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Mar 7 2019

Structure, Function and Parameter of Transformer

Warm hints: This article contains about 3500 words and reading time is about 20 mins.


Regarding the transformer, I think everyone knows it better. There are two types of transformers, one is buck-boost and the other is impedance matching. Let's talk about the buck-boost. Usually we use a variety of voltages, such as 220V for living lighting and 36V for industrial safety lighting. The voltage of the welding machine needs to be adjusted. These are inseparable from the transformer. The transformer is electromagnetically driven through the main and auxiliary coils. The principle of mutual inductance can reduce the voltage to the voltage we need. In the long-distance voltage transmission process, we need to raise the voltage to a high level to reduce the voltage loss, usually rising to several thousand volts or even several tens of kilovolts, which is the role of the transformer. Impedance matching: The most common is in the electronic circuit, in the connection between the output and the input, in order to make the signal unobstructed, usually the transformer is used for impedance matching, such as the old-fashioned broadcast, because the constant voltage output is used, the speakers are all high-resistance speakers. Therefore, it can only be matched with an output transformer. Therefore, daily life is inseparable from transformers, and industrial production is inseparable from transformers.

What is a Transformer And How Do They Work?

Article Core



Introduce what structure, function and parameters of transformer are.


Semiconductor industry.






ⅠStructure of Transformer

1.1 Dry-type Transformer Structure

1.2 Oil-immersed Transformer Structure

1.3 The Main Components and Functions of the Transformer

II The Role of the Transformer

2.1 DC Transformer Function

III Transformer Characteristic Parameters

IV Transformer Use

4.1 Voltage Transformation

4.2 Impedance Transformation

4.3 Phase Transformation


Ⅴ Common Transformers in Life and Their Applications

5.1 IF Transformer

5.2 Power Transformer

5.3 Switching Power Transformer

5.4 Input and Output Transformers

5.5 line Output Transformer

ⅠStructure of Transformer

The transformer consists of two basic parts, the core and the winding, as shown in the figure below, which is its schematic diagram and symbol.

structure diagram and symbol of transformer

structure diagram and symbol of transformer

This is a simple two-winding transformer with two windings on a closed core, insulated between the windings and the windings and between the windings and the core.

The windings are usually wound with insulated copper or aluminum wires. One of the windings is connected to the power supply and is called the primary winding. The other winding is connected to the load and is called the secondary winding.

In order to reduce the hysteresis loss and eddy current loss in the iron core, the iron core of the transformer is mostly laminated with a silicon steel sheet of 0.35-0.5 mm thick. In order to reduce the magnetic resistance of the magnetic circuit, a staggered stacking method is generally adopted, that is, each layer of silicon steel sheet is used. The seams are staggered. The following figures show several common core shapes.

several common core shapesseveral common core shapes

several common core shapes

The transformer can be divided into two types, a core type and a shell type, according to the combination of the iron core and the winding, as shown in the following figure.

structural forms of transformers

The core of the core transformer is surrounded by windings, which use less iron and are used for large-capacity transformers, such as power transformers.

The winding of the shell-type transformer is surrounded by a core lock. It uses a lot of iron, but does not require a special transformer casing. It is often used in small-capacity transformers, such as transformers in various electronic equipment and instruments.

1.1 Dry-type Transformer Structure

The dry type transformer is mainly composed of a core composed of silicon steel sheets and a coil made of epoxy resin. An insulating cylinder is placed between the high and low voltage coils to increase electrical insulation, and the coil is supported and restrained by the spacer, and the fasteners of the parts are overlapped. Both have anti-loosening properties.

Construction performance: (1) solid insulation enveloping winding (2) does not enclose winding.

Winding: Among the two windings, the higher voltage is the high voltage winding and the lower is the low voltage winding. From the relative positions of the high and low voltage windings, the high voltage can be divided into concentric and overlapping types. Concentric windings are simple and easy to manufacture, all using this structure. The overlapping type is mainly used for special transformers.

1.2 Oil-immersed Transformer Structure

The main structure of the oil-immersed transformer is composed of a body, a fuel tank, a cooling device, a protection device and an outlet device. The body includes a core, a winding (winding), an insulation, a lead and a tap changer; the fuel tank includes a fuel tank body and a fuel tank accessory (a drain valve, a ground screw, a trolley, a nameplate, etc.); the cooling device includes a radiator and a cooler; The protection device comprises an oil storage cabinet, an oil standard, a safety air passage, a moisture absorber, a temperature measuring element and a gas relay; the outlet device comprises a high and low pressure sleeve.

1.3 The Main Components and Functions of the Transformer

(1) Iron core: The iron core of the transformer is the path of the magnetic flux line, which concentrates and strengthens the magnetic flux and is used to support the winding.

(2) Winding: The winding of the transformer is the path of current, the current is passed through the winding, and the induced electromotive force is generated by electromagnetic induction.

(3) Fuel tank: The fuel tank is the outer casing of the oil-immersed transformer. The main body of the transformer is placed in the fuel tank, and the tank is filled with transformer oil.

(4) Oil pillow: The oil pillow is also called auxiliary oil tank. It is a round barrel container made of steel plate. It is horizontally installed on the transformer fuel tank cap. It is connected with the fuel tank by a curved connecting pipe. The oil pillow is equipped with oil level indication at one end. The volume of the oil pillow is generally 8% to 10% of the volume of the oil contained in the transformer tank. Its function is that the inside of the transformer is full of oil, and because the oil level in the oil pillow is within a certain limit, there is room for manoeuvre when the oil expands and contracts at different temperatures, and the position of the air pillow is small, so that the oil and air are in contact with each other. The possibility of oil moisture and oxidation is reduced. In addition, the oil in the oil conservator is much lower than the oil in the upper part of the tank, and hardly convects the oil in the tank. A gas relay is installed on the connecting pipe of the oil pillow and the fuel tank to reflect the internal fault of the transformer.

(5) Respirator: The respirator contains a desiccant, silica gel, to absorb moisture from the air.

(6) Explosion-proof pipe: The explosion-proof pipe is installed on the fuel tank cap of the transformer. The top end of the explosion-proof tube is equipped with a piece of glass. When a fault occurs inside the transformer, a high pressure is generated, and the gas inside the oil breaks through the glass sheet and is discharged to the outside of the tank to release the pressure, thereby protecting the transformer tank from being damaged.

(7) Thermometer: The thermometer is installed in the side temperature cylinder on the fuel tank cap to measure the temperature of the upper oil in the fuel tank.

(8) Bushing: The bushing is an insulating device that leads the leads of the high and low voltage windings of the transformer to the outside of the tank. It is both the insulation of the lead to the ground (the outer casing) and the role of the fixed lead.

(9) Cooling device: The cooling device is a device that dissipates the heat generated by the transformer during operation.

(10) Oil purifier: also known as temperature difference filter. Its main part is a cylindrical net oil tank welded by steel plate, which is installed on one side of the transformer tank. The tank is filled with adsorbent such as silica gel and activated alumina. In operation, due to the temperature difference between the upper oil and the lower oil, the transformer oil flows from top to bottom through the oil purifier to form convection, and the oil is in contact with the adsorbent, wherein the water, acid and oxide are absorbed, so that the oil Get purified. Extend the life of the oil. The oil purifier of the strong oil circulation transformer relies on the oil flow pressure difference to make the transformer oil flow through the net oil pump to achieve the purpose of purification.

II The Role of the Transformer

a. Ensure the safety of electricity consumption and meet the needs of various electric appliances.

b. Use a transformer to lower the high pressure.

c. The transformer also has the function of transforming current.

d. The transformer also has the function of transforming the impedance.

2.1 DC Transformer Function

The role of DC transformers is mainly in the following aspects:

First: Raise the DC voltage so that the voltage is more suitable for the voltage required by the electronic device.

Second: DC transformers can boost voltage when the current voltage is too small to meet the needs of electronic equipment.

Third: It also plays a vital role in the coupling of signals.

Fourth: DC transformer can replace the size of the resistance.

Fifth: DC transformers also have the function of isolation.

III Transformer Characteristic Parameters

Working Frequency

Transformer core loss has a large relationship with frequency, so it should be designed and used according to the frequency of use. This frequency is called the working frequency.

Rated Power

At a specified frequency and voltage, the transformer can operate for a long period of time without exceeding the output power of the specified temperature rise.

Rated Voltage

Refers to the voltage that is allowed to be applied to the coil of the transformer and must not be greater than the specified value during operation.

Voltage Ratio

Refers to the ratio of the primary voltage to the secondary voltage of the transformer, which has the difference between the no-load voltage ratio and the load-to-voltage ratio.

No-load Current

When the secondary of the transformer is open, there is still a certain current in the primary. This part of the current is called no-load current. The no-load current consists of magnetizing current (generating magnetic flux) and iron loss current (caused by core loss). For a 50 Hz power transformer, the no-load current is substantially equal to the magnetizing current.

Load Loss

Refers to the power loss measured at the primary when the transformer is open secondary. The main loss is the core loss, followed by the loss (copper loss) of the no-load current on the primary coil copper resistance, which is small.


Refers to the percentage of the ratio of the secondary power P2 to the primary power P1. Generally, the higher the rated power of the transformer, the higher the efficiency.

Insulation Resistance

It indicates the insulation performance between the coils of the transformer and between the coils and the iron core. The insulation resistance is related to the performance of the insulating material used, the temperature and the degree of humidity.

IV Transformer Use

4.1 Voltage Transformation

A transformer is a component that converts voltage. In practical applications, we often need to change the voltage of the AC to adapt to different needs. For example, large generators emit tens of thousands of volts of AC voltage; long-distance transmissions require hundreds of thousands of volts; household appliances such as electric lights, rice cookers, and computers require 220V; lamps on machine tools require 220V; The above lighting requires a safe voltage of 36V; the filament of a general tube only needs a low voltage of 6.3V, etc., which requires a power transformer to obtain a suitable voltage.

The power transformer consists of a closed core and two coils (also called windings) wound around the core. One coil is connected to the power source, called the primary coil (also called the primary coil); the other coil is connected to the load, called the secondary coil (also called the secondary coil). Both coils are wound with insulated wires, and the core is laminated with silicon steel sheets coated with insulating varnish. It can be roughly assumed that, at no load, the ratio of the terminal voltage U1 of the terminal of the transformer primary coil to the terminal voltage U2 of the secondary coil is equal to the ratio of the number of turns N1 of the primary coil to the number of turns N2 of the secondary coil, that is, U1/U2= (N1/N1) 2. When the number of turns of the secondary coil is greater than the number of turns of the primary coil, the transformer raises the voltage. This transformer is called a step-up transformer; when the number of turns of the secondary coil is smaller than the number of turns of the primary coil, the transformer lowers the voltage. A transformer is called a step-down transformer.

When the transformer is operating, the currents in the primary and secondary coils are inversely proportional to their turns. The high-voltage coil of the transformer has a large number of turns and a small current can be wound, and the wire can be wound with a thin wire; the low-voltage coil has a small number of turns and a large current, and should be wound with a thick wire.

4.2 Impedance Transformation

The transformer has an impedance transformation effect. The primary and secondary turns of the transformer are different, and the impedance from the secondary to the primary is also different. The ratio of the secondary impedance Z2 of the transformer to the primary impedance Z1 is numerically equal to the square of the ratio of the secondary winding number N2 of the transformer to the primary winding number N1, that is, Z2/Z1=(N2/N1)2. By using the impedance transformation of the transformer, the impedance at both ends of the circuit can be well matched to maximize the signal power. For example, the 300Ω balanced TV signal output by the TV antenna is converted into a 75Ω unbalanced signal by an antenna impedance converter (a high-frequency transformer) and transmitted to the TV set to match the input impedance of the tuner for amplification. . For example, through the impedance transformation of the audio output transformer, the higher output impedance of the audio amplifier can be matched with the lower impedance of the speaker, so that the output power of the audio amplifier is maximized and the distortion is minimized.

4.3 Phase Transformation

The transformer has a phase change function, and the junction of the transformer coil can change the phase of the signal voltage. For example, using an audio input transformer in a push-pull power amplifier, the original signal and the inverted signal can be distributed to two power amplifier tubes, causing them to alternately amplify the positive and negative half-cycle signals.

Ⅴ Common Transformers in Life and Their Applications

5.1 IF Transformer

Open the back cover of the tape recorder, you can see a few silver-white square metal shells on the circuit board with a "one" slot on the circuit board. You can use a non-inductive screwdriver to gently rotate, there are red, white, green and other colors, this is IF transformer. The intermediate frequency transformer used in the transistor radio has two types of single tuning loop and double tuning loop.

The IF transformer is mainly used for frequency selection and interstage coupling in the radio. Frequency selection refers to the selection of useful signal frequencies among many signals and the transmission of useful signals. In the AM radio, the 465 kHz useful signal is selected by the IF transformer. And coupled to the next level to zoom in. At the same time, the signal outside 465 kHz is suppressed so that it cannot be transmitted to the next stage.

IF Transformer

5.2 Power Transformer

In everyday life, the voltages used by various household appliances are different. Household appliances are all operated with a low-voltage DC power supply. The power transformer is used to convert 220 V AC mains into low-voltage AC, and then through diode rectification and capacitor filtering to form a DC power supply. Moreover, in this transformation process, its own energy loss is small, thus achieving convenient and economical purposes. TV picture tubes require tens of thousands of volts to operate and are supplied by a "row output transformer."

In view of the wide variety of electronic devices, the required voltage volts and the required power delivered are different, and the installation location is as small as the space inside the machine. The type and shape of the transformer are also different.

Power Transformer

5.3 Switching Power Transformer

The switching power supply transformer is an important component in the color TV switching power supply. It is a kind of pulse transformer. Its role is to carry out power transmission, to provide the required power supply voltage for the color TV set and to achieve reliable electrical isolation of the input and output.

Switching Power Transformer

5.4 Input and Output Transformers

The main functions of the input and output transformers are: to achieve impedance matching; to transmit signal voltage and signal power without distortion; to isolate from DC.

The purpose of the input and output transformers is that the output transformer couples the output power of the transistor's final stage power amplifier to the speaker, matching the optimal load of the power amplifier tube to the voice coil impedance of the speaker. In the recorder and TV set, the pot coupling of the power tube and the speaker is also inseparable from it. The input transformer is typically a coupling transformer between the last stage and the last stage of the low frequency circuit.

5.5 line Output Transformer

(1) Black and white TV line output transformer.

The line output transformer is referred to as FBT, and is sometimes referred to as a line reverse transformer. Black and white TV line output transformers are mainly "separated" and "integrated". Separation is now rarely used.

The integrated line output transformer is composed of high-low voltage coils, high-voltage rectifier tubes, plastic skeletons, high-voltage coils and outer casings, which are packaged together with non-toxic, non-corrosive, high-temperature resistant and high-insulation chemical materials.

(2) Color TV line output transformer (FDT).

Color TV sets use a multi-stage primary boost line output transformer. The high-voltage winding of the line output transformer is divided into multiple sections, and high-voltage rectifier diodes are respectively connected between the sections, and the output DC super-high voltage is generated by multi-stage rectification and then connected in series, so it is called Multi-stage primary boost mode. Since the high voltage winding, low voltage winding and high voltage rectifying diode of the line output transformer are all packaged in one housing, they are often referred to as an integrated line output transformer.

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