 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's core heating transformer. The loss caused by "eddy current" is called "iron loss". In addition, a large amount of copper wire is needed to wind the transformer. These copper wires have resistance. When the current flows through, the resistance will consume a certain amount of power. This loss is often consumed as heat. We call this loss "copper loss" . Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss.
Since the transformer has iron loss and copper loss, its output power is always less than the input power. For this reason, we introduce an efficiency parameter to describe this, eta = output power/input power.

5. Transformer Materials
To wind a transformer, we must have a certain understanding of the materials related to the transformer. For this reason, I will introduce this knowledge here.

1. Core material:
The core materials used in transformers mainly include iron sheets, low silicon sheets, and high silicon sheets. Adding silicon to steel sheets can reduce the conductivity of the steel sheets and increase the resistivity. It can reduce eddy currents and reduce losses. . We usually call the steel sheet with silicon added as silicon steel sheet. The quality of the transformer has a great relationship with the quality of the silicon steel sheet used. The quality of the silicon steel sheet is usually expressed by the magnetic flux density B. Generally, the B value of the black iron sheet is 6000-8000, low silicon wafer is 9000-11000, high silicon wafer is 12000-16000.

2. The materials commonly used for winding transformers include
enameled wire, sand-covered wire, silk-covered wire, and the most commonly used enameled wire. The requirements for wires are good electrical conductivity, sufficient heat resistance of the insulating paint layer, and certain corrosion resistance. Under normal circumstances, it is best to use Q2 model high-strength polyester enameled wire.

3. Insulating materials
In winding transformers, insulating materials must be used for isolation between coil frame layers and windings. Generally, transformer frame materials can be made of phenolic cardboard, and polyester film or telephone paper can be used for isolation between layers. , yellow wax cloth can be used to isolate the windings.

4. Impregnated material:
After the transformer is wound, the last step is to impregnate the insulating paint, which can enhance the mechanical strength of the transformer, improve the insulation performance, and extend the service life. Under normal circumstances, cresol varnish can be used as the impregnated material. Material.