News Category

Electrical knowledge

How the transformer works?

Views : 145
Update time : 2020-03-12 15:31:00

Transformer is the use of electromagnetic induction principle to change the AC voltage device, the main components are the primary coil, the secondary coil and the core (core). The main functions are: voltage conversion, current conversion, impedance transformation, isolation, voltage regulator (magnetic saturation transformer) and so on. Can be divided according to use: Power transformers and special transformers (EAF, rectifier, power frequency test transformers, voltage regulators, mine variable, audio transformers, IF transformers, high frequency transformers, impact transformers, instrument transformers, electronic transformers , Reactor, transformer, etc.). Circuit symbols commonly used T as the beginning of the number. Example: T01, T201 and so on.

working principle

Transformer consists of iron core (or core) and coil, the coil has two or more windings, which is connected to the power supply called the primary winding coil, the other called the secondary winding coil. It can transform AC voltage, current and impedance. The simplest core transformer consists of a core made of a soft magnetic material and two coils of different turns wound around the core, as shown in the figure.








The role of the core is to strengthen the magnetic coupling between the two coils. To reduce eddy currents and hysteresis loss in the iron, the core is made by lamination of painted silicon steel sheets; there is no electrical connection between the two coils, and the coil is made of insulated copper (or aluminum) wire. A coil connected to the AC power supply is called the primary coil (or the primary coil) and the other coil is called the secondary coil (or secondary coil). The actual transformer is very complex, there is inevitably copper loss (coil resistance fever), iron loss (core fever) and magnetic flux leakage (air-closed magnetic induction line), etc., in order to simplify the discussion here to introduce the ideal transformer. Ideal transformer is established conditions are: ignore flux leakage, ignoring the original, the resistance of the secondary coil, ignoring the core loss, ignoring the no-load current (the secondary coil coil in the coil of the coil current). For example, the power transformer in full load operation (auxiliary coil output rated power) that is close to the ideal transformer situation.
Transformer is the use of electromagnetic induction principle made of static appliances. When the transformer primary coil connected to the AC power supply, the core will produce alternating flux, alternating flux φ said. The original, vice coil φ is the same, φ is also a simple harmonic function, the table is φ = φmsinωt. According to Faraday's law of electromagnetic induction, the induced electromotive force in the original coil and the auxiliary coil is e1 = -N1dφ / dt and e2 = -N2dφ / dt. Where N1, N2 is the original, the number of turns of the secondary coil. The figure shows that U1 = -e1 and U2 = e2 (the physical quantity of the primary coil is denoted by the lower subscript 1 and the physical quantity of the secondary coil is denoted by the lower subscript 2). The complex valid values ​​are U1 = -E1 = jN1ωΦ and U2 = E2 = -jN2ωΦ, Let k = N1 / N2, said the transformer ratio. U1 / U2 = -N1 / N2 = -k can be obtained from the above equation, that is, the ratio of the effective voltage of the primary and secondary windings of the transformer is equal to the ratio of turns and the phase difference between the primary and secondary windings is π.
Then come to:
U1 / U2 = N1 / N2
In the no-load current can be ignored, I1 / I2 = -N2 / N1, that is, the primary and secondary coil current RMS value is inversely proportional to the number of turns, and the phase difference π.
And then available
I1 / I2 = N2 / N1
Ideal transformer original, the secondary coil power is equal P1 = P2. Describe the ideal transformer itself without power loss. The actual transformer total loss, its efficiency η = P2 / P1. Power transformer high efficiency, up to 90%.

The main classification

General classification of the transformer can be summarized as follows
1, according to the number of points:
1) Single-phase transformers: for single-phase load and three-phase transformer group.
2) three-phase transformer: for three-phase system, the voltage drop.

2, according to the cooling method points:
1) Dry-type transformers: rely on air convection to naturally cool or increase fan cooling, used for high-rise buildings, high-speed toll site electricity and local lighting, electronic circuits and other small-capacity transformers.
2) Oil-immersed transformers: relying on oil as cooling medium, such as oil immersed cold, oil-air-cooled, oil-immersed water-cooled, forced oil circulation.

3, according to the use of points:
1) Power Transformer: used for power transmission and distribution system, down voltage.
2) instrument transformers: such as voltage transformers, current transformers, for measuring instruments and relay protection devices.
3) Test transformer: can produce high voltage, high voltage test of electrical equipment.
4) Special Transformers: such as electric furnace transformers, rectifier transformers, adjusting transformers, capacitive transformers, phase transformers and so on.

4, according to the winding form points:
1) Double winding transformer: Used to connect two voltage levels in the power system.
2) three-winding transformer: generally used in power system substation, connecting three voltage levels.
3) Autotransformer: used to connect different voltage power system. Can also be used as a normal boost or drop transformer.

5, according to the core form points:
1) Core transformer: Power transformer for high voltage.
2) amorphous alloy transformer: amorphous alloy core transformer is a new type of magnetic material, no-load current decreased by about 80%, is the energy-saving effect of the distribution transformer is more ideal, especially for rural power grids and developing regions such as load rate Lower place.
3) Shell transformers: special transformers for high current, such as electric furnace transformers, welding transformers; or for power transformers for electronic equipment, televisions and radios.

Characteristic Parameters

working frequency
Transformer core loss and the frequency of a great relationship, it should be based on the use of frequency to design and use, this frequency is called the operating frequency.

rated power
In the provisions of the frequency and voltage, the transformer can work long hours without exceeding the specified temperature rise of the output power.

Rated voltage
Refers to the transformer in the coil allows the applied voltage, the work shall not exceed the specified value.

Voltage ratio
Refers to the transformer primary voltage and secondary voltage ratio, the no-load voltage ratio and load voltage ratio difference.

No-load current
Transformer secondary open, the primary still have some current, this part of the current is called the no-load current. No-load current by the magnetizing current (to produce flux) and iron loss current (caused by the core loss) composition. For 50Hz power transformers, the no-load current is essentially equal to the magnetizing current.

Load loss
Refers to the transformer secondary open circuit, measured in the primary power loss. The main loss is the core loss, followed by no-load current in the primary copper resistance loss (copper loss), this part of the loss is small.

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

Insulation resistance
Said the transformer between the coils, the insulation between the coil and the core performance. The level of insulation resistance depends on the performance of the insulation material used, the temperature and humidity level.