Function and principle of transformer
Transformer is a kind of electrical equipment used for electric energy conversion. It is an essential and important device in the power grid. It can convert the alternating current energy of one voltage and current into another of the same frequency. Transformer is used in almost all electronic products. Its principle is simple, but according to different application (different purposes), the winding process of transformer will have different requirements. The main functions of transformer are voltage conversion, impedance transformation, isolation and voltage stabilization (magnetic saturation transformer), etc. the common core shapes of transformer are E-type and C-type core.
In different environments, the uses of transformers are also different, such as:
1. Increasing the voltage during long distance transmission can reduce the power loss on the line
2. Reducing the voltage when reaching the area of use can meet the power demand of different users
3. Transformer connection can change impedance when matching impedance
4. The use of isolation transformer can isolate the two-phase electricity and prevent the occurrence of electric shock
The classification of commonly used transformers can be summarized as follows:
1. By phase number:
1) Single phase transformer: used for single phase load and three phase transformer group.
2) Three phase transformer: used for voltage rise and drop of three-phase system.
2. According to cooling mode:
1) Dry type transformer: rely on air convection for natural cooling or increase fan cooling, mostly used in high-rise buildings, high-speed toll stations, local lighting, electronic circuits and other small capacity transformers.
2) Oil immersed transformer: depending on oil as cooling medium, such as oil immersed self cooling, oil immersed air cooling, oil immersed water cooling, forced oil circulation, etc.
Working principle of transformer
The transformer is composed of iron core (or magnetic core) and coil. The coil has two or more windings. The winding connected to power supply is called primary coil, and the other winding is called secondary coil. It can transform AC voltage, current and impedance. *A simple core transformer consists of a core made of soft magnetic material and two coils with different turns on the core, as shown in the figure: [picture] the role of the core is to strengthen the magnetic coupling between the two coils. In order to reduce the eddy current and hysteresis loss in the iron, the core is made of coated silicon steel sheets; there is no electrical connection between the two coils, and the coils are made of insulated copper wire (or aluminum wire). One coil connected with AC power is called primary coil (or primary coil), and the other coil connected with electrical appliance is called secondary coil (or secondary coil). The actual transformer is very complex, there are inevitably copper loss (coil resistance heating), iron loss (core heating) and magnetic leakage (magnetic induction line closed by air), etc. in order to simplify the discussion, this paper only introduces the ideal transformer. The conditions for an ideal transformer are: neglecting the flux leakage, the resistance of the primary and secondary coils, the loss of the core and the no-load current (the current in the primary coil with the secondary coil open). For example, when the power transformer operates at full load (rated power output of secondary coil), it is close to the ideal transformer.
Transformer is a static electrical appliance which is made of electromagnetic induction principle. When the original coil of the transformer is connected to the AC power supply, the alternating magnetic flux will be produced in the iron core, which is represented by φ. In the primary and secondary coils, φ is the same, and φ is also a simple harmonic function, which is shown as φ = φ MSIN ω t. According to Faraday's law of electromagnetic induction, the induced electromotive forces in primary and secondary coils are E1 = - n1d φ / dt and E2 = - n2d φ / dt. N1 and N2 are primary coils. It can be seen from the figure that U1 = - E1, U2 = E2 (physical quantity of original coil is represented by subscript 1, physical quantity of secondary coil is represented by subscript 2), and its complex effective value is U1 = - E1 = JN1 ω Φ, U2 = E2 = - jn2 ω Φ, let k = N1 / N2, and the transformer ratio is called. From the above formula, U1 / U2 = - N1 / N2 = - K, that is, the ratio of the effective value of the voltage of the primary and secondary coils of the transformer is equal to its turn ratio, and the phase difference of the voltage between the primary and secondary coils is π.
It is concluded that:
When the no-load current can be ignored, there is I1 / I2 = - N2 / N1, that is, the effective value of primary and secondary coil current is inversely proportional to the number of turns, and the phase difference is π.
Then we can get
The power of primary and secondary coils of ideal transformer is equal, P1 = P2. The ideal transformer itself has no power loss. The actual transformer always has loss, and its efficiency is η = P2 / P1. The efficiency of power transformer is very high, up to 90%.